Tele-Dave
Active member
jim p said:
Please Do. looks like a good mod.
Crate V5 mods... anyone?
- Thread starter Tarin
- Start date
Please Do. looks like a good mod.
Step by step modifications done to Crate V5 amp
Step by step modifications done to Crate V5 amp
1) Remove the amplifier from cabinet. First remove speaker jack then the six screws holding back cover of amplifier then with amplifier upside down remove four screws two on each side of cabinet that hold amplifier chassis in cabinet. (Note: you may want to enlarge the holes slightly in sides of cabinet and on back cover to make reinstallation and future removal easier)
2) If you haven’t already done so (you do have this unplugged right?) get out a dc voltmeter the highest voltage in the amplifier is approx. 320 volts DC measure from chassis to R25 should be 0 to 15 volts if so you are safe. This amplifier has a bleeder resistor R25 so supplies should drop after power down.
3) Remove preamp (12AX7) and power tube (EL84 6BQ5) put them some where you wont break them.
4) Next remove knobs on front panel may require flat blade screwdriver and a thin piece of wood to work against to prevent scratching front panel. The pots are D shaft type walk then off gently. Then remove nuts holding pots and ¼ inch jack on front panel.
5) Now make note of all the connections to the PC board because you are going to remove them. If you have a digital camera now is a good time to use it. Also if you print out the schematic note wire color and location on it. The faston connectors J4 J5 J1 and J2 may be hard to remove don’t break them of pcb by working them back and fourth.
6) Next remove the six screws that hold PCB into chassis and take it out. (hope you are putting parts someplace you wont loose them)
PC Board modifications
(See attached pictures of mods and changes to schematic also pic DSCF0947.jpg in previous post is the pre emphasis mod the tone mod is incorrect in that post tie unused side of pot in tone control to wiper not ground or unconnected up to you)
1) You are going to be removing approx 10 parts. You will need solder, solder wick, soldering iron, needle nose pliers and desoldering tool. In manufacture the compnet leads have been bent so desolder a componet lead then straighten it before removing or you may damage PCB plated feedthrough.
2) This modification will use the two Op Amp stages for an input buffer and small gain to over come loss due to pre emphasis circuit that will be added. Desolder and remove volume pot from PCB. Next remove R13 this makes IC1-A a unity gain buffer. Remove C26, C24 and C27 replace C24 with a 47nF 50-100 volt poly cap.C24 sets the low frequency limit at input to the amplifier with 47nF this is approx 125 HZ 3db down (for more bass use 100nF). In the place of C27 install a 33k ¼ watt resistor this equals a gain of 1.22 for IC1-B (want to keep gain low so first stage tube will clip before the Op Amp with overdrive).
ERROR Value of C24 should be 100nF lowest frequency from guitar is 82Hz.
3) Pre emphasis circuit For the tone control to be able to cut, boost and be set for flat response the high frequencies need to be boosted first. So C25 needs to be removed if you want replaced or use it again for this mod. Install one lead of a 100nF 50-100 volt cap into plated hole (via) that is connected to pin 7 of IC1 in the second hole of C25 insert lead of 33k ohm ¼ watt resistor with a 10nf capacitor accost it then connect the other lead of the 33k resistor to the second lead of the 100nF capacitor. If you want full treble brighter you can lower value of R2
4) Tone stack and volume control Here we will put volume control after tone stack and first tube stage so you will be able to overdrive input tube and still control volume. Remove C5 C6 C28 and C4 Change C6 to a 22nF 50-100volt poly capacitor. (Here a lower value cap will raise where the tone control will cut the highs higher value will lower it.) Now the volume control remove R15 and R27 using the volume pot removed from the board you will need to bend the leads 90 degrees towards the back of the pot to attach 1 ½ inch wires to the three terminals of the pot. Also need to insulate PCB were pot was attached to prevent shorting (three layers of electrical tape works) looking at the front of the pot as use to be connected to the PCB the right side terminal connects to the plated hole C28 that connects to R29 the center terminal (wiper) connects to the plated hole of R27 that connects to R7 and control grid (pin 2) of the 12AX7. The last terminal goes to the plated hole of R27 that goes to ground. (note be careful the leads soldered to pot do not short to metal back shell of pot no sound out of amp)
5) Oscillating power tube in testing the amplifier found output did not clip symmetrically and oscillated when clipped. Based on the tube data sheets the cathode biasing is to high in value. Change R17 to 270-300 ohms 2-5 watt or parallel the 330 5 watt with another 1.5k ohm 2 watt (270 ohms)
6) That’s it now put it back together and be careful around high voltage always follow one hand rule working on tube amplifiers. The one hand rule is make sure other hand and body are not tied to ground other hand does all the probing. So tie meter or scope ground to the chassis ground then do all your measurements.
Other tweaks
From the way this amplifier is set up it may have had gain, tone and volume controls?
And they took the volume control out instead of the gain control so if you add a third 250k pot across TL01-B same as in the stock schematic this will be a gain control you should add a 27k resistor in series to set minimum gain at unity (1).
