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Old 10th September 2012, 01:30 AM   #11
kevinkr is offline kevinkr  United States
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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.
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Old 10th September 2012, 11:18 AM   #12
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Quote:
Originally Posted by kevinkr View Post
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, TT21īs have never been produced by anyone than GEC and the KT88īs 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 havnīt a clue how to tell.

Thanks
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Old 10th September 2012, 11:28 AM   #13
AJT is offline AJT  Philippines
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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...
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Old 10th September 2012, 12:44 PM   #14
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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 ?

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Old 10th September 2012, 04:38 PM   #15
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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.
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Old 10th September 2012, 04:44 PM   #16
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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


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I wish I had a scope.....
Anyway, if this is the culprit then why only the old tubes. I have many brands of modern KT88īs and even the KT90īs of different versions dont behave this way giving cherry plates??

Itīs just so strange. Anyway, the GECīs sound great at 43mv bias so I will just live with it. Itīs just annoying not knowing why....

Cheers
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Old 10th September 2012, 09:52 PM   #17
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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
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Old 10th September 2012, 11:08 PM   #18
6L6 is offline 6L6  United States
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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.
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Old 10th September 2012, 11:15 PM   #19
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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.
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Old 10th September 2012, 11:18 PM   #20
6L6 is offline 6L6  United States
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The questions never asked are those that cause problems...

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