Waveform shifts during loud notes?

[Matt T.]

I rebuilt a BF Bassman into the preamp circuit below (Bass channel is a tweeked-for-guitar version of the 6G6-B circuit...Normal channel is just Normal BF Fender without reverb/tremolo) with a 'typical' BF Fender PI and power amp (there is a 220K resistor to ground just before the PI input cap not shown in that schematic).

When I 'scope the amp, the waveform at the grid of V2B (third stage of Bass channel) shifts when played hard or loud-ish notes are picked...then the waveform slowly returns to it's original position on the screen (I can't remember if it shifts up or down). The amp can sound pretty darn clean when this is happening. It doesn't really happen at all at really low volumes...happens to some degree at pristine clean settings...happens quite a bit when the amp is on the verge of breakup and just gets worse as I increase the volume. This channel has about the same amount of gain as the Normal channel btw. This is some sort of dc shift...correct? Is it most likely that the grid is pulling current? If so, why would this grid pull current where the grid of the second gain stage of the Normal channel doesn't? Or could it be a leaky cap? I used Solen fast caps in this amp. If the amp sounds good should I even worry about it? Doesn't sound that good cranked but I only intend to use it for sparkley clean tones...which btw at anything but quite loud volumes this channel actually does the BF thing better than the BF Normal channel (more twangy/spanky but still very similar...surprisingly to me). Thanks for any help.

[MWJB]

You say it sounds OK, but you can't turn it up? Is ythis not slightly contradictory? What are you going to do if you play with a louder band and you have to get "quite loud"? If it's a clean amp, it should stay reasonably clean at high volume.

Plate voltages on the Bass channel look a little low to me, bumping them up (reduce value of dropping resistor between #4 & #5) will help that channel stay cleaner for longer.

[Matt T.]

Thanks for the response.

I can turn it up but it just doesn't sound that good when it gets distorted. But that's the case for virtually every BF-style amp I've ever played...I never expect them to sound good when cranked into distortion. Up until it starts getting fairly distorted (which is quite loud) I love the tone.

After the choke the dropping resistors go 4K7-10K-36K. I had forgotten I had used such a big dropping resistor for that last supply node. I'll try lowering it.

I just thought it was a bit strange that that grid would shift as much as it does.

[Enzo]

Just sounds to me like the scope is finding a different trigger point. Alter the trigger setting and see if the shift goes away. All it takes is a little distortion and once you turn it up enough, that wrinkle in the waveform becomes large enough for the trigger to notice.

[Matt T.]

I'll try that Enzo, thanks. But I guess I thought of the triggering as more of a left-right (i.e. horizontal displacement) thing as opposed to a up-down (i.e. vertical displacement) thing. It's set to 'auto' so I guess I didn't think the adjustment did anything (unless it was in manual).

Question that I just considered: I haven't looked at what the B+ is doing. Is it possible that since all 4 stages use the same B+ node and maybe the phases are such that 3 stages are all pulling down the B+ for the same half of the waveform while only one stage is pulling for the other half? I remember reading in TubeCAD that a direct-coupled gain stage/cathode follower pair tugged on the B+ 'evenly' for each half of the waveform (provided the Ra for the gain stage and Rk for the CF were both the same value). However, I have a 220K Ra and a 100K Rk on that first tube.

[Enzo]

OOoooooohhhhhhh.

I read it as the waveform moving over. YOu mean it moves vertically, as if you turned the position knob.

Is the scope on DC or AC coupling? Watch on DC. On AC if the DC offset changes it will move at first then recenter. ON DC, any offset just moves the trace and it stays there. Easier to see.

Do indeed monitor the B+ while this happens. I could see some serious B+ sag affecting charge on the tone caps