New tubes --> New Oscillation (6eu7)

[Dreams]

Hey guys. Schematic attached.

Got this Gibson GA-200 to fix up. Last step is replacing the microphonic, but still good sounding/working 6eu7 preamp tubes (V1+ V2). Put a pair of new Sovtek brand tubes in there, and channel 1 (V1) starts oscillating. The frequency and amplitude of the oscillation is affected by the settings of the volume pot and the treble pot.

All other tubes have been pulled, and the connection between the volume pot and the rest of the amp has been disconnected, and no change. The problem is sufficiently isolated.

Channel 2 works great with either type tube; the old GEs or the new Sovteks. Channel one ONLY behaves with the old GEs. I've tried a total of 5 different tubes in the V1 position; 3 old ones, 2 new.

I suspect there is a problem with the treble pot. When disconnected, or full off, the stage behaves. This is just a suspicion. There could be some grounding issue: seems like everything is grounded at the closest available point, rather than any considered scheme.

Now, I haven't played around with it too much, and I'm not exactly asking for solutions, I'm just trying to understand what is going on here. Is the circuit juuuuuust on the edge, and a newer (stronger?) tube pushes it over? Is there something broken, or is the new part revealing bad...layout?

Gonna play around a little, but if anyone has any ideas, anecdotes, etc... to contribute, please do. Any questions, please ask.

For clarity: V1 is a dual triode; plates, grids, and cathodes tied to each other. "500k RA" is the bass pot, "500K A" is the treble pot, and "250k A" is the volume pot.

[tubenit]

I've rewired 6EU7 sockets to 12A_7 socket wiring so that I can choose between 12AX7, 5751, 12AT7, 12AY7, 12AV7 or 12AU7.

Food for thought.

With respect, Tubenit

[PRR]

It aint easy to make one stage oscillate.

What frequency, growl or squeal?

I'd be checking those grounds--- actually bus them together instead of relying on old chassis contacts.

Also verify as many part-values as possible.

[Dreams]

Grounding it is. Lifted the three ground points that I could (not gonna mess with the grounded jacks) and busssed them back to the input stage's filter cap. No more oscillation.

Starting with the volume pot fully ccw and rotating it cw, the oscillation was first a growl, then a squeal.

The components were mostly fine (within tolerance anyway...) save the cathode caps, which trippled in value. Replaced 'em.

Tubenit, I went over that with the owner, and he wasn't into it. Whatever: it would have been a giant pain to get to those sockets... Thanks though.

PRR, could yo please elaborate on what you said about one stage oscillating? Part of the reason I posted about this was because it did seem strange; at least out of the ordinary for me.

Thanks!

[Dreams]

Sorry drgonzonm, I meant 'volume pot' not 'input jack'. It was pretty late; I edited my reply.

I actually did compare the datasheets; it looked like Sovtek just copied the specs!

Actually the schematic was nowhere to be found on the internet, luckily, Gibson had the good sense to glue a copy onto the side of the amp's chassis. I posted a picture of the full schematic here:

http://www.el34world.com/Forum/index.php?topic=15108.0

[PRR]

> one stage oscillating?

Oscillation requires non-inverting gain.

Our basic gain-stages are all inverting.

There's tricks.

A transformer can flip the polarity. This is very happy in radio, where transformers may be small. Uncommon in audio.

One C-R stage approaches 90 degrees phase shift. Three C-R networks exceeds 180 degree shift. Bring that back around to the input of the inverting gain stage, it will oscillate where the C-R-C-R-C-R network hits 180 degree shift. This is the Fender Tremolo oscillator.

If one stage inverts, two stages makes non-inverting. So sneakage back from stage 2 output to stage 1 input is likely to squeal. I found this in a Champ. Moving wires around increased it or killed it. Variations (with a more stable frequency control network) are very common audio oscillators.

A special case. If cascaded first and second stage share a cathode resistor, there's positive feedback. (Cathode is out of phase with grid. Plate-cathode sneakage won't oscillate: there's voltage gain but no current gain. But a second tube's cathode may give enough current gain to bring the system into oscillation.)

I can't figure what you had, since you had it with most tubes out. But it didn't growl when new, it just-barely growled now depending on the tube: what changed? By age and description, my guess was a "ground" didn't have a low-enough resistance due to tarnish, the circuit was not what it appeared to be. Maybe if I threw 5K darts at different "ground" points and pondered what-if, I would see an oscillator. Easier to suggest you try some soldering work.

[Dreams]

Makes sense. Thanks!

I believe the problem was the treble pot's grounding, but it's all good now, so I don't care to worry about the exact details, dig?

I found that I could increase or decrease the level of the oscillation by adjusting the proximity of the (unshielded) grid wire in relation to the cathode. I briefly (for about ten seconds) considered shielding the grid wire and changing the layout to make it shorter, but I figured that if the original layout has worked for the past 50+ years, I should probably actually just Fix the thing, not band-aid it.

So the hell with it; busssed all the grounds = no more squealy.

All that's left to do now is replace a plate resistor or two (hissy), source a new switch for the compression, throw in some new speakers and call it a day. In that order.

I'm pretty charmed by this thing, by the way. Pick one up, give it all of the love. If you ever see one, that is...