Red plate KT88

[h00hbt]

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

[artosalo]

Originally Posted by h00hbt

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.

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 ?

[Tony]

what is your plate voltage at 50mv bias, can i assume that the cathode resistor is 1ohm and so cathode current is 50mA?

[h00hbt]

Originally Posted by Tony

what is your plate voltage at 50mv bias, can i assume that the cathode resistor is 1ohm and so cathode current is 50mA?

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

[Tony]

at a bias of 60mA and 450 volts plate, your tube dissipates 27 watts.....at 43ma 19 watts...if they sound fine with this bias, i see no point aiming for 60mA, your tube will live a happy longer life this way...

[h00hbt]

Originally Posted by Tony

at a bias of 60mA and 450 volts plate, your tube dissipates 27 watts.....at 43ma 19 watts...if they sound fine with this bias, i see no point aiming for 60mA, your tube will live a happy longer life this way...

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

[richwalters]

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

[h00hbt]

Originally Posted by richwalters

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

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.....

[Tony]

that's strange, i heard the opposite with new production tubes.....but i can not argue with your results...

you can pm Jim Mcshane maybe he has an explanation...

[Zekk]

Originally Posted by h00hbt

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