Hughes&Kettner Triamp MK1 with kt66

[vahtar123]

Hi! I'm having some problems with my amp.
I've just put some kt66 genalex gold lions into my triamp and the tubes start redplating.
the plate voltage is 485 volts and i've set the bias even to 25ma but they still redplated
I've heard that it could be parasitic oscillations so i've increased the grid stopers from 1k5 to 10k but they still redplate.
here's the schematic the amp originaly comes with 6l6gc tubes
http://www.freeinfosociety.com/media/pdf/3216.pdf" onclick="window.open(this.href);return false;
I hope you guys could give me some help.


Cheers

[novosibir]

You can't stop PO by simply increasing the grid stopper values.
It's just one of many methods sometimes necessary.
Find the cause of the PO!

Larry

[vahtar123]

it seems I've didn't read the schematics right and put bigger resistors on the control grid instead on the screen grid
well that explains the high bias
I'll try to get the screen grid from 470 ohms to 1k
and the 6l6gc work without a problem when i put them in

[vahtar123]

I've just increased the screen grid resistor from 470 ohms to 1k and i've tried the 6l6gc tubes back in and they glow blue like hell when i hit the standby. the glow is hitting the glass and and i get a nasty hum from my speakers
any suggestions?

[BillERoach]

You confused the screen and control grids. I think you're in over your head, honestly. Just put it back the way it was and stick with the 6L6's, or take it to a professional.

[Lefty Lou]

Your heater current will be in the 1.3 amps per tube range as opposed to 900mA commonly found in a 6L6. Do not assume your amp will be as stable. If you do not know if your amp can handle the additional heater current draw, contact the manufacturer before installing KT66 tubes.

Remember, even though KT66 tubes will bias with a range of voltage used for a 6L6 tubes, each KT66 draws about 1.3 amps of heater current. Your power supply’s heater tap will use 50% more current compared to a pair of 6L6 tubes. Check the heater voltage to see if it sags down to 6 volts or less. If it drops like this you are likely pulling too much current from the heater winding.

[alexradium]

Triamp accepts 6L6 and EL34,which draw up to 1,5A filament current per tube.
Anyway red plating is not a function of starving filament circuit,in any case tubes work even with less than 6V.
Sometimes i have seen Marshalls with 6550 tranny get crazy oscillations with EL34s,so that's one reason,OT.
Why did you choose those tubes,whats your intention?

[Lefty Lou]

Triamp accepts 6L6 and EL34,which draw up to 1,5A filament current per tube.
Anyway red plating is not a function of starving filament circuit,in any case tubes work even with less than 6V.

True about the red plating, although some don't prefer to run KT66's in a "starved filament circuit" for much the same reasons as those same people don't prefer to power a Marshall with a variac at lower voltages. It's always preferable to stay within tube datasheet specs for a particular tube in order to get the best performance from said tubes. The filament consideration is just that, something that should be considered as well when utilizing KT66 tubes.

Sometimes I have seen Marshalls with 6550 tranny get crazy oscillations with EL34s,so that's one reason,OT.
Why did you choose those tubes,whats your intention?