Crate V5 mods... anyone?

[stoneattic]

Thanks for pointing out my error on the schematic; I’ll have to fix that. Still confused on the need for the diodes but they can’t hurt except on the no bleeder resistor aspect.
The last part of the post was just commenting because of another post in regard to feedback and master volume together being a problem. From what I gather you are just adding master volume and variable voltage regulator those two together should be no problem. Hope you are having no problems making the modifications. Probably would be good to post pictures of the mod and details for anyone else considering installing the VVR themselves.

I didn’t go in to detail on changing R25 to a short it will give you a filter cap after the VVR. If you look through previous posts you will find that C16 should have been a filter cap for the screen grid voltage so one side of R18 should have been hooked to it. Figure that the EL84 is ok without the lower screen grid voltage and C16 is really serving no purpose so may as well take advantage of it for this mod.

By changing R18 to 4.7k that will lower the screen grid voltage relative to the screen grid current through it. Also the higher value will be safer for the tube if the plate voltage goes lower than the screen grid voltage and the screen grid current rises. The higher value resistor should provide greater negative feedback and limit the maximum current that might pass through the screen grid.

Hope your mod is going well let us know how it works out.

Now I don't feel too bad about not following how the feedback related to what I was doing.

I'm just adding the MV and VVR to your IC removal mod from post 168. The 168 mod and a different speaker REALLY improved this amp. It's actually enjoyable to play now.

I was just digging though my various caps, pots and resistors and of course I don't have what I need so I'm gathering parts right now. I believe 1/2w on the resistors is all I need. On cap C1 there is an electrolytic there now, I really don't know when you use the various caps (elec, ceramic, poly, etc) over another in different applications. I assume I would use an electrolytic since that's what's there already, correct?

I'll post some pics when I finish, although I'm sure they will be embarrassing. I plan on putting the MV where the light is now and either drill a new hole for the VVR, or (I'm leaning toward) moving the switch and use that hole for the VVR.

Thanks for all your help.

 

[jim p]

Re: wattage rating and capacitors

I was just digging though my various caps, pots and resistors and of course I don't have what I need so I'm gathering parts right now. I believe 1/2w on the resistors is all I need. On cap C1 there is an electrolytic there now, I really don't know when you use the various caps (elec, ceramic, poly, etc) over another in different applications. I assume I would use an electrolytic since that's what's there already, correct?

On the wattage rating of the resistors the only one I know off hand that you need is R26 to a 25k you can just get by with a ½ watt there but a 1 watt would be better.

On the construction or type of capacitor film caps are your best then they vary by what they are made of Mylar, polyester, ect. problem being size and cost. You want your better capacitors in the signal path, the coupling capacitors such as C2 and C7. At the lower capacitance values mica caps are good ceramic capacitors capacitance value may change greatly with temperature and they can be microphonic. Electrolytic capacitors are usually just used for power supply filtering and at times if a large value is needed in a signal path such as the nopolar electrolytic capacitors used in crossover networks in speakers.

So if the capacitance value required is less than 1uF and the voltage rating is not too high you could probably find a film capacitor for that location.

Another thing about electrolytic capacitors is they can dry out so you should keep them away from the hotter components in the amplifier. I fixed a JVC TV and the basic cause of the failure was an electrolytic capacitor mounted inside of the u-shaped heat sink for the vertical deflection amplifier. Also good to keep high wattage resistors raised above the PCB for air flow.

 

[stoneattic]

On the wattage rating of the resistors the only one I know off hand that you need is R26 to a 25k you can just get by with a ½ watt there but a 1 watt would be better.

I also need a 1Meg resistor for the master volume, originally a 1/2 watt.

On the construction or type of capacitor film caps are your best then they vary by what they are made of Mylar, polyester, ect. problem being size and cost. You want your better capacitors in the signal path, the coupling capacitors such as C2 and C7. At the lower capacitance values mica caps are good ceramic capacitors capacitance value may change greatly with temperature and they can be microphonic. Electrolytic capacitors are usually just used for power supply filtering and at times if a large value is needed in a signal path such as the nopolar electrolytic capacitors used in crossover networks in speakers.

