Crate V5 mods... anyone?

[jim p]

what mod to do?

First off I would get a clean copy of the schematic either from a post or Crate. Then go through all the posts here that should give you an idea of what is going on in and with the amplifier. As stated way back this amp by design is a solid state preamp (op amp) with a tube output so not ideal. Also it is based off an amp that had an 8 inch speaker that they limited the bass response on. Most of the parts on the amplifier are good except the speaker, screws and nuts. You have a double sided PCB with plated holes I was just looking inside a Epiphone Valve Jr. that has your chepo single sided no plated holes PCB. You also get that 1 Hennery choke in the power supply, iron being a big expense to a manufacture in cost and shipping weight. So anyway go through the posts and figure what mod makes the most sense for you in terms of skill level and the tone you want. For the most bass response adding feedback will extend bass by lowering output impedance therefore reflected impedance at the primary. Also the triode mode will increase bass response by lowering the tube impedance to 2 K ohms from the pentode mode impedance of 35 to 50 k ohms but with loss of output power 1.5 watts and darker sound.

 

[timothymegg]

In response to post 207 being wrong with Jim P's adjustments

Caution the schematic mark up of the volume pot in post #207 is wrong. Where it shows the red wire connected on the pot should be where the pot should be grounded. Then the red wire should be connected on the side of the pot that used to be connected to pin 7 of the op amp. The little dot on the schematic symbol is probably indication of clock wise direction so at full clock wise you want the wiper of the pot at the R29 side(when mod completed). There is also a cut not shown on the schematic the wiper needs to be cut free from the other terminal of the pot. If you leave R1 in this will give you some feedback for high frequencies on the input triode and if over driving the input to amp sounded shrill it would give a plate to grid feedback cap something to work against. The diodes are a few Pico farads the guitar cable is more capacitance then they are so no big need to remove them they were in there to protect the op amp input so your choice. The location of the 1 Meg to ground is not that critical the difference in distance is what an inch or so this is inside of a shielded enclosure and these are audio frequencies. If you remove the op amp C25 and R2 all you need to do is run a jumper from the feedthrough hole of pin 3 of the op amp to feedthrough hole of C25 or R2 to connect input signal to first stage.

I am no expert. I appreciate all that you do to help me figure this stuff out. Thanks Jim P. After I looked at the pictures supplied by deafelectromark after his mod I checked and rechecked where he hooked up the jumpers that he used. I don't think I missed anything and, I drew it the way he jumped it from the photos and his description. So, I have looked at it again after you said what you did - I still think that I have it correct. (Ill give you the trace cut between the wiper and the right side of CW1-1). The idea to leave in R1 and jump from Pin 3 to C25/R2 is awesome. If it was jumped the way you said above then it looks backwards from the way the pot was used originally with the signal going in the exit and being grounded in the input side? Below is the schematic that way you suggested. If you look where it is jumped compared to before, the R29 side is the input and the R27 side is the output. Maybe it doesn't matter which side is the input or output? But, the way it was jumped was the way I had it originally based on deafelectro's pictures (above). Looking right at the board where C27/R28 are placed on the board, the schematic and where the jumper was is why I put it that way on the schematic. I say all this because I believe the schematic I am posting here to be not what has been already tried and tested to work but, it is here to take a look at:

Anyone have any comments?

 

[timothymegg]

Bass Q and Clean Schematic

First off I would get a clean copy of the schematic either from a post or Crate. Then go through all the posts here that should give you an idea of what is going on in and with the amplifier. As stated way back this amp by design is a solid state preamp (op amp) with a tube output so not ideal. Also it is based off an amp that had an 8 inch speaker that they limited the bass response on. Most of the parts on the amplifier are good except the speaker, screws and nuts. You have a double sided PCB with plated holes I was just looking inside a Epiphone Valve Jr. that has your chepo single sided no plated holes PCB. You also get that 1 Hennery choke in the power supply, iron being a big expense to a manufacture in cost and shipping weight. So anyway go through the posts and figure what mod makes the most sense for you in terms of skill level and the tone you want. For the most bass response adding feedback will extend bass by lowering output impedance therefore reflected impedance at the primary. Also the triode mode will increase bass response by lowering the tube impedance to 2 K ohms from the pentode mode impedance of 35 to 50 k ohms but with loss of output power 1.5 watts and darker sound.

