Replacement of KT88's. No mods' necessary?

[taotao]

Hello,

I've got an DIY Tubeamp (Ella). Schematics see here:
http://home.netvigator.com/~goneill/diags.htm

I want to put in EL34 (now I've got KT88), but one is broken (see here: Help, strange glowing in KT88 )

Can I just replace the KT88 without further modifications (except lowering bias to around 32 mA)?

The voltages you see here:
http://i852.photobucket.com/albums/a...afzen/ella.jpg

Thank you very much for input.

Olaf

Oh, forget: OPT has 3,5 or 5 k (don't know exactly :-)

[DigitalJunkie]

It doesn't look like any mod's are needed,just re-bias.

[taotao]

Nice, thank you

[taotao]

Hi Forum,

today I received my EL34's. Changed Bias to 23mA.
No my speakers are singin again, but also my tubes: They give a high pitched sound, it must be some mechanical stuff, 'cause you don't hear from speakers. And it's not low enough in volume to ignore. In other words: IT'S ANNOYING!
I opened the amp and measured again the voltages, and Oh, B+ is 590 V. Although EL34 are rated with 800V as maximum, mine don't like that voltage (they are JJ).

My questions:
1) Could it be the high voltage is reason for the tube making this noise?

and 2)
How do I lower B+, I guess a resistor is working, but what wattage?

Thank you.

Ah, once again, here the schematic (note that 409V should read 580V):
http://i852.photobucket.com/albums/...lafzen/ella.jpg

Olaf

[ChrisA]

My questions:
1) Could it be the high voltage is reason for the tube making this noise?

and 2)
How do I lower B+, I guess a resistor is working, but what wattage?

1) Just a guess... Maybe the amp is oscillating at some ultra-sonic frequency (that you can't hear in the speakers) and the tube is ringing at some multiple like 1/2 or 1/3rd of that. The cure then would be to get rid of the oscillation or mechanically damp the tube. (with a silicone o-ring or whatever?) Would be nice to look at the signal inside with a scope or spectrum analyser. Maybe I'm wrong. I don't think high voltage alone can cause a tube to mechanically "ring" the voltage is causing something else that is then causing the sound.

Can you remove a tube or disconnect a stage and find which exact place on the circuit that is causing the problem? The first step is to figure out where it is not failing and where it is. Is it one both left and right? the power tubes, driver tube???

2) What wattage? Multiply the current through the resistor times the voltage drop across the resistor. For example 10 volts at 200ma or 10 x 0.200 = 2W But then if the power is 2W you want to have a safety margin over that, I'd double the wattage to 4 and then pick the next highest standard size which is 5W. Use you own numbers. 10 and 200 are just my example.

[taotao]

Hi Chris,

thanks for reply.

Unfortunately I haven't got a scope.
It's the powertube making this noise. If I put it in another socket, the noise moves with it.
So it's definately this one tube, making this noise.

I'm not shure, if the amp oscilates, it's a diy-kit and beside some screen diodes, I didn't change the stock circuit.
So all kits could have this problem, but I never heard about it.

Only one thing, my amp differs: You've got a feedback loop with a resistor and a cap in parallel. In my version this cap is missing (I lost it some time ago).
Maybe this could cause oscilation?

Thank you for support.

Olaf

[taotao]

I found one cap (100 pF like in the kit) and soldered it in.
No change.
Oh well.
I ask vendor to replace that tube.

Olaf