Lowest frequency amplified the stock value of C24 is to low 3db down at approx 270Hz the value in the mod is 47nF for 125Hz if you want you can go bigger 100nF but this may increase that annoying 60 Hz hum we have all come to love.
ERROR Should go with 100nF lowest frequency from guitar is 82Hz
On the pre emphasis mod you can change the value of R2 to increase or decrease the brightness of the maximum tone. Higher value for R2 (stock 100k) will reduce the maximum lower value increase it. If you lower the value you may want to increase the gain in the Op Amp stage to offset the loss.
Tone control pot from my results the tone is flat at the 9 o’clock position so if you use a 100k (125K) pot for the tone control this will move it to the 12 o’clock position.
Changing where the tone control operates you can reduce the value of C6 to raise or lower where the tone control operates but you should change the value of the cap in the pre emphasis circuit also (10nF across 33k ohm).
Step by step modifications done to Crate V5 amp
1) Remove the amplifier from cabinet. First remove speaker jack then the six screws holding back cover of amplifier then with amplifier upside down remove four screws two on each side of cabinet that hold amplifier chassis in cabinet. (Note: you may want to enlarge the holes slightly in sides of cabinet and on back cover to make reinstallation and future removal easier)
2) If you haven’t already done so (you do have this unplugged right?) get out a dc voltmeter the highest voltage in the amplifier is approx. 320 volts DC measure from chassis to R25 should be 0 to 15 volts if so you are safe. This amplifier has a bleeder resistor R25 so supplies should drop after power down.
3) Remove preamp (12AX7) and power tube (EL84 6BQ5) put them some where you wont break them.
4) Next remove knobs on front panel may require flat blade screwdriver and a thin piece of wood to work against to prevent scratching front panel. The pots are D shaft type walk then off gently. Then remove nuts holding pots and ¼ inch jack on front panel.
5) Now make note of all the connections to the PC board because you are going to remove them. If you have a digital camera now is a good time to use it. Also if you print out the schematic note wire color and location on it. The faston connectors J4 J5 J1 and J2 may be hard to remove don’t break them of pcb by working them back and fourth.
6) Next remove the six screws that hold PCB into chassis and take it out. (hope you are putting parts someplace you wont loose them)
PC Board modifications
(See attached pictures of mods and changes to schematic also pic DSCF0947.jpg in previous post is the pre emphasis mod the tone mod is incorrect in that post tie unused side of pot in tone control to wiper not ground or unconnected up to you)
1) You are going to be removing approx 10 parts. You will need solder, solder wick, soldering iron, needle nose pliers and desoldering tool. In manufacture the compnet leads have been bent so desolder a componet lead then straighten it before removing or you may damage PCB plated feedthrough.
2) This modification will use the two Op Amp stages for an input buffer and small gain to over come loss due to pre emphasis circuit that will be added. Desolder and remove volume pot from PCB. Next remove R13 this makes IC1-A a unity gain buffer. Remove C26, C24 and C27 replace C24 with a 47nF 50-100 volt poly cap.C24 sets the low frequency limit at input to the amplifier with 47nF this is approx 125 HZ 3db down (for more bass use 100nF). In the place of C27 install a 33k ¼ watt resistor this equals a gain of 1.22 for IC1-B (want to keep gain low so first stage tube will clip before the Op Amp with overdrive).
ERROR Value of C24 should be 100nF lowest frequency from guitar is 82Hz.
3) Pre emphasis circuit For the tone control to be able to cut, boost and be set for flat response the high frequencies need to be boosted first. So C25 needs to be removed if you want replaced or use it again for this mod. Install one lead of a 100nF 50-100 volt cap into plated hole (via) that is connected to pin 7 of IC1 in the second hole of C25 insert lead of 33k ohm ¼ watt resistor with a 10nf capacitor accost it then connect the other lead of the 33k resistor to the second lead of the 100nF capacitor. If you want full treble brighter you can lower value of R2
4) Tone stack and volume control Here we will put volume control after tone stack and first tube stage so you will be able to overdrive input tube and still control volume. Remove C5 C6 C28 and C4 Change C6 to a 22nF 50-100volt poly capacitor. (Here a lower value cap will raise where the tone control will cut the highs higher value will lower it.) Now the volume control remove R15 and R27 using the volume pot removed from the board you will need to bend the leads 90 degrees towards the back of the pot to attach 1 ½ inch wires to the three terminals of the pot. Also need to insulate PCB were pot was attached to prevent shorting (three layers of electrical tape works) looking at the front of the pot as use to be connected to the PCB the right side terminal connects to the plated hole C28 that connects to R29 the center terminal (wiper) connects to the plated hole of R27 that connects to R7 and control grid (pin 2) of the 12AX7. The last terminal goes to the plated hole of R27 that goes to ground. (note be careful the leads soldered to pot do not short to metal back shell of pot no sound out of amp)
5) Oscillating power tube in testing the amplifier found output did not clip symmetrically and oscillated when clipped. Based on the tube data sheets the cathode biasing is to high in value. Change R17 to 270-300 ohms 2-5 watt or parallel the 330 5 watt with another 1.5k ohm 2 watt (270 ohms)
6) That’s it now put it back together and be careful around high voltage always follow one hand rule working on tube amplifiers. The one hand rule is make sure other hand and body are not tied to ground other hand does all the probing. So tie meter or scope ground to the chassis ground then do all your measurements.
