1969 Fender Twin Reverb Amp

[HarryMowrey]

Hey All,
I own a 1969 Fender Twin that I rescued out of a buddys garage. I also know there is a ton of knowledge and experience here. This twin that when I turn it up about 1/2 way on the second channel it distorts. At first I thought it was a speaker going south but that is not the culprit. Here is what I have done,
Changed all the preamp and power tubes with known good ones,
disconnected the internal speakers and connected to a speaker cab with the same results.
isolated the 1st channel by pulling all the tubes but the v1 and v6 with no issues, it is only on the second channel that this unwanted distortion occur's. But I noticed the 1st channel has a lower output than the second even though they both have the exact same tubes in the v1 and v2 spots.

Little history on this amp, I received this amp and it was unplayable. I serviced the jacks and pots and it immediately responded. I then retubed it and replaced the filter and coupling caps. It was at this point that I noticed this issue and I observed 1 power tube redplating. I then changed out the power tube sockets and screen resistors. the red plating stopped but the distortion persisted. I then took the amp head to a competent tech over in Sacramento CA. He looked everything over changed a couple of resistors and caps (installed some blue bottle type) and said he thought it was good to go. The issue did not rear up again until I had a outdoor gig and turned it up to 6. Even right now if I keep the volume knob below 4 it is clean as can be. I have attached an MP3 of the amp distorting please listen and let me know if any of you have encountered that before and what was the solution. Any suggestions / Ideas. Thanks

BTW The Amp tech has told me next time I am in Sacramento CA. to bring it to him and he will troubleshoot and repair at no cost to me (I live in Reno NV.)

[Leo_Gnardo]

First of all congratulations on doing all the right things to restore your Twin. About the distortion: "Cosi fan tutti" aka "They're all like that." There's plenty enough gain in the pre stages to drive the power amp well into distortion at volume 5 with ordinary guitars and typical gain/eq settings. Although Twins are accepted as a go-to clean sounding amp, they do have their limits, even with the most efficient speakers (JBL, EV, Altec, some Celestions like G12-80).

You could push that volume knob up a couple points if you used lower-gain pre tubes like 12AY7 or 5751 but expect you'll be back in clip zone by volume 6 or so. A super low gain pre tube like 12AU7 may get the response you're looking for, though generally not recommended. The AU is intended for higher power purposes like driving difficult output tube grids or low-impedance tone stacks. In one instance a customer asked me to modify his Deluxe Reverb (a junior size Twin if you think about it), he wanted to walk onstage, spin his volume up to 10 and get nothing but teen-kleen tone. It worked all right, he was satisfied, and to return the amp to stock all he needed to do was swap a 12AX7 back into the preamp tube socket.

A different trick one of the local blues-meisters showed me: dial all your eq knobs to minimum, volume at 10. Then just barely crack open those EQ's, there's a whole different tone palette down there, and if you're careful you can stay in the clean zone. Costs nothing but a couple minutes experimentation, certainly worth a try.

About having to crank it up outdoors: nothing unusual there either. There's no room to echo your sound back at you. It just goes out and keeps going. You'd need a big pile of Twins to make up the difference. Some alternate solutions: A) set up your amp in front of you, like a monitor wedge, instead of behind as usual. Radical, I know, but it does work. A2) Extension speaker cab, same thing, use as a monitor. Of course if you step out of the speakers' focus you'll hear less volume, gotta expect that. B) if there's a monitor system, get your amp in your wedge. I've seen guys show up with tiny 5 watt amps, of course they'd be lost in the wind, but with a mic on the amp and dialed into the monitor they're happy as could be, no sore arms & backs from hauling around big guitar amps either. Let the PA do the heavy lifting when at all possible.

[HarryMowrey]

Fender Twin

First of all congratulations on doing all the right things to restore your Twin. About the distortion: "Cosi fan tutti" aka "They're all like that." There's plenty enough gain in the pre stages to drive the power amp well into distortion at volume 5 with ordinary guitars and typical gain/eq settings. Although Twins are accepted as a go-to clean sounding amp, they do have their limits, even with the most efficient speakers (JBL, EV, Altec, some Celestions like G12-80).