Jim P - looking at Deafelectromark's mod, since C24 is cut out because the OP Amp is removed, what would be a way to increase the bass response or is there no need to do so?

Clean Schematic

 

[jim p]

Schematic correction to post #212

Just need to correct Timothy’s mark up in post #212. You show one cut to many if you use the vias (holes) from where the op amp was do not cut the trace to pin 3(see attached schematic). On bass response with the op amp in C24 is a high pass filter that is 3db down (half volume) at 270Hz so if you removed the op amp bass response will be fine. If you use the op amp should change C24 to 100nF to lower bass to approx 50Hz. Only other way to extend bass response would be to replace transformer with one that has higher primary inductance (for the most part one that is bigger and weighs more). Use feedback this lowers the output impedance which lowers the reflected impedance at primary which lowers bass response. Use output tube in triode mode this lowers the plate impedance that is in parallel with the reflected impedance on the primary to increase bass response. The calculated numbers for these modes using an 8 ohm speaker, stock transformer turns ratio of 20:1 and primary inductance of 8 Hennery (a value I got from testing with AC signal no DC applied)
Pentode mode (stock) approx 60Hz 3db down
Feedback equivalent to ½ output impedance 30Hz 3db down
Triode mode 24Hz 3db down

Note to go as low as 24 Hz will need to replace C1 2.2uf with higher value 10uf or better. Also connecting to a 4 ohm speaker would increase bass response by halfing the reflected impedance.

 

[letter109]

Celestion Tube 10 speaker. . .yummy!

:rotflmao: Hi all, just a note to say I have installed an 8 ohm Celestion Tube 10 speaker in place of the stock Crate speaker in one of my V5s. Very nice, and heartily recommended: more bottom end, less brittle plasticky high-end, and when in distortion mode, when two notes heterodyne (like on a major third interval) I get this really creamy, warm break-up that sounds frighteningly like Jimmy Page sted of just garbage. And the speaker isn't even broken in yet! My other two V5s will be getting their Tube 10 speakers in the near future.

FYI I got my speaker from Antique Electronic Supply. I play mainly a stock MII Squier Standard Tele I bought new through the V5. . .I can't believe the sound I am getting for a total $350 (for the guitar, the amp AND the speaker)!:rotflmao:

Next I will be trying some of the mods posted twoards the top of the thread. . .I'll let you know what I do and what happens when I cross those bridges. Thanks all for a great thread -Mike:rockon:

 

[Rampant]

Hi All

Just replaced my standard V5 speaker with a Jensen 8 ohm R10Q.
Vast improvement over stock.
Still deciding whether to risk modding...?

 

[goonrick]

Hi All

Just replaced my standard V5 speaker with a Jensen 8 ohm R10Q.
Vast improvement over stock.
Still deciding whether to risk modding...?

If you're at all familiar with soldering and the risks of probing into a tube amp, then it's well worth it. I couldn't stand the sound of mine before the mods, but now I love it. I did all the non-committal mods (speaker, tube changes) before I routed around the op-amp and it still sounded terrible.

There is a broad range of changes you can make, and the posts in this thread give you a lot of knowledge and options. I took a rather easy route with mine, combining some of the suggested mods based on my own tastes and mods I've done in the past.

This little amp is an excellent platform to get into modding. The PC comes out of the chassis quite easily and it's got a lot of room on the board for component changes.

I highly recommend doing a mod of some form or another. Check Jim P's notes on the output and power sections. Then pick and choose from the preamp mods based on the various posts and approaches.

