Help! new Tung Sol 6l6gc Tubes make terrible"sqwaking" distortion sound on loud notes

[bbrownsbrick1]

yes, it does! it actually has two outs in parallel and a switch that chooses between 8 and 4 ohms.

[bnwitt]

yes, it does! it actually has two outs in parallel and a switch that chooses between 8 and 4 ohms.

Ok, what impedance is the speaker or speakers you are using? 8 ohms?

[bnwitt]

Ok, the more I read this post the more confused I get. It seems the builder is saying that he built your amp for 6L6's. The plate voltage you cited is not only too low for EL34 class AB PP but 6L6's too. So I'm wondering if your meter is correct when you are taking the reading. This is an odd duck for sure.

[bbrownsbrick1]

yes, is it an output transformer issue? maybe thats it. my amp tech says its either the output transformer or the B +. im really not sure. all i know is that these amps, when operating correctly sound terrific. itd be a shame to give it up like that!

[bnwitt]

I'm afraid at this point I don't have enough info to answer your question.

[tubeswell]

Having quickly scanned this thread, I'm not sure if this has been covered yet but, have you tried running a 4R speaker (or wiring two 8R speakers in parallel) on the 8R tap ? (This will halve the reflected load - for running the amp with 6L6s)

[bnwitt]

Having quickly scanned this thread, I'm not sure if this has been covered yet but, have you tried running a 4R speaker (or wiring two 8R speakers in parallel) on the 8R tap ? (This will halve the reflected load - for running the amp with 6L6s)

That was going to be my next suggestion but I wanted to wait for the techs investigation on the B+.

[soundguruman]

I'm surprised that the point is missed. It's the bias voltage causing the squawk.
The bias is too high. Bias requirements vary from tube to tube. When you changed the tubes, you could not adjust the bias low enough for the different tubes.
lower the bias voltage all the way, and the squawk will get better, raise it and it will get worse.Your bias circuit does not accommodate the wide range of tubes.

OK theoretically, if you lower the bias low enough you can make any output tube work, mismatched or not. EL34, 6L6, 6V6. You gotta change the screen resistor prehaps.
Sounds to me like changing the cathode resistor to a different value, to accommodate whatever output tube.
Then the squawk will be gone. The squawk IS crossover distortion.
But one drawback, does the heater current of the power transformer accommodate 1.5 amp for each output tube filament? For EL34, you will need that.
THAT's where you are going to get stuck. 6V6 heater current, if I am not mistaken IS lower.

It's time to get the specs of the power tranny, and see how much heater current you have. THAT is the deciding factor.


Why couldn't an EL34 be made to work on 245volts?

[bnwitt]

Too late me thinks

Soundguruman,
This is a very old thread and the poster hasn't come back in a while now.