Transformer Getting Hot on Cathode Biased Power Amp

[mwelch55]

When I designed this amp, I think I might have used a transformer that is under-powered. The transformer is rated at 300-0-300vac@144ma. The 2 Tungsol 5881 tubes consume a total of around 120ma.  That seems like it is pretty close to the max current rating of the transformer.
It is supposed to be 25-30 watts, cathode biased. I borrowed the cathode values from Fender.  They used a 250Ω cathode resistor with a bypass capacitor.  Fender used a 25 uf and I used a 100uf bypass capacitor.
The problem is that the power transformer is getting hot. I have heard that you should be able to hold your hand on the power transformer for about 10 seconds without discomfort. I can leave it on the transformer for 10 seconds, but it gets very uncomfortable.
According to my calculations, the current going through the tube is close to 100 percent of the max plate dissipation.  If I converted it to fixed bias and set it to 70 percent of max dissipation, would the transformer run cooler?
Would it be worth the effort to convert it to fixed bias, or do I need to get a beefier transformer?

[sluckey]

It's OK for the PT to get hot. Fixed bias will allow it to run cooler while the amp is just idling but it will get plenty hot when you play loudly. I would not convert to fixed bias just to run cooler at idle.

[mwelch55]

What about the transformer rating?  Is 300-0-300@144ma enough or should I get a bigger transformer?

[sluckey]

The PT is running close to it's rating. How long has the amp been operating with that PT? You can certainly put a higher capacity PT in it but do you really need to? I've had amps that ran a very hot PT, especially when I was gigging.

Maybe try a 500Ω cathode resistor and see if that works for you.

[mwelch55]

It is a new build.  I got it running a few weeks ago.  Sounds great, but I noticed the transformer was getting hot.  I was worried that increasing the cathode resistor value would change the tone of my amp.  I don't want that because really like the way it sounds now.

For future reference: How close can you get to the max current of the transformer before it begins to overheat?  If my current draw is 120 ma, what would be the recommended transformer current rating for a 300-0-300 transformer?

[sluckey]

I would like 180 to 200mA for 6L6s or 5881s. 140mA sounds like 2x6V6 to me.

[mwelch55]

Thanks Sluckey. Your responses are very helpful.  I'll keep that in mind on my next build.

I haven't decided what I am going to do about this.  Do you think the transformer is far enough out of spec that it would be in danger of burning up?  If so, I'll need to replace that transformer with a bigger one.

[sluckey]

Why don't you just try running a pair of 6V6s? Much cheaper and easier than replacing a PT.

[mwelch55]

I have tried running 6V6s and they sound great, but I don't like the sound as much as I like 5881s or EL34s in that amp.  I especially like the Tungsol 5881.  That is a great sounding tube in this amp.  I also like the Electro-Harmonix EL34 in this amp.  The preamp is a Hot-Rodded Marshall circuit inspired by the Ceriatone Chupacabra or the Fortin Cali.

Sounds like if I want to safely run 5881 or EL34, I will need to change the power transformer.  The transformer I have in there now will be good for a 6V6 amp build (in the future), so it won't go to waste.

I want to make sure this is solid because I might sell it and don't want it to fail.

[shooter]

change the power transformer

IF, you change it out, you might notice/make a tonal change.
One of the "tricks" I like is double the current draw spec, so 100mA draw, I use a 200mA PT
add big caps and PS related things like sag go away. 
fwiw

[NewYorker]

"I want to make sure this is solid because I might sell it and don't want it to fail."

If you do sell it, then I'd suggest you either upgrade the PT or "downgrade" the power tubes to 6V6's.  Think about the aggravation that could result from a post-sale PT failure, the impact on your reputation, etc.

On the other hand, if you keep it, then you know what you are dealing with, and it sounds like you like how it works now.  Maybe 1) swap-in a bigger PT and see if you still like the sound, or 2) keep a spare PT, or 3) make sure you have a spare amp with you when you're gigging.

Other opinions?

Ed

[alerich]

I have tried running 6V6s and they sound great, but I don't like the sound as much as I like 5881s or EL34s in that amp.  I especially like the Tungsol 5881.  That is a great sounding tube in this amp.  I also like the Electro-Harmonix EL34 in this amp.

Another consideration is heater current. 5881/6L6 pull 900ma of heater current each from the PT 6.3VAC winding. EL34 is 1600 ma each. Just popping in EL34s adds an extra 1.4 amps of heater current draw. That may be what is causing your PT to run hot - an underspec-ed heater winding or one that was not designed to support EL34 comfortably. My JCM 2000 100W head runs 4 EL34 from the factory but the PT was running hot. I pulled the two inner tubes and re-biased and now the PT runs comfortably warm. The tone and volume difference is negligible and now I feel that the amp will run more reliably. I think Marshall cut corners on the heater winding and really didn't engineer it to be robust.

Does it run just as hot regardless of what power tubes are in it? Do you know what the spec is for the 6.3VAC heater winding? Heater current seems to be the most common culprit for hot iron in an amp in my experience. We are talking several amps of heater current versus a few hundred milliamps of B+ current. I have disconnected the heater winding on amps before and tapped in an auxiliary 6.3VAC transformer and then the PT runs almost at room temperature.