 

[jim p]

Conjunctive Filter (corrective filter)

From a RCA Receiving Tube Manual:

"A corrective filter can be used to improve the frequency characteristic of an output stage using a beam power tube or a pentode when inverse feedback is not applicable. The filter consists
of a resistor and a capacitor connection in series across the primary of the output transformer. Connected in this way, the filter is in parallel with the plate load impedance reflected from
the voice-coil by the output transformer. The magnitude of this reflected impedance increases with increasing frequency in the middle and upper audio range. The impedance of the filter,
however, decreases with increasing frequency. It follows that by use of the proper values for the resistance and the capacitance in the filter, the effective load impedance on the output
tubes can be made practically constant for all frequencies in the middle and upper audio range. The result is an improvement in the frequency characteristic of the output stage.

The resistance to be used in the filter for a push-pull stage is 1.3 times the recommended plate-to-plate load resistance; or, for a single-tube stage, is 1.3 times the recommended plate load
resistance. The capacitance in the filter should have a value such that the voltage gain of the output stage at a frequency of 1000 cycles or higher is equal to the voltage gain at 400 cycles.
A method of determining the proper value of capacitance for the filter is to make two measurements of the output voltage across the primary of the output transformer: first, when a
400-cycle signal is applied to the input, and second, when a 1000-cycle signal of the same voltage as the 400-cycle signal is applied to the input. The correct value of capacitance is the
one which gives equal output voltages for the two signal inputs. In practice, this values is usually found to be in the order of 0.05 microfarad."

This is like reading old English or Shakespeare they even have that cycles per second thing going on what’s it all mean?
It’s gotten quiet out there and thought people were running out of things to do so thought I would bring up another mod. As it states above the impedance of the speaker you are driving is increasing in impedance with increasing frequency. Because for the most part it is an inductor whose impedance is equal to 2 Pi (2 * 3.14) times frequency times inductance. Now if you are using feedback as it states above the feedback will correct for this and you should be fine. But what if you like that overdriven sound then feedback is a problem and maybe things are a little shrill or a bit of ice pick tone is happening. Well then if we level off the high frequencies maybe life will be good and that is what putting a resistor in series with a capacitor across the primary will do. From what I am finding on the web the values of choice for this are a 600 volt 47nF cap and a 5k 10 watt resistor so things should start to happen at approx 700 cycles per second (Hz).from above the resistor should be 6.5k(1.3 times the reflected impedance) so? Also instead of power going to the speaker you will be dropping some of it across the watt wasting 5k so maybe just dropping some will work say 10k instead. Also instead of 700Hz starting at 1 kHz might be better then the cap would be 33nF instead of 47nF or starting at 350Hz might be better so a 100nF cap would be your choice. Bottom line if you don’t have a waveform generator and scope you might need to play around with values and if you do have a scope you may still wind up playing with values. Another thing to consider is the turns ratio on the Crate transformer is 20:1 so the primary with 8 ohm speaker is 3200 ohms.

After all this I would just try what everyone else has done before the 5k 10 watts and 600 volt 47nF for a start and see how it goes. Be careful with wiring one side is the plate supply at approx 300 volts and the other is the plate of the output tube that can go to peaks of approx 600 volts at high signal levels. Also if you plan to have a switch to cut the filter in and out I would put it on the plate supply side but still there will be 600 volt peaks on switch contacts with the switch off(so don’t use a little itsy bitsy switch). Also there will be big peak to peak signal voltages on the leads so keep them away from the preamp section (12AX7). If you use a switch the plate supply lead is at AC ground so twist the wires to the switch so this lead will act as a shield. Also check things carefully because you could short the plate of the tube to the plate supply so use your ohmmeter and check before powering up.

Oh and another thing on the web some people think this filter is to correct for the output transformer that is not what the old guys at RCA were saying above it’s the speaker.

It would be nice to know if things are working for people out there so? Thanks

 

[rock_mumbles]

Conjunctive Filter

Hi Jim,

I have put Conjunctive Filters (CF's) on two amps. I've used a 1nf capacitor on one amp and a 2.2nf capacitor on the other, the 2.2nf is used by a high end amp builder. You just want the CF to take out some of the EL84 harshness without changing the overall sound.

