My Pignose G40V is a Bright Pig that could use some Dimming--Suggestions?

[Platefire]

I used this amp in the past with mostly a strat or tele. In order to get close to my tone, I haveto turn the treble and mids completely counterclockwise and the bass completely clockwise, Presence about 6 or less. This gets it close but still a little too bright. The tone stack on this amp is the most UN-responsive I've seen. The only changes I did to it was used a 12AY7 in V1 to clean it up and replaces the original 10" Speaker/Baffle with a 12" Baffle/speaker for a fuller sound. I love the sound of the amp and it's response using the MV. If I could just get more tone control to get more lows out of it, that would be the cat's meow!

This amp circuit is based roughly on the 5F6A but with Dennis Kager's mods with Pignose. I see a lot of the values of caps, resistors and pots have been changed/added different from the original 5F6A. The 47pF bright cap on the first volume control is the first thing I see that could probably be cut or made switchable--is like having the bright switch on all the time. I never use bright on! To be honest I've avoided doing mods to this because of the PCB board, I didn't want to mess with. However if I could do some effective stuff without getting to deep with good results might be worth a shot. I welcome your input! Schematic is attached. Platefire

[Willabe]

Yeah that volume control treble bypass cap but R3, 510K (?) is making things worse. Even with the volume all the way up there's still 510K of resistance before the full up volume control that the treble cap is bypassing.

V1b's K bypass cap is really small, I'd try a larger value there. Maybe 2uF to 10uF? And V2a has no K bypass cap, if you still need more bottom end you could add a bypass cap their too.   

I'm suspicious of R34, 51K in the TS. I'd lift 1 end see how that sounds. Even with the 50K mid pot full up the tone stack see's only ~25K to ground because R34, 51K is in parallel with that mid pot. (They might have wanted 25K for the mid pot but got a deal on the 50K pot so they put the 51K R in parallel with it to end up with ~25K?)

[Platefire]

Thanks Willabe for having a look!

On R3, I'm thinking if you remove this resistor, will increase the gain pretty drastically. I'm very happy with how the gain structure is presently working and don't really want any more gain.

Guess the first step would be to disconnect C21(bright cap) first to see what that does. If more needs to be done, then proceed to your suggestions on the bypass caps.

On C12 in the tone stack, I guess .001uF/1000pF is about high as that should go?? in the 5F6A it's .00025uF/250pF. Platefire

[plexi50]

Also I was wondering about C12(.001uF/1000pF) in the tone stack? Wondering if changing that to a lower value might help---say 500pF?

I was wondering the same thing about C12. 1000pf seems very high. Try 500pf. Lift R34 at one end and see how it differs. I don't like when they put in fixed resistors in series or parallel with pots. The mid pot itself is not fully in control of the mids.

[Willabe]

On R3, I'm thinking if you remove this resistor, will increase the gain pretty drastically. I'm very happy with how the gain structure is presently working and don't really want any more gain.

Agree, but I'm saying the way it's set up, the R & C, the C is making that amp very bright. The 510K is only knocking down the gain of the frequencies below the treble bypass cap, those frequencies stay at full gain.

Guess the first step would be to disconnect C21(bright cap) first to see what that does. If more needs to be done, then proceed to your suggestions on the bypass caps.

Yes.

[2deaf]

V1b's K bypass cap is really small, I'd try a larger value there. Maybe 2uF to 10uF? And V2a has no K bypass cap, if you still need more bottom end you could add a bypass cap their too.

A K bypass cap. on V2A will not increase bottom end and may well decrease it depending on the value of the cap. 

The 0.047uf at C9 is the only thing I see that would explain the absence of bass response.  If increasing this cap. by an order of magnitude does not fill in the bottom end, the problem may be with the amp and not the schematic.

[Willabe]

You may have overlooked the 10K cathode resistor on V1B.

 

Yes I did miss that.

As I am sure you are aware, frequency effects of the cathode R and C are inversely proportional to the product of R multiplied by C.  Therefore, a 10K resistor in parallel with a 0.047uf capacitor is the equivalent of a 1.5K resistor in parallel with a 0.31uf capacitor as far as frequency response is concerned.  If you want the frequency response of a 1uf C and a 1.5K R, you would use a 0.15uf C with a 10K R.

That's above my pay grade but I think I understand you. But that cap is still way small.

He's having to set the TS controls;
 

"In order to get close to my tone, I have to turn the treble and mids completely counterclockwise and the bass completely clockwise, Presence about 6 or less."

 

A K bypass cap. on V2A will not increase bottom end and may well decrease it depending on the value of the cap.

Why?

