Fender Twin Reverb, Running Hot - Thoughts Anyone?

[NoMoreMKIV]

Hey there-

I've got a late 70's silverface Twin Reverb, with JBL's. This amp has been modified to blackface specifications, but still has master volume.

The issue is this: the amp seems to run really hot, and I am wondering if this is normal. The tubes in it are Philips 7581A, and they are drawing 45mA cathode current each, and dissipating just under 19W per tube. I know that these tubes are able to run at higher power than standard 6L6 with no ill effects (to the tube).

You can feel the heat coming off the amp from a few feet away, and after rehearsing for 2 hours, the entire metal chassis is literally too hot to touch.

Is this typical for the amp?

There's no fan on this amp, either. I'm guessing this is typical for the amplifier model, but has anyone thought about or tried installing a fan?

Any suggestions or reassurances would be appreciated.

Thanks!

[Pandemonium]

Re: Fender Twin Reverb, Running Hot - Thoughts Anyone?

Take it to a competent fender tech before you play it anymore, something is wrong.

[Curly]

Re: Fender Twin Reverb, Running Hot - Thoughts Anyone?

I think I run my 6L6's at 30ma in my Bassman, although I don't know the spec on your tubes .... I'm sure that info is readily available on one of the tube sites ..

I suspect a bias is in order, although if it was OK, then the problem appeared, then you need to have someone look at it - even balanced pairs can go off after some usage

some amps run pretty hot, but if you're heating up a Twin, that's a problem

[kaknight]

Re: Fender Twin Reverb, Running Hot - Thoughts Anyone?

I can never feel heat coming off my twin more than about 6in away from those damn big kt66hp's it's got. Something is wrong my friend.

[59paul]

Re: Fender Twin Reverb, Running Hot - Thoughts Anyone?

I would have to agree with the other posts. If you're feeling that much heat, something is wrong. My 1st inclination would be that the bias is too hot. Do the tubes glow Red??? Whether they do or not, I would have it looked at by a Tech !!!......

[NoMoreMKIV]

Re: Fender Twin Reverb, Running Hot - Thoughts Anyone?

The tubes are not glowing red at all. The 7581A's are rated for 35W, and I have them biased such that they're running at about 19W. That number is definitely in specification for the tube.

But what I don't know is:

a) I don't know the actual temperature of the chassis. It could be 160F, or it could be 220F - I know it's too hot to touch, but it's not like a barbecue.

b) I don't know how much power is being dissipated in the heater filaments. If that is also higher than a typical 6L6, then I'd have that adding on top of the delivered power.

c) I don't really know "how hot is hot".

I'm going to reduce the bias so that the cathode current is down to 30-35mA, which will put me in the ~15W range, and see if it is noticeably cooler.

Anyone have any thoughts on why some amps have fans built in, but others don't? Do the newer (reissue) Twins have fans?

Thanks.

[CsaRvoX]

Re: Fender Twin Reverb, Running Hot - Thoughts Anyone?

I've got the same amp with jbl's but 6L6's are in it...after about 15 min it runs hot but not that much that I can feel the heat in front...when I touch the chassis where the tubes are located under it is naturably warm but not very hot...there should be a problem with your amp...