From what I've read an amp will sound muffled (blanket over the speaker) with a 47nf or 22nf cap.

On the Gear Page Steve Ahola has posted about putting a CF on a Crate V18 here: http://www.thegearpage.net/board/showpost.php?p=5183840&postcount=63

 

[jim p]

RE: Conjunctive filter

What I posted is what the old guys at RCA had in the handbook and approx the same values in the “sewatt” web site octal mod I found on the web the other day. Also it tracks the impedance curve of the Jensen MOD speaker I posted. As I said you may want to play with values especially if you are just trying to kill a little high frequency ringing. Good chance a 10k to 20k resistor would do fine and not waste as much output power (need to increase cap values to stay in same frequency range). You will loose high frequencies but there is the tone control and boost in preamp section to make up for that. With a value like 2.2nf and 10k that starts to kick in at approx 7.3kHz so there you must just be knocking off some high frequency ringing, so it all depends on what you are after. Also the problem could be before the output and you are correcting it after the fact at the output transformer. But if you are doing mods with just a meter, soldering iron and assorted parts no way to really tell. Even with a scope and signal generator things may not be obvious. So all food for thought, you actual mileage may vary see dealer for details.

Also what is with conjunctive I thought that is a verb that ties two things together or another name for pink eye?

 

[rock_mumbles]

Conjunctive Filter question ...

Hi Jim, since you know and really understand this stuff and I'm just a hack ...

I read a post on sewatt.com about using a conjunctive filter to keep the grid at the proper voltage in reference to the plate voltage. Can you explain this a bit more???

Thanks!

 

[jim p]

Re: Screen grid voltage level

If the post was by Fondue then I am the one behind the post.

What I was getting at if that is the case is that you may be using the conjunctive filter to mask an oscillation that is occurring due to the screen grid being more positive then the plate during the negative swing of the output signal. What will happen then is the screen grid current will increase so its voltage drops then current flows back to the plate then screen grid voltage rises and the cycle repeats so you have an oscillation.
Two ways to try to stop this are to lower screen grid voltage which is probably the best way to go. Second is to add a grid stopper resistor such as the 470 ohm resistor already on the V5 (but not on stock Valve Jr) connected to the screen grid. If needed the value of this resistor could be raised as high as 2.5k

The screen grid should only attract the electrons and have little current flow, it is meant to increase the gain of the tube and isolate the plate from the cathode for better bandwidth. Any current flow to the screen grid is a loss of signal to the output (speaker) of the amplifier. Lowering the screen grid voltage will lower the gain of the output tube but only marginally.

The grid stopper resistor will increase the voltage drop of the screen grid with grid current which may just be changing the frequency of oscillation in the output tube.

The values you posted for the conjunctive filter were in the 7 kHz range from what I recall which would be in the range of masking an oscillation. Although if the speakers impedance does keep rising with frequency (which it probably does) it would put a limit on the highest impedance the primary of the transformer will see which is probably a good thing. Also no reason not to roll the amplifiers output off out side of the range of the guitar and its harmonics, and I doubt the speaker could reproduce a frequency that high.

The only way to find out for sure what is happening would be to check the amplifier out with a scope while it is driving a speaker at volume and hope to catch the transients.

PS: I am just in the process of replacing the output transformers on a couple of Valve Jr combos and found that I added a filter on the first one I am modifying. It has a 1nf with a 4.7k ohm in series which is 34 kHz not sure I needed it. The amp also has FET voltage regulators for plate, screen and preamp section but I can’t find a marked up schematic for what I did. A good thing to do is mark up a schematic of how your amp is wired and keep it with the amp. In the old days they use to have a schematic glued to the inside of the cabinet on radios, amps ect.

 

[jim p]

Low frequency response stock output transformer

While replacing the output transformer on a Valve Jr combo with an Edcor GXSE 10-6-5K then measuring it’s low frequency response (-3db@ 100Hz) I decided to check the low frequency response of the stock Crate output transformer was amazed to find it is -3db @ 40 Hz with a 6 ohm load. It is just -1db at 80Hz which is the low end of the guitar, so any loss of low end on this amp is in the preamp section.

