Guitar power amp proposal

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Last week I made some PCB’s and put the whole amp together. There were some alterations made to the schematic:
- Quasimodo filters in the PSU. For the high voltage 10n || 100n-470r, for boas and MOSFETs 10n || 100n-220r. All secondaries are snubbed. All rectifiers UF4007. On the scope I observed some ringing on both power supplies, around 2 V p-p without the rc snubbed in place. After snubbing all ringing (which wasn’t much to begin with) is gone. The PSU is clean now, as far as I can see.
- no snubbers on the OPT yet. Don’t have a 100x probe yet, so can’t really measure the primaries safely. Could probably measure through a high voltage cap though.
- Volume control is placed after the first parallel triode and in front of the phase splitter. Input is now a 10k grid stopper with a switchable 470k or 1k grid leak after. Input impedance/sensitivity is this switchable without much effect on the high frequencies.
- added a 220r resistor after the choke in series with the rest of the PSU. No more low frequency ringing/overshoot in the screen supply.

Stability of the amp is pretty good. I observed some oscillations when testing different cap values in the negative feedback path. There’s a small 100pF cap over the 22k feedback resistor. Leaving long leads on that cap for testing purposes caused the oscillations. Short leads:problem gone.

With some old el34’s biased at 60% I tested numerous times at full output. I observed somewhere between 26 and 27 Vrms in a 7.8r load at around +20V peak grids of the EL34’s. Output shows some flat topping, but no hard clipping. Good! 90W at full tilt, not bad.

Now the bad news: screen grids are just holding up. With some winged C’s the were already glowing at idle. Some old Marshall’s were fine, but started to faintly glow when approaching grid conduction. I’d like to be able to lower the screen voltage. Would a Maida regulator work?

More testing results will follow.
 
Actually, I do use a Maida Regulator now. Next up is regulating the bias too with a simple 2 transistor regulator. Provisions have already been made on the psi pcb.

The screens are now at 310V so bias voltage went up a bit. There lies the new limitation: as I use 1k grid stoppers on the output tubes I can barely push to class AB2 and thus have safe running tubes delivering less power than before (still around 60W at clipping). Need to do some more measurements to see if the grid stoppers are actually the culprit. Probably they are. If so, could I use an anode stopper of 10 turns 18 AWG wire // to a 100 Ohm resistor? Or is there a better way?

What do I gain and lose by dropping the screen resistors to 47R Ian? Stiffer screen supply at which tradeoff?

Here's my 'safe' work bench with the amp in it's current form.
 

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The output Tube current is largely set by the screen voltage and not the anode voltage so when you reduced the screen voltage the required bias voltage for the same idle current should have reduced too.

Lower screen resistors will impart "tighter"/"punchier" sound (less compression).
The 47R is the minimum "screen grid stopper" resistor for its grid stop function, that is to supress parasitic oscillation, larger screen resistors are for tube protection

The 1K (control grid) gridstops will reduce drive when grid current is drawn (the Class AB2 condition) but I would hesitate to drop them much below that 1K value.

I have never found the need to use anode stoppers except on a couple of 807 Amps. Usually those few turns of wire around a 10R resistor was OK for the 807.

I went looking for the 2 transistor bias tracking regulator from the London Power Standard. In this case you would track the screen supply.
I know I posted a 2 page extract from TUT5 showing the powerscaling regulator and bias tracking regulator schematics as a pdf but could'nt find it.
Maybe it was to another forum.

Cheers,
Ian

P.S. Your test setup pretty good ("safe") to me. I've seen and used much worse.
 
Would the effect of less compression be noticeable or prominent in say the first 1 or 10 Watts of output power? Testing is easy enough. Probably have got some low value MOX resistors in the boxes. Now, will it be safe? El34 screens tend to glow, even on a lower voltage and with 1k screen stoppers. Screens not aligned with the control grid, right?

I’ve got the same basic question regarding grid stoppers. Are you hesitant because it could fry them? The MOSFETs will happily deliver 200mA to the grids...

I try to keep the tubes and transformers as safe as possible. I’d like to discuss that in a broader sense a bit later.
 
The compression effect from the screen resistor will be noticable only on screen current peaks, that is at the upper end of output power. Little effect at low power.

Not an expert on the EL34 screen grids glowing, it may have something to do with alignment but we need a more expert opinion.
EL34 have a reputation (whether deserved or not) of having fragile grids BUT that may just be due to the fact that being a true pentode rather than a beam power tetrode the "screen" draws a little more current than say a 6L6

Control Grid Stoppers are there to suppress parasitic oscillation. Not too worried about frying them but about the possibility of parasitics if you reduce them too much. The low output Z of the driver may be sufficient to damp any tendency to oscillation but if you start swinging 200mA into the control grid from the driver the grids WILL melt.

See what others have to say on this.

Cheers,
Ian
 
Oh, those screens can glow pretty badly in EL34's and EL84's, especially when they are at B+ and there are just 1k screen resistors. Old Marshalls and Vox amps just have one way of producing distortion: turning up the volume of the whole amp and make the PI and output tubes clip. Did just that last week with a 1973 Super Bass. Grids start glowing red/orange at heavy clipping.No wonder service life of output tubes was/is rather short when used that way. High voltage sagged from 460 to 360V during clipping. Quite some compression, but that's the nature of the beast.

Now, this amp is a different beast altogether. PSU is capable of 450mA @ 350Vct. MOSFETs driving the grids. There shouldn't be much sagging or compression at all.

The goal of grid stoppers is clear to me. I'm looking for a way to achieve that same goal while ditching or considerably lowering the grid stoppers. There is a way to do that, I just don't know it yet. I probably could make something to limit the drive capability of the MOSFETs.
 
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