Pignose G40V tone nuance question...

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chikov
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Pignose G40V tone nuance question...

Post by chikov »

I met a fellow who asked me to check out his Pignose G40V amp. Some tech a couple of years ago did a cap job on it and replaced an imput jack. His complaint was that it sounded weak and too distorted. After looking into it, i noticed that his 1M resistor on the imput jack was not grounded ( the lug that grounds the hot tip of the jack was connected to 1M resistor but was not connected to any ground at all). After I fixed that, the noise and distortion went away, but, the owner says that it lacks the bottom end ( the thump) that it used to have before it worked on a couple of years ago. I checked all the tubes on my tube tester- all good. Tonight I checked the polarity on all caps - all good. Does anyone know what can cause for his amp to loose the kick, the thump on the lower frequencies?
PS I tried to use a different speaker, but the guy said it is still not there. To me it sounds ok, but it goes into overdrive pretty soon ( not enough head room, really)...
Stevem
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Re: Pignose G40V tone nuance question...

Post by Stevem »

If you have the type of Tube tester that only checks for emission then that will not tell you much!
A tester needs to tell you what the Tube does Gm wise.

I would say you need to try out a known good kickin pair of output tubes in the amp an if they do the trick measure the current they idle at for future reference.
I would bet that if you have done this when you first worked on the amp and then compare the idle current to what it is now you would find it's much lower, and this inturn means the output tubes are going south!

It's also possible some of the new power supply filters are not still in tip top shape so jumping the main ones out would be a quick test to make.

The first production run of these amps where also known to burn up power transformers so I would check to see how hot that's running and what the B+ on the output Tube plates is.

Note also that even though the preamp Tube sockets in the are a nice ciramic , the receivers are just Aluminum not Steel , so poor connections and noises are common out of these if you are not very careful taking tubes in and out of them.

Also note that these are not 40 watt amps, the best I have been able to muster out of my GV40 with real strong output tubes is 32 watts RMS!

If you go to the Blue guitar page he has a nice blue's mod for this amp that I did to mine that you might want to check out.

Even with all of this it's hard to beat a amp like this that cost all of 220 bucks when they first came out!

Nice find on the input jack resistor by the way!
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Not screaming like the passengers in his car!😊

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chikov
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Re: Pignose G40V tone nuance question...

Post by chikov »

Thank you, Stevem, for such a detailed help. That gave me several directions and the first one I chose was to check all the plate voltages. Wel, well, well, rightaway I found that on the plate of V1a it had only 185VDC, on the V1b 237V, on V2a 147V(!) on V2b 277V, on the phase inverter for some reason pin 1 was 182V and pin 6 was 180VDC...on the nodes Va 407VDC, Vb 396V, Vc 319V, Vd 273VDC... According to the schematic, I should be running about 280VDc on both plates of my first preamp tube... I wonder what could be causing such a voltage drop?...
BTW, the tone controls hardly make any difference in the sound...
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Re: Pignose G40V tone nuance question...

Post by Stevem »

Here's what I see from the info you posted.
You main power supply node Va thru Vc are all good as they are only 2% off from what the schematic states, however your Vd node is 20% down from what it should be,

In regards to your posted plate voltages vs the schematic I see this.
V1a 16 low.

V1b 7% low

V2a 10% low

V2b 6% low

V3a 28% low

V3b 20% low

What this all shows to me is that the amp Phase inverter section is drawing far more current then it should and in turn loading down the amps power supply.

I would first place a known good 12ax7 in that socket and check that it's not a 12at7 , 12au7 or 12ay7 in there now as all of these tubes will draw more current then a AX7 will.

If that checks out then check all of the resistors on the Cathode side of the PI tube .

I would bet that the PI Cathode voltage is far above the 37 volts stated on the schematic!?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Stevem
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Re: Pignose G40V tone nuance question...

Post by Stevem »

Here's what I see from the info you posted.
You main power supply node Va thru Vc are all good as they are only 2% off from what the schematic states, however your Vd node is 20% down from what it should be,

In regards to your posted plate voltages vs the schematic I see this.
V1a 16 low.

V1b 7% low

V2a 10% low

V2b 6% low

V3a 28% low

V3b 20% low

What this all shows to me is that the amp Phase inverter section is drawing far more current then it should and in turn loading down the amps power supply.

I would first place a known good 12ax7 in that socket and check that it's not a 12at7 , 12au7 or 12ay7 in there now as all of these tubes will draw more current then a AX7 will.

If that checks out then check all of the resistors on the Cathode side of the PI tube .