So if the capacitance value required is less than 1uF and the voltage rating is not too high you could probably find a film capacitor for that location.

I failed to mention that when I did your post 168 omp amp removal I didn't mod the tone control that you mentioned. I wanted to hear what just the omp amp removal sounded like. But now the tone control doesn't do much so I was going to replace C1 with a 200nf as you recommended. C1 is currently a 2.2uf (50v) electrolytic and after a quick check on mouser.com it looks like I might be limited to eletrolytics if I need the 50V.

So if cost it no object which order of film caps would you recommend?

Thanks again!

 

[jim p]

Re: Voltage rating of C1

C1 has no more than two to three volts across it so you do not need a 50 volt capacitor there. A 6 volt or greater voltage rating will be fine. If you need the 1 Meg for R16 ½ or ¼ watt will be fine.

 

[SynySter_SlaSh]

This is the no op amp modification with the op amp removed a continuation of post #160. How you get the op amp out of the board is up to you, basic way is to cut off all pins and remove them one at a time. Also can desolder all the pins and remove op amp in one piece.
Steps involved in modification after removing PCB from chassis

1) Remove op amp IC1, C25, R28, R14, C5, C4, R27, R15 and R2
2) Replace R15 with a jumper
3) Jumper from op amp pin 3 to junction of C25 R2 and R1 (amplifier input jumper)
4) Jumper from pin 7 of op amp to ground via of R27 (black wire in picture)
5) Jumper from op amp pin 6 to junction of R27, R15, R7 and pin2 (grid) of 12AX7 (orange wire in photo near volume pot)

Options:
Remove R7 for higher maximum signal on grid of second triode.
Short R30 or R29 and remove C28 for more maximum signal at grid of second triode.
Remove C27 and use the vias (plated holes) in the PCB to connect rheostat (pot) to location of R27. Makes for shorter jumper runs on PCB.
Replace loss of pre emphasis for tone stack by replacing C1 with a 200nf cap so tone control will be treble boost, flat and treble cut. Or instead change R9 to approx 220k and add a Resistor and capacitor in parallel with it for treble boost.

See attached pictures.
Picture of tools used to remove op amp, resistors and capacitors. Also to remove solder from PCB vias.
If you get a vacuum desoldetring tool best to use fresh solder to make better thermal contact with solder joint to be desoldered. Also allow enough time to heat joint before pulling the trigger on the desoldering tool count in your head 1001, 1002… for 10 to 20 sec. Remember ground plane connections and large component leads will require more time for good results. Keep in mind too much time will lift the copper trace from the PCB making for a rework job.
If you see any errors please let me know, thanks.

isnt the orange wire on r14 ?
I cant see anything on pin 6 of the op amp ???

Im confused, are there jumpers on the other side of the board ?

Do you have a picture please.

EDIT!!!

I have done the mod as to how i can see it on the photo, I put it back together just enough to test it but have a couple of concerns

1. with the plastic knobs off and a lead plugged in i was holding the guitar side of the lead with one hand and turning the vol pot with the other to test the signal, it seemed a bit noisy and as i turned it up i got a bit of a shock, i cant remember in which hand tho. i put it pack together and played through it a bit with a guitar. tone is definately improved and i am yet to change the bias resister to 180 ohms. although when i touch any metal part of the guitar there is a slight pop noise ??? sort of what you get on cheaper strats where the hum goes away from touching it, it wasnt there before the mod.

any idea what ive done wrong ? (shock, pop noise)

2. it appears that the tone knob goes from 0 - 5 (like a guitar tone control) and then doesnt realy do anything from 5 - 10

sugest me some easy improvements forthis please

thanks

 

[jim p]

Re: post #168 mod

isnt the orange wire on r14 ?
I cant see anything on pin 6 of the op amp ???

Im confused, are there jumpers on the other side of the board ?

Do you have a picture please.

EDIT!!!

