A friend of mine dropped of this amp for repair.
On first inspection I see R10 cathode resistor is burnt and is open circuit.
Is there any reason this can happen.
I have metered the tube and it shows no shorts .
The power transformer meters ok and so does output
On my diy 5e1 I was recommended by Dlab to use a ik
On first inspection I see R10 cathode resistor is burnt and is open circuit.
Is there any reason this can happen.
I have metered the tube and it shows no shorts .
The power transformer meters ok and so does output
On my diy 5e1 I was recommended by Dlab to use a ik
Oldest reason in the world, a bad tube. Resistors burnt up from excess current, and the only path for current is through the tube. Your meter does not test tubes with their heaters running and with 300-400 volts on the elements and real currents flowing.
I see no reason to change from the 470 ohm stock resistor.
I see no reason to change from the 470 ohm stock resistor.
> ...put in a higher-wattage one.
Why? Mu of 6V6 is 10, so Rk dissipation must be <1/10th of plate dissipation.
Yes, you can work a tube closer to rating than a resistor you want to live long. But actual Rk dissipation is 1W or less. Except when the poor abused 6V6 shorts-out. (And no, you can't always tell on a ohmmeter: lost a good VT-40 to a tube that was fine cold and short when fully hot.)
Why? Mu of 6V6 is 10, so Rk dissipation must be <1/10th of plate dissipation.
Yes, you can work a tube closer to rating than a resistor you want to live long. But actual Rk dissipation is 1W or less. Except when the poor abused 6V6 shorts-out. (And no, you can't always tell on a ohmmeter: lost a good VT-40 to a tube that was fine cold and short when fully hot.)
When I looked up old Tung Sol data sheets, the original North American 6V6 is rated at 12 watts maximum plate dissipation, while the 7591 is rated at 19 watts. Presumably the 7591 had a bigger plate structure.
But, as you can see in the two attached photos, the JJ "6V6S" seems to have identical electrode shapes and sizes as the JJ "7591". Presumably both are re-pinned versions of some former Soviet-era tube, that originally carried an entirely different Russian/Soviet tube designation.
No wonder the JJ "6V6S" is more robust than a real 6V6 - its a 19-watt (or more) tube, masquerading as a 12-watt one.
I have a reissue '65 Princeton Reverb with 440 volts B+ (!!). I bought it second-hand, with JJ 6V6S output tubes. They can take the abusive B+ voltage, but seem to sound much blander (lower THD) than the other-brand 6V6 tubes in my other Fender amp, a Super Champ XD.
-Gnobuddy
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