Very weird reason for blowing heater fuse JTM 60

[Chuck H]

Ah. I wondered about that.

If it's not too much trouble, could you do the test again measuring one power tube at a time? Because if it's just one of the new power tubes that is surging grossly that would be something to worry about.

[torcamaniac]

Ah. I wondered about that.

If it's not too much trouble, could you do the test again measuring one power tube at a time? Because if it's just one of the new power tubes that is surging grossly that would be something to worry about.

I could do it.
Does it matter in which tube socket I put each?
And wouldn't the current at the fuse socket be less if one tube socket is empty?

[Chuck H]

It doesn't matter which socket. The current will be a little different, but what we're looking for is if one of the new tubes is surging grossly and the other is not. So it's not important.

[Dave H]

Five extra amps at turn on (and it's hard to know if you caught the worst of it) seems significant.

I read somewhere that an EL34 heater is 4.2R (6.3/1.5) hot but only 1R cold so worst case that's 12A at turn on for two EL34s. If the heater voltage is a little high you could put a small resistor in series with each side of the heater winding to reduce the turn on surge.

[torcamaniac]

I read somewhere that an EL34 heater is 4.2R (6.3/1.5) hot but only 1R cold so worst case that's 12A at turn on for two EL34s. If the heater voltage is a little high you could put a small resistor in series with each side of the heater winding to reduce the turn on surge.

Thanks Dave,
I'll check the heater voltages anyway.
Ordered a few 0.22 Ohm 2 Watt resistors just in case...

[Tom Phillips]

It doesn't matter which socket. The current will be a little different, but what we're looking for is if one of the new tubes is surging grossly and the other is not. So it's not important.

Igor,
If you are going to do the test with one tube then I suggest that you do the test with all the other tubes removed from the amp. That will isolate the data to just the one tube under test and will be the best way to measure the absolute differences, if there are any, between the tubes.

[Enzo]

You have to calculate the hot rsistance of a heater with Ohm's LAw, but you can measure the cold resistance with your meter.

[torcamaniac]

You have to calculate the hot rsistance of a heater with Ohm's LAw, but you can measure the cold resistance with your meter.

I was actually thinking of checking the heater voltages of all tubes and also preamp tubes.
If they are over 6.9Volt, I would install 0.27 Ohm resistor to drop the heater voltage a bit.

[Dave H]

You have to calculate the hot rsistance of a heater with Ohm's LAw, but you can measure the cold resistance with your meter.

Done that.

6.3/1.5 = 4.2ohm and the two EL34 (Svetlana) in my cupboard measure 1ohm

[torcamaniac]

So measured heater voltages for preamp and power amp eith all tubes installed.
They seem fine.
Preamp measured from PI heater to signal ground on standby 6.6 - 6.7 Vdc. Standby off 6.5 - 6.6 Vdc.
Poweramp measured from heater pin of one EL34.
6.5 - 6.6 Vac.
No need for dropping resistors then.

[Enzo]

Heaters only need to be within 10%.