Guitar Amp question

[thecr4ne]

I've got a Pignose G40V which I think doesn't sound right. (Schematic below)
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It's not humming or hissing or making noise, it just doesn't sound full in the low, low-mid area and the high end is murder on the ears. I haven't gotten out a meter or scope to do any real troubleshooting, but I did swap the tubes to rule that out as a possible cause, and no change.
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Any thoughts on where to start? Issues in the Tone stack? A bad cap somewhere? Nothing obviously wrong upon visual inspection, and the pots all seem to work (though could stand to be cleaned) it just sounds shrill to me. As always any wisdom or guidance is appreciated.

 

[Matador]

Did it suddenly change at some point, and was sounding "good" before?

If it's always been "bad", the simplest way to change the overall high frequency response of the amp is to add a grid stopper on V1A - try 68k as a starting point.

The schematic is (mostly) based on a Marshall JCM800 2203 topology, so typical treble voicing changes for that circuit would work here too. I notice that the stage between V1B and V2A is missing the typical Marshall 'pad' (or 50% gain reduction), so it might be there is just too much clipping making it sound 'bright'.

 

[thecr4ne]

I couldn't tell you, I got it years ago and only recently pulled it out of storage to try it out. My buddy was playing a danelectro through it, so I thought it was just that for a while, but it sounds shrill with everything.
Been trying to find more info in the amp, and the few youtube vids I've found seem to sound fuller than mine, not that that's a great way to compare, but it's not a super common amp it seems.

If it were a sudden change, what could cause this? Some component failing in the tonestack or something?

 

[thecr4ne]

Looking at the 2203 schem I see what you're talking about. The G40V is way too gain heavy so the Pad between V1B and V2A seems like a reasonable addition regardless. I just want to make sure nothing's wrong with it now before I start changing things.

 

[pucho812]

Might need a recap…

 

[Matador]

I just want to make sure nothing's wrong with it now before I start changing things.

It's difficult because 'shrill' is in the ear of the beholder.

For voicing I find it helpful to start at the output and work backwards: for example, you can lift the center of the master volume and use the amp as a power amp, and get that voicing what you want, then go 'backwards' through the circuit until you get to the input. I don't recall if pignose used PCB mounted tube sockets so some of these adjustments might be difficult.

 

[thecr4ne]

Tube sockets are chassis mounted with eyelets.

 

[mr coffee]

I'm with Pucho - it prolly needs a re-cap. They are good hi-gain guitar amps.

 

[thecr4ne]

Might need a recap…

I'm with Pucho - it prolly needs a re-cap. They are good hi-gain guitar amps.

Can I confirm by measuring the voltages coming off the Power supply? Or is it more involved to actually check if this is needed?

 

[transient]

Is that schematic correct at the input jack? The break contact is lifting the resistor that should be biasing the grid? The 1M0 resistor should stay connected, and maybe a grid stopper as mentioned above. Compare it with a Fender Princeton, for example.

https://schematicheaven.net/fenderamps/princeton_aa964_schem.pdf

 

[pucho812]

Can I confirm by measuring the voltages coming off the Power supply? Or is it more involved to actually check if this is needed?

Yes, check for shorted or leaky caps.

 

[Tubetec]

The Ht voltages listed in the schematic are ball park and measured values tend to vary a bit from case to case .

If the input is wired as the schematic says , you rely on whats connected to provide a grid leak resistance , it just looks like a problematic thing to do .
Maybe its just a mistake in the drawing , I dont know .

Pignose arent especially known for having a clean sound , they do seem to be trying to squeeze as much gain as possible out of the input , I agree going back to the conventional widom of the FenderBassman or Marshall .

You might have a leaky coupling cap somewhere , that will mess up the biasing on the next tube in the circuit and really make things sound like dirt .

The other thing is Im not familiar with this model of Pignose ,amp companies makes some strange choices in the way they voice their products these days , it sounds like the marketing department are telling the the engineers how to make an amp sound good ,

 

[thecr4ne]

The other thing is Im not familiar with this model of Pignose ,amp companies makes some strange choices in the way they voice their products these days , it sounds like the marketing department are telling the the engineers how to make an amp sound good ,

Yeah, it was produced briefly in the late 90's/early 2000's. It's 40W with a 10" speaker in a compact combo.
Will try to find some time to actually work on it. Will check caps for leakage/shorts and change the input at the very least. I have another amp to recap too so hopefully I can get moving on them sometime soon.
Thanks for all the input!

 

[Matador]

It's not always easy to definitely prove a new set of caps is needed: treat it like an oil change (preventative maintenance).

 

[musipol]

That schematic looks like about the schlockiest 2203/2204 implementation I have ever seen. But "if" it did sound good once, then leaking coupling caps or bad filter caps would be a place to start looking. Also check if it is oscillating due to lead dress or lack of shielded leads.

 

[gepicaster]

Small things to change: Connect the 1Meg at the input with a 68k as shown. Change the former contact from the 1M to ground. Reduce the cathode C8 to 25uF and reduce the anode C10 to 0,01 that reduces the deep at the input. Other things as to bridge the V1B - are tricky. Have fun

 

[thecr4ne]

So based on suggestions here and elsewhere I'm planning to do the following:
First and foremost, take some measurements. I need to dig out my meter and scope and set up a space to work on this, but I'll go through and see if I can find anything wrong. Depending on the outcome of that, I think I'll then

-Recap (electrolytics) as preventative maintenance. Made in China~20 years ago doesn't instill confidence in quality control.
-Replace input jack with shorting switchcraft, and add the 68k grid stopper to V1A
-Remove C9 bypass cap from cold clipper.
-Add 470K attenuator resistor between C11 and R15
-Replace R2 with 50K pot for variable bias, and add 1R cathode resistors to the power tubes for bias measurement.
-Generally inspect and touch-up solder joints as they don't look great at first glance.

Small things to change: Connect the 1Meg at the input with a 68k as shown. Change the former contact from the 1M to ground. Reduce the cathode C8 to 25uF and reduce the anode C10 to 0,01 that reduces the deep at the input. Other things as to bridge the V1B - are tricky. Have fun

Sorry if I'm misreading your suggestion, but I'm not trying to reduce deep, I'm trying to restore the lows and low mids, or at least tame the treble. I'll likely go ahead with the changes to the input though.

Thanks for the input everyone.

 

[CJ]

Probably the weird C core output transformer they used.

But here is a page on some mods:
https://remychan.com/projects/pignose-g40v-mods/

 

[MagnetoSound]

Be worth checking the speaker cone is moving freely.

 

[thecr4ne]

Always with the transformers CJ... (I kid. I'm sure that's a significant part of the sounds, but I'll give it a chance with some of the other changes)

I did see that page, and have also been looking at this one:
Pignose G40V

Probably the weird C core output transformer they used.