Other tweaks
From the way this amplifier is set up it may have had gain, tone and volume controls?
And they took the volume control out instead of the gain control so if you add a third 250k pot across TL01-B same as in the stock schematic this will be a gain control you should add a 27k resistor in series to set minimum gain at unity (1).
Lowest frequency amplified the stock value of C24 is to low 3db down at approx 270Hz the value in the mod is 47nF for 125Hz if you want you can go bigger 100nF but this may increase that annoying 60 Hz hum we have all come to love.
ERROR Should go with 100nF lowest frequency from guitar is 82Hz
On the pre emphasis mod you can change the value of R2 to increase or decrease the brightness of the maximum tone. Higher value for R2 (stock 100k) will reduce the maximum lower value increase it. If you lower the value you may want to increase the gain in the Op Amp stage to offset the loss.
Tone control pot from my results the tone is flat at the 9 o’clock position so if you use a 100k (125K) pot for the tone control this will move it to the 12 o’clock position.
Changing where the tone control operates you can reduce the value of C6 to raise or lower where the tone control operates but you should change the value of the cap in the pre emphasis circuit also (10nF across 33k ohm).
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tunghaichuan
Well-known member
Cool, thanks for posting that. The Best Buy near me has these for $100, I may have to pick one up and try the mods.
Have you tried adding additional capacitance to kill some of the 60hz hum?
tung
Have you tried adding additional capacitance to kill some of the 60hz hum?
tung
jim p said:
the Amplifier is quiet
When I was talking about 60 Hz hum I meant pick up from the guitar not in the amplifier. The amplifier has 1 H choke pi filter and DC filament voltage on the 12AX7 its quiet compared to the Valve Jr combo I have.
When I was talking about 60 Hz hum I meant pick up from the guitar not in the amplifier. The amplifier has 1 H choke pi filter and DC filament voltage on the 12AX7 its quiet compared to the Valve Jr combo I have.
crate V5 with reverb pentode/triode sw Webber spk
Posted pictures of close to final modification of Crate V5 I bought. The vector board is the reverb drive and recovery amplifiers. Replaced the stock (crap) speaker with a Webber ceramic 10S for 38 bucks with shipping. The reverb tank is a Accutronics box with Ruby OEM guts (2 spring) from a Crate V33H. On the back pannel is the reverb pot and pentode/triode switch. May do a better job of running the RCA lines to reverb in future. I see the Crate V5 is 85 bucks at Best Buy dot com with in store pick up so the shipping is free.
Posted pictures of close to final modification of Crate V5 I bought. The vector board is the reverb drive and recovery amplifiers. Replaced the stock (crap) speaker with a Webber ceramic 10S for 38 bucks with shipping. The reverb tank is a Accutronics box with Ruby OEM guts (2 spring) from a Crate V33H. On the back pannel is the reverb pot and pentode/triode switch. May do a better job of running the RCA lines to reverb in future. I see the Crate V5 is 85 bucks at Best Buy dot com with in store pick up so the shipping is free.
Tele-Dave
Active member
jim p said:
Can you post schematic changes and step by step for these mods?
Question on previous mods. In step 2 of board mods where you remove C26 C24 and C27, C24 and C27 are replaced with other components but C26 is not. Is this left open or is it jumpered? If not jumpered then I assume it is O.K. to remove R28 since it is not connected to anything?
Also I want to have Gain and Volume setup. Can I just leave existing pot connected in its place on board and use another pot for the volume mod such as you have it? I understand that I will have to lift a leg on pot or cut trace on board to put in 27k resistor in series with pot. Is this right?
Thnx---Dave
re: R28
Yes if you want you can remove R28 without C26 it has no function. If you want gain and volume CW1 the 250k pot accost TL01-B can be used as a gain pot. In that case do not install a resistor in the location of C27. You may to set a minimum gain by putting a 10k resistor in series with the 250k gain pot. If you search the web or call Crate for a VC508 schematic this will give you an idea of the gain, tone and volume set-up. I don't like the values of capacitance they used for the 508 also the low supply rails on the op-amp (+7 -7). This amp is better in the fact you have +/- 15 volts (more within limits if you reduce R19 and R20) for the op-amp a 1 Hennery choke in the plate supply and DC filament supply on the 12AX7. Bad thing is they took out the wrong pot to remove the gain function did not change bandwidth for a 10 inch speaker (more bass) and used a crappy speaker.
