Red plate KT88

[kevinkr]

I'm thinking there is more to this - I had GEC KT-88 in a PP UL amplifier running 550V on the plates and more than 70mA of plate current with no red plating.

VHF oscillation in the output stage? Could explain why the TT21 also red plate, vintage NOS KT88 generally seem to have higher transconductance than current production IMVLE..

My general experience has been that the GEC KT88 is far more rugged than modern production under any guise.

Are you certain the KT-88 are absolutely genuine? Some high value vintage tube types sold on eBay (not all) turn out to be counterfeit. Can you post some really good pictures and someone here should be able to verify that.

[h00hbt]

Originally Posted by kevinkr

I'm thinking there is more to this - I had GEC KT-88 in a PP UL amplifier running 550V on the plates and more than 70mA of plate current with no red plating.

VHF oscillation in the output stage? Could explain why the TT21 also red plate, vintage NOS KT88 generally seem to have higher transconductance than current production IMVLE..

My general experience has been that the GEC KT88 is far more rugged than modern production under any guise.

Are you certain the KT-88 are absolutely genuine? Some high value vintage tube types sold on eBay (not all) turn out to be counterfeit. Can you post some really good pictures and someone here should be able to verify that.

Hi kevinkr

Im 110% certain they are absolutly genuine GEC tubes, TT21´s have never been produced by anyone than GEC and the KT88´s are GEC. I know this to be a fact.

Someone mentioned grid leak, but this seams a bit far fetched because of the same result for all tubes. VHF oscillation could be an explanation but I havn´t a clue how to tell.

Thanks

[Tony]

one quick way to test for vhf oscillations, put a small am radio near the tubes, if the reception is disturbed in any way, then you have oscillations...

[richwalters]

The sil diodes in poorly designed power supply can also create broadband hash...the proper way is squarewave doctoring....inject a 1kHz into amp input and set for at modest o/p level, say 5-10W into a dummy load and observe waveform purity on scope.

On a well designed amp, it should look the same as the input signal. If good, then substitute the loudspeaker for the dummy load and observe again. Any difference.. i.e Ringing ?

richy


richy

[h00hbt]

Originally Posted by Tony

one quick way to test for vhf oscillations, put a small am radio near the tubes, if the reception is disturbed in any way, then you have oscillations...

I wil try this, thanks.

[h00hbt]

Originally Posted by richwalters

The sil diodes in poorly designed power supply can also create broadband hash...the proper way is squarewave doctoring....inject a 1kHz into amp input and set for at modest o/p level, say 5-10W into a dummy load and observe waveform purity on scope.

On a well designed amp, it should look the same as the input signal. If good, then substitute the loudspeaker for the dummy load and observe again. Any difference.. i.e Ringing ?

richy


richy

I wish I had a scope.....
Anyway, if this is the culprit then why only the old tubes. I have many brands of modern KT88´s and even the KT90´s of different versions dont behave this way giving cherry plates??

It´s just so strange. Anyway, the GEC´s sound great at 43mv bias so I will just live with it. It´s just annoying not knowing why....

Cheers

[richwalters]

I'm not kidding....the enclosed pics illustrate what an orig KT88 can handle.
45W dissip with tester on UL setting. In the 1960's these conditions weren't unusual on amps that drifted over time.

The cheap pocket cameras are quite blue sensitive; but the unequal glowing Svetlana will soon running into destruction with exactly the same conditions as the GEC KT88.

Take ones choice: NoWay can one run todays tubes at the same candle power that the oldies could. For longer life; A derating factor is imperative.

richy

[6L6]

h00hbt - Do you have grid stopper resistors on the output tubes? If not, that might be a very easy way to stop any RF oscillations. Try a 1K 1W carbon comp resistor very close to the grid pin.

[h00hbt]

Originally Posted by 6L6

h00hbt - Do you have grid stopper resistors on the output tubes? If not, that might be a very easy way to stop any RF oscillations. Try a 1K 1W carbon comp resistor very close to the grid pin.

Hi 6L6

Yes 1W CC resistor grid stopper is there.....

Thanks anyway.

[6L6]

The questions never asked are those that cause problems...

Cheers!