I have done the mod as to how i can see it on the photo, I put it back together just enough to test it but have a couple of concerns

1. with the plastic knobs off and a lead plugged in i was holding the guitar side of the lead with one hand and turning the vol pot with the other to test the signal, it seemed a bit noisy and as i turned it up i got a bit of a shock, i cant remember in which hand tho. i put it pack together and played through it a bit with a guitar. tone is definately improved and i am yet to change the bias resister to 180 ohms. although when i touch any metal part of the guitar there is a slight pop noise ??? sort of what you get on cheaper strats where the hum goes away from touching it, it wasnt there before the mod.

any idea what ive done wrong ? (shock, pop noise)

2. it appears that the tone knob goes from 0 - 5 (like a guitar tone control) and then doesnt realy do anything from 5 - 10

sugest me some easy improvements forthis please

thanks

The orange jumper (pot to grid and input signal) in the picture is in the via (PCB hole) for R14 but if you look there is a PCB trace that ties that to the pin6 via of the op amp so it is the same as pin 6. So all the jumpers required for the modification are in the picture of post #168.

You can have more hum from line pickup (60Hz) because the bass response of the amplifier was rolled off at 250Hz by the values of C24 and R14 that were used by the op amp. If you touch the strings of the guitar and the hum stops that is a ground problem somewhere (usually the guitar).

Nothing in the modification changed the grounding of the amplifier and from the noise, pop and shock you are asking about a bad ground would be the likely cause. Have you reinstalled and tightened all the screws that hold the PC board into the chassis? Are you plugged into a three prong wall outlet that has a known good ground prong? Do you have florescent lights near by if so turn them off and see if noise stops?

As far as the tone control to get treble boost you will need to change C1 to a 200nF 6 volt or greater capacitor instead of the stock 2.2uF. The op amp provided treble boost before the tube stages without it you have no boost so tone control will only cut the treble.

Before you change R17 to 180 ohms you should read up on how to measure the power dissipation of a power tube in a single ended amplifier, it is probably in these posts and out on the web (how to bias a Valve Jr) for the Valve Jr (also a SE amplifier). You will need a voltmeter and have to make voltage measurements in the amplifier with it running. You have to be careful because you will measure the plate voltage which is approx 330 volts.

Hope this helps you figure things out.

 

[SynySter_SlaSh]

Thanks for all your answers guys.

Now juat wondering about this:

"Options:
Remove R7 for higher maximum signal on grid of second triode."

I what way will this effect the tone of the amp ?

thanks

EDIT

Im in the uk and cant seem to find a 6v 200nf cap, only the 50v big ones. suggest me somewhere online please ?

 

[jim p]

Re: post #168 treble boost and volume level

You can use a capacitor (or condenser) with a higher voltage rating it would be best to use a film cap and they usually are found in the lower voltage rating. As far as removing R7 it will only have an effect at a high volume setting and it is a little safer for the control grid of the triode to have it installed. Shorting R30 and R29 would be the better choice to increase the maximum signal into the second triode (but it will reduce the gain of the first triode). If you have plenty of range on the volume control as it is now I would just leave the volume control alone. Also I have noticed that some pots are touchy at the low setting and having R7 as a 220k should help. The way the volume pot is setup with the mod of post #168 is not the best way to go just easiest way to move the volume control to between the triodes. The volume control would be better to be a potentiometer and not a rheostat as it is in this mod. A rheostat is a variable resistor so the resistance the first triode is driving changes with the volume setting. If the volume control was set up as a potentiometer the second triode would be connected to the wiper so it is connected to a variable voltage divider. Then the first triode would see a close to constant load independent of the volume setting.
As a quick temporary capacitor change if you removed C25 in one piece it is a 100nF cap so you could replace C1 with that to get some treble boost starting at 1kHz instead of 500 Hz.
Also you could change R3 to two 750 ohm resistors in series then put 2.2 uf across one of the resistors and 100nF across the other for just +3db treble boost instead of +6db as you have with 200nf across the 1.5k stock R3. (see schematic) With the split resistor there is more low frequency gain.

 

[stoneattic]

jim_p,

Maybe I'm over thinking this, but I'm trying to decide on what capacitor to order to replace C1 and there a quite a few different types of film capacitors (polyester, polypropylene, metalized, etc). On top of that there seems to be few choices that have 200nf. I see plenty of 220nf, is that close enough?

