Fender Style tube guitar amplifier: hard clipping distortion from reverb driver tube

[raketenmann]

Hey folks
Probably some of you can help me because this is probably a common issue or an easy fix - I don't understand electronics too much, but I can do some minor stuff probably.... So here's my problem:

I bought a used Gomez The Tahitian Amp 2 years ago. It is a handwired amp, built in the USA. I don't know what circuit style, some say it it sort aof a Fender Princeton Reverb amp, only with 100W and as a head unit, no combo. The amp was working very good. After some months I did was swapping all the tubes for NOS RCA 6L6 power tubes and also all the preamp tubes for NOS ones. I also just wanted to have 50Watts, running it on only 2 6L6s.

After swapping the tubes, I did get some bad, hard clipping distorted sounds with higher input levels on the input. The distortion only occures when I hit the strings very hard on a high output guitar. After some tests I realised that the more reverb there is, the more distortion. But also with the reverb off the amp did distort sometimes. So I experimented with removing the reverb driver tube. Without the reverb driver, the distortion was gone. So I inserted the old tube again, wich previously worked perfect, but it did distort again. So I thought probabyl th connectors on the tube sockets could be faulty and insterted some contact spray, but it didn't change anything.

So in short: After tube swapping, distortion is here, but it is not a faulty tube, cause it appears with all tubes.

Do you have any ideas what could be the problem? I am pretty sure that it needs to have something to do with the tube replacement, because before there was no issue...

Every help is really appreciated!

Kind regards,

Fabian

 

[user 130681]

Put the old valves back in and start from there.
Probably put a 12AX7 in as reverb driver instead of the correct type.

 

[raketenmann]

Put the old valves back in and start from there.
Probably put a 12AX7 in as reverb driver instead of the correct type.

Hi there
As I already explained, I did put in the old tube again but the problem remaind, also with the exact same old tube back in place.... But I can try out the 12AX7 instead, I do have some of them here!

best reagrds,

Fabian

 

[Walrus]

I think Jon S meant put all the old tubes back in to check no distortion, then swap them one by one.

 

[raketenmann]

I think Jon S meant put all the old tubes back in to check no distortion, then swap them one by one.

OK, so far I understand. Well, I didn't do that exact thing, but for sure I can play the amp without this distortion with all tubes in there except the missing reverb driver tube. I thought this maks clearly that the problem occures somewhere in the reverb driver section - what do you think?

 

[mad.ax]

"NOS RCA 6L6"...
Which version precisely? 6L6 nothing (Metal case)? 6L6G? 6L6GC?

 

[user 130681]

Thank you Walrus, the joys of my dislexia, it all made sense to me.

A schematic would be useful but I understood that there was no distortion until you replaced the valves. So, put the valves back and check what the difference is.
It could be as simple as a good valve, draws more current and makes the HT (high tension {voltage} supply to the pre amp stage) fluctuate. That may be the root of your issue.

 

[raketenmann]

Hey there
OK, so I did some experiments with swapping valves. Right now, I am using all the new tubes, also on the reverb driver side and I am having no issues. I think it could have been a contact issue on the tube socket, so for now I am happy. Except for the Vibrato: When I go into deep Vibrato setting I also get some weird distortion, but this has been the case since I own the amp and I know at least from one other owner of the same amp that he has the same thing. Probably this has to do how the Vibrato is designed, I remember reading about that somewhere else. But on moderate Vibrato setting, it is working good.

As powertubes I am using 6L6GC's from the 50ies or 60ies, they are RCA black getters.

best regards,

Fabian

 

[TDM]

You would have wanted to rebias after putting in the power tubes and would have wanted/hoped they were reasonably
matched by current draw to start with. They would have black PLATES, getters are bare metal/shiney/silver.

But that wouldn't cause the issue with the distortion in the reverb.

Bias wobble vibrato is effected by the bias, so proper power tube bias is part of having it perform the best.
On a princeton it does sometimes get fuzzy as the driver stage is different than most of the other blackface amps
(cathodyne vs LTP) I change a couple of resistors and a cap in the vibrato circuit and it gets you slower deeper while keeping it cleaner.

 

[raketenmann]

Hi @TDM
Thanks for you reply. Well, I just realised that I didn't take enough time for giving a full explanation of what I did in my first post.

Of course I am totally aware that a tube amp needs to be biased (at least class AB, some of my amps don't need to be biased). When I did the tube replacement, a friend of mine who has more experience with amps come over and adjusted the bias on the amp. However, we just had time to check the amp shortly and we did it without connecting the tube tank and so we didn't hear any distortion. I just realised after he went, and then he said, I need to bring the amp over to his workshop, but I just wanted to get some ideas first to where to find the problem or - as I managed to do so - fix the problem myself.

So yes: The 6L6GC (and well, you see that I don't know exactly about getters and plates... are a matched, NOS pair and were measured and tested by a professional on a Hicock tube tester. I paid a good amount of money for them, but the guy who sells them is an expert in tubes and knows the stuff exactly. He also assured me, that he tested the power tubes for some minutes in his HiFI tube amplifier, so this is also the reason why I was very sure that everything is OK with the tubes.

I just tried the same trick as with the reverb driver yesterday: I removed the 12AU7 which is the Vibrato tube, brushed the pins, put on some contact spray and put it back in: The distortion in extreme settings is almost gone, it can get audible a bit, but almost inaudible, so now everything is OK.

Thanks so far for your help!

 

[pkuchar]

Hi @TDM
Thanks for you reply. Well, I just realised that I didn't take enough time for giving a full explanation of what I did in my first post.

Of course I am totally aware that a tube amp needs to be biased (at least class AB, some of my amps don't need to be biased). When I did the tube replacement, a friend of mine who has more experience with amps come over and adjusted the bias on the amp. However, we just had time to check the amp shortly and we did it without connecting the tube tank and so we didn't hear any distortion. I just realised after he went, and then he said, I need to bring the amp over to his workshop, but I just wanted to get some ideas first to where to find the problem or - as I managed to do so - fix the problem myself.

So yes: The 6L6GC (and well, you see that I don't know exactly about getters and plates... are a matched, NOS pair and were measured and tested by a professional on a Hicock tube tester. I paid a good amount of money for them, but the guy who sells them is an expert in tubes and knows the stuff exactly. He also assured me, that he tested the power tubes for some minutes in his HiFI tube amplifier, so this is also the reason why I was very sure that everything is OK with the tubes.

I just tried the same trick as with the reverb driver yesterday: I removed the 12AU7 which is the Vibrato tube, brushed the pins, put on some contact spray and put it back in: The distortion in extreme settings is almost gone, it can get audible a bit, but almost inaudible, so now everything is OK.

Thanks so far for your help!

If you removed two power tubes you should up the impedance. If your at 4 ohms with four tubes go to 8 ohms with two tubes.

 

[raketenmann]

If you removed two power tubes you should up the impedance. If your at 4 ohms with four tubes go to 8 ohms with two tubes.

Yes, I know this as well, but thanks for taking care!