'79 Fender Deluxe Reverb

[garcia]

I have a 1979 Fender Deluxe Reverb that I just had serviced. All the tubes were replaced (the old ones were all original), bias adjusted, and speaker tightened (apparently, it was quite loose). And I have to say, it made an unbelievable difference - this amp sounds amazing now! The problem I'm having, (and I should emphasize that I was also having this problem before all this work was done), is an intermittent loud crackling sound. It's not a buzz or hum so much as it seems to crackle. I tried unplugging my guitar completely, and it still makes this sound, so pretty sure it's not the guitar. And the amp doesn't always make this sound.
Since I'm not an electronic technician by any stretch of the imagination - but my dad is - are there any specific things I could ask him to check on my amp? Is this a common problem with old tube amps?
Some things I've tried are:
- unplugging guitar, still makes noise
- unplugging stock footswitch for Rev and Vib, still makes noise
Any suggestions would be much appreciated.

LC

[Enzo]

It could be a lot of things. A competent repair tech should be able to solve the problem. You can download the schematic from Schematic Heaven or maybe fender's Mr Geargead.

In any case, isolate the problem. Ball up your fist and whack the amp. Does the crackle respond? If so, there is a loose connection somewhere - could be solder or wiring, or could be loose hardware. Wwiggle each tube in its socket. ANy of them make noise when you do? Loose tube socket then.

Now move on to the controls. Do any make static noises when you turn them? They need attention then - clean them, or tighten their nut, or repair their solder. Turn the reverb control to zero. Still noisy? The reverb sort of is off by itself so it is good to eliminate it early or to zero in onit.

Turn all the conrols down to zero. Still noisy? If so, it is in the power amp. Possibly a weak plate load resistor in the phase inverter, or maybe a failing cap. If the noise is gone, then it is from earlier in the amp. ANy control that affects the noise - loudness, tonal quality, whatever - is after the source of that noise.

we can also pull tubes to isolate things. If pulling a tube kils the noise, then the source of the noise was either at that tube or before it. So you need to pull tubes in meaningful order, because any tube you pull will quiet the noise from ANY earlier tube.

[garcia]

Thanks, Enzo! Much appreciated. I'll definitely take your advice and see if I can't locate this very annoying sound.

LC

[TD_Madden]

Didn't I read somewhere that the 100K resistors in the preamp section of most Fenders tended to become noisy?

[Enzo]

I don't believe that, otherwise most Fenders would have the noise problem.

I might accept the reverse - if a Fender DOES GET noisy, it is quite likely a 100k resistor.

[garcia]

A clue?

One thing I notice, and this may be a clue, is that if I crank it up and play it for a while, the sounds tend to go away. Does that eiminate or point to anything in particluar?

[stokes]

I think TD may be refering to an issue with the Bassman RI,they used 1/4 watt plate load resistors instead of 1/2 watt.They stress and get noisy before they actually open up and fail.Sometimes the carbon comp plate load resistors get noisy in older amps,but this is true of many components in older amps.