[2deaf]

As I am sure you are aware, frequency effects of the cathode R and C are inversely proportional to the product of R multiplied by C.  Therefore, a 10K resistor in parallel with a 0.047uf capacitor is the equivalent of a 1.5K resistor in parallel with a 0.31uf capacitor as far as frequency response is concerned.  If you want the frequency response of a 1uf C and a 1.5K R, you would use a 0.15uf C with a 10K R.

That's above my pay grade but I think I understand you. But that cap is still way small.

He's having to set the TS controls;

Upon further consideration, I don't think that's right and I deleted it from my post. 

A K bypass cap. on V2A will not increase bottom end and may well decrease it depending on the value of the cap.

Why?

With no bypass cap., the frequency response is flat.  With a large bypass cap., the frequency response is still flat, but the gain will increase.  With a 0.68uf cap., the gain will increase and the lower frequencies will have less gain than the higher ones.

[Willabe]

With no bypass cap., the frequency response is flat.  With a large bypass cap., the frequency response is still flat, but the gain will increase.  With a 0.68uf cap., the gain will increase and the lower frequencies will have less gain than the higher ones.

Mmmm, yes but, I have a Champ 5F1 on my bread board with a K bypass cap 6T rotary switch. The 1st position is open/no bypass cap, 2= 0.68uF, 3= 2.2uF, 4= 4.7uF, 5= 10uF, 6= 25uF.

Yes the gain does go up on the high end 1st but by the time I get to the 3rd/2.2uF (or above) it gives a much fatter/fuller bottom end. So as I switch to a larger value bypass cap I don't hear as much top end compared to the bottom. The balance in what I'm hearing shifts?     

[DummyLoad]

I get to the 3rd/2.2uF (or above) it gives a much fatter/fuller bottom end.

you're hearing the overall boost, so the lows seem enhanced. like 2deaf states, still flat, but has a roll-off.

try a 1-2uF under the v1b and bypass v2a with a 2.2-4.7uF. you may not like the v1b with a bypass at all. just bypass v2b. the .047uF is a treble peak tweak.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

as you already state - c21.

removing r3 will give you LESS gain, slightly less, but less nonetheless.

switch in another 22nF in parallel to c14 or just replace with a 47nF for a bit more bass boost.

c12 as a 1nF takes a significant amount of mid-scoop out. try 470pF or 250pF if you like more mid scoop.

with mid control as 50K pot, overall you have about 2dB LESS loss than with 25K pot as mid control. that could also make amp seem brighter overall.

--pete

[2deaf]

Ok, I thought about it a little more.  The proportional effect on the frequency response is going to be the same as long as the product of R multiplied by C is the same, but the gain changes with R.  So with the 10K/.047uf combination, the frequency where the gain is halfway between the minimum and maximum is the same frequency where the 1.5K/0.31uf combination is at its halfway point.  But the difference in gain between a fully bypassed and unbypassed 10K cathode resistor is much larger than the difference with a 1.5K cathode resistor because the 10K has a lot more feedback effect on the grid that is being destroyed when bypassed.

As an example, let's say that the unbypassed signal is 12 db lower than the bypassed signal for the 10K/.047uf combination and the unbypassed signal is 6 db lower than the bypassed signal for the 1.5K/0.31uf combination.  Both are halfway through their range at, say, 700Hz, but the 10K/.047uf is down 6 db and the 1.5K/0.31uf is down 3 db.  In order to get the 10K/0.47uf combination to only be down 3 db at 700Hz, it would require a bypass capacitor that is significantly larger than 0.15uf.

When you factor in gain like this, the .047uf at C9 is really whacking the low frequencies.

Happy Halloween
     

[Willabe]

you're hearing the overall boost, so the lows seem enhanced. like 2deaf states, still flat, but has a roll-off.

Ok, I do hear it and it sounds good/better to me.

removing r3 will give you LESS gain, slightly less, but less nonetheless.

Why, impedance the next gain stages grid see's?

[Platefire]

I' be been reading what I can find off the net on the G40V. I think this amp re-design was tweaked to sound good overdriven. So the brightness I'm getting for my clean tones probably sounds more balanced when overdriven------so I think all the diversions from the original 5F6A were to sound good on a Kranked amp, not clean. Platefire

[PRR]

I'd snip C9 right out.

It boosts above 333Hz, which is not what -you- want.

Using no-cap here reduces gain and small distortion, which is where you were going with the 12AY7.

C21 looks dubious for your taste.

For a more-classic "clean" amp, I'd bypass the whole V1B stage. This is adopted from Marshall's high-gain plans, and may be too much for some.

> The tone stack on this amp is the most UN-responsive I've seen.

The 0.001u cap on the end of the tone-stack is just "wrong". It loads the thing down (perhaps why they had to boost-back the highs with the other mods). It "might be right" if you work V2B to the MAX, then C13 takes some of the harsh off the sound. But if you are not flogging V2B past heavy clipping, C13 just makes the tone-stack not do much.

> tweaked to sound good overdriven.