Yes if you want you can remove R28 without C26 it has no function. If you want gain and volume CW1 the 250k pot accost TL01-B can be used as a gain pot. In that case do not install a resistor in the location of C27. You may to set a minimum gain by putting a 10k resistor in series with the 250k gain pot. If you search the web or call Crate for a VC508 schematic this will give you an idea of the gain, tone and volume set-up. I don't like the values of capacitance they used for the 508 also the low supply rails on the op-amp (+7 -7). This amp is better in the fact you have +/- 15 volts (more within limits if you reduce R19 and R20) for the op-amp a 1 Hennery choke in the plate supply and DC filament supply on the 12AX7. Bad thing is they took out the wrong pot to remove the gain function did not change bandwidth for a 10 inch speaker (more bass) and used a crappy speaker.
Tele-Dave
Active member
jim p said:
So no resistor in place of C27. Do I remove C27 or leave it in place?
Also in step 5 you change R17 to 270ohm 5W, but in tung's post next looks like it was changed to 180ohm 5W. Which one is correct?
Thnx----Dave
Tele-Dave
Active member
Jim, please excuse all of my questions, I'm quite new at this. Want to do the mods, but I want to understand everything that is being done so I will learn something, instead of just replacing things like a robot.
Thnx again,-----Dave
Thnx again,-----Dave
using CW1 as gain pot
If you keep the volume pot as a gain pot you can leave C27 installed, it will just limit the maximum frequency the op amp will amplify at full gain to 13 kHz. Also may prevent possible oscillation of the op amp.
If you keep the volume pot as a gain pot you can leave C27 installed, it will just limit the maximum frequency the op amp will amplify at full gain to 13 kHz. Also may prevent possible oscillation of the op amp.
Tele-Dave
Active member
jim p said:
Thnx, Thats just what I was wondering.
So, if I just leave the existing pot in place, leave C27 in place, And not put the 10k in series with the pot, I can get max gain with good breakup because I'm not limiting gain on second stage of op amp. Without the 10k in series would probably be too much and go into clipping, but all I have to do is not crank gain up so much so as to control it, correct. I just want to limit amount of changes if necessary
What would the effect be if I just leave C26 and R28 in place also? Too much bass injection, Treble limiting into second stage of op amp?
Thnx again
Re: volume pot as gain pot
Let me explain how the gain of an op amp works for an inverting amplifier. The 2nd stage op amp TL074-B is an inverting amplifier with the gain equal to the feedback resistor (CW1 the 250k pot) divided by the input resistor R14. So with the pot set at 250k the gain is equal to 250k divided by 27k equal to -9.3 if you figure that a strummed cord is 1 V rms (2.8 V p to p) the output will be 26 V p to p just within the supply rails of the op amp ( +15/-15). The minimum gain if you do not put a resistor in series with the 250k pot is 0/27k ohms or zero that is why I recommended to put say 10k ohms (27k would give you unity -1) in series with the pot so you wont have to guess where to set the gain pot if you don’t want to use it. Where are going to put the volume pot on the chassis with this mod?
Regarding the value of R17 I retested the amplifier after adding the reverb looking at the cathode current vs voltage across the EL84 found the cathode current to high giving 14 watts on the plate of the tube so I backed off that value and wound up with a slight tweak to the stock value. It looks that the stock value is not so bad. What I do have now is a 150 pf cap in parallel with R7 (I might try to lower this value to 100-50pf) the reverb required this to prevent feedback and this may have cured the oscillation problem I saw before.
Let me explain how the gain of an op amp works for an inverting amplifier. The 2nd stage op amp TL074-B is an inverting amplifier with the gain equal to the feedback resistor (CW1 the 250k pot) divided by the input resistor R14. So with the pot set at 250k the gain is equal to 250k divided by 27k equal to -9.3 if you figure that a strummed cord is 1 V rms (2.8 V p to p) the output will be 26 V p to p just within the supply rails of the op amp ( +15/-15). The minimum gain if you do not put a resistor in series with the 250k pot is 0/27k ohms or zero that is why I recommended to put say 10k ohms (27k would give you unity -1) in series with the pot so you wont have to guess where to set the gain pot if you don’t want to use it. Where are going to put the volume pot on the chassis with this mod?
Regarding the value of R17 I retested the amplifier after adding the reverb looking at the cathode current vs voltage across the EL84 found the cathode current to high giving 14 watts on the plate of the tube so I backed off that value and wound up with a slight tweak to the stock value. It looks that the stock value is not so bad. What I do have now is a 150 pf cap in parallel with R7 (I might try to lower this value to 100-50pf) the reverb required this to prevent feedback and this may have cured the oscillation problem I saw before.
Tele-Dave
Active member
Jim, I'm going to set up the volume pot just like you have shown it on the schematic. Will mount pot on other side of power switch towards edge of panel. Am going to use shielded wire since it's a long run.
O.K., understand about the op-amp now. (Just learned something-Thnx) Will place a 10k in series with pot.
Will leave value of R17 alone for now.
Am very interested in the Pentode/Triode switch setup
Going to order parts from Mouser tonight for the mods.
Went out today and bought another V5 at Guitar Center to use as a "Control" so I can see how much of a difference this makes. GC had them on sale for $85, plus I had a 10% off coupon, so I got it for $77. Very good price!!
Thnx-----Dave
O.K., understand about the op-amp now. (Just learned something-Thnx) Will place a 10k in series with pot.
