Excessive heat - SF Twin Reverb...

[Bob M.]

I renovated and set up a Fender Twin Reverb amp for a friend - an early 70s silverface version. It needed the usual stuff - tubes, electrolytic caps, some resistors replaced. When finished, the amp sounds very good, really good, but it seems to generate excessive heat after about a two hour running. The area around the 4x 6L6 power tubes gets very hot, hotter than normal IMO. Here's some specs:

B+ = 450 Vdc
on the power tubes:
Plates = 448 Vdc
Grids = -52.0
I set the bias for 30 mA per tube, 60 mA per side.
The guy likes clean guitar sounds, he plays a Gretsch.
I re-grounded everything, especially the filter grounding and power transformer grounding at the usual area, at the power transformer bolt on the chassis. The amp is very quiet. These numbers are pretty typical for a Twin of this era.

What else could I try or address to try and get this heat situation under more control (besides add a fan)?

I'd appreciate any on-point advice,

Bob M.

[The Dude]

Bias seems reasonable, but does it hold? In other words, if you recheck bias after playing the amp for a while, is it still 30mA? I've never had to add a fan to one of these, so something must be amiss.

[Jazz P Bass]

I would lower the bias to 25ma's.

[Justin Thomas]

Is it one of those "mixed-bias" deals? Maybe something is needing adjustment if it is. What with modern tubes being well, you know...

Not that any of those amps have survived the blackfacing craze...

Justin

[g1]

The heat is not from a transformer?

[Chuck H]

What with modern tubes being well, you know...

That was my first thought. New tubes are always suspect anymore

I would do as suggested and check the bias for drift. Then determine the cause. It could be the bias supply (did you replace the electrolytics in the bias supply too?) or it could be the new tubes are misbehaving.

g1 also made a good observation. If it's a transformer you may have some shorted winds or such reducing a transformer ability to deliver the necessary current.

[Mick Bailey]

I bias these amps pretty cool - usually around 20mA or so. I see them sometimes where they're playing just fine with a bias of around 18mA. As bonus is the tubes have a very long life. Most of the guys are looking for an ultra-clean amp or are getting all of their drive from pedals.

[pdf64]

...If it's a transformer you may have some shorted winds or such reducing a transformer ability to deliver the necessary current.

My understanding is that in most cases, even just 1 shorted turn renders a transformer non-working.
This is because the shorted turn will 'suck up' the energy in the magnetic circuit, causing the winding inductances to collapse.
In a PT, the primary fuse will blow.
In an OT, the lack of primary inductance etc will cause the amp's power output fall to a tiny fraction of the normal level.

[Bob M.]

Thanks for all your replies and suggestions,

The bias does hold and it is stable. I have re-capped the bias circuit. In my opinion, in a 4x power tube amp, it is false economy not to re-cap the bias circuit. About the first thing I do when I bench any tube amp is examine the bias circuit and replace anything that is iffy. I use a circuit I came up with years ago using the stock bias balance with an added bias amount pot. I've used this for years on many amps with complete reliability.

I did change out some tubes but none of the 4x GE power tubes that looked, tested, measured and sounded great. These tubes all measured pretty close on their own (between 27mAs and 33mAs) and I just moved them around in pairs so I was within 1 mA for each side, then a final correcting with the bias balance pot, pretty straight ahead. Also, each of the four power tubes seemed to be generating about the same amount of heat - no outliers here.

The heat seems more from the tubes than the power transformer. The power transformer seems to be a quite normal temperature for a Twin Reverb after a 2 hour play. I think lowering the MAs for the power tubes is a good next step. Also, I did re-ground the cathodes of the power tubes from pin 8 to ground.

All that silverface stock power reduction garbage in and around the power tubes had been previously removed by some former owner or tech. I left the silverface PI values alone but I did do some minor blackfacing to other parts of the circuit. I will pay some attention to the output transformer.

Thanks for the replies,

Bob M.

[cjenrick]

how hot is hot? you have a lot of heat being generated in an enclosed area, and heat rises. so the chassis is gonna get pretty cozy.

if it is real hot, look for ultrasonic oscillation, this might only occur while the guy is playing. put a scope on it and look for gremlins, they will show up at a hi freq setting on the scope, but not all the time, so you can also check for this by looking at the sine wave on the output, if it is being modulated by the hi freq osc, you will see a crappy wave form. this can be heard also.

[TimmyP1955]

Get a Harbor Freight IR thermometer - sometimes they have a coupon, making it $20. Then you can see what's up.

[Bob M.]

How hot is hot? Well, this is pretty un-scientific, pretty anecdotal but having said that I've probably owned 20~25 Twin Reverbs in my time (I have three currently) and I've probably worked on two hundred or so over the years. I'd say the heat on this one is in the top 2% of all that I've seen. Considering the amp is working perfectly, spec-ing perfectly and sounds great, it's now probably in the top 1%. That's why I posted.

Bob M.