Ugly distortion on tremolo channel of Pro Reverb clone

[Sansteeth]

Hi everyone,
me again trying to get my Guyatone Reverb Jazz (GA-1050) running properly. 
It's been an adventure: https://el34world.com/Forum/index.php?topic=25912.msg281824#msg281824
Tremolo was acting up, the tremolo channel was a lot less gainy than the normal channel even though it has one more gain stage. I did a full cap job (well almost full, waiting for the bypass caps on the cathodes on the preamp tubes and one or two odd values here and there) and it solved all those issues.

Now I would like to tackle the ugly distortion I'm getting on the tremolo channel (and only on the tremolo channel) when the gain is on 10. I'm getting a trailing distortion that sounds a lot like a rubbing loudspeaker cone but I checked the stock speaker for a brand new 15" Eminence and I'm still getting that ugly trailing distortion.
It disappears when I remove the reverb circuit (by unplugging the RCA plugs to and from the reverb tank or by pulling out V3).
Here's the sound it makes with the reverb circuit (but with the reverb knob totally turned off):
https://soundcloud.com/user-390669199/guyatone-reverb-jazz-with-reverb-circuit/s-Jp6KobGGmXt
that's without the reverb circuit:
https://soundcloud.com/user-390669199/untitled-201guyatone-reverb-jazz-without-reverb-circuit/s-8x9OwXfWW08

First thing I did was a tube-by-tube tube swap with brand new tubes and I'm not getting any improvement.
I doubt it is the reverb tank as the reverb knob is turned all the way down and I would except the ugly distortion to get more present as I add more reverb but it doesn't.
My research for ugly distortion gives me a whole lot of material on blocking distortion, I never really heard what it sounds like but I was told it's like those whacky velcro-sounding fuzzes that sounds almost gated and that's not really what I'm hearing. But who knows, maybe grid-stoppers could solve that?
Let me know what y'all think I dont really know where to start here
Schematics of the 100W version of my amp (my amp, even though 50W has the same component values/schamtics as the 100W) and the Fender AA165 I based my blackfacing on are attached to this post

[pdf64]

How about if the V3 reverb driver cathode bypass is lifted?

[nandrewjackson]

Do you mean the gain at 10 o' clock position?


Or do you mean dimed, turned all the way up?


If you mean that you're getting distortion when the tremolo channel volume knob (preamp gain knob) is dimed, turned up all the way, then that's why you're getting distortion. 

[Sansteeth]

How about if the V3 reverb driver cathode bypass is lifted?

Hey thanks for the lead, I'll try that tomorrow!
What's the idea behind it? If the cathode bypass is lifted then it changes the bias of that stage to something somewhat colder? Thus maybe reducing that ugly distortion?

Do you mean the gain at 10 o' clock position?


Or do you mean dimed, turned all the way up?


If you mean that you're getting distortion when the tremolo channel volume knob (preamp gain knob) is dimed, turned up all the way, then that's why you're getting distortion. 

The knobs on that amps are copies of the ones Fender traditionally uses for their Pro Reverb, the ones numbered from 1 to 10. So when I say "on 10" I don't mean at 10 o'clock, I mean turned all the way up, I'll try to be clearer next time, sorry about that.
As to whether I missed the memo that guitar amplifiers produce distortion when pushed, I'll just let you guess ;)

I heard that those Fender can sound pretty farty when pushed, but that's not farty that I'm hearing, it's a trailing distortion behind the main signal. I think the sound samples show the difference between the distortion you are expecting from the amp (when the reverb circuit is out) and the ugly one decaying in the back (when the reverb circuit is on).

[pdf64]

For any reasonable control setting, the earliest point to clip in a Fender reverb channel preamp is the V3 g-k diode. Removing the bypass cap kinda bootstraps the diode, so same signal Vac appears at either end, which significantly increases the signal required to overdrive it.

[finkaudio]

....obviously, the cathode bypass capacitors are still the old ones. So what is the point? Nobody knows the conditions of the old caps. Those bypass caps are the first one that I change together with the power supply capacitors.


