"Re-Forming" Caps on Old Tube Amp

[DreadPirate]

I've got my "dim bulb" setup to attempt a power on an old tube receiver. I have 7.5, 40, 75 watt bulbs, which should I use? How long should I leave the unit on for?

[kevinkr]

Safer to replace all those caps if possible... If not start with the smallest bulb and see what voltages are developed. Knowing the power consumption of the amplifier would help here.

Reforming caps works well in units that are just a few years old and reduces the likelihood of component failure under those specific conditions. (Limits leakage current in the caps to a value that results in insufficient self heating to damage them.) IMO this should not be necessary with modern caps.

Older caps are another question and should be replaced.

[DreadPirate]

The unit is a Pioneer SX-110, early '60s I'm guessing. I agree at some point all caps would need replacing, but what I'm trying to do is determine basic functionality first to see if that is worth doing. If there are just too many things not working, then I may abandon the project altogether. This unit looks fine cosmetically, but it spent some time outside and the chassis is rusted. Some wires look gnawed on. Therefore, I'd like to take it along slowly. I did plug it in 110V and turn it on (yes, complete lunacy I know better now) for about a minute or two and enjoyed some very nice sounding FM about a year ago. The tubes are not original, so it may have been operated in the last 20 years, can't say, it's a curbside find.

So if I go with the 7.5 watter, what measurements should I take to see how things are going? I do have the schematic. I do plan to go through the caps with an ESR meter before power on.

Do you think going straight to a re-cap on a unit like this is a good idea?

[anatech]

Hi DreadPirate,
The first thing I'd do is recap it. Main filters and coupling caps. You may find many resistors are way out of tolerance too. 100K and up are the worst, although I've seen all values shift greatly.

If nothing else it will be a great project and a funky computer speaker amp.

Forget the ESR meter. Once high voltage comes up a small leakage can possibly destroy output tubes and transformers, or the power transformer. It's avoidable, so avoid it.

Everything you know about low voltage semiconductor failure modes is invalid at higher voltages. If not completely invalid, then substantially modified.

-Chris

[DreadPirate]

I guess I'll be hunting those down. So for now, these caps only?

[AndrewT]

Hi,
7.5W will be too low.
The filament heater current alone will turn on the bulb.

40W may be too low as well, but will do no harm to the valves/amplifier.

You may have to go upto 100W or even 150W to get a valve amp to run at near full voltage when it's OK. It's due to all the quiescent current it draws in the ClassA stages and in the filament heaters.

[anatech]

Hi DreadPirate,
For now, check everything. Often resistors and capacitors connect to the same terminal. To avoid doing the same work over, replace all the filter and coupling caps plus any resistors that are out. It's easy to run around with a multimeter.

Do not replace the mica caps or ceramics in the RF section. Only change the paper / wax type caps and any electrolytics.

You may have to go upto 100W or even 150W to get a valve amp to run at near full voltage when it's OK.

That is true. You can allow damage to occur so don't even power it up with replacing the main filters and coupling caps at a bare minimum. You would probably end up replacing more caps and resistors, so you may as well do this in one go.

-Chris