800B Power Supply Question

[Joseph W Strickland]

The 800B power supply uses two 5Y3 rectifiers. Each tube has its plates in parallel for double the current capacity of a single tube. The RCA RC-16 tube manual says that up to 10uF input filter can be used with a single tube. Since there are two tubes with the plates of each in parallel I am thinking that 20uF would be feasible in the power supply for the input capacitor. Have any of you tried this and had experience with such an approach? Alternatively a 5AR4 could be substituted for the 5Y3 and has slightly lower filament current requirements and a lower internal impedance and more DC current capability. Typically they are used in circuits with 30 to 40uF input capacitors. Has anyone tried them?

Joe

[Norman S Braithwaite]

I don't know of anyone who has tried alternative rectifier tubes in the 800B but a 5AR4 would certainly work as would a 5U4.  The 5AR4 tube is a bit pricey unless using a new and likely short lived Chinese or Russian version.  I am unaware of anyone experiencing problems with the use of the pair of 5Y3 tubes hence a lack of motivation to seek alternatives.

Norman

[Joseph W Strickland]

My only issue would be as to the inrush current to a 30 or 40mFd input capacitor instead of the specified 4mFd that was originally in these units. I suspect that since two 5Y3 tubes were capable of twice the current of a single tube that it would not be a problem. The 5AR4 has a slower warm-up time than some other rectifiers and gives output tubes and others in the set time to begin to draw current before the rectifier tubes begin to apply voltage, so in general the 5AR4 treats the power supply electrolytics more gently than some others. I think the two 5Y3 tubes may be able to handle 30 to 40mFd at the first input capacitor and as a consequence the power supply filtering would be even better.

For now I will go ahead with the existing tubes and try a larger than 4mFd capacitor for the first electrolytic and see how things go.

Thanks for your input!

Joe

[Norman S Braithwaite]

Joe:

Increasing the value of the first capacitor in a pi filter will increase the B+ voltage above the design value.  Combined with our higher line voltages than present at the time the radio was sold, the high B+ may cause problems.  Be careful how much you increase the value of the first capacitor.  If concerned with inrush current, install a thermistor.

Norman

[Joseph W Strickland]

Norman;

Good point. I plan to experiment with that aspect. It might become necessary to increase the value of some dropping resistors if it rises too much. This particular unit only has the one choke, not the later two-choke design.

Thanks.

Joe