Understanding the ToneKing Gremlin power supply

[janou195]

Hi all,

I reproduced the schematic of a Tone King Gremlin. There may be some little errors and I have to double check everything, but it seems to be something like in the file attached.

I am quite a newby, and some part of the power supply are quite obscure to me:
* The R42, C26, C21, R43 and Q1 MOSFET: it seems that the gate get a slightly reduced and filtered (with C21) voltage with respect to the drain. Is this some kind of cathode follower like with R45 as cathode resistor? What is the purpose of the small C26 capacitor?
* I feel that Q2 and R34 acts as a cathode follower to provide a low output impedance supply to the screen grid of the KT66. Am I correct?
* I feel that Z1 and Z2 act as voltage regulator to set a stabilized 260V voltage 'C'. Am I correct?
* I don't understand anything about R32, C18, D8, D9 and their connection to the KT66 plate. What is that?? I have never seen something like that!

I would be very grateful if someone could clarify me on these points (especially the last one)!

Thanks

[shooter]

A = PA plate VDC
B = plate VDC for preamp tubes

C = Driver plate VDC
D = regulated vdc to G2's PA
E is there when things go bad 

[Merlin]

* The R42, C26, C21, R43 and Q1 MOSFET: it seems that the gate get a slightly reduced and filtered (with C21) voltage with respect to the drain. Is this some kind of cathode follower like with R45 as cathode resistor? What is the purpose of the small C26 capacitor?

Exactly. It is an active smoothing stage called a capacitor multiplier. I don't know what C26 is for, it seems redundant.

* I feel that Q2 and R34 acts as a cathode follower to provide a low output impedance supply to the screen grid of the KT66. Am I correct?

Yes

* I feel that Z1 and Z2 act as voltage regulator to set a stabilized 260V voltage 'C'. Am I correct?

Yes

* I don't understand anything about R32, C18, D8, D9 and their connection to the KT66 plate. What is that?? I have never seen something like that!

D8 and D9 appear to be there to prevent negative flyback across the transformer, which you sometimes see in PP amps. I don't think they're necessary in a SE amp.  R32 and C18 appear to be applying feedback to the screen grid; a kind of quasi-ultra-linear operation. It's a bit strange. The KT66 is missing a grid leak, so the schematic may be suspect in other areas.

[Jonas]

Looking at the Tone King Gremlin rectifier circuit, are caps C43-C48 there to ensure even voltage across the diodes or are they more for smoothing?

[shooter]

spike suppression, when sand conducts it has a tendency to throw out a transient spike

[tubeswell]

^what shooter said^


(Being change in current/time) any diode switching transients are connected straight to the HT winding, and therefore end up being induced (back through the PT core) into all the other PT windings (including the heater winding- thus ending up as heater hash noise).

[Jonas]

ok appreciate the help. What is the "spike math" for determining the voltage rating of the cap? Or just match the cap rating to the diode rating?

[janou195]

Exactly. It is an active smoothing stage called a capacitor multiplier. I don't know what C26 is for, it seems redundant.

D8 and D9 appear to be there to prevent negative flyback across the transformer, which you sometimes see in PP amps. I don't think they're necessary in a SE amp.  R32 and C18 appear to be applying feedback to the screen grid; a kind of quasi-ultra-linear operation. It's a bit strange. The KT66 is missing a grid leak, so the schematic may be suspect in other areas.

Thanks for these valuable informations. In the end this thing about R32 and C18 is more a question about the design of the output stage than of the PSU. I fixed some little mistakes in the schematics (yes, I missed the grid leak resistor of the KT66) and reorganized it a bit, and will open a dedicated topic about the output stage!

[shooter]

I'll chime in here to keep things simple;
UL is like a 1/2 way between a pentode PA n a triode PA, most of us use a UL OT, has a tap for G2, look at the loadline for a pentode vs a triode.  typically used where you want a cleaner amp, think jazz, classical, where the "tone" comes before the PA section, the PA simply amplifies without adding "flavor"
here's an example I built;