Also, you suggested either 500K or 1M for the MV pot. What are the advantages/disadvantages between the two? Audio taper I assume?

Any specific type of resistor (metal, carbon, etc) for the 1 watt to replace R26?

Sorry for all the questions. I can handle the mechanical end of things (mounting, soldering, etc) but I'm just starting to learn the electronics end.

thanks

 

[jim p]

Re: resistor and capacitor type and pot value

On the capacitor metalized polyester should be fine and 220nf is an OK value. Regarding R26 the composition is not that important it is just a series dropping resistor in the power supply circuit. On the tone front legend is that you would want carbon composition in the signal path.
The master volume pot needs to be an audio (log taper) pot with 1 Meg you will get a little more gain from the triode that is the input to the output pentode say by calculation 59 with 1 Meg and 57 with 500k ohms.

 

[SynySter_SlaSh]

hello again. which of these is most suitable for replacing c1

http://cgi.ebay.co.uk/4-X-MALLORY-0-2uF-600VDC-POLYESTER-FILM-CAPACITOR_W0QQitemZ350114259294QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item51846fa15e&_trksid=p3286.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1|72%3A2136|293%3A1|294%3A50

http://cgi.ebay.co.uk/5-pcs-MP-Axial-Capacitor-0-2uF-600V-10_W0QQitemZ140301844908QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item20aaa441ac&_trksid=p3286.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1|72%3A2136|293%3A1|294%3A50

http://cgi.ebay.co.uk/10-condos-polyester-mylar-0-2uF-200nF-200V-10_W0QQitemZ280348133654QQcmdZViewItemQQptZFR_YO_MaisonJardin_Bricolage_ElectroniqueComposants?hash=item41460d0916&_trksid=p3286.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1|72%3A2136|293%3A1|294%3A50

thanks

 

[rock_mumbles]

You could make any of them work...

(my opinion)

The third one would probably be the easiest to fit into the PCB I would expect it to be the smallest of the three caps and it has radial leads.

The second and third are probably both higher quality caps than the third one but installing them may be more difficult.

The second one with axial leads (leads come out of each end) has to be mounted vertically (staked) I used a Mallory 150 series cap for C1 in my V5.

The first cap "an Orange Drop" style had radial leads but is probably a much larger cap, the large Orange drops I've used have large diameter leads which can be difficult to install into the pcb

Here's a picture of my C1 "staked" cap

 

[jim p]

Re: Caps in the UK

As stated in the previous post you can use any of the three, the first ones are large and usually used for coupling caps between tube stages. Also I surfed on electronic components at yahoo.uk where I go to see the latest on F1, The tour of France what Lady Ga Ga is up to, if Amy Winehouse has fallen into a pool in a drunken fit and also the swine flu status. Found there some distributors you can buy from RSH Electronics, Rapid Electronics and ESR Electronics are a few. Found a Web site for hobbyists Called The Electronics club they have a links page with distributors on it http://www.kpsec.freeuk.com/links.htm This one http://www.bowood-electronics.co.uk/ charges 1 Pound 65p for shipping with a minimum order of 5 Pounds. So instead of just caps for your shipping you could get some other parts you may need.

 

[rock_mumbles]

RE: post #278 :: Split Resistor Treble Boost

...
As a quick temporary capacitor change if you removed C25 in one piece it is a 100nF cap so you could replace C1 with that to get some treble boost starting at 1kHz instead of 500 Hz.
Also you could change R3 to two 750 ohm resistors in series then put 2.2 uf across one of the resistors and 100nF across the other for just +3db treble boost instead of +6db as you have with 200nf across the 1.5k stock R3. (see schematic) With the split resistor there is more low frequency gain.

Hi Jim, I just did this on my "Home Bake" (5F1 ???) scratch built amp and I really like it. I previously had a 1.5K cathode resistor bypassed with a 2uf film capacitor. With the split resistor treble boost the tone was evened out, it helped to tamed the (ugly) mid-range.

Thanks, for the great tip!!!!

