64 Fender Pro Reverb Problems

[andyjazz]

This was my old amp when I last played in a R&R cover band. It blew a fuse at the last gig for the band 5 years ago and it's been sitting in the basement until a couple of weeks ago. I changed the rectifier tube which got it to run ok, set the bias voltage to the schem specs (It was way too hot), played it for awhile. Seemed ok. Brought it to the show (I had a back-up). 5 songs into the show it starts making horrible scratchy sounds (Like a bad pot) with no input and the volume turned down. Reverb knob, treble knob, any knob, had no effect.

I have since replaced all of the caps shown in the layout (Funny the layout view schematic doesn't show the 5 electrolytic caps in the power section) Anyway, I tested these 5 electrolytics (That I didn't replace) with the backyard-mechanic 9-volt battery-let's-see-if-they-hold-a-charge-test and well, they hold a charge but....they're freakin OLD.

I played through the amp for about 20 minutes, everything sounded ok, I left the amp turned on and went on to another chore and noticed some intermittent scratchy sounds this time less audible than before but still there. And I get this sinking feeling........I should have just spent the extra $ on the electrolytics (times are tough, moneys tight) instead of trying to milk some more life out of them. Or am I missing something?

[drewl]

Start simple by swapping the preamp tubes to see if one is bad, or even a bad output tube can make strange noises.

[CharlieP]

It sounds like you are doing drastic maintenance before simple. I usually replace spark plugs and oil before I start tearing the whole engine apart! Start with the simple first. Does it have an EFX loop? Clean loop jacks and tube sockets! You left it set 5 years, what do you expect? Tarnish and corrosion! If that doesn't help then it is time for some known good tubes to swap around! If no results go on a step at a time. You can replace all the caps and neglect the simple stuff and guess what! You will be scratching your head!

[Chuck H]

+++ It's probably dirty tube sockets. That amp doesn't have an effects loop.

And you absolutely DO need to replace those five caps under the pan for reasons that may or may not be related to your problem. Those caps do wear with age, have a life of about twenty years and are most likely to become bad after sitting unused for a long period of time. With the exception of the aluminum electrolytic caps in the layout the film caps were probably not remotely related to the problems your having and you only made the amp less stock and collectible by replacing them. If you didn't mangle them during removal you should probably put them back in.

Further, if there has been a power supply failure and high voltage, electrolytic goo and soot has mad it's way onto your old black paper circuit board it is very likely to become conductive now and require replacement. It happens to those old black paper boards ALL THE TIME when they are exposed to excessive voltage on the wrong circuit, excessive heat or deeply embedded grime. Replacing an entire circuit board is a PITA.

If those five caps haven't failed yet they are on borrowed time. Replace them and your amp will hum less, perform better and more reliably.

Chuck

[andyjazz]

Yeah I'll just bite the bullet and pay the $ for the new electrolytics. As far as making the amp "less stock" by changing out those original 400v caps.....I don't buy it. I mean maybe some collector is going to consider this a value but I don't. The amp is not for sale, never will be as long as I'm alive. In my book 45 year old filter caps are suspect. I'll hang on to them but I don't ever seeing me selling this amp. Oh, and the tube sockets have been cleaned. Thanks for the ideas.

[Chuck H]

It could be a preamp tube going bad then. Tap on the preamp tubes with the amp at gig volume and see if any misbehave.

And if you don't plan to sell the amp then I say do whatever revoicing or component changes you want to. With the information age more players and collectors have become aware that old filter caps need to be replaced so they're less of an issue now. But original film caps are still a plus.

Chuck