You could push that volume knob up a couple points if you used lower-gain pre tubes like 12AY7 or 5751 but expect you'll be back in clip zone by volume 6 or so. A super low gain pre tube like 12AU7 may get the response you're looking for, though generally not recommended. The AU is intended for higher power purposes like driving difficult output tube grids or low-impedance tone stacks. In one instance a customer asked me to modify his Deluxe Reverb (a junior size Twin if you think about it), he wanted to walk onstage, spin his volume up to 10 and get nothing but teen-kleen tone. It worked all right, he was satisfied, and to return the amp to stock all he needed to do was swap a 12AX7 back into the preamp tube socket.

A different trick one of the local blues-meisters showed me: dial all your eq knobs to minimum, volume at 10. Then just barely crack open those EQ's, there's a whole different tone palette down there, and if you're careful you can stay in the clean zone. Costs nothing but a couple minutes experimentation, certainly worth a try.

About having to crank it up outdoors: nothing unusual there either. There's no room to echo your sound back at you. It just goes out and keeps going. You'd need a big pile of Twins to make up the difference. Some alternate solutions: A) set up your amp in front of you, like a monitor wedge, instead of behind as usual. Radical, I know, but it does work. A2) Extension speaker cab, same thing, use as a monitor. Of course if you step out of the speakers' focus you'll hear less volume, gotta expect that. B) if there's a monitor system, get your amp in your wedge. I've seen guys show up with tiny 5 watt amps, of course they'd be lost in the wind, but with a mic on the amp and dialed into the monitor they're happy as could be, no sore arms & backs from hauling around big guitar amps either. Let the PA do the heavy lifting when at all possible.

Thanks for the reply Leo Gnardo,
I have quite a few different amps and frankly none will put out the arching distortion sound that the twin is currently displaying on the vibrato channel. The clean channel I can crank to 10 and it just gets the typical muddy distortion not that arching sound.. Listen to the clip again and you should be able to hear what I am describing. Thanks again

[überfuzz]

It's a pity you don't like fuzz then. :-) Seriously though, you did try known good power tubes, yes? I had a quad of 6L6 tubes turning into something similar once, mine distorted a bit harsher while being pushed...

This being said, check the circuit yourself (UNPLUGGED and don't touch any circuitry) and compare to the schematics. Take some gut-shots and post here. You'd never guess how good the members of MEF are at spotting wonky circuitry.

[52 Bill]

Hey Harry, welcome to the place.

I listened to your mp3 and it definitely sounds like you have a problem. It would help us all if you can tell us exactly which model Twin you have. If you look at the tube chart inside the case, there may be a model number printed there. If not then perhaps a description of the front panel controls, like Master Volume, Push Pull Master Volume, etc.

It seems only to happen when you hit a hard chord with some low end. It does sound like something is arcing. When it happens, where are the controls set? Do you have the reverb turned on?

[Chuck H]

Three possibilities I can think of...

There is an oscillation that only affects the vibrato channel.

The reverb circuit is wonky somehow. Anyone here ever listen to the reverb on a fender amp with the pan unplugged and the reverb knob turned up? The reverb is only functional on the vibrato channel.

There's an intermittent fault or short that is affected by vibration when the volume is up.

None of these things is likely to be easy troubleshooting

[Drewline]

The vibrato channel has an extra gain stage, V4a, which is why it is louder than the normal channel. What do you have the reverb set to? It should be off for comparison. I like to bypass the reverb tank by plugging a 470K resistor into the reverb tank rca jacks to test the circuit. At 1 there is no gain boost, by 10 you've added another stage of gain. This just serves to eliminate the tank. Make sure that V4 is a good tube. When you changed the filter caps did you replace the bias cap(s) & the bypass caps? One or more could be leaking. Did you convert the stock bias balance to bias level & properly bias the output tubes? Some pictures would be helpful.

[Leo_Gnardo]

There's an intermittent fault or short that is affected by vibration when the volume is up.

I'm thinkin' along that line now. Maybe a soldered connection has come unstuck, I'd concentrate around pre tubes V2 & V4, and whatever's connected to them. Also the ground leads that go to the brass bus sheet. Although it would affect both channels, one favorite loose-connection spot is the 100 ohm feedback node resistor, and its connection to that brass bus.