I would bet that the PI Cathode voltage is far above the 37 volts stated on the schematic!?

This large current draw is what's pulling that Vd node way down!
Last edited by Stevem on Wed Jul 26, 2017 10:42 am, edited 1 time in total.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
chikov
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Re: Pignose G40V tone nuance question...

Post by chikov »

I did some checking around and first thing I found was R26 on NF just 4.5 K instead of needed 100K. I replaced it with 100K. Next, I am not positive but I think one of preamp tubes was 12AT7 and I made sure all 3 of them are 12AX7. I swapped 6L6 tubes with a known good pair, but it did not make any difference. After that the voltages went up. Not quite here yet, but close. On V1a plate it is 205V ( 10V less), V1b 244V ( also 10V less). The cathode is 1.8 on pin 2 and 2.6v on pin 8 which is very close. On V2a it is 150V on pin1 ( 11V less) and 283v on pin 6 (10V less). The cathodes are 1.6V (close) and 152V ( 10 V less).
The V3 has 217V on pin 1 and 207 on pin 6 ( schematic shows 235V and 216V)... Cathode voltage is 38.9. On Va 408, Vb 397, Vc 320, Vd 283, Vc 273VDC. I can still see about 10V drop all around. Is it normal? I played guitar through it and it sounded good, but the overdrive kicks in half way up on the volume dial ( little too early IMO)...
I cheched all the preamp resistors - seem to be all good... my secondary on the PT reads about 326VAC - normal...
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Re: Pignose G40V tone nuance question...

Post by Stevem »

I think your good now, but do you have,or can you get access to a O scope and a 8 ohm 25 watt load resistor to check where the output stage clips at?
Tonight I will forever dome check on mine and give you some info.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
chikov
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Re: Pignose G40V tone nuance question...

Post by chikov »

Yes, I do have O scope and I have not used it much, so I will be looking forward to learn more about actually diagnosing the problem spot with it. I do have 8 ohm dummy load resistor and the wave generator too. All your help is very appreciated.
chikov
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Re: Pignose G40V tone nuance question...

Post by chikov »

I generated 1000Hz sine waive and plugged it in into the amps imput, next, i scoped the grid on the pin 2 of Va1 ( straight signal) - perfect waive, but when I put my probe on the next grid of V1b ( pin 7) I get funny looking upper wave - see photo:

I wonder is t normal? Where should I look for a problem?
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pompeiisneaks
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Re: Pignose G40V tone nuance question...

Post by pompeiisneaks »

Looks like typical compression from the tube, it's getting too much signal and being distorted. It may be a bit more than you want, so you may need something to attenuate it prior to hitting the next stage, there are a couple ways to do that: 1. add a split load on the anode of the previous stage and pull the signal out there to the coupling capacitor, say you've got a 100k, change it to a 47k and 68k and take it out in the middle, or 2. put a resistor inline between phases, this does cause some tonal change though too, you may also want do either try NFB or adjusting the bias of the previous stages.

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Stevem
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Re: Pignose G40V tone nuance question...

Post by Stevem »

That's the way these amps compress in stock form and is the reason for the mod that the Blue guitar site has to change it.

But back to the amps lack of punch deal, is it better now?

If not I would have to go back to looking at the filters that where changed before you got your hands on it !
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
chikov
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Re: Pignose G40V tone nuance question...

Post by chikov »

Stevem wrote: Thu Jul 27, 2017 1:02 pm That's the way these amps compress in stock form and is the reason for the mod that the Blue guitar site has to change it.
I will google this mod, i think I should perform it too, because this amp needs a little more of a head room.
The only way I could check those filter caps was with my Fluke meter and all of them ( while attached to the PCB) showed about 130 uf, but the last one showed about 99 uf. I know that the most accurate way to do so is by desoldering them first, but what are the other methods of testing capacitors?
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Re: Pignose G40V tone nuance question...

Post by pompeiisneaks »

Using an esr meter to see if the capacitor is starting to fail

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Re: Pignose G40V tone nuance question...

Post by xtian »

You're over driving the tube with your test signal. Play with the guitar and listen with your ears before you decide what to do.
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chikov
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Re: Pignose G40V tone nuance question...

Post by chikov »

pompeiisneaks wrote: Thu Jul 27, 2017 3:05 pm Using an esr meter to see if the capacitor is starting to fail
To my shame I do not have ESR meter, but I heard about them on different videos and I think I am ready to obtain one. Which one should I get? Any suggestions? Thanks.
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