 

[timothymegg]

I'm happy now - Thanks guys

On to getting more treble by changing C1 do you have two 100 nf (0.1 uf) caps if so two in parallel is equal to 200nf so you could use that at C1 for a treble boost. Using 200 nf (0.2 uf) the boost will start at 550 Hz. In the case of voltage rating 50 volts is plenty you could get away with 12 volt rating easy.

I'm so happy !! I finally got around to getting a couple 100nf caps which I wired in parallel like you said replacing C1. The difference is night and day. Before the amp sounded too bassy and muffled without an eq in front of it. Now it sounds like a guitar amp again!

On another note: I re installed the R7 resistor to allow a little more headroom because the amp was breaking up too early. Now it starts to get a little crunch with hummbuckers around 4 and gets more driven from 6.5 on. Before it was breaking up as low as 2.5. So, I don't recommend taking out R7 in the mod I did in post # 240.

Thanks again to all the geeks and guitar amp freaks that put their knowledge out for adventurous guys like myself. Especially thanks to jim p and deafelectromark (manoteal) !!

Peace out!

 

[jim p]

Fet input stage for more gain for overdrive.

The attached schematic is to make the first stage of the amplifier a JFET gain stage for overdrive. It has a minimum gain of two and a maximum gain of 10 to 20 depending on the gain of the FET installed. So if you have or are modifying the amplifier for preamp overdrive by using a master volume pot between the second triode and the output tube this will give you more gain to maybe get closer to the sound you want. I would also add a high level of brightness to the amplifier to go with this. Let me know if you need ideas on how to do that and I will post them. Also if more gain is needed the positive supply for this circuit could be increased by using a zener diode and series dropping resistor allowing for maximum gains of 30 to 60 and giving more head room for the JFET.

These changes are for an amplifier that has had the op amp removed from the amplifier.

 

[timothymegg]

Tubes - Replacements ??

Hey all you guys who know stuff about these things that I don't!

What tubes have you guys tried with good results for replacements to get a better sound?

I'm looking at Ruby Select tubes and JJ tubes right now so, if anyone has a recommendation with their experience on what they have done to change the sound then I am all ears.

I have already modded the amp and, the last stage seems to upgrade the tubes. First step was changing the speaker. Second, I bypassed the op amp. Third, I increased treble response in the tone circuit. Now the final step of getting some quality tubes in there.

Peace

 

[Rampant]

Hey all you guys who know stuff about these things that I don't!

What tubes have you guys tried with good results for replacements to get a better sound?

I'm looking at Ruby Select tubes and JJ tubes right now so, if anyone has a recommendation with their experience on what they have done to change the sound then I am all ears.

I have already modded the amp and, the last stage seems to upgrade the tubes. First step was changing the speaker. Second, I bypassed the op amp. Third, I increased treble response in the tone circuit. Now the final step of getting some quality tubes in there.

Peace

Hi matey

The valves were the first things I changed when I got mine!

I'm running a Mesa Boogie EL84 tube and a Mesa Boogie 12AT7 at the moment. I noticed an immediate improvement in tonal quality when I changed the EL84... not the night and day difference that changing the speaker makes, but a definite improvement nonetheless. Note that I'm using an alternative 12AT7 and not the standard 12AX7 - which gives a slightly lower output, but my practice room is a small box-room and has a massive mirrored wardrobe on 1 side - so I don't need the oomph. Even then I only normally play on volume 1 to 2. That's un-modified, of course, apart from the Jensen speaker...

HTH

Cheerz

Rampant

 

[jim p]

Re: tubes

I don’t have an opinion on which is the best tube or tonal qualities of one vs. another. But one thing to look for is it really a different tube. In a lot of cases such as Grove tube or Mesa the tube is a Sovetek it may have been tested to meet there standards but the construction is the same. Electro-Harmonics and JJ tubes are at least a different construction then a tested and relabeled Sovetek so that is something to look for. If you search on this site on the subject of tubes you will find a survey on favorites.