On rare occasions I've found a tube socket that's gone bad, again suspect V2 & V4 mostly in this case. Other suspects: are the pots tightly socked down? Ground connection usually made by pot tab soldered to pot body - if the pot's loose then a little shaking from loud playing will make volume pop up & down, cracking noises, etc.

Lastly, all too often I find input jacks not tightened down, outputs too. That will make unexpected noises, as you might expect.

[Chuck H]

FWIW there have been a lot of suggestions about checking conditions and circuits that would affect both channels. Only one poster made any indication of a reason. which was that the vibrato channel has more gain and therefor would be more prone to induce a problem in a peripheral circuit used by both channels.

I'm not buying it. Once you get either channel clipping there is plenty enough going on to cause a problem with the bias, a bad power tube socket, the bias balance, etc. I'm going to take HarryMowrey's at his word, since he owns and uses the amp, and try to stick with troubleshooting problems that would be exclusive to the vibrato channel.

JM2C

[Enzo]

Does the vibrato channel really have more gain, or just more stages? I don't remember one channel of a Twin being lots louder than the other.

[Chuck H]

You know.?. I've never owned or played a BF extensively and I never thought to actually evaluate it for myself when I did play them since I was nearly always just plugged into the vibrato channel (for the reverb). But this is what I've read from a lot of players. Such as even the players that don't use reverb or vibrato prefer the small amount of extra gain from the vibrato channel.

[Justin Thomas]

It's a very tiny amount more gainy in the Vibrato channel... WAY less than switching between Hi & Low inputs. But to me it's a matter of fidelity. Maybe it's because we've come to equate "gain" with "distortion," "dirt," etc. I think the Normal channel is more "direct" sounding - more hifi. Not a bad thing, but maybe a bit cleaner than some (apparently most?) would prefer. I do know a few squeaky-clean players who use the Normal channel. And I prefer it for fat jazz.

Different beast, but on my Concert if I'm not using the "Harmonic Vibrato," I plug into the Normal channel. Much more in-the-face than the Vibrato channel. It's sharper. I'd compare the effect of either of these beasts to the difference between plugging into a couple effects boxes (but turned off) in front versus plugging straight in. It's subtle, but there. But not as radical a difference as it seems to be in the amp in question.

Not that I've played hundreds of vintage Fenders, just a few tens...

Justin

[Enzo]

I just meant that the extra stage didn't really increase the overall gain of the channel - other than the tiny bit reported - and was unlikely to be the source of the issue.

[Leo_Gnardo]

It's a very tiny amount more gainy in the Vibrato channel... WAY less than switching between Hi & Low inputs. But to me it's a matter of fidelity. Maybe it's because we've come to equate "gain" with "distortion," "dirt," etc. I think the Normal channel is more "direct" sounding - more hifi. Not a bad thing, but maybe a bit cleaner than some (apparently most?) would prefer. I do know a few squeaky-clean players who use the Normal channel. And I prefer it for fat jazz.

Different beast, but on my Concert if I'm not using the "Harmonic Vibrato," I plug into the Normal channel. Much more in-the-face than the Vibrato channel. It's sharper. I'd compare the effect of either of these beasts to the difference between plugging into a couple effects boxes (but turned off) in front versus plugging straight in. It's subtle, but there. But not as radical a difference as it seems to be in the amp in question.

Agree with all that ^^ Justin. The slight gain advantage in vib channel can be easily offset by pre tubes with less gain, not especially because they're chosen that way like 5751 or 12AY7 but simply luck of the draw. So on occasion we find more gain in the normal channel. No biggie, fraction of a turn of the volume control takes care of the difference. Also true what you say about brown Concert, goes for the Super too, tone seems to suffer because the signal is split into separate hi & low frequency signal paths then added back together. Makes for a great sounding vibrato, not so much when the vibrato's off.

[Chuck H]

I just meant that the extra stage didn't really increase the overall gain of the channel - other than the tiny bit reported - and was unlikely to be the source of the issue.

That's what I was saying in post #9

I guess I didn't realize you were backing it. I'm pretty sure if the OP say's the problem is only on the vibrato channel, the problem is only on the vibrato channel. The guy is playing outdoor gigs. I'm sure he's used the normal channel plenty in a pinch or to get by in tough situations. I think we need to focus on circuitry exclusive to the vibrato channel.