1966 - Fender - Pro Reverb - Repair

[ianmccallum]

Hey Guys,
When playing one of my main amps, it started making some hissing sound and a bit of popping.

When the amp is turned on but with standby on, there is no hiss or extra sound, only when the amp is trying to generate sound.

We took the tube covers off and noticed the 2nd furthest right 12AX7 tube was no glowing and that the rectifier looked a little bright.
I took a tube out of my 1966 Super Reverb to try to check to see if the tube had just failed, however even using a new tube it still wouldn't light up.

We took out the chassis to check the caps or to see if we could notice anything odd about the amp and the caps don't look like they're bulging or leaking at all.
T he filter caps have definitely been replaced at some point which is great.

We were hoping to be able to easily identify where the amp was failing but are kind of at a loss at the moment.
My amp tech is only open during times that make it almost impossible for me to get to due to my work schedule.

Our next logical approach is going to be to try cleaning the tube socket with some iso.

Does anyone else have any suggestions or have had this issue?

Thanks in advanced!!

[Enzo]

Do both channels work? And all the functions like reverb? If so, then your tubes are glowing, whether you can see it or not.

You have a noise, so isolate the problem. Do ANY of the controls affect the noise in ANY way?

[The Dude]

^^^^ Agree and add: If you suspect the tube is indeed not lighting. Check for filament voltage at the socket pins. You could have a connection problem with the filament wires or a socket issue. Your meter will tell you.

[ianmccallum]

Do both channels work? And all the functions like reverb? If so, then your tubes are glowing, whether you can see it or not.

You have a noise, so isolate the problem. Do ANY of the controls affect the noise in ANY way?

I immediately turned off the volume and flicked the amp off, I didn't want to blow a speaker or make the matter worse.

When I tried to fire the amp back on there was hissing, white noise followed by popping so I turned it off again then opened it up.

Should I try powering it on with a guitar plugged in with a ringing chord and quickly to see if there is sound then go from there?

[g1]

If it's second preamp tube from right looking at front, that will be the tremolo tube.
Hissing and popping could be caused by one of the other preamp tubes, but it won't be caused by a tube that doesn't light up.

[nevetslab]

When I'm checking out a amp on the bench, I use its' self-noise as a source. I first listen to what's coming from the speaker(s) with both channels volume controls CCW, tone controls mid-position, Reverb & Vibrato controls CCW. If just minor or no noise from the speaker (hum included), I'll then turn the Normal Ch Volume pot up. Does the pot make noise when rotating? With that volume up fully CW, I'll turn the tone controls, one at a time up and down. Do those make noise when rotating? If all ok, turn Volume back to CCW. Then turn up Vibrato Ch Volume. Does turning that control make noise when rotating? With it at full CW, I rotate that channel's tone controls. Do those make noise when rotating? Then, I turn that volume fully CCW. Turn up the Reverb control Does that circuit make noise? With the control up maybe to 2 or 3, I tap on the reverb tank to see if I can get the reverb to respond. Normally you don't need a foot pedal to get it to work. It's volume control comes into the amp after the two channels, so neither channel volume control is involved. For hearing Vibrato, I turn up the that channel's volume fully CW, turn the treble fully CW, then turn the intensity up. You normally DO need to have a pedal engaged to get the vibrato to work.

With you hearing noise, and haven't found where, with the amp on standby, disconnect the preamp tube next to the power tubes (driver tube V6). Usually with it removed, you won't hear anything thru the amp. switch back to S/B, plug that driver tube back in. Remove the 4th tube from the right rear (facing the rear). That's the mixing stage, which also passes the Reverb circuit on to the power amp. Switch out of S/B. Has the noise stopped? If so, back to S/B, restore V4 tube, then remove 2nd tube from the right. That's the Vibrato Ch preamp tube. back out of S/B....noise gone? Then Vibrato Ch is source. If noise still there, switch back to S/B, restore V2 tube, and remove V1 (Normal Ch preamp tube). Back out of S/B. If you had been having noise all along, this should have remove it, if the noise was tube-based. The Vibrato tube wasn't removed....that's V5...between the Driver tube V6 and the Mixing tube V4. V3 tube is the driving tube to the Reverb circuit, fed from Ch 2. If you had noise from it, you would have heard it when testing the Reverb control.

All this testing is just using the self-generated noise from the tube circuits in the amp. I spot noisy pots, noisy tubes, non-functional tubes, reverb problems, vibrato circuit problems fairly quickly prior to removing the chassis for more definitive testing.

[Enzo]

Yes, please play through the amp. Noise and hiss and hum are not going to damage the amp. If anything, they are symptoms of damage already done. I use the term damage very loosely. A part that goes noisy is not really damage.

It is really quite difficult to repair an amp without powering it up.

[sbluestubes]

Replace the carbon comp plate load resistors and the dropping resistors on the filter cap board.

[vintagekiki]

Firstly schematic diagram
www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Pro-Reverb-AB668-Schematic.pdf

Question
Are there exist any sound sensations (hissing or popping sound) when normal, vibrato and reverb volume pots are at min (0)

Repair
Forget repair any amp with tubes "like new" (as-as), because by changing tube place you will not find out which tube is defective.
New tubes 12AX7 (2 pcs), 12AT7 (1 pcs) and 6L6GC (2 pcs) is not some big $$ expense even for someone who plays music as hobby.

To determine which tube is causing sound sensations, remove old V2, V3, V4, V5 tubes and install new V1, V6, V7, V8 tubes.
Are any sound sensations, or you hear good old clean Fender sound. If you hear clean sound, continue with testing existing (old) tubes.
First remove new V6 and in his socket install old 12AT7 (V3, V6). Are any sound sensations?
Examine existing (old) tubes 12AX7 in the same way by removing new V1 in his socket install old 12AX7 (V2, V4, V5). Are any sound sensations?

From your attach pictures it can be seen that amp is orderly serviced (changed caps in power block, in preamp cathodes ...)
Since the amp is decade-old timer, most likely oxide caught under jacks (input or RCA) and chassis. Carefully remove them, and clean any oxide.
Do not treat tube sockets with any chemistry, it is enough to put and remove tube in his socket several times (be sure to remove the AC main cable from the wall)