Nice to here if anyone’s mods are working out there.

 

[SciHi]

low frequency response of the stock Crate output transformer...

I have played and listened to similar "SE Charming Leah" mods to the crate v5 and valve jr done by Rock Mumbles. The tone and amp response is very good and similar for both amps. But the value jr puts out more volume for equivalent gain and distortion levels. Rock attributes it to the iron in the valve jr, Hammond 125ESE versus stock v5 OT. We haven't tried any other OT in the v5, but the 125ESE are a major upgrade for the valve jrs. Do you think it would help the v5's?

 

[jim p]

Re: Hammond 125ESE

Well you will be down on output volume with the stock Crate output transformer because of the 20:1 turns ratio. Not sure why Crate used this ratio maybe so you can just plug in anywhere from 16 ohm to 4 ohms for the speaker load with just one secondary output. I never checked out the Hammond transformer did not want to spend around 60 bucks to upgrade the Valve Jr’s I have. So I went with the Edcor GXSE 10-6-5k for 25 bucks each for two with shipping. I would think the bass response of the stock Crate transformer should be close to or equal to the Hammond ESE.
The secondary to primary load for the Crate is
4 ohms : 1600 ohms
8 ohms : 3200 ohms
16 ohms : 6400 ohms
Having only 3200 ohms on the primary (with 8 ohm load) will also improve the bass response. But you will take loss of around 35% in output power. So you will get more output with the Hammond for a 60 buck investment or you could change to a 16 ohm speaker.
I didn’t think anyone is going for output volume with these 5 Watt amps anyway?

PS You said that the tones and distortion are similar for both amps that would mean the ideal primary impedance what is it 5k for EL84 6.5K for a 6V6 rule may not be true?

 

[SciHi]

I didn’t think anyone is going for output volume with these 5 Watt amps anyway?

Thanks. A pair of valve jrs is the stage setup for the blues trio Rock plays with. He admits the v5 is a great bedroom amp, and as you say, is not ready to put $60 (another 125ESE) into a $80 amp. We have been discussing some other OT options, more in the $25 range. He had the version 2 valve jrs and had to upgrade the OTs. I will likely mod my v5 for my son-in-law that likes heavy distortion drop-D, and stick with valve jrs...

 

[jim p]

transformer choices

The ones that I can think of are the Hammond _SE models guess can go by rule of thumb more they weigh better the bass. There is the OT8SE but I think I would spring for the OT10SE few bucks more should have better bass they have two primary taps for 5k or 6.5k I think and 4, 8 and 16 ohm secondary. Then Edcor they just have single secondary tap the primary has an ultra-linear tap that I might try out. I got the GXSE 10-6-5k so it would be between 4 ohms and 8 ohms bass was -3db at 120Hz with 6 ohm resistor. For 30 bucks instead of 20 could jump up to GXSE 15 they are a pound bigger then the GXSE 10 (1.75 lb) so the bass should be better guess there are others to choose from.

 

[SciHi]

... the ideal primary impedance what is it 5k for EL84 6.5K for a 6V6 rule may not be true?

I think they say ~6.5K can be used for either EL84 or 6v6, but not the best for either. Maybe it is 4-5K for EL84s and 7-8K for 6V6?

 

[tunghaichuan]

I think they say ~6.5K can be used for either EL84 or 6v6, but not the best for either. Maybe it is 4-5K for EL84s and 7-8K for 6V6?

Just a few thoughts from modding Valve Juniors and building tweed Princeton circuits:

I've used the Hammond 125ESE with both 6V6s and EL84s and it seems to work well using the 5K setting.

In one Valve Junior rebuild (chassis gutted and rebuilt with an eyelet board and NOS tubes) I used a massive Heyboer OT. It was rated conservatively at 160mA on the primary. It had 2.5k, 4k, and 6k primary impedances. I tried both 4k and 6k for use with the EL84 and 6k sounded best to me.

I've used a 125CSE in a Valve Junior circuit and I thought it sounded too bright and gritty, the 125ESE was a major improvement to my ears.