Will leave value of R17 alone for now.
Am very interested in the Pentode/Triode switch setup
Going to order parts from Mouser tonight for the mods.
Went out today and bought another V5 at Guitar Center to use as a "Control" so I can see how much of a difference this makes. GC had them on sale for $85, plus I had a 10% off coupon, so I got it for $77. Very good price!!
Thnx-----Dave
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Re: location for volume pot
Looking at the front panel I would consider putting the volume pot where the power indicator lamp is. Then put the power indicator above the power switch. You might be able to get a Knob from Crate tell them one of yours is broken or buy it from them. I don't think shielded cable is necessary for the volume pot you can just twist the ground wire around the other two wires this should be sufficient.
I bought another amp from MF told them Best Buy had them for 85 with in store pick up so they matched the price and shipped for free I saved the sales tax.
You can see the pentode/triode switch in the last pictures I posted on the back panel and inside the cabinet in the picture that shows the reverb vector board.
Looking at the front panel I would consider putting the volume pot where the power indicator lamp is. Then put the power indicator above the power switch. You might be able to get a Knob from Crate tell them one of yours is broken or buy it from them. I don't think shielded cable is necessary for the volume pot you can just twist the ground wire around the other two wires this should be sufficient.
I bought another amp from MF told them Best Buy had them for 85 with in store pick up so they matched the price and shipped for free I saved the sales tax.
You can see the pentode/triode switch in the last pictures I posted on the back panel and inside the cabinet in the picture that shows the reverb vector board.
Tele-Dave
Active member
jim p said:
Great suggstion on pot and light locations. Wasn't thinking about chassis in cabinet. See there is no room for pot with chassis bolted into cab. Glad you caught me on this. Would of drilled holes and then knob would of interfered with cab.--------Thnx
Will call Loud Tech today and see about knobbbbbbs.
Cant distinguish hookup locations for Pent/Tri switch on pic. Cant see wiring too good. Looks like you lifted leg on R18 and wired DPST switch between R18 and Pin 7 on power tube Can you do a simple drawing/diagram showing install points for hookup and switch wiring for Pent/Tri?
Also what is in R23. My board has nothing in it there. Is that a different value used for Pent/Tri setup???? Dont know too much about Pent/Tri Differences. Will google for info so I will understand it better than only that it changes power output of tube.
Jim, you don't know how much I appreciate all your help here. Thnx much!!
----------------Dave
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Tele-Dave
Active member
Just got off of phone with Loud Tech (Crate)
Pots are $1.50 (Volume--- Part designator CW-1)
Knobs are $1.50
Surprisingly they had them in stock!
Ordered 2 of each so will have extra incase I want to mod other V5 I have.
Pots are $1.50 (Volume--- Part designator CW-1)
Knobs are $1.50
Surprisingly they had them in stock!
Ordered 2 of each so will have extra incase I want to mod other V5 I have.
Re Pentode/triode switch
For the pentode/triode switch I lifted R18 where it connects to the HV supply (J4) and hooked this side of the resistor to the wiper of the switch. Connected one contact of the switch to HV supply (plated hole resistor was connected to) this is the pentode connection. The other contact to the plate of the EL84 pin7 for the triode connection. I still want to check operating current and voltages of the tube in triode mode to see that maximum ratings are not exceeded, forgot to do that the last time I had amp opened up. Along with the drop in output power to approx 1/3 that of pentode mode the brightness of the amp also drops. You would need to lower the impedance of the stage driving the EL84 to over come the Miller capacitance to not have this drop in brightness I am going to go as is for now but in future may try a JFET or op amp between the 12AX7 and the EL84. A pentode has little to no Miller effect cannot get a good number on what the capacitance from plate to control grid is on an EL84 as a triode.
With the miller effect in mind you may want to limit the high frequency range of the first stage tube that you will be over driving with the op amp so I suggest putting a 20 to 33pf cap (300 volts minimum rating) from the plate (pin 6) to the grid (pin 7) of the 12AX7 to roll the high frequency off.
That is as great price for the knobs and pots from Crate did they charge shipping?
I called them for the schematic for the VC508 and the V5 is a variation on the 508 but as I said before done the wrong way and with bad values for a 10 inch speaker.
Oh I forgot that other resistor that you see installed, the board has a location for a resistor in parallel with the 1 H choke I installed a 1k ohm resistor there so a transit would not cause a slight oscillation also if the choke opened it is a little safer.
For the pentode/triode switch I lifted R18 where it connects to the HV supply (J4) and hooked this side of the resistor to the wiper of the switch. Connected one contact of the switch to HV supply (plated hole resistor was connected to) this is the pentode connection. The other contact to the plate of the EL84 pin7 for the triode connection. I still want to check operating current and voltages of the tube in triode mode to see that maximum ratings are not exceeded, forgot to do that the last time I had amp opened up. Along with the drop in output power to approx 1/3 that of pentode mode the brightness of the amp also drops. You would need to lower the impedance of the stage driving the EL84 to over come the Miller capacitance to not have this drop in brightness I am going to go as is for now but in future may try a JFET or op amp between the 12AX7 and the EL84. A pentode has little to no Miller effect cannot get a good number on what the capacitance from plate to control grid is on an EL84 as a triode.
