ShortScale.org

I've always been a bit ashamed to bring this up, but a while ago I had a gig in August....while loading my 1970 silverface reverb and trying to carry the thing myself, I slipped on a step and dropped it on my foot. it didn't fall over or anything and the tubes weren't broken.

Well when I brought it to the gig and plugged in, my reverb channel wasn't working. everything was hooked up right, to the reverb tank, etc. I was getting SLIGHT signal through it, but it was quieter than 1 even all the way up and reverb/tremolo wasn't working.

I then plugged into the normal channel and things have been fine since on that channel. I went and bought a quartet of Groove Tube 6L6's, but that turned out not to be the issue. I kept the tubes because it made the normal channel sound better anyway. All the preamp tubes are glowing and nothing looks abnormal. I took a look inside to see if anything was unsoldered or burned out and nothing looked odd to my untrained eye anyway.

Any ideas? I feel weird paying $40 just to have an amp tech tell me what's wrong with it.

I'm not sure, but I think one of those preamp tubes just drives the reverb and trem, it's possible that got damaged - anyway, it would be a cheaper fix. IDK if there's anything other then tubes that would break that easily.

I work at a guitar store and can get the tubes at cost but they're still pretty expensive if I'm re-tubing the whole amp. The tubes are all intact and glowing, and I've swapped the ax7's around enough times trying to see if I can pinpoint a bad tube.

Part of me wants to just take the plunge and get new preamp tubes too, the other part wants to be cheap and keep using the holy stain for tremolo/reverb. It just isn't the same though.

Nick wrote:I've always been a bit ashamed to bring this up, but a while ago I had a gig in August....while loading my 1970 silverface reverb and trying to carry the thing myself, I slipped on a step and dropped it on my foot. it didn't fall over or anything and the tubes weren't broken.

Well when I brought it to the gig and plugged in, my reverb channel wasn't working. everything was hooked up right, to the reverb tank, etc. I was getting SLIGHT signal through it, but it was quieter than 1 even all the way up and reverb/tremolo wasn't working.

I then plugged into the normal channel and things have been fine since on that channel. I went and bought a quartet of Groove Tube 6L6's, but that turned out not to be the issue. I kept the tubes because it made the normal channel sound better anyway. All the preamp tubes are glowing and nothing looks abnormal. I took a look inside to see if anything was unsoldered or burned out and nothing looked odd to my untrained eye anyway.

Any ideas? I feel weird paying $40 just to have an amp tech tell me what's wrong with it.

If you dropped a Twin Reverb on your foot, the first thing I'd check is your foot!

I understand the issue to be that you are getting barely any signal at all through the Vibrato channel, regardless of reverb and vibrato knob settings. If so....

There are six preamp tubes in your Twin. These are often referred to as V1 through V6 (V for "valve"). When you look at your Twin from the back, V1 is way over on the right, and you number leftwards from there, with V6 being the last preamp tube next to your big power tubes. V1 is the Normal channel preamp tube, and V2 is the Vibrato channel preamp tube. If your Normal channel's working correctly, then swap V1 and V2. Check to see that the Normal channel still works OK and the Vibrato channel is still broken.

Now unplug your footswitch and your reverb tank, set the Reverb, Speed, and Intensity controls to 0, and check both channels again. Now set Reverb to 10 and check again. (You won't hear reverb of course because the tank's unplugged... we're just checking for dry signal.) I can't say that these steps will make any difference at all, but you might as well remove those items from the picture as much as possible to isolate the problem.

If both of the tubes you tried in the V1 position worked OK, you can assume both those tubes are good. Leave one of them in V2 and plug the other known-good tube into V4. (V4 amplifies what comes out of the reverb tank, and also the mixed dry-plus-reverb signal.) What to do with the tube you pulled from V4? Plug it into the empty V1 socket. Try Normal and Vibrato channel again. If Normal works, you now have verified 4 good tubes, currently in positions V1, V2, V4, and V6. V6 has been working fine all along; this is the phase splitter tube, and nothing would sound right if it were broken.