Perhaps spot-on.

If you really want clean, no ear-bleed scream, then you might snip C8. Gain goes down and V1A's non-clipping distortion goes from 5% to 2%. However the huge values suggests they may have a hum problem, somewhat controlled by super-bypassing V1A cathode.

Is R25 really 68K? A more traditional value is 82K. It "should be" equal to R24, minus the loss through V3A V3B cathode coupling, times various other parameters. A slightly low value leads to slightly increased even-order distortion (below clipping) in V4 V5. That may be the intent, it may be fine; but you might try tacking 470K across R24 (to make that 100K lower in about the "right" proportion). I don't expect any "Wow!" difference.

[DummyLoad]

Why, impedance the next gain stages grid see's?

yes, Rload is 500K without r3. it's 1M with r3. the lower the resistance of load the harder the tube has to work, so less gain. make r3 1M and you have a load of 1.5M so gain will rise slightly.

--pete

[Willabe]

Ok, thanks Pete! 

[DummyLoad]

> tweaked to sound good overdriven.

> Perhaps spot-on.

one trick pony? much like the thunderbolt & trainwreck.

make the treble tweaks and bypass caps switch [pull switches on controls] selectable? it would be a more versatile amp then.

--pete

[VMS]

removing r3 will give you LESS gain, slightly less, but less nonetheless.

Yes, but by removing R3 you lose the voltage divider it makes with volume so overall gain increases.

[DummyLoad]

removing r3 will give you LESS gain, slightly less, but less nonetheless.

Yes, but by removing R3 you lose the voltage divider it makes with volume so overall gain increases.

no, it does not.


--pete

[VMS]

what I meant was that V1b grid gets bigger signal without the R3, so the gain of the first gain stage increases.

[DummyLoad]

what I meant was that V1b grid gets bigger signal without the R3, so the gain of the first gain stage increases.

again, not it doesn't.


--pete

[alerich]

You can do some interesting things with that voltage divider, too. I have an JCM800 style amp that was modified and the first gain stage V1A is really hot. They then used a voltage divider like we see here between V1A and V1B but they used a 1M resistor and a 100K pot. The total effective resistance is 1.1M but the signal is only sampled across 100K of that. So you maintain the gain (by not loading down V1A) while still managing the level sent to the next stage. You have elements of texture and amplitude to play with. You can manipulate the tone by retaining the texture without slamming the next stage.

[2deaf]

what I meant was that V1b grid gets bigger signal without the R3, so the gain of the first gain stage increases.

If I understand you correctly, you are using the word "gain" to mean maybe something like "amplified signal presented to the succeeding stage".  The GAIN of the first stage will decrease by a fraction of a db when R3 is eliminated and the load is reduced from 1M to 500K.  The amplified signal at the grid of V1B will increase by around 6 db with the elimination of R3.   

[2deaf]

They then used a voltage divider like we see here between V1A and V1B

That voltage divider serves another function in addition to signal reduction.  In general, and especially in amps designed to overdrive, I don't want a grid to be directly connected to a coupling cap.  The unwanted grid current will mess with the coupling cap. to give a type of distortion that I don't like and it can also mess-up the sustain.  Even though the signal is not all that large between V1A and V1B, the effect still occurs enough to irritate me.  If another circuit also gets its signal from the plate of V1A, the direct connection to the grid of V1B will also mess with that signal.   

[Platefire]

This next week if it don't get to busy, I will pull the chassis and start looking at the pig works to see what I must do. Working on a cheap PCB to me is like taking a bitter herb. Give me a good old classic fender any day. Hopefully if I take my time and be patient, I can work my way around all the board issues and do some good. Thanks to all for your input, we'll see where it goes! Platefire

BTW-I got the pix off the net, a pig gut shot! Peeuuwee

[VMS]

If I understand you correctly, you are using the word "gain" to mean maybe something like "amplified signal presented to the succeeding stage".  The GAIN of the first stage will decrease by a fraction of a db when R3 is eliminated and the load is reduced from 1M to 500K.  The amplified signal at the grid of V1B will increase by around 6 db with the elimination of R3.   

Yes, in my mind gain stage means the whole block before the next grid. Like in this slo demonstration:

https://ampbooks.com/mobile/classic-circuits/soldano-slo-preamp-1/

[2deaf]

I tested that 10K/.047uf cathode with a 100K plate and it was down 6db at 850Hz and over 13db at 100Hz.  A bite like that is going to take a lot of tone stack to counter.  It might be a good idea to try some larger bypass cap.'s and then tweak the tone stack.

I saw another gut shot online that had six knobs and three pre-amp tubes like the schematic.  Same general idea, but not quite so busy.  You could easily tack the 2uf or 10uf cap. that Willabe suggested right to the tube socket for the cathode of V1B for testing purposes.