Reverb issue

[miltown]

Hey all. I have an Gibson amp from the 70's. The reverb just isn't there. When you bump the amp you can hear the springs through the speaker. When I inject a tone into the circuit there seems to be some reverb there (kinda like a hall echoy sound) but not very loud. Is this what happens to a tank when it fails or does it just quit working altogether? Because there is something happening it just doesn't have much effect left.

[Chuck H]

I would guess there's a problem with the input coupling circuit from the pre amp. If not then it could be the reverb drive circuit is weak. It could be that the tanks input transducers are weak (not likely but it happens). The way you describe the problem there are a couple of things to check before damning the tank.

Have you tried a new tube in the reverb drive socket?

Have you tested the output of the reverb drive transformer?

Have you tried a new tube in the recovery stage?

Have you checked for drifted component values in either the input or recovery reverb circuits.

I mention all this because you do have some sound from the tank. This isn't to say the tank isn't bad but do all the easy and free stuff first before buying a tank and waiting for shipping only to fine it wasn't the tank after all.

[miltown]

I should have made it clearer this is not a tube amp. It is from '73 but is all transistors. When I inject the signal there sounds like there is reverb there although weak. When a guitar plays through, it does not sound like any reverb is there. I have checked the input stage and components are good. Duh I have not checked the output. Though it shares the same circuit with the output of the tone stack . There are two outputs from the pre-amp one that goes to the recovery stage and one that goes to the reverb. Then out of the reverb to the recovery stage where the tone stack goes in. I can post a schematic in a few. There is only a couple of components at the output before it meets back up at the recovery stage or mixer as it is called in the schematic. One question I would have is if I hook up a speaker at the input to the reverb op-amp would I hear guitar sound or is the level still to low at that input. I hear tone when I inject it in but I can never hear guitar and I can inject tone all the way back to the output of the pre and still have tone at the input to the reverb. Hope that makes sense. Like I said give me a few moments and I'll post a schematic.

[Chuck H]

+++ on the schematic.

There may be a transistor in the circuit somewhere that's kaput. The schem will help. I'm still going with the coupling circuit.

If the reverb has a footswitch, have you tried it? Some amps require that he footswitch be plugged in to trigger or un trigger the reverb. If so then it's also possible the jack for the footswitch is dirty and is not making an internal switch contact.

[miltown]

Here is the schematic. The output circuit is operational. I'll bet your right on the footswitch being required to operate this reverb. Check it out .G105,G115.pdf

[52 Bill]

Did you test the input coil on the tank? Read across the RCA jack with your ohmmeter. Do you get a resistance reading?

[miltown]

The resistance is 173 ohms each jack.

[Chuck H]

I still think the problem is somewhere from the junction of the dry/reverb drive signals and the tank input transducers. Have you signal tested the reverb drive amp to see if it's putting out?

[miltown]

If you mean MC1741 yes . I applied tone at pin 3 and it comes out of the tank and you hear it with reverb at the speaker. I should note that the IC was changed. Which I think might also be an issue. I have ordered a cross referenced one to the MC (of coarse it stopped being made in '75) because the one that is currently in there TL071cn is not in the list of cross referenced IC's. One other note cap C8 the first one in the chain. When I apply tone there it has no reverb just the tone which I think is just then backfeeding thru the circuit. When I disconnect it from the pre-amp side and apply tone at the input (for lack of a better term) of C8 there is reverb in the tone that is being injected. I don't have a footswitch so I just plugged in a 1/4" and jumped across the R&T still no reverb also. I need time to step back and think.

[J M Fahey]

That 071 will happily replace your old 741.
Anyway you don't need an "MC" 741; *any* 741 you can get will do, just coming from other manufacturer.
Trace audio along its path to see whether it gets to the 741 input, then that it puts out a couple volts to drive the reverb tank, then that you get at least a couple hundred mV at Q4's collector.
Check those trimmers, they may have dirty contacts or just be open.
The actual signal from the reverb transducer may be too weak to be (comfortably) measured.
Nice classic SS guitar amp, don't understand why they were neither successful nor remembered.
Of course, built-in distortion is cr*p, nothing a period correct MXR Dist+ couldn't help.

[Chuck H]

I might be missing something but it seems to me the problem is between C8 and pin 3 of the driver. Could be a bad solder contact, a bad cap, a broken circuit trace or a bias faiure at the amp input that was somehow corrected temporarily when you injected signal.

[miltown]

Yeah Chuck I think I'm gonna pull the board, resolder. Everything checked out. The voltage at the input to tank was 2.3 vac . The voltage at the collector was .375 vac . I cleaned the pots and they check out fine, the audio path is right there, the only thing I think I can't check out really is the input transd. except for continuity. I think it my be the tank after all, I probably hooked up the leads and induced my tone into that part of the circuit. I noticed I would just hold my tone probe over parts of the circuit and I would here tone. The way that tank is with everything having continuity with everything else I think some freaky sh&t happens in there and my tone was somehow picked up with the output transducer and really was not coming from the input. Basicaly I think I messed up hooking up my tone probe. I'm gonna see if someone has a tank I could borrow, I have the word out. Do I need to worry about the in/output impedance? Well see .

[J M Fahey]

Hey !!: if tou have 2.3VAC at the tank input, you *are* driving it, so everything before it works; if you have 375 mV on Q4's collector, you have recovered reverb signal, you have no tank problem !!
To check that the tank really does its thing, and you are not measuring some stray feedthrough signal, press your fingers slightly on the springs.
2 results: you should feel the springs twitch in rythm with the music, and Q4's signal should die.
If all well, set trimmer TR1 to maximum, and trace signal through R25, P6 and C21.

[miltown]

Thanks JM I'll get on that later today. I appreciate the help.

[miltown]

OK here is the deal. I cranked both trims to maximum. I hear reverb now. If I put back the feedback op-amp trim back to it's original position the reverb is there just very slight. If I put back the trim on the transistor at the output the reverb is not there. That is both in original position. If I reverse that so that output trim is in orginal and op-amp trim is max I have reasonable amount of reverb and then if I crank the output trim there is heavy reverb. So can I safely conclude that it is an issue in the feedback circuit at the op-amp? I've checked those components and the only one that seems weird is C10 with an ESR of 6. That is in circuit. I'm going to assume that is not good. I know it should be in the .1 range. I paralleled another cap on but did not make a difference in reverb with the trims put back at original spot.