If the Vibrato channel is still broken, you can assume it isn't a tube issue, because you used known-good tubes all the way along the Vib channel's dry-signal path: V2 => V4 => V6 => powahTubes. The 6L6 tubes work together on everything, so if they work OK with either channel then they aren't the problem.

Be sure to at least put the amp on standby before swapping tubes, better yet power it down completely. Try all the above nonsense, and that will probably settle the tube issue. (Reverb driver V3 and trem tube V5 shouldn't affect the dry signal, even if they've gone wrong. So you can ignore them for now.)

In spite of your inspection turning up nothing amiss, I'd suspect a cracked solder joint someplace. Hopefully the fix would be cheaper than a full retube anyway.

EDIT: Still mulling this over. You get a signal thru the Vibrato channel, but it's quiet.... AND the reverb doesn't work at all AND the vibrato doesn't work at all?

filtercap wrote: In spite of your inspection turning up nothing amiss, I'd suspect a cracked solder joint someplace. Hopefully the fix would be cheaper than a full retube anyway.

Thats kind of what I was thinking. It sounds kind of like the signal bleed if you turn your one channel way up, but you're plugged into the ohter one.

filtercap wrote: If you dropped a Twin Reverb on your foot, the first thing I'd check is your foot!

EDIT: Still mulling this over. You get a signal thru the Vibrato channel, but it's quiet.... AND the reverb doesn't work at all AND the vibrato doesn't work at all?

Hahaha my foot was in a bit of pain throughout the gig and the morning after, it sucked but I don't think anything was broken thankfully.

Anyways, yeah that's the gist of it. I just tried everything out that you said about the tubes. Channel 1 sounded fine no matter what, channel 2 had the same symptoms. Just for shits and giggles I also switched 3 and 5, still nothing.

Also, I think I underestimated the signal that's actually going through channel 2. I'd say it's more like 1-1.5 when the amp is on 10. Probably a decent enough volume to record a pedal demo with, but nothing else really.

Any ideas what specific area might be weak in solder? I know I'm going to probably have to just have it looked at but I think it'd help to know what's up.

I was puzzling over how you could be getting a wee bit o' the signal without hearing any reverb or vibrato at all.

Aen may well be onto something. In the schematic diagram, I see one place where signal going through V2 (Vib channel preamp) will likely cause the voltage across V1 (Normal channel preamp) to fluctuate, possibly causing V1 to create its own "copy" of what V2 is doing. I don't know why these two tubes are tied together this way, but schematix for several different Fender amps show the same thing.

So it could be that when you plug into the Vibrato channel, everything works OK through V2, the tone controls, and the Vib channel's volume control... and then it hits a roadblock, but not before V1 gets a whiff of the signal and sends its reduced-volume copy thru the Normal channel. That would give the kind of bleed-through Aen's talking about, and it would bypass the reverb and vibrato.

If all the above holds true, it would suggest that the break -- if there is one -- is somewhere downstream from V2 and before V6.

A rather rude way to test more of the Vibrato channel would be to make sure your reverb tank is plugged in the correct way, unplug the footswitch, turn your Reverb knob up to 10, and then make like Dick Dale and shake your amp hard enough to get the reverb springs to vibrate. If you hear anything, then you've likely got an OK circuit from V4 onwards, and (if the above stuff is true also) the problem is between V2 and V4. If you have a quarter-inch-to-RCA cable or adaptor plug, an even better way to test is to plug your guitar right into the reverb-output jack and see if you get any sound. It could be that the drop broke one of the wire connections inside your reverb tank (likely easy to fix). The Dick Dale reverb-shaking test doesn't exactly extend the life of those wires, so don't get too violent, eh?

Also.... you do have a footswitch, right? The vibrato effect won't work without one, so that could provide an easy explanation for vibrato not working.

Just telling the tech the same stuff you've posted on this thread should be enough info to let them zero in on the right chunk of amp circuitry.