With the miller effect in mind you may want to limit the high frequency range of the first stage tube that you will be over driving with the op amp so I suggest putting a 20 to 33pf cap (300 volts minimum rating) from the plate (pin 6) to the grid (pin 7) of the 12AX7 to roll the high frequency off.
That is as great price for the knobs and pots from Crate did they charge shipping?
I called them for the schematic for the VC508 and the V5 is a variation on the 508 but as I said before done the wrong way and with bad values for a 10 inch speaker.
Oh I forgot that other resistor that you see installed, the board has a location for a resistor in parallel with the 1 H choke I installed a 1k ohm resistor there so a transit would not cause a slight oscillation also if the choke opened it is a little safer.
Tele-Dave
Active member
When in triode mode, how dark does it get? It isnt too bad, is it? Darker than amp was before tone mods? It is liveable, isn't it?
Going to do Pent/Tri mod anyway. When you come up with a fix, let me know.
Have to dig thru parts box for cap for Plate/Grid. Will try it if I can find one.
Poly cap? Correct?
Was better price for pots and knobs than I thought it would be. Shipping $6
Forgot to ask for schem for 508. Will call them back in few minutes.
As per R23, circuit looks like 1/2 watt or larger would be o.k., Correct?
Question, I want to put a standby switch in also. Would it be that all I would have to do is to install switch to break power at J1 and J2? Correct?
Thanx Jim----------------Dave
Going to do Pent/Tri mod anyway. When you come up with a fix, let me know.
Have to dig thru parts box for cap for Plate/Grid. Will try it if I can find one.
Poly cap? Correct?
Was better price for pots and knobs than I thought it would be. Shipping $6
Forgot to ask for schem for 508. Will call them back in few minutes.
As per R23, circuit looks like 1/2 watt or larger would be o.k., Correct?
Question, I want to put a standby switch in also. Would it be that all I would have to do is to install switch to break power at J1 and J2? Correct?
Thanx Jim----------------Dave
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Problem with stand by switch
Because of the 1 H choke in the power supply you cannot just put a switch in the HV power supply and expect it to last. In a mechanical analogy a choke is like a flywheel it wants to maintain a constant rate it resists change at start up (building magnetic field) and if interrupted (clasping magnetic field). A choke is what is used to generate a spark on a spark plug in a gas engine. If you put the switch at pin 2 J6 (C15 side) when you open the switch the voltage on the choke will go positive to the point of arcing across the contacts in the switch and at some point the switch will fail. If you put the switch on the other side of L1 (J1 pin1) when you open the switch the voltage on the inductor will go negative but this side will work if you put a reverse biased rectifier diode to ground that can handle a minimum of 150mA and reverse voltage of 400 volts (1N4007 should work). Better is to put diode with a low value resistor in series (50 ohms 1 w) in parallel with the choke (cathode on pin 1 side) it will be forward biased when the choke goes negative with the switch opening. Also consider a bleeder resistor on the C15 side for safety.
Me I don’t believe in standby switches so much I don’t think cathode striping happens at these voltages and currents plus the amp is quiet.
If R23 is the one in parallel with the choke on the PCB 1 to 2 k 1 watt is fine.
For the plate to grid cap minimum 300 volt rating mica or poly is fine.
Because of the 1 H choke in the power supply you cannot just put a switch in the HV power supply and expect it to last. In a mechanical analogy a choke is like a flywheel it wants to maintain a constant rate it resists change at start up (building magnetic field) and if interrupted (clasping magnetic field). A choke is what is used to generate a spark on a spark plug in a gas engine. If you put the switch at pin 2 J6 (C15 side) when you open the switch the voltage on the choke will go positive to the point of arcing across the contacts in the switch and at some point the switch will fail. If you put the switch on the other side of L1 (J1 pin1) when you open the switch the voltage on the inductor will go negative but this side will work if you put a reverse biased rectifier diode to ground that can handle a minimum of 150mA and reverse voltage of 400 volts (1N4007 should work). Better is to put diode with a low value resistor in series (50 ohms 1 w) in parallel with the choke (cathode on pin 1 side) it will be forward biased when the choke goes negative with the switch opening. Also consider a bleeder resistor on the C15 side for safety.
Me I don’t believe in standby switches so much I don’t think cathode striping happens at these voltages and currents plus the amp is quiet.
If R23 is the one in parallel with the choke on the PCB 1 to 2 k 1 watt is fine.
For the plate to grid cap minimum 300 volt rating mica or poly is fine.
Tele-Dave
Active member
jim p said:
Tele-Dave
Active member
jim p said:
That was my concern, and why I thought of puting it there. On some other amp schematics I was checking for switch placement, most of them placed switch on transformer secondary leads prior to any other electronics and if there was a choke farther downstream, and i noticed on some schematics that did not have chokes that they put the switch anywhere downstream prior to feeding plates.