 

[tunghaichuan]

I don’t have an opinion on which is the best tube or tonal qualities of one vs. another. But one thing to look for is it really a different tube. In a lot of cases such as Grove tube or Mesa the tube is a Sovetek it may have been tested to meet there standards but the construction is the same. Electro-Harmonics and JJ tubes are at least a different construction then a tested and relabeled Sovetek so that is something to look for. If you search on this site on the subject of tubes you will find a survey on favorites.

Have to agree with Jim here. Tubes are only made in a few places these days. There are two versions of the Sovtek EL84, a Slovak version made by JJ, a Chinese version and maybe another made by EH, although it could be a tested/rebranded Sovtek. EL84s are cheap enough to buy one of each and let your own ears be your guide.

tung

 

[stoneattic]

jim_p,

I finally got around to starting the tone, master volume and VVR mods this evening. I started by swapping C1 with a 220nf cap and then did the master volume. At that point I put it back together to test it. The tone mod really helped and made the tone pot much more useful.

The result of the master volume is kind of strange. 0 to about 4 acts as expected, but from 4 to 6 the volume doesn't increase, but actually drops slightly. From 6 to 9 the volume increases but after 9 the volume drops quickly to nothing.

i believe I have everything wired correctly per your schematic and the pot is a 1meg, audio tapper, R31 is pulled and R16 was replaced with a 1meg.

Any thoughts?

thanks!

 

[jim p]

Re: master volume pot

The solution would be to add a resistor in series with the control grid of the EL84 or a minimum value of resistance in series with the master volume pot. The idea of the master volume pot is to use the existing volume pot as a gain pot to overdrive the second triode for preamp overdrive then use the master volume pot to limit the signal to the output tube. So you would normally not turn the master volume pot up past ½ to ¾. If you want to allow for the master volume pot to be turned to maximum you will need to add a resistor in series with the pot to the coupling cap C7 say 50k to 100k ohms to set a limit. The way it is now if you turn the pot to maximum on the positive going signal to the control grid of the EL84 if the amplititude is high enough you will get grid current from the EL84 that will look like a low impedance to the 12AX7 driving it and it will not be able to supply that through R6. But as I said you should limit the master volume to ¾ max and use the gain pot to increase the volume if that is not enough.

As you change the volume the load on the 12AX7 is changing at the high volume settings this reduces the gain of the 12AX7 and the loss of signal when you set the master volume pot at a high level.

So basically you need to supply enough signal to the master volume pot by where you set the gain pot so it has something to work with. Every pot in the amplifier tone, gain and master volume are killing signal when you turn them down and what the master volume pot has to work with is dependent on what is before it that is the tone and gain pot settings.

To give you a better idea of what I am talking about check out this article.
http://www.tgpwebzine.com/?page_id=808

 

[jim p]

Speaker Directivity Modifier

Some people have stated that the Crate V5 is a little boxy sound wise. In an early post I showed a beam blocker to reduce this and cut down on the ice pick tone from the speaker. Since then I have run across this article http://www.tgpwebzine.com/?page_id=424 and this looks like it may make for a better solution. They talk about reflections of the sound back to the speaker with a beam blocker which is something I thought might be a problem when I tried it. What is neat is this creates the effect in a way of a dual cone speaker by blocking the outer part of the cone from emitting sound at higher frequencies. The higher frequency signal from a 10 inch speaker are not as pure a tone as the lower frequency sounds. By physics from what I have found in other articles the frequency range of a speaker is relative to the cone diameter divided into the speed of sound and a factor of ten greater in frequency from that point. So for a 10 inch speaker this is 13584 inches/sec divided by 10 inches for 1358 Hz to 138 Hz so all those frequencies above 1358 Hz are less and less pure in tone. If you look at the response curve for 10 inch guitar speaker you can see some of this in the peaking of the curve at higher frequencies.

Anyway food for thought and maybe something you may want to try out.

 

[SciHi]

Any thoughts?

I posted this earlier: http://www.thefret.net/showpost.php?p=140280&postcount=261
Maybe a litttle more conventional SE master volume and it worked great. I know Jim has had the discussion earlier about the grid resistor before or after the pot (or grid load), but after the pot is a little more conventional. Does it make any difference? I don't really know but the 250K pot with 6.8k grid resistor, post pot, sure added a lot to my amp. I did a half dozen other mods that didn't last as long as it took to put them on, but this master volume on my v5 is a real keeper.

