Fender Twin Died, HELP!

[-Ty-]

Hi everyone, I have a bad problem with my amp. I was playing it the other day and it sounded fine, then I went to play it today and it made a frying pan sound and the tubes glowed red and blew the fuse (I should mention that it is a 1979 Fender 135w twin reverb with the UL transformer and these tubes are only a year old-Winged "C" 6L6GC). The filter caps and the caps on the board have never been replaced, so I'm thinking that maybe they went bad. I ran some tests before the fuse blew though, I started by pulling one preamp tube at a time and then turning it on. The buzz only went away when the power tubes were pulled. I am going to order some new caps and put them in and see if it will work.
Thanks in advance
Tyler

[elronhoover]

Hi everyone, I have a bad problem with my amp. I was playing it the other day and it sounded fine, then I went to play it today and it made a frying pan sound and the tubes glowed red and blew the fuse (I should mention that it is a 1979 Fender 135w twin reverb with the UL transformer and these tubes are only a year old-Winged "C" 6L6GC). The filter caps and the caps on the board have never been replaced, so I'm thinking that maybe they went bad. I ran some tests before the fuse blew though, I started by pulling one preamp tube at a time and then turning it on. The buzz only went away when the power tubes were pulled. I am going to order some new caps and put them in and see if it will work.
Thanks in advance
Tyler

Hi there, I had one of these, brutally loud and tight it was..IIRC it ran 500v+ on the plates...

First up, be very careful working around inside of the amp. I only say this because it sounds like you may not have a lot of experience, based upon your troubleshooting approach to your problem. Lethal voltages can really mess up your day..

Second, do not turn the amp on any more, new fuse or not. You only stand a chance of doing more damage to what may already have been done. A frying sound indicates shorting, and shorting ain't good...

If the filter and bias electrolytic caps are original, yes it's time to replace them all. Do you know how to test the current caps (actually the entire B+ supply) to see if they shorted? Do you visually see any evidence of venting on any of the caps (or a small BB sized dimple bulging out of the cap end, indicating it's getting ready to fail)?

Did all 4 of the power tubes start to redplate? This may indicate problems with the bias supply (particularly it's electrolytic cap). If it wasn't all 4 tubes, which?

You also want to check all 4 of the output tube 470r screen grid resistors (soldered across the top of the tube pins) to see if they have been burned/stressed from excessive screen current if 1 or more power tubes have failed.

If you're not comfortable with any of this, take it to a tech, but above all, be careful!! If you find something, post back here..

Dave

[-Ty-]

Hey Thanks for your suggestions. I have had experience before in amps. I've built two of George's kits and done work on my other amps. I think that I have discovered the problem though, I guess when I was putting the chassis back in I touch one of the heater lines which move to the pin to make contact with the pin beside it on V10. I also have looked at the filter caps and one of them does have the BB sized bulge that you indicated, I am going to replace them all. I also measure the screen resistors and all of them where in the 470-490 range but one was at about 510. I was wondering though with the caps I can get everyone but the 70MFD 100VDC. They only make 80MFD 100VDC, is this safe to run? And what is the best brand to go with, I've heard a lot about the Sprague, but I also heard that Mallory are good to, any thoughts?
Thanks again,
Tyler

[elronhoover]

I also have looked at the filter caps and one of them does have the BB sized bulge that you indicated, I am going to replace them all. I also measure the screen resistors and all of them where in the 470-490 range but one was at about 510. I was wondering though with the caps I can get everyone but the 70MFD 100VDC. They only make 80MFD 100VDC, is this safe to run? And what is the best brand to go with, I've heard a lot about the Sprague, but I also heard that Mallory are good to, any thoughts?
Thanks again,
Tyler

For the bias cap, yes you can use the 80u/100v (my schem for the 135w fender shows a 80u/75v bias cap actually). You could go up in cap voltage if size permits too, but you dont want to go too far up in capacitance.

I've only been using F&T axials and cans as far as new caps, and my experience in current prod. is limited to those. You can still find sprague atoms around (AES still has some values), but they are getting pricey and I'm not so sure that it's worth it. In general, stay with a name brand, and avoid cheaper generics (I had some chinese weber axial caps a couple years ago that were actually triple-nested radials of wrong cap/voltage with ext. legs tacked on and then rewrapped twice with new and different cases/values on them, absolute junk).

