Don't Know What I'm Hearing

[boots]

I'm not sure if I'd know a parasitic oscillation if I heard it. Maybe one of you folks can recognize this problem. I've done some searching on this site but not sure how to describe the problem.

I'm still working the bugs out of this 5F6A/JTM thing I just finished. I pretty much followed the Bassman layout, but I'm using EL34's.

The amp sounds great! Until I crank the volume pot past about 8 or so and drive it hard. Then I get a "Thup Thup Thup Thup" sound that continues on its own until I back the volume pot down. The thupping is rhythmic, about 2 times per second. The sound is accompanied by flashing inside both EL34's - I can see it through the holes in the plate. Could it be the screen?

Does this sound like "motorboating"? Could it be blocking distortion? I don't know what that sounds like. The flashing in the tubes is disturbing. I have tried a different set of tubes, and the problem persists. The problem seems to be independent of bias - it still happens even when biased very conservatively.

Any ideas?

[xtian]

Sounds like motorboating. Not enough filtering? Power tube grid wires running too close to OT primaries?

[joeboo88]

Hi boots. Did you set up the power tubes to us 1k resistors across pin 4 and 6? Also a 5k resistor off pin 5. The bassman with 6l6s uses a 470ohm.
The reason I'm saying is I had the same issue with my JTM.
Let us know and maybe post a picture if I'm wrong.
Plus the more experienced guys know a lot and can help further. Cheers

[Stevem]

If all you did was build it as a Fender then you likely also have far too much drive signal hitting the 34s!

[pdf64]

Yes, likely its motorboating, ie oscillation via the HT; does the bass control setting affect it?
How about a lower value of V1 cathode bypass cap, eg 22uF?
I guess you're using a 12AX7 in V1, so much higher gain than a stock 5F6A?
And EL34 have perhaps over twice the gm of 6L6 types, so even more gain in the system.

Oscillation can't be parasitic if it exhibits the issue without a regular signal, ie if it starts thumping if you just turn it up with no input signal; parasites need a host!

Check that the HT string has been properly implemented, the components are good and grounds connected; but it may be that the barely adequate HT decoupling of the 5F6A is inadequate with the tube swaps you've made.

And yes, grid stoppers mounted at the socket terminals are a good thing, especially with all that extra gain.

[tictac]

Why are you turning up the volume past eight? Do you plan to actually use the amp that way? If so get ready for various part of the amp to fail starting with power tubes....

It like driving a car, sure you can floor it but most cars aren't made to be operated that way and somethings going to fail....

There are things that can cure parasitic oscillation but most of them change he tone of the amp, so if you're not going to play fully cranked why not be happy with what you've got?

Flashing in the power tubes is not a good sign, 1K screen resistors will help lower screen current but it's not going to cure what's going on in a fully cranked amp....

Might want to stock up on EL34's and output transformers.....

TT

[boots]

Thanks guys, that's all good info. This amp has been a real learning experience for me. I've never had troubles with any of the smaller SE amps I've built. This is a different animal.

I do have 1k screen resistors - they are only 1 watt resistors though. Could they be trying to break down when pushing the amp hard?

I did not have additional grid stoppers on the EL34's. I added a 5.6k to each socket. Did not cure the problem (actually, it seems worse now).

I popped in a pair of 6L6's and the problem is much less severe, but still occurs a little bit at full volume.

As for pushing the amp, I don't really play loud music these days except when testing amps! But I figure there is something wrong if the amp can't deliver the goods at full volume. This is more an issue of: I think something is wrong and I want to figure out what's going on.

I'm suspecting something more along the lines of a lead dress issue. I'm somewhat ashamed of how sloppy this build has become, so I haven't posted any pictures.

Thanks for all the input guys!

[tictac]

It's like you said, "This is a different animal..." you're used to SE amps which can be cranked up full volume without going into oscillation but Push Pull amps, especially fixed bias type have more than double the gain than a single tube provides, are not going to behave the same way as you are experiencing.

This doesn't mean there's a problem with the amp; in fact, since none of the Fender amp designs were made to be cranked up to full volume but designed for clean undistorted audio output, you could say your amp is behaving exactly the way you'd expect for the design....

Which is not a bad thing or a problem, especially since you say you play at lower volumes.

The symptoms you describe sound like motorboating, and the problem is due to you turning the volume up too high....

