Looks like a half fast implementation of an active plate load. I've seen the same thing donw, but with FETs. Probably more of a solid state guy who doesn't understand that triodes don't make good constant current sources the same way FETs do. As for the rest of the design decisions, I have nom idea.
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Looks like the schemo from the first post was incorrect, and it was an SRPP after all. Interesting article.
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Looks like the schemo from the first post was incorrect, and it was an SRPP after all. Interesting article.
Troubleshoot.
How much hum at the output secondary?
100uV, 1mV, 25mV?
First ground the output of the capacitor in the RC that drives the parallel 300B tubes.
Still have hum?
Are the 300B tube filaments AC powered.
Did you put in just one 300B, and set its hum pot? And then put in only the other 300B and set its hum pot?
Do they each singly put out the same hum?
Then put in both 300B and slightly change only one hum pot.
How much hum is there with the RC cap shorted to ground (no signal and no hum to the 300B grids)?
Did it get better after setting one tube hum, and the other, with only one tube at a time, and then combining both?
You get the idea.
Then short the secondary of the interstage transformer. Does the hum get better?
Etc.
Then short the secondary of the input transformer. Does the hum get better?
You have some cathodes that are not bypassed to ground. You can do that just to troubleshoot.
A tube that has bad filament to cathode leakage can introduce hum.
Are the filaments of the top tube in the input stage elevated?
What is the B+ circuitry like.
Does it have wiring that allows for ground loops.
The most powerful ground loop is the B+ secondary (full wave with center tap; or bridge), that drives a capacitor input filter.
That creates very large transient currents, and lots of harmonics too. That transient current is much much larger than the current draw of the output stages and input stages combined.
A choke input filter reduces those current transients, but sprays magnetic hum fields.
How much hum at the output secondary?
100uV, 1mV, 25mV?
First ground the output of the capacitor in the RC that drives the parallel 300B tubes.
Still have hum?
Are the 300B tube filaments AC powered.
Did you put in just one 300B, and set its hum pot? And then put in only the other 300B and set its hum pot?
Do they each singly put out the same hum?
Then put in both 300B and slightly change only one hum pot.
How much hum is there with the RC cap shorted to ground (no signal and no hum to the 300B grids)?
Did it get better after setting one tube hum, and the other, with only one tube at a time, and then combining both?
You get the idea.
Then short the secondary of the interstage transformer. Does the hum get better?
Etc.
Then short the secondary of the input transformer. Does the hum get better?
You have some cathodes that are not bypassed to ground. You can do that just to troubleshoot.
A tube that has bad filament to cathode leakage can introduce hum.
Are the filaments of the top tube in the input stage elevated?
What is the B+ circuitry like.
Does it have wiring that allows for ground loops.
The most powerful ground loop is the B+ secondary (full wave with center tap; or bridge), that drives a capacitor input filter.
That creates very large transient currents, and lots of harmonics too. That transient current is much much larger than the current draw of the output stages and input stages combined.
A choke input filter reduces those current transients, but sprays magnetic hum fields.
@jan.didden: Have you modelled SRPP? Have you had correspondence with John brosky on SRPP? You do not want to bypass the top cathode, distortion goes up considerably.
Back to the issue: Is the filament of the ECC82 elevated? You may have exceeded the Vhk and there is h-k breakdown.
Back to the issue: Is the filament of the ECC82 elevated? You may have exceeded the Vhk and there is h-k breakdown.
Unless you also bypass the bottom cathode, then the low distortion is preserved.
Rayma's post # 22 above also offers confirmation.
Jan
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Thanks for this explanation. I’ll try swapping V1 and V2 with ones in the 2 good amps that I also have. There are 4 of them. Only 2 are humming.
One other thing I haven’t seen before is that the builder had custom mains transformers built with 400V-0V-400V and 350V-0V-350V windings. Each of those is rectified by a GZ37 and CLC filtered with 47uF/10H/47uF. 442V from the first one feeds the 300Bs. 372V and subsequently 256V from the other feeds the 6SL7 and 12AU7. Usually the voltages for the first and second stages are derived from one set of windings.
I wondered whether the heater of the 12AU7 might need elevating but but it has a Vhk max of 200V.
Jan, late reply. Engage brain before engaging mouth. Just model it and then you'll discover that the distortion is still present. (I just confirmed it by modelling)
Wow! Have I been working on these amps for that long?
The 2 of the 4 amps that hummed really badly had bad joints in the very badly constructed earth bus.
After fixing them and re-wiring the earth bus in all 4 amps, I realised that all the amps were noisier than they should be. The original builder had put these huge fancy caps in the power supply that were actually only 47uF. I've added 560uF in parallel and all 4 are now much quieter.
Not only were the cases of the line input transformers screwed to the chassis but the white case wires were also connected to the earth bus creating a couple of loops.
The 2 of the 4 amps that hummed really badly had bad joints in the very badly constructed earth bus.
After fixing them and re-wiring the earth bus in all 4 amps, I realised that all the amps were noisier than they should be. The original builder had put these huge fancy caps in the power supply that were actually only 47uF. I've added 560uF in parallel and all 4 are now much quieter.
Not only were the cases of the line input transformers screwed to the chassis but the white case wires were also connected to the earth bus creating a couple of loops.
As connected the upper triode of the first stage is just a diode. No Rk.😉
There should be at least a One K cathode resistor to make it a mu follower.
There should be at least a One K cathode resistor to make it a mu follower.