Hi
I have a quad of GEC KT88´s that show red plates when I bias them at 50mv. If I bias down to 43mv they are fine, sound great. I also have a quad of TT21´s and they are the same, have to be just over 40mv.
When I use newly produced KT88´s like Penta or EH I can bias them ut to 50mv without any red plates.
Are GEC´s or the older tubes more sensitive or vice verca? My amp says that the OP tubes should be biased at 50mv.
Any ideas?.......
Cheers
I have a quad of GEC KT88´s that show red plates when I bias them at 50mv. If I bias down to 43mv they are fine, sound great. I also have a quad of TT21´s and they are the same, have to be just over 40mv.
When I use newly produced KT88´s like Penta or EH I can bias them ut to 50mv without any red plates.
Are GEC´s or the older tubes more sensitive or vice verca? My amp says that the OP tubes should be biased at 50mv.
Any ideas?.......
Cheers
What do you mean with "50 mV" ? How do you measure this ?
What is the anode voltage and current in case the plates are red ?
Hi Thank you both for answering.
Well I have a 1 ohm cathode resistor, that´s right. The amp has a fixed bias, so I can adjust the bias. I measure from pin 8 (cathode) to ground. The schematics say to adjust for 60mv. B+ is 470V, havn´t measured the actuall plate voltage. OPX are 4K. This is a PP UL amp.
Cheers
Yes, they sound absolutly fantastic at 43mv. I was just wondering why both the old KT88 and TT21 take a lower setting at 40+mv and newer tubes can cope with 60mv without getting red plates.
Cheers
Bias at 50mA per tube at 470V+ is a dream run come true for KT88's.....red plates.. I question if they are really KT88's ? You suggest ~27W dissipation.....6L6 can swallow that. .....Measurements correct ?
I've been running KT88's at 580V at 60mA (~35W diss) and then one will see a hint of cherry red. This may appear to be a hammering; but I've got alot of older designs that suggested KT88's are capable of even harder running.
richy
I've been running KT88's at 580V at 60mA (~35W diss) and then one will see a hint of cherry red. This may appear to be a hammering; but I've got alot of older designs that suggested KT88's are capable of even harder running.
richy
Of course they are KT88´s and also TT21´s. The TT21 could take more plate voltage than KT88´s and still they cherry at 50mv cathode bias. These tubes where true NOS, never used and from sealed boxes.....
My TT21 have 550v at plate > 57v bias > 680ohms > 84mA > 41watts.
Sounds full bloom and no red plate nor hum
But... it's hot! Am thinking designing a heat sink attached to metal base of tube.
Good Luck to you.
Zekk
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.
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
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
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
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
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
Attachments
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.