Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Joeriz on July 20, 2019, 12:49:34 pm
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I have a Deluxe Reverb clone that I built about two years ago that has worked fine since that time. However, for at least the majority of that time (perhaps all of that time), the 6L6GC output tubes have been getting VERY hot very quickly. I have had at least 7 or 8 guitar tube amps over the years (all Fender), still have another Deluxe Reverb (DRRI), and several tube hifi amps. On none of those amps have the tubes become this hot. Nor have they become hot so quickly. I have no temperature data to go on other than what I am used to. But, typically, even after gigging all night I could touch the output tubes for perhaps a second before taking my hand away. With this amp, the tubes become too hot to touch for more than a fraction of a second within 3-5 minutes without even playing a note. Likewise, after an hour or two the power transformer becomes so hot that I can't touch it for more than a second. This behavior happens with every set of output tubes I have tried.
The amp uses up-rated transformers for a Deluxe (Allen TP40D power and TO40MT output) so these should have no issues handling 6L6's.
Here are the measurements I took this morning:
Power transformer red leads = 632 across, 314 to ground
Rectifier pin 8 = 405v
6L6GC pin 3 = 400v
Bias = 42ma per tube
I'm not seeing anything above that seems out of line.
Any thoughts?
Thanks,
Joe
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What is your heater voltage
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What is your heater voltage
6.2VAC
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got 2 kids, one is normal like most, one isn't, but every time I took them to the Doc, he couldn't find anything :icon_biggrin:
play it till it burns
learn to measure in-circuit current and check ALL loads
or except it's just and odd nut
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play it till it burns
lol...and i'm using a small fan in the meantime to delay that day...
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It's hard to say without data to compare but I know my power tubes get extremely hot pretty damn quick. Sometimes I tube roll to compare tones of different brands and within a minute or so all of my power tubes get so hot a touch of more than a second or 2 can cause a 1st degree burn. I am talking about multiple amps on multiple occasions. I got some 6973 tubes that were used and had their as tested values written on a sliver of paper and scorched taped on. Within 30 to 40 seconds each one the paper blackened and the glue on the tape melted and the label fell off.so to me it's pretty normal. What you need to worry about is if you plate dissipation reads within tolerance and the tubes are not red plating I think you are good to go.
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Only thing I can come up with other than making sure of your bias values as noted above.. is the load on the OT.. are you using the 8 ohm tap with an 8 ohm load? A mismatch could make the power tubes run a little warmer possibly.
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Only thing I can come up with other than making sure of your bias values as noted above.. is the load on the OT.. are you using the 8 ohm tap with an 8 ohm load? A mismatch could make the power tubes run a little warmer possibly.
Yes. I built an impedance switch into the amp so I use the 8 ohm tap when running the internal speaker by itself and the 4 ohm tap when using my extension cab.