New blackface-ish circuit with 6BM8 tubes, motor boating and some kind of ringing.

[reddart]

With the master (PPIVMV) full up, and the normal volume up full, or interestingly down almost all the way, there is some kind of ringing I can detect (and tube current goes higher).

The amp has a 12AX7 for the 2 stages, BMT and volume in between, and goes into a LTP using the triode halves of the 6BM8's like a 12AT7.

Some points of the amp that might contribute: plate V is 310V, screens are 304V (I think these should be below 300V).
Presence control ala feedback into the tail of the PI with a pot (like a Marshall)
Shared grid load resistor for the pair of 6BM8s vs the conventional resistor on each tube.

Could any of these contribute to instability?

I might also work on the lead dress of the presence circuit as well.

[nickb]

Sounds like instability might be an issue. Try disconnecting any feedback including presence to see what effect that has. Also look at the layout to see if there could be places where output signals could be coupling back to earlier stages as this seems to be a problem when you turn the volume up.

What grid stoppers are use using? Please post a schematic of your amp so that we are not guessing.

[Chuck H]

What Nick said, and...

The motor boating is indicative of some sort of decoupling failure and not necessarily (or even likely) related to the ringing or HF oscillation. Both conditions can be related to decoupling though. It will be hard to move forward effectively at this point without a proper schematic that includes the power supply, HV circuit and grounding scheme.

[reddart]

Yes, I agree a schematic would be helpfull. I did have one at one time....this amp was originally abandoned because there was another problem I couldnt solve (which I recently solved), add 8+ years, a divorce, moving across the country, all my stuff just in storage, and finally getting back to the amp...the schematic has been "misplaced" somewhere along the road.

I will draft a schematic and post before proceeding further.

[reddart]

I haven't messed with the amp yet, but I think you may have something there on the feedback...this problem seemed to get worse after fixing another problem (cold solder joint in preamp ground) and rearanging the feedback components slightly. Lowering the Bass (or any of the tone controls) has little effect on the motoboating and ringing, so it seems to be something downstream from them.

[Chuck H]

It's not lost. It's one of a dozen boxes (though you don't know which one) full of stuff you're not using but don't want to part with. Yet never unpacked because you know it may not be long before you need to pack them again (How'd I do? ).

I didn't intend to put you on the spot. More like a "How to post this sort of question" suggestion.

I alluded to the decoupling. That should be a good place to start checking. Make sure your HV filters are wired correctly, working well and grounded, make sure you don't have two like phase circuits sharing a filter node and make sure to ground each node only with the circuit it's filtering. Make sure that you either star ground or at least ground the power amp and main filter separate from the preamp and it's filters. NO DAISY CHAINED GROUNDS!!!

In case the oscillation isn't related to decoupling you want to make sure grid leads are as short as practical and power amp leads shouldn't route through preamp territory at all in the layout.

With pictures and an accurate schematic WE could look all that stuff over. Then you'd have many pairs of eyes (and brains) on it instead of just yours.

[reddart]

I have followed Aiken's grounding (pretty religiously) for all of my builds, taking care to have the grounding of each stage at the appropriate filter nodes, and then referencing them to a star point on the chassis...though that doesnt rule out a wiring error.

I think layout is where there could be problems.

[reddart]

Question on the feedback loop: in terms of oscillation, is the feedback feed (from the OT secondary) likely to cause oscillations to other sensitive parts of the circuit, or receive signals from the amp circuits, causing the oscillation?

[nickb]

Question on the feedback loop: in terms of oscillation, is the feedback feed (from the OT secondary) likely to cause oscillations to other sensitive parts of the circuit, or receive signals from the amp circuits, causing the oscillation?

The intended negative feedback can become positive under certain conditions. It is the responsibility of designer to check that there are sufficient margins to ensure that never happens. We don't have enough info to determine if that is a problem in your case. So, the easy test is just to disconnect the feedback and see if it helps.

[Chuck H]

What he said^^^^^^^^^^^^^^^^^^^^^

And I think you're asking about proximity of the lead as well. So, yes. The location of that lead can be important. Remember that it's coming right off the OT secondary. If that lead passes near any preamp circuits or leads it is very prone to cause oscillation if the proximal circuits are of the same phase. That lead should be routed well away from the preamp. It can also help to place the series resistor for the feedback loop right at the impedance switch (or output jack depending on where that end of the lead terminates) rather than on the board. That reduces current in the lead and it's radiant field BEFORE it's routed rather than after.

[reddart]

Update (I know you're all holding your breath on this one)

Disconnected the feedback. Doesn't motarboat now, but still has ringing when the master and volume is all the way up. I think looking at the input grid resistors (or lack there of) as mentioned earlier may be in order.

I finally got the ExpressSCH program installed, and am starting to map the schematic.

[reddart]

It can also help to place the series resistor for the feedback loop right at the impedance switch (or output jack depending on where that end of the lead terminates) rather than on the board. That reduces current in the lead and it's radiant field BEFORE it's routed rather than after.

I was trying to wrap my brain around this...isn't the current of the entire circuit the same, no matter if its before or after the resistor?

And.....if it's not, wouldn't the current be higher after the resistor, since really electrons flow from ground to + (even though we like to think in terms of positive current flowing to ground)?

Edit...I forgot, it's AC, so maybe this is all a moot point?

[Chuck H]

I miswrote earlier and should have said voltage rather than current. Though I think it's actually both because you do need voltage and current to cause parasitic induction. And you're right that AC is the issue. Look at it like this... Say you want to ground a 10V source through a lead and a 10R resistor. If you place the resistor at the source and run the lead to ground there is 0V on the lead. If you place the resistor 10R at ground and run the lead to the source you have 10V on the lead. You are perfectly right to say that there is 1A through the lead either way (I think?). But 0V won't emit a radiant field. Since the FB loop works like this, though the termination is less grounded, you can still limit output signal interacting with earlier stages by reducing output signal voltage in the lead. Well, that's how I picture it anyway.