1. Change all the filter caps. Sprague makes the 8uf-450v, I think they are the only ones with that exact value. When changing the cathode bypass cap on the power tube, increase the voltage rating to 50 volts is a good idea. Also, mounting it away from the cathode resistor helps a lot. That cathode resistor gets pretty hot, and mounting the cap away from it keeps it safe.
2. The coupling/blocking caps in these are usually yellow or red Astrons (all both of 'em!). These will frequently be leaky. I just worked on a '61 (still tweed in those years) and the yellow Astrons were indeed leaking about .4 volts, which is no good at all. I use Sprague Orange Drops usually as replacements.
3. A 5V4 is a nice sounding rectifier tube in this amp. Well, actually the rectifier tube creates no sound on it's own, but the effect it has on the rest of the amp is nice. It raises voltages a bit. If you do decide to use one, it's a good idea to monitor the sound of the amp for a while before you use it for a critical application. Sometimes the 5V4 will cause the carbon comp power supply resistors to become noisy. Actually, just installing a new 5Y3 or installing new filter caps could do this. But you MUST do the other things after time passes so...
4. If the amp does go noisy, you'll need
to change the resistors anyway. It's always a good idea to listen to the
amp for a while before taking it to the studio. I'd hate to see a $$$ an
hour session go to crap because somebody didn't get the amp serviced properly!
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