An amp that melts 6L6s?

[tubehead123]

Hey y'all, I'm working on a buddy's fender twin (4x6L6 mono) and it melted a quad of power tubes - the amp got waaaay hot, the internal structure started to sag and the plastic base melted a bit...thing is it was still working (passing signal) when he noticed it getting really hot...seems like it was doing this for a bit and the heat melted the power tubes - has anybody ever seen this?? I'm going thru it trying to see what may be wrong, the supply voltages look OK and i'm trying to get a handle on measuring the currents from the AC line, the heaters and the plate (and bias) supplies but i haven't finished that yet...i did replace the phenolic power tube sockets with ceramic ones so i had to rewire all of the output tubes, but i've been checking them over and over again and they seem to be OK as far as i can tell...is there any possiblity the output transformer is going bad and giving the tubes a higher load?? could the heaters be too hot?? i'm getting to the end of the things i can think of if you can give me some suggestions i'd appreciate it...l8r...Nick

[karma]

you just about covered everthing.

im Guessing its the bios or the output tranny

and a nice set of matched high current tubes

[cerrem]

Make sure to replace ALL the tube sockets....
Sometimes the bias on pin 5 gets leakage from the SCREEN voltage on the next pin #4....

YOu have a run-away bias condition....SO you will need to turn the amp on with meters to see WHAT is causing the bias to go POSITIVE... You have a small amount of time to find the problem before things gets cherry red...so shut-down the STANDY-BY switch before things get RED...
There are just a few many things that can do this..

1. The phase inverter coupling caps can sometimes leak DC to the bias pins.... So you can "lift" the coupling caps to check for this..

2. Bad tube sockets....you already replaced...BTW: use Micanol sockets in the future not ceramic...

3. Check or replace the BIAS filter caps...

It would be rare for the Output Transformer to cause this...yes I have seen OPT's with shorted windings cause this, but usually they have very low volume and distorted when this occurs....
The quick and dirty way to do a check on the OPT is to apply the heater voltage to the secondary....Then check the primary voltage on each side of the Center-Tap to see if you have symmetry on each Push-Pull half....

Chris

[Giaime]

I've seen this... shorted diode on the bias supply ( how can happen???) and 350VAC on the control grid...

And... you say 6L6... I hope you're using good quality 6L6GC...

[gingertube]

Yep,
Bias Supply almost for sure. Whenever you have a Fender to work on its worth doing the following things as ROUTINE.
Replace the bias supply diode
Replace the bias supply carbon resistors with 1W metal films (same value)
Replace the bias supply filter capacitor with 105 degrees rated cap
Replace the 1K5 carbon output tube grid stoppers with 1W metal films
AND ESPECIALLY
Ditch those 2W 470 ohm carbon screen resistors and replace with 1K 5W wirewound. This last step will improve the longevity of the output tubes and allow the amp to "break up" at slightly lower volume - that is you'll get "full tilt" sound a little earlier on the volume control. I've done this screen resistor mod to at least 6 or 7 Fender Amps, mostly Twin Reverbs but including a Super Twin (the 180W job with 6 RCA Blackplates) and the owners have always liked the result.
Cheers,
Ian

[anatech]

Fender Twins are the acid test for a new 6L6GC. Most will run with red plates and possible hum. 7581A's work very well in these, as do Electroharmonix 6L6EH's. Of course, real KT-66's should be fine also.

Sovteks will run red as many other tube brands will. I've spent a lot of money finding these things out. I use only 7581A's and 6L6EH's now.

-Chris