by Peter W. Fri Sep 22, 2017 5:35 pm
jrethorst wrote:I have the chance to buy one with:
The power transformer is a Custom Coil PA-211, manufactured as a higher capacity direct replacement for the original Dynaco transformer.
The tube rectifier has been replaced with two 1N4007 diodes and the original four-section power supply capacitor has been replaced with two NOS two-section capacitors of higher value. The plate power supply resistors have been replaced with 3 watt metal oxide resistors of the original values. Plate supply voltage is about 10 volts higher than original when running on a 120V AC line.
The filament supply has been replaced with an adjustable integrated (LM-317) DC regulator. I have set the filament voltage to the original 22 volts, but the filament supply is capable of up to 24 volts.
Most resistors have been replaced with metal film or carbon film types of the original value. Most capacitors (other than in the power supply) have been replaced with polypropylene or polystyrene types of original value. The output capacitor is a polyester film type.
Are these well-chosen mods?
Thanks,
John R.
OK - I am walking into the world of *opinion* here. The PAS3X is >the< PAS to have, so you are off to a good start.
The OEM transformer is a bit of a weak link and is purported to sag under some conditions. BUT - replacing the 12X4 with silicon diodes is pretty much sufficient to overcome this problem. Replacing the OEM carbon resistors is also a good idea.
I am not sure where anyone got the idea that small-signal tubes *LIKE* being starved, or that starvation somehow extends their effective life. No, they do not like being starved. No, a decent 12A*7 will not suffer if run at its rated voltage - and I have examples that are now-50 years old with tens of thousands of hours on them that test just fine on my big Hickok. From the factory, that 22V was under load. I would suspect that the open-value would be somewhat higher (C.F. Transformer sag). Writing for myself, my filament supply is also a regulated circuit, and is operating at 24V - and has been for years. I would like to see how the other parts of the filament supply are managed.
If one is going to replace the power-supply filter caps, I would strongly suggest that they all be individual caps. Dual-caps are still multi-caps.
What you have gotten is a better transformer (Good) and no 12X4 (also Good). And the individual components (resistors/caps) replaced with better stuff (Good). The rest of it can be cured with any of several after-market options, all less than $50, all-in, that neatly address filament, bias and B+, and multi-section caps in a tidy single-board solution. And, remember, you are starting with the X-version, the last and rarest of the series. For that reason alone, it may be a good deal - depending on cost. Lastly - there is NO comparison between a standard 2 or 3 and the 3X. Full Stop.