Safe PSU capacitor values for old Dyna amp?

[jgf]

I wouldn't get too carried away with large increases in supply capacitance despite the obvious sonic benefits because the power transformer in the SCA-35 is running right on the hairy edge of self-immolation. (I've seen a few fried even with the stock capacitor values) Larger capacitors will increase the peak current flowing in the HV secondary significantly and the additional heating in that winding might be enough to push it over the edge. I wouldn't go more than 20% or so higher in value at most.

That's only a factor for the first capacitor, right? Or am I missing something... The second one (where the power tubes draw from) can safely go higher, 1 or 2 hundred uF no problem.

 

[rayma]

I would need to put two 47 µF caps, two 180-200R resistors (as opposed to the original 100 µF / 95R arrangement)?

Yes, that's right.

 

[crispycircuit]

Justradios.com has 600v electrolytics (axial) for a reasonable price. I've been happy with them....

 

[ChrisA]

UF4007s or, even better, Cree C4D02120A 1200 PIV Schottkys.

Play safe and install a CL-130 inrush current limiting thermistor between the junction of the rectifying diodes and the PSU filter....

If you must use a thermistor in a tube amp, place it on the primary side of the transformer.

A thermistor needs a high steady state current so that it can heat up and lower it's resistance. If the AC mains voltage is less then the Amp's B+ voltage there is more current on the primary side, especially when you think about the heater current.

And yes, limiting current on the primary also limits secondary current.

That said, I think you can simply depend on the transformer's internal resistance to do what you want unless you are doing something silly like quadrupling the filter cap size.

 

[Eli Duttman]

If you must use a thermistor in a tube amp, place it on the primary side of the transformer.

Putting the thermistor on the B+ side does not slow filament heating down. If the amp is "fixed" bias, a thermistor on the primary side could cause a catastrophe. Instant on is very much the order of the day, for bias supplies. If filament/bias power come from a trafo not also associated with B+, inrush current limiting on the B+ trafo's primary side is fine.

Try to pick a NTC inrush current limiter that comes close to fully heating. A few ohms of hot resistance are unlikely to create trouble. The NTC inrush current limiter data sheet is here.