I've probably built over a dozen amps using tweed Princeton circuit and various combinations of tubes. About half were built using the 125ESE and 6V6 tubes. On one amp I substituted an Edcor XSE15-8-5k OT for the Hammond. To my ears the Hammond sounded better when the amp was pushed into distortion. The Edcor sounded better clean.

FWIW

tung

 

[Fonzie]

Eminence Speaker

A few pages back, there was some discussion about which speakers will fit in the V5. I just put an eminence Patriot Ragin Cajun in mine and it fit with no problem; which surprised me since I measured everything first and thought that no way it would fit.
I also put in JJ EL84 and ECC81 while I had it apart.
Noticeable improvement in sound especially with my overdrive and distortion pedals. The stock speaker sounded like an angry can of bees, the Eminence sounds, well, like a speaker should sound.
I also plugged the V5 into my Deluxe Reverb cabinet with a Weber 12F150; The 12" speaker has some better warmth, but, considering that the Weber cost more than the whole V5 amp (I got mine at BestBuy for $50) i think the V5 is just fine. I might improve the board if I get brave enough, or just ship it to Mark Deafelectro because he is cool.

Mike

 

[SciHi]

The secondary to primary load for the Crate is
4 ohms : 1600 ohms
8 ohms : 3200 ohms
16 ohms : 6400 ohms
...

Jim, on the 6v6 mod earlier, http://www.thefret.net/showpost.php?p=124456&postcount=58, did you stick with the stock v5 OT? Are you saying running a 16 ohm speaker will make the OT more 6v6 friendly? How did that 6v6 mod turn out? As you say with the extra heater capacity you could power another preamp tube for a cascaded preamp, similar to the new Plexi Junior SE build.

 

[jim p]

RE output power and 6V6 mod

What I was getting at with a 16 ohm load you won’t loose the watts at the output you are with an 8 ohm load. Power is equal to current squared times impedance (resistance) so do to the fact that with 8 ohms you are only 3200 ohms reflected instead of 5000 you are loosing power. While with 16 ohms you would have 6400 reflected and not have the loss. I don’t know about the whole 7k is better with this tube 5k with that thing I used the stock transformer with the 6V6 tube when I moded that amp. As I stated the heater current is less for a 6V6 by a factor equal to the current required for a 12AX7 so I may add a third tube to the third amp I have. But I may go with a signal pentode that I will overdrive before the output to see how that works.
I am thinking of dropping the plate supply on the V5 to up the plate current so it might give you greater output power with the stock transformer. But first I need to replace the stock (junk) output transformer on a VJ combo ver 1 amp and may give the ultra-linear mode a try with that.

 

[SciHi]

As I stated the heater current is less for a 6V6 by a factor equal to the current required for a 12AX7 so I may add a third tube to the third amp I have...

So we aren't running the opamp, so can we gain enough current from the existing 12.9 volt preamp heater circuit to run the second preamp tube ? The PT has 6.3 volt for the EL84 separate from the opamp and preamp heaters (+14.5,0,-14.5).

 

[SciHi]

...I used the stock transformer with the 6V6 tube ...

So the stock setup has R4-C23, Zobel (?) network, across the output to the speaker. So isn't this to correct an issue between the speaker and OT? So is the problem with the speaker or OT?

When you do your variable nfb you mentioned over on SEWATT, do you just connect at R4?

 

[jim p]

Re: feedback and third tube

That RC network (R4, C23) on the secondary of the output transformer by my calculation doesn’t do anything till approx 160 kHz so don’t know the point in it. As far as variable negative feedback check post #201 I have a schematic and description of how to do it there.

Regarding heater current I was going to hook up the third tube to the 6.3 volt AC heater supply. I am running a tacked in 6CB6 pentode to try things out as far as adding a pentode for overdrive with the stock EL 84 and so far no problems. If things work out will switch to an EF86 for the pentode (wish they were cheaper). The heater supply is on the high side (should be 6.3 volts +/- 5%) so in future may look into adding a resistor in series.