Best KH

[Sansteeth]

For any reasonable control setting, the earliest point to clip in a Fender reverb channel preamp is the V3 g-k diode.

Oh I did not know that! Considering how are biased the two first stages of that channel (with a 12AX7 with a 1.5K and a 820 bias resistor on the first and second diode respectively) compared to the 12AT7 of V3 biased with a 2.2K, I really thought that the 12ax7 making up the two first stages would be the earliest points to clip. Or maybe I shouldn't compare the absolute values of the bias resistor when comparing a 12ax7 and 12at7 bias?

Removing the bypass cap kinda bootstraps the diode, so same signal Vac appears at either end, which significantly increases the signal required to overdrive it.

I'm not familiar with what you mean by bootstrapping in this context (bear in mind I'm not a native speaker). I know of Merlin's "bootstrapping" of a cathode follower stage which is about reinjecting signal from a DC-coupled cathode follower back to the plate of the previous stage to increase the gain. However I'm not yet entirely sure what a bypass cap really does, I think it's going to be a great way for me to find out! 

....obviously, the cathode bypass capacitors are still the old ones. So what is the point? Nobody knows the conditions of the old caps. Those bypass caps are the first one that I change together with the power supply capacitors.

I don't remember why those 25uF didn't appear in my last capacitor order, maybe they were not available in the form factor I needed and eventually decided to order them on another website. But I'm not sure what you mean by "what is the point?"

You two seem to be thinking "bypass cap" that's really useful, I'll dig deeper into that. I was aware that the filtering cap should be given special attention when they reach a certain age considering their role in the circuit and the voltages they are subjected to, I didn't know the same kind of treatment was recommended for bypass caps. After all all they are both electrolytics in my case so that makes sense, thank you two for the lead!

[Willabe]

Removing the bypass cap kinda bootstraps the diode, so same signal Vac appears at either end, which significantly increases the signal required to overdrive it.

From Merlin's web site, scroll down to > page 25 > 1.18/K bypass cap;

http://www.valvewizard.co.uk/gainstage.html

I was aware that the filtering cap should be given special attention when they reach a certain age considering their role in the circuit and the voltages they are subjected to, I didn't know the same kind of treatment was recommended for bypass caps. After all all they are both electrolytics in my case so that makes sense, thank you two for the lead!

Yes, because they are the same, same internal construction, just different voltages and uF values. The negative grid bias supply for the power tubes has 1 or 2 electrolytic's also. When you do a cap job, you change ALL the electrolytic caps in the amp, power supply filter caps, K bypass caps and -grid bias caps.

[finkaudio]

....obviously, the cathode bypass capacitors are still the old ones. So what is the point? Nobody knows the conditions of the old caps. Those bypass caps are the first one that I change together with the power supply capacitors.

I don't remember why those 25uF didn't appear in my last capacitor order, maybe they were not available in the form factor I needed and eventually decided to order them on another website. But I'm not sure what you mean by "what is the point?"

You two seem to be thinking "bypass cap" that's really useful, I'll dig deeper into that. I was aware that the filtering cap should be given special attention when they reach a certain age considering their role in the circuit and the voltages they are subjected to, I didn't know the same kind of treatment was recommended for bypass caps. After all all they are both electrolytics in my case so that makes sense, thank you two for the lead!

What I wanted to say is that without some fresh bypass caps, it's difficult to know what's going on. Those caps in GA-1050 are easy to get.....Mouser, Digikey, Farnell, RS. As I normally work with HiFi-electronic, I have lots of different ones available. Elna, Nichicon, Phanasonic.....but I'm aware this does not help you :-)
 

[Sansteeth]

Hey thanks everyone for taking the same to direct me to the good sources, things got in the way and I just had time today to implement what pdf64 was recommending trying, that is, to lift the bypass cap on V3. Unfortunately, by doing that I'm getting more distortion, just as ugly but slightly different, that one does sound like what people describe when they talk of blocking distortion.
So I'm going to do the next thing you good folks recommended that is changing all the bypass caps on all the cathodes. I still have the "Made In Japan" ones so a cap job on those is past due. Let's hope that's what the issue is. It might take some time as I'm waiting on a potential customer's decision on a custom-made pedal and I'm trying to keep supply's shipping costs reasonably low by grouping my orders.