 

[stoneattic]

jim_p, I tried the 100k resistor in series with the pot as you show on the schematic but have essentially the same issue. The control seems a little better but it still drops to nothing above 9 or so and there's still the dead spot/drop though the middle of the sweep.

SciHi, on your schematic there are a few other mods. Is the only thing relevant to the MV mod the pot and resistor shown in blue?

 

[jim p]

Remove C3

Looking at the markup I posted and doing some calculation if you still have C3 installed remove it. As you turn the pot up the capacitor could be working against you.
Also it may pay to check the pot with an ohmmeter sometimes they go bad where either the wiper looses contact to the resistive element or they wrap around and connect to the lower side which is ground in how this pot is wired.

The more I think about this the more I think your problem may be the pot is bad. I do think the 220p cap can work against you so it may be best to remove it. But as far as grid current that should not happen until full volume out of the amplifier so I think I was wrong on my earlier post. It is probably a good idea to add a series resistor with the pot but I thought I would make the mod as simple as possible. As far as how I marked up the first schematic it is the same as the master volume mod for the Valve Jr over at the Sewatt web site with exception of pot value which is 250k on the Vj mod. But higher value pot will give you greater gain from the 12AX7 driving the EL84 so that is why I suggested 500k to 1 Meg. As far as a grid resistor the pot is a resistor in series with the grid and connected to ground so you could add one but no real need.

 

[SciHi]

SciHi, on your schematic there are a few other mods. Is the only thing relevant to the MV mod the pot and resistor shown in blue?

That is correct. The other mods are for other reasons...

 

[stoneattic]

jim_p, I pulled C3 but the only difference it made is that now the volume change from 0-4 is minimal, and the volume change from 6-9 is more dramatic. It still drops to nothing from 9-10 and there's still the dip in volume from 4-6, but it's not quite as noticeable, which could just be because the overall volume is lower at 4 now.

 

[jim p]

ohmmeter the pot?

jim_p, I pulled C3 but the only difference it made is that now the volume change from 0-4 is minimal, and the volume change from 6-9 is more dramatic. It still drops to nothing from 9-10 and there's still the dip in volume from 4-6, but it's not quite as noticeable, which could just be because the overall volume is lower at 4 now.

Have you tried checking the pot with an ohmmeter? The overall circuit is pretty basic and as far as others posted main difference is the value of the pot used. Just to check R16 the 220k resistor has been removed, yes? If not it should be. Another thing I noticed is that for some reason R6 in the schematic looks like it is 10k but it should be the stock value of 100k just in case you thought that was a change you needed to make. Have no idea how the screen shot made that come out that way. So if you ohmmeter the wiper of the pot to ground with power off of course should be 100k at the mid point to 1 Meg ohm at full clockwise position and should be the opposite if you test the C7 100k ohm resistor side 1 meg at full counter clockwise to zero ohms at full clockwise. Hope this helps.

 

[stoneattic]

Have you tried checking the pot with an ohmmeter? The overall circuit is pretty basic and as far as others posted main difference is the value of the pot used. Just to check R16 the 220k resistor has been removed, yes? If not it should be. Another thing I noticed is that for some reason R6 in the schematic looks like it is 10k but it should be the stock value of 100k just in case you thought that was a change you needed to make. Have no idea how the screen shot made that come out that way. So if you ohmmeter the wiper of the pot to ground with power off of course should be 100k at the mid point to 1 Meg ohm at full clockwise position and should be the opposite if you test the C7 100k ohm resistor side 1 meg at full counter clockwise to zero ohms at full clockwise. Hope this helps.

And the big d'oh, take his soldering iron away, award goes to me.
I had the screen grid and ground connections reversed on the pot. :thwap:

Thanks again for all your help and patience. I'm thrilled with this little living room practice amp right now. For want I plan on using this for it's perfect. I still have the VVR to add to it but honestly if I had done the MV first I may not have bothered ordering the VVR but since it's sitting here I'm going to (try to) install it.