Maybe one of the guys who work more with the new stuff can give you some cap recommendations..

One thing I'll add also is that those 135w fenders really run the 6l6's hard; they shipped with STR387's originally, and that was the unofficial recommended-only replacement tube back in the day for these amps. If you are using the stock 'tube balance' circuit and not an adjustable bias voltage, definitely check your bias and make sure you are OK. I wouldn't go more than say 65%/idle for the bias with those Wing C's. JJ's can probably be run a little hotter..

Dave

[-Ty-]

So I figured out what was wrong, I guess when I went to put the head back up in the chassis I push one of the heater lines on V10 into the bias line. Now with new fuse it does not make the frying pan sound, but I now have another issue. When I plug into the vibrato channel I get no sound and then when I plug into the normal input I get sound, but no where near as loud as it was. I have confirmed that all the tubes are good, could this be related to the filter caps, and the caps on the fish eyed board? Also when I turned up all the knobs it makes the same level of hiss and buzz as it did when I setup the hum balance, and output tube matching pot a while a go (It was working perfectly fine then). I am stumped, I don't think that its the OT because I am getting sound from the normal input.
Oh and I pulled up the schematic and measure my voltages, on average I was reading +20v above what the schematic says it should be. Again filter caps or possibly screen resistors? I do have new caps coming in tomorrow, so I will swap them out and try again.
Should also mention that I am getting voltages on all of the pins that are suppose to (According to the schematic).
Thanks Again,
Tyler

[elronhoover]

So I figured out what was wrong, I guess when I went to put the head back up in the chassis I push one of the heater lines on V10 into the bias line. Now with new fuse it does not make the frying pan sound, but I now have another issue. When I plug into the vibrato channel I get no sound and then when I plug into the normal input I get sound, but no where near as loud as it was. I have confirmed that all the tubes are good, could this be related to the filter caps, and the caps on the fish eyed board? Also when I turned up all the knobs it makes the same level of hiss and buzz as it did when I setup the hum balance, and output tube matching pot a while a go (It was working perfectly fine then). I am stumped, I don't think that its the OT because I am getting sound from the normal input.
Oh and I pulled up the schematic and measure my voltages, on average I was reading +20v above what the schematic says it should be. Again filter caps or possibly screen resistors? I do have new caps coming in tomorrow, so I will swap them out and try again.
Should also mention that I am getting voltages on all of the pins that are suppose to (According to the schematic).
Thanks Again,
Tyler

Dont worry about the 20+ voltages, the wall VAC is higher today than when the amp was spec'd..

Have you measured your bias voltage and your idle cathode current on all the 6l6's? What are you getting for both?

Your screen R's should be OK, they tend to drift high over time, and aren't too far off spec.

You can trace your signal backwards from output to each channel's V1, touch a metallic probe (meter set accordingly if using a meter probe) to the control grids, when you get to the point that you hear no pop out of the speakers, there's the problem in the circuit.

Is your reverb send/return working? Turn it up, bump the amp, get that crashing sound?

Not sure what you mean re. the hum balance and tube matching adjustments, are you saying it hums the same no matter how you adjust it?

Dave

[-Ty-]

Ok I will test the reverb later on today, and by hum balance I mean the pot on the back of the amp for tuning it up. It just controls the amount of hum and noise you have in the circuit. On the power tubes, I was reading bias voltage evenly across all of the tubes, and it was the same as when it was working fine. I think I remember it being -57 vdc, somewhere around there (I will measure and post back later on today).
Thanks again
Tyler

[-Ty-]

Hey everyone, I have an update. The amp is now working, After replacing the filter caps it was still doing the same problem, So I got out the chop stick and went over each and every wire looking for anything not normal. I didn't find anything, but I plugged in and long and behold it was going again. The only thing was that the normal channel was still to low, so I replace the V1 preamp tube and now it works, reverb and all. I dont know if it took the new filter caps and bias caps a chance to go through the amp, but everything is now cool. Thanks for all of your suggestions, Im hoping that it stays this way (At least I know my transformers and tubes are okay.
Thanks
Tyler

[Mr. Mullard]

With modern tubes being unreliable junk for the most part, they are what I ALWAYS check first (swap them out with known goodies) when an amp goes funky