From Merlin's web site, scroll down to > page 25 > 1.18/K bypass cap;

Sometimes the answer is just sitting on my bookshelf, thank you Willabe for reminding me! 

What I wanted to say is that without some fresh bypass caps, it's difficult to know what's going one. Those caps in GA-1050 are easy to get.....Mouser, Digikey, Farnell, RS. As I normally work with HiFi-electronic, I have lots of different ones available. Elna, Nichicon, Phanasonic.....but I'm aware this does not help you :-)
 

I understand your frustration, but let me explain my approach. I'm not "just" trying to fix my amp, I'm trying to learn, with your help, how to fix amps.
The context here being, I always play this amp clean, partly because I know it has that weird crying distortion in the back, but also because that's my clean amp, so eventually I sort of forgot about that problem. The recap job brought the tremolo channel back to life and fixed the tremolo, great, I got a little enthusiastic and decided to crank it a little, and there it was still, that ugly distortion.
If, indeed, the problem is the bypass capacitors and I had changed them with the whole cap job, I would not have had the problem anymore but I would also have no clue as to what I did that fixed it and which was the incriminated capacitor. At this point in my journey, the process to get the amp to work is almost more important than having a working amp. A fixed amp is useful to the guitarist that I am, it is utterly useless to the amptech I would like to become if I have no idea how I got it to work.
I understand it's annoying to have yet another thread popping up about problems on the same amp after the last problem got fixed while the symptoms might all be related but I need to understand the cause of each individual symptom.
For all I know, my problem could be too much signal entering a following gain stage, could be the bias of one of the tubes, could be subsonic oscillation, could be an open resistor somewhere. This is my first amp on the bench, some people here have seen thousands, I'm not asking them to fix my amp, I'm asking if they've encountered this problem before, what comes to their mind when they hear those symptoms and what was the thought process to that hypothesis.
So that is my approach. I certainly do not wish to waste anyone's time or patience.

In any case, your help has been precious and I have learned something yet again, about amptech workflow and standard procedure, which is extremely precious to me. So thank you very much, I will report back when I get those last caps changed!

[finkaudio]

I understand your frustration, but let me explain my approach. I'm not "just" trying to fix my amp, I'm trying to learn, with your help, how to fix amps.

No worry, I'm not frustrated  . It's just so difficult to say anything about an amp with unknown condition of the parts. Once you got it working again, it's a lot easier to find a possible problem. I also had some serious crackling noise on my amp and it took me a while finding it. The first problem was that I soldered in two wrong capacitors (10n instead of 1n and 22n instead of 2.2n). That did not fix the problem, but it was nice to find out my stupid mistakes. 

In my case, I changed the Master volume to a PPMV (LaMar) and after I switched back to the standard MV, the crackling noise was gone. Guess I screwed up the somewhat "different" ground layout.
I also soldered a 560p capacitor on parallel to the A/K of the driver tube (used in newer Fender). The Reverb transformator of the Guyatone is 8k/8 Ohm in comparison to the 22k of the Fender one - at least I could find that in one of the schematics. Maybe one day I measure it and find out the truth.

If you cannot get the Bypass caps, I can send you some.....it's not complicated from Germany to Netherland.

ATB KH

[Sansteeth]

I also soldered a 560p capacitor on parallel to the A/K of the driver tube (used in newer Fender). The Reverb transformator of the Guyatone is 8k/8 Ohm in comparison to the 22k of the Fender one - at least I could find that in one of the schematics. Maybe one day I measure it and find out the truth.

ATB KH

When you mean A/K, you mean Anode/Cathode? What do you achieve doing that? I'm really curious
the reverb transformer on my Guyatone is a 10k/8 Ohm, at least that's the value stamped on mine! haha

If you cannot get the Bypass caps, I can send you some.....it's not complicated from Germany to Netherland.

I really appreciate but I will make that order pretty soon, so don't worry about me, thanks a lot though!