Clapton played his early JTM 45's cranked up to full and when played live he always had two amps. One was the backup amp since he blew up amps on a regular basis... Unless you copy the same parts, layout and lead dress you're not going to be able crankup and blowup amps like he did...

TT

[boots]

Maybe you're right, Tictac. I haven't played around with a lot of push-pull amps, and maybe I don't know what's "normal" for them. However, I have repaired a few Fender amps, and I have never seen this symptom with any of them. They all sound great at full volume. Although - all those amps had 6L6's, not EL34's.

I just can't help thinking that there is an actual problem with this one. Shouldn't it function properly at full volume? Or is that asking too much from this circuit?

[pdf64]

I think it's reasonable to expect that the amp should be stable and viable at all settings; motorboating indicates there's a HT decoupling problem.

Does the bass control setting affect it?

If flashing means the regular 'blue glow' of power tubes, then it's fairly normal for that to be modulated by large signals, so the thump of the motorboating, in flipping the power tubes between off and heavy conduction, may be expected to do that.

[boots]

Yes, the bass control does affect it. I can eliminate the problem by turning the bass control down to about halfway. Then the amp is stable at full volume.

The flashing is visible through the holes in the plate and from one angle, it looks like an orange glow that throbs in time with the motorboating. From a different angle, with the lights off, I can see a white flashing. It gives me the willies - I don't like it.

I can set the motorboating into action just by touching the strings sightly with the volume at max (and the bass control turned up). And it will continue on its own until I back off either the volume or bass.

By insufficient decoupling, do you mean that I need larger value filter caps? Or are you talking about a grounding problem?

[tictac]

I have repaired a few Fender amps, and I have never seen this symptom with any of them. They all sound great at full volume.

I've seen a few Fender amps myself being a former amp repair tech; had a sweet Blackface Pro Reverb that sounded great but if you cranked it past 8 it would completely shut off....

I would have never found this out except that I loaned the amp out to a studio musician who cranked it up past eight then calls me telling me "the amp has a problem, it keeps shutting off..." well then turn the volume down and remind me never to loan you my amp again!

I'll say it again, turning this type of amp up past 8 is not a normal operating condition, and if you're not going to use the amp at that volume and the amp sounds great at the volumes you like to play then why do you care if it motorboats when the volume is on ten?

I don't think I have anymore to say about this if you're convinced you have a problem when I don't think you do. If you're willing to spend the time and effort to make the motorboating go away that's your call but even then you may end up not liking the way your amp sounds...

CBS Fender put a .002uF cap on each plate to prevent oscillations, you could try it but many techs remove those sooner or later...

TT

[drew]

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

How does the power supply in your amp compare to the one in Merlin's 2 EL34 example, boots?

[vibratoking]

I am in the, "it should not motorboat at any setting" camp. There are different opinions on this obviously. Even if you can't hear the motorboating, there still could be some underlying oscillation that will negatively influence the tone of the amp. By the way, I have experienced amps that broke into HF oscillations only at the peaks of the waveform. I always use a scope to check. I have been able to fix this with lead dress and cap values. I like a well behaved amp.

I believe, I could be wrong, that Kevin O'Connor mentions that oscillations can be cured by increasing and sometimes decreasing the power supply caps. I think this is mentioned in TUT 3 with reference to the JTM. But I am at work and can't reference it at the moment. I have no experience with reducing the capacitance, but I am sure I read about it.

I find the volume knob is essential, especially Fender type circuits, to realize the full range of tones. There is a big tonal difference between 8 and 10 in my experience.

[tictac]

I am in the, "it should not motorboat at any setting" camp....

I agree with that in the context of a well designed and constructed amp shouldn't have oscillation problems at full volume setting.

But we're talking here, about a fella who says his build is so sloppy he's "ashamed" of it... he's asking for help and maybe the help is: gut it and start over. (Without actually seeing the amp we don't know)

Boot says he's happy with the sound and that he doesn't actually play with the volume past 8 so my .02 is if it ain't broke don't fix it and be happy with what you have...

2) My recollection was that Darr, O'Connor and others suggest that motorboating is caused by poor filtering which could be due to bad or under sized filter caps. This would mean that replacement or increasing capacitance would be the fix.... Again we don't know the cause on boots case because we don't know if he followed the 5F6-A schematic exactly, what parts did he use? What values are they? What is the layout?

TT