Know about collapsing field in coils, that is why didnt want to break either side of coil, and thought that with R24 biased to ground, would bleed off pulse when field collapses. Should be just like turning off main power switch as far as coil is concerned, or that is what I thought.
Thnx Jim, just wanted to verify. Don't know too much about electronics, but was trained in automotive mechanics when there was Points, Plugs and condensers, (and coils) And understand about a collapsing field in a coil of wire producing high voltage surge.
Called Loud Tech again today 3 times, but message says "Our personel are in Training presently, please call back later." I will keep trying to get thru, would like to have copy of schem just so I can compare to V5.
Cathode bias with pentode/triode switch
Cathode bias with triode mode needs a higher value of cathode bias resistor vs. the pentode mode. On the amplifier I have modified have found that the stock value of 330 ohms is fine for either modes this results in a cathode current of 38mA in either pentode or triode mode. The result is a maximum plate dissipation of 11.3 watts in the triode mode 10.3 watts in pentode mode.
Driving the EL84 with a signal generator have calculated the capacitance from plate to control grid to be approx 2.2pf in triode mode with the Miller effect this becomes approx 30pf this in parallel with the 220pf has a slight effect on the high frequency roll off of the EL84 (20KHz in pentode mode 19KHz in triode mode) Some of the major changes between the two modes is output power 5 watts pentode mode and 1.5 watts in triode mode. Also plate impedance 38k ohms approx for the pentode mode 2k ohms in the triode mode the result the amplifier is more dampened in the triode mode closer to a solid state amplifier sound then in the pentode mode.
Cathode bias with triode mode needs a higher value of cathode bias resistor vs. the pentode mode. On the amplifier I have modified have found that the stock value of 330 ohms is fine for either modes this results in a cathode current of 38mA in either pentode or triode mode. The result is a maximum plate dissipation of 11.3 watts in the triode mode 10.3 watts in pentode mode.
Driving the EL84 with a signal generator have calculated the capacitance from plate to control grid to be approx 2.2pf in triode mode with the Miller effect this becomes approx 30pf this in parallel with the 220pf has a slight effect on the high frequency roll off of the EL84 (20KHz in pentode mode 19KHz in triode mode) Some of the major changes between the two modes is output power 5 watts pentode mode and 1.5 watts in triode mode. Also plate impedance 38k ohms approx for the pentode mode 2k ohms in the triode mode the result the amplifier is more dampened in the triode mode closer to a solid state amplifier sound then in the pentode mode.
Tomko
Member
Ok I now have a tube amp with a new/better speaker. This thing sounds great distorted but I can't get any clean headroom out of it. Can anyone help me? The op-amp circuit is gone. The tone circuit is gone replaced with a Marshall 18 watt tone circuit. The input resistor was changed from 1.5K to 47K. The resistor into the input of the power section was changed from 4.7K to 47K. I'm using a 12AY7 instead of a 12AX7. Still no clean headroom. I think all the distortion is power amp distortion. Can anyone give me some voltages to look for at the pins of the EL84? The bias is stock on the EL84 and I changed it to a Tesla tube. Any ideas?
Thanks for the schematic Tarin!
My first post here folks. . .just wnted to say thanks to Tarin for the hi-res schematic of the V5. I tried calling the Loud Technology toll-free number but they're still out on training or something. . .been that way for several weeks now, eh? Anyway, I got the schem from this group and is greatly appreciated. I have three of those little V5s sitting around here, and I actually like 'em OK out of the box. But I also agree with deafelectro that this may be one of the next big things in little amps, and a great beginning place for a heck of a mod. So, if I have success with anything (or even failure) I'll try to let you all know how it came out. Thanks for a great group. I'm glad to be a member of it. -M
My first post here folks. . .just wnted to say thanks to Tarin for the hi-res schematic of the V5. I tried calling the Loud Technology toll-free number but they're still out on training or something. . .been that way for several weeks now, eh? Anyway, I got the schem from this group and is greatly appreciated. I have three of those little V5s sitting around here, and I actually like 'em OK out of the box. But I also agree with deafelectro that this may be one of the next big things in little amps, and a great beginning place for a heck of a mod. So, if I have success with anything (or even failure) I'll try to let you all know how it came out. Thanks for a great group. I'm glad to be a member of it. -M
re: problem with 12AY7 mod
The most important voltage on the EL84 would be the cathode voltage to calculate the cathode current. With the 330 ohm resistor this should be approx 14 volts max equal to 42mA. Ballpark you would then have 38mA plate current and 4mA screen grid current. Not sure the 12AY7 may be problem free by the data sheets it is calling for a higher plate current (3mA) and a lower value grid resistor 100k ohms (may have high grid leakage current vs. 12AX7). I also noted on one data sheet a higher heater current so check to see that you have 12.6 volts across the heater. What are the cathode, grid and plate voltages for both locations of this triode? Have you tried the Sovetek tubes that were stock in the amplifier?
