Raising high voltage / v. drop resistors / capacitors

[boroman]

Since I started "building" small amps with existing parts on my table I have many questions that I can't find answers to.. Or partially... See this example:

Champ 5F1 calls for 325-0-325 power transformer that lead to around 375V on the first stage in this circuit (measured on the first cap with 10K resistor, SS rectifier---!!!). I had only 235-0-235 but seeing some other amp examples I saw that you can actually make 375V out of similar design putting smaller resistor on the first cap (and raising its value). I experimented a bit at found out that something around 300 ohm range (vs 10k) and 47uf (vs 16uf) gave me exact 375V from this transfomer. So, is there any golden rule I can use when calculation both? Tried to find out more about voltage drop resistors, but it's all like a black magic to me. Simple - any calc that I can use for that and downsides going up from that lower-HV pT (it will draw more amperes, right?)

(SS-rectifier in all cases)

[Helmholtz]

Please post a real schematic.

[Mick Bailey]

I use Duncan's PSU designer which shows peak currents etc. It's pretty straightforward to model a PSU and the results are close to what you get with the actual circuit.

[TomCarlos]

5F1 Schematic ...

[Helmholtz]

Thanks to TomCarlos for posting the schematic!

I could have found it myself, but generally expect the OP to post it - if available.

It is not possible to get more than around 330VDC with a 235V-0V-235V secondary and full-wave rectification; actual loaded DC will be more like 300V.
Lowering the 10k dropper will not change the power stage B+.

For power supply design I like and recommend this tool: https://www.duncanamps.com/psud2/

[J M Fahey]

That.

235VAC can only put out 332V peak, best case (what "helmholtz" said)

You only got somewhat higher because manufacturers typically overwind by 5% or so, to compensate for losses, and I guess you are measuring it unloaded , but it will drop under load.

That said, your amp can probably work fine, just putting out slightly less power.

[boroman]

Sorry for my 2 day absence. I mesaured it again with load and it's 325V, on B+ you're 100% right. Maybe I did it wothout load back then.
Tom ,thanks for the schematic.( I thought I did insert schematic but posted layout insetad...)

Lowering the 10k dropper will not change the power stage B+.

I deafinitely tested it with higher and lower drop resistor values and voltage changed.. hm???
But I changed caps too the same time.
So, what they do for the stage? I see similar design works o 5F1 and those vdrop resistors on the 1st stage/cap vary from 0,5-22k?

Thanks for the Duncap app link, I'll give it a try today

[Helmholtz]

I deafinitely tested it with higher and lower drop resistor values and voltage changed.. hm???
So, what they do for the stage? I see similar design works o 5F1 and those vdrop resistors on the 1st stage/cap vary from 0,5-22k?

The 10k dropper in the original circuit drops supply voltage between the first and second filter cap and improves ripple filtering for the screen supply.
It has no influence on the first B+ which feeds the OT primary and determines power output.

[g1]

The 10k dropper in the original circuit drops supply voltage between the first and second filter cap and improves ripple filtering for the screen supply.
It has no influence on the first B+ which feeds the OT primary and determines power output.

Changing screen voltage should have some impact on 6V6 plate current. Maybe not a huge impact on B+, but I would expect some variation?

[Helmholtz]

Changing screen voltage should have some impact on 6V6 plate current. Maybe not a huge impact on B+, but I would expect some variation?

Auto-bias tends to stabilze plate current. Even if a large increase of screen voltage would raise the plate current by 5mA, B+ might drop by only 2.5V assuming a power supply source impedance of 500 Ohm.