70's Deluxe Reverb reverb squeal

[ampeg_lover]

So here is an interesting problem! I have a 1973 Deluxe Reverb which has high feedback squeal whenever the reverb knob is turned up past 7. It also has constant static which does not change despite the volume level. One of the tubes was microphonic but when replaced it did not change anything. I removed the reverb tube, reverb tank and pedal which isolated both the hiss and the squeal within the reverb circuit. I removed the chasis and found that there was a 470 ohm, 1/2 watt resistor which was cracked (now broken as I finished the break) which I have replaced and has done nothing to remedy either problem. Also, the tube is hotter than all of the other preamp tubes. I can touch the other tubes but this one gets hot enough to remove skin! Does anyone have any options for me?

[jcollins]

Did you try swapping out the reverb recovery tube? That's the one to the left of the reverb driver tube (the one that got hot). Pretty normal for that driver tube to get really hot. Much more so than the rest of the pre-amp tubes.

Looking at a DR schematic. I think that 470 ohm resistor is for the powertube screens. The schematic calls for a 1 watt resistor there. Which may explain why the 1/2 watter burned up.

For your static sound. You say the level is independent of volume. That means that it's coming after the volume controls. If you can still hear it when
you've disconnected the reverb tank, that means your suspects start at the
reverb driver circuit. You can remove the preamp tubes starting there one by one and listen for the static. It could even be one of the power tubes. It could also be a cap or a resistor. Pulling the tubes should help you localize the part of the circuit that's causing the problem.

I'd be really suspicious of V4, the reverb recovery tube.

[Rob Mercure]

Thoroughly clean the reverb tank phono jacks, the chassis jacks, and the plugs at each end - DeOxit on each end, twist them in place, remove and flush will probably do the job. Fender reverb connections are notorious for being noisy after a few decades of use/abuse. The reverb drive tube - 12AT7 - is for all practical purposes - a one watt +/- power output tube and not a preamp tube. The reverb drive is a single ended Class A output circuit which will drive an 8 ohm speaker if you connect one there. So the tube normally gets hot in use.

Rob

[ampeg_lover]

Rob
I removed the pedal and tank from the circuit and still have the problem. I have no clue where else to go after that. I have replaced the broken resistor but it is still an issue and now I am having trouble with the vibrato clicking away when it is turned on. I am hoping that this is the same problem so that it solves it. Could it be a problem in the reverb pot?

[Rob Mercure]

Well you could have a bad pot but it sounds like something else is going on. First of I don't understand your tremelo symptoms - could you describe them a bit better as I could interpret your comments as they are working.

What have you done so far - JC offered some suggestions and I wonder if you've tried them. As s/he noted the 470 ohm resistor should be the screen grid current limiting resistor one one of the output tubes and should be of higher power dissipation. I usually replace both of them with 1K ohm 2 watt flame proof units. If one's cracked the other one may be and these are known to melt the solder when hot and create a bad crackly-noise joint. Have you gone over the output section and cleaned/retensioned the output tube sockets?

Check the forum links and find the "Tube Amp FAQ" on R.G. Keene's site - this should help introduce you to systemtic trouble shooting.

And, due to the age, has your amp ever been recapped - had filter and bypass caps replaced? These parts have a 15 year, or so, design life and when they fail they can cause all sorts of "gremlins" that cut across they spectrum in funky symptoms.

Report back after some work and we'll keep on workin' on her.

Rob

[52 Bill]

I removed the pedal and tank from the circuit and still have the problem.

Try shorting out the return RCA jack and see if it stops oscillating.

[Bruce / Mission Amps]

Assume no bad tubes, cables or RCA jacks, etc., here's what I would do if not done already.
Locate lugs 3 and 8 of the 12AT7 reverb driver tube socket and follow the wire up to the biasing resistor and cathode bypass cap.
The resistor could be anything but if it is a newer silver face amp, probably 680ohms.
Anyhow. yank it and use a 2200 ohm 1/2 watt resistor here with a new 22uF/50v cap... 22uF/25v is OK too.
No go over to the next tube socket and find lug 3 and find where it's wire goes, replace that cathode bypass cap with a new 22uF/50v cap, 22uF/25v is OK too.

[mac1amps]

Some of the earlier 70's SF's used a 470 Ohm cathode resistor in the [12AT7]reverb drive network, which OFTEN got burned up, sometimes taking the drive xformer along with it as a result of the tube operating too hot. Way back when I was doing those under warranty, Fender advised changing them to 680 Ohm. The squealing factor can oftentimes be lead placement inside the chassis.
One question.........WHO would ever use a [normal operating] Fender reverb set @ 7! ANY Fender that has a "normal" operating reverb network would be TOTALLY "washed-out" at a setting of seven, even too much for "surf" players. (not trying to sound critical. 'just an observation)

[ampeg_lover]

Mac, Now I know it sounds weird to have it up to 7 but I just didn't want this thing going buggy. Especially when it was not giving problems before now even when turned to 10.

[Chuck H]

I'm siding with replacing the recovery tube. Not because theres something wrong with recovery...Because the same tube acts as a mixer for the wet and dry signals. If the squeal is volume dependant it is probably a microphonic tube. The mixer stage may be microphonic. I would try replacing that tube. sometimes you can get away with swapping a microphonic tube within the same amp by swapping with a less sensitive stage. But in your case there isn't a less sensitive stage. Unless you have a 12AX7 in the PI socket. Swap with that one if you do.

Otherwise I think it may be a ground problem.

Chuck

[ampeg_lover]

So ladies and gents;

I have done everything that you have told me to do except for what Bruce said about replacing the caps. That is about to be my next step but I did find a few cold solder joints that needed fixing which I too care of and that broken resistor has been replaced. I did find that the squeal continues even when both the in and out reverb tubes are removed but quits when I remove the last preamp tube. That perplexes me. One of the solder joints that I fixed in that area was a resistor going from one stage to another just opposite that last 12AT7 socket. Repairing it did little good other making the hiss go away. Don't know what that had to do with anything.