The most important voltage on the EL84 would be the cathode voltage to calculate the cathode current. With the 330 ohm resistor this should be approx 14 volts max equal to 42mA. Ballpark you would then have 38mA plate current and 4mA screen grid current. Not sure the 12AY7 may be problem free by the data sheets it is calling for a higher plate current (3mA) and a lower value grid resistor 100k ohms (may have high grid leakage current vs. 12AX7). I also noted on one data sheet a higher heater current so check to see that you have 12.6 volts across the heater. What are the cathode, grid and plate voltages for both locations of this triode? Have you tried the Sovetek tubes that were stock in the amplifier?
Cathode bias with 12AY7
Looking further into the data sheets for the 12AX7 and the 12AY7 based on the plate impedance and the plate current vs. plate voltage you need to change the cathode bias resistors when using a 12AY7. The 12AY7 requires 4 to 3 volts at the cathode relative to ground so a resistor value of 2500 to 3300 ohms should be used for both R3 and R8. With R3 and R8 changed you should have approx 3.5 volts at the cathode and with no signal a DC voltage of approx 170 volts at the plate of each triode (pin 6 and pin 1) hope this helps.
Looking further into the data sheets for the 12AX7 and the 12AY7 based on the plate impedance and the plate current vs. plate voltage you need to change the cathode bias resistors when using a 12AY7. The 12AY7 requires 4 to 3 volts at the cathode relative to ground so a resistor value of 2500 to 3300 ohms should be used for both R3 and R8. With R3 and R8 changed you should have approx 3.5 volts at the cathode and with no signal a DC voltage of approx 170 volts at the plate of each triode (pin 6 and pin 1) hope this helps.
6V6GT output tube
My second V5 modification is changing from EL84 to 6V6GT for the output tube. So far with the exception of taking out the 9 pin socket and replacing it with an octal socket the only change is value of cathode resistor R17 to 380 ohms. I am looking into the value of the screen resistor R18 and may increase it to 5 to 10 k ohms. If someone was into a total rebuild with a 6V6 instead of an EL84 you gain 300mA of heater current capability so you could have two 12AX7 tubes and a 6V6GT output tube within the limits of the power transformer. With that you could have input stage, cathode follower driven tone stack, gain stage, volume stage and output tube. Now will start on adding reverb to this amp and maybe a Weber AlNiCo 10S speaker if it will fit.
My second V5 modification is changing from EL84 to 6V6GT for the output tube. So far with the exception of taking out the 9 pin socket and replacing it with an octal socket the only change is value of cathode resistor R17 to 380 ohms. I am looking into the value of the screen resistor R18 and may increase it to 5 to 10 k ohms. If someone was into a total rebuild with a 6V6 instead of an EL84 you gain 300mA of heater current capability so you could have two 12AX7 tubes and a 6V6GT output tube within the limits of the power transformer. With that you could have input stage, cathode follower driven tone stack, gain stage, volume stage and output tube. Now will start on adding reverb to this amp and maybe a Weber AlNiCo 10S speaker if it will fit.
Tomko
Member
Jim P
Thanks for the good suggestions. I will give them a try. Unfortunately, both the vol. and tone pots have failed mechanically- be careful with them. I can't get in touch with Loud Tech. Anyone know where else I can get them? So the amp is down until I get some new pots. I don't think I tried the stock Sovteks after the mod but I believe I tried a different 12AX7. I had the same results- no headroom. I tried a 12AY7 to lower the gain to get more headroom. Maybe it's the Tesla EL84. So I will proceed with the stock Sovteks after I get some new pots. Thanks again.
Thanks for the good suggestions. I will give them a try. Unfortunately, both the vol. and tone pots have failed mechanically- be careful with them. I can't get in touch with Loud Tech. Anyone know where else I can get them? So the amp is down until I get some new pots. I don't think I tried the stock Sovteks after the mod but I believe I tried a different 12AX7. I had the same results- no headroom. I tried a 12AY7 to lower the gain to get more headroom. Maybe it's the Tesla EL84. So I will proceed with the stock Sovteks after I get some new pots. Thanks again.
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rock_mumbles
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No Clean Headroom
Hi Tomko, as far as your lack of clean headroom.
What mods have you done, you said you took out the op amp circuit, changed the 1.5K to a 47K and the 4.7K on the EL84 to a 47K, is that all you have done?
Did you leave R11 (1M) resistor to ground after the input resistor and remove R2 (100K)?
What I did to clean up one of my Valve Juniors was to decrease the value of the resistor to ground between the two triodes of the 12ax7.
Where did you put the tone stack and volume control? Did you put the volume control in place of R7 the 220K to ground into the second triode?
I know a lot of questions...
Hi Tomko, as far as your lack of clean headroom.
What mods have you done, you said you took out the op amp circuit, changed the 1.5K to a 47K and the 4.7K on the EL84 to a 47K, is that all you have done?
Did you leave R11 (1M) resistor to ground after the input resistor and remove R2 (100K)?
What I did to clean up one of my Valve Juniors was to decrease the value of the resistor to ground between the two triodes of the 12ax7.
Where did you put the tone stack and volume control? Did you put the volume control in place of R7 the 220K to ground into the second triode?
I know a lot of questions...
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