Removing the op amp only frees up 10mA of current at best if you were going to use the +/- 15 volt supply for heater current. If I used one of those supplies I would go with the -15 to balance the load on the secondary. I see at Sewatt that they look to be adding all kinds of loads on the stock VJ transformer I wonder how they determined that the transformer is ok with the added loads?

 

[SciHi]

As far as variable negative feedback check post #201 I have a schematic and description of how to do it there.

Thanks. The schematic posted at SEWATT has the pot on the ground side of the the cathode bypass cap (cap needs lifted for nfb to work). I think that is correct, not as shown in the schematic in post #201???

 

[SciHi]

cheaper pentode?

If things work out will switch to an EF86 for the pentode (wish they were cheaper).

Has anyone tried the Russian tube equivalents? " NOS RUSSIAN tubes (6J32P)/6267/EF86 " We have had good results with some of the Russian EL84M's.

 

[SciHi]

What I was getting at with a 16 ohm load you won’t loose the watts at the output you are with an 8 ohm load...

Jim, I think you really have something here! So I hooked up to my 2-12 cab, 2 - 16 ohm speakers in parallel. Put one on a switch so I could go from 2-12 at 8 ohms to 1-12 at 16 ohms (Jensen mod 12-70). Sounds much, much better with the single 16 ohm speaker, clearer, fuller (bottom end), and louder!!! I will have to try the reverse with 2 - 8 ohm speakers hooked up in series to really nail this one.

OK, just tried it with 2 - 8 ohm 10's in series. Sounds better at 16 ohms. So I think I will shop around and try to find a mod 10 in 16 ohm, I checked out the weber sig 10s and they are only available in 8 ohm.

So what's the downside with the 16 ohm load, is it harder on the EL84? (nothing died yet)

 

[jim p]

Re: SciHi posts

In the schematic for the negative feed back there is an X through the cap not to clear in the picture. Removing it is also in the text describing the mod. As far as the order of cap and pot and which is tied to ground doesn’t matter much.

On rethinking if the pot is at ground then the wires to the pot will be a twisted pair with one lead at ground so may be better for noise reduction wired with pot at ground.

I figure to go with the EF86 pentode because it looks to be an available standard part now, plus the mesh screen inside of the tube looks cool. If the pin out of the EL84 is the same I might look for that.

On to a 16 ohm load on the amp I don’t see that it should be a problem the primary impedance will be 6400 ohms instead of the standard 5000 ohms which should not be too bad. I forget where the plate voltage is at this point but at most may pay to keep it at approx 300 volts but without scoping it out that is just speculation.

 

[timothymegg]

Tone Stack seems to do VERY little

Post #168: No OP Amp, No Jumper Mod...

Options:
Short R30 or R29 and remove C28 for more maximum signal at grid of second triode.
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.

I did the mod last night and, WOW!! The options above are the ones I haven't done. My only complaint is that the tone stack seems to do VERY little. I did remove C4 and C5. And, with a Route 66 pedal on the compression side the tone knob seems to give me back all the treble that I want. I would like to be able to get some treble with the tone stack on the amp. So, my question is about the C1 replacement to 200nf... I don't have one. Can I use a different one of say 0.47uf (same as 470nf?) or 0.1uf (100nf?) because those are what I have? (both are 50V) What would be the tonal difference or effect?

I am very happy with the mods because now my amp sounds like a guitar amp. It is my first guitar amp and first time modding a circuit board. I am very thankful to deafelectromark and jim p for all there ideas and support!

Peace, Timothy

Edit: I realized I forgot to remove C25 :thwap: as I was marking up the schematic. After I did there was a lot more volume overall (guessing I was dumping some of the input signal to ground before I removed C25?). After I removed C25 I still wish the tone knob would do more so the same question above stands. Thanks

Schematic as I modded it:
PS: I didn't cut a trace between the wiper and the output of the volume pot and it works fine. jim p suggested before that it be cut but, I don't no where to cut it? deafelectromark didn't cut it either (from what I could gather?).