suggestion for KT88 in super lead

[martin g]

Hello I'm going to put JJ Kt88 to my super lead, what elements should I change to bias them properly? I've got 1 ohm bias resistor instaled in all tubes' socket, shuold I remove them? How about negative feedback resistor and NFbw I've got 27k negative feedback resitor to 8 ohm tap. How to measure bias in kt88 -across 1 ohm like on el34? what should it be for cleand bell strato sound?
best wishes and thanks

[Dax]

Leave the 1 ohm resistors in, all you will need to to is throw in the KT88's and check your bias. If you can't get it, you will need to change the resistor above the bias pot to give you the range you need, 47k or 56k, whatever it takes. Bias formula for KT88's:

6V6 12 watts MAX
6L6GC 23 watts MAX
5881 (American) 18 watts MAX
5881 (Russian) 24 watts MAX
EL34 20 watts MAX
KT66 24 watts MAX
6550 27 watts MAX


KT88’s can be treated as 6550s
EL34’s are about 20 to 25 watts

Example: Plate Voltage is 400 volts. EL34’s run at about 20 watts. You want to bias to about 70 percent dissipation.
Take 20 times 70 percent which equals 14. divide this by the plate voltage of 400. This equals .035 or 35 millivolts. This is your bias setting needed. Use the bias chart for min and .max

KT88’s, EL34’s and 6550’s are interchangeable with a bias adjustment.

[Flames1950]

Dax, I take it you already figured those at the 70% mark? I show KT88's as rated between 35 and 42 watts heat dissipation. EL34's are typically a 25 watt dissipation tube (except for the GT E34Ls which is a 30 watt tube.) 6L6GC's are usually good for 25 watts too.

[martin g]

Hi thanks I've got no rotate bias pot but slide pot about 100k and I couldn't get less bias than 50mV on 1 ohm resist. where is the problem? How could I measure plate voltage? With speakers or without and what pin. When I try measure pin3 and ground I've got buzz on tube.

[flemingmras]

You have to change the stock 47K resistor in the bias supply to either a 56K or a 68K to raise your bias voltage(make it more negative). The more negative voltage you have available, the less bias current your power tubes will draw. KT88s and 6550s require more negative bias to get them to bias around the recommended bias current draw.

Since these tubes are rated for a 42 watt plate dissipation, 70% of this would be 29.4. So perform this equation to find your max bias current rating:

(29.4/Plate Voltage)x1000=Max Bias Current in mA(milliamps) per tube

Start with a 68K ohm resistor and rotate the bias pot. If you cannot get it to bias at the max bias current(current draw too low) raise the 68K to a 56K. If you can't get it to bias at max bias current draw(current draw still too high) raise that 68K to an 82K.

Then as you're playing through the amp, play with the bias control until you find the setting that sounds the best, but DO NOT go above the max bias current draw. And yes you measure it across the 1 ohm resistors just like on an EL34.

Hope this helps.

Jon

[Dax-The-Ax]

Yeah, I found the info from lord valve and got a litttle confused, my bad.

[martin g]

Hi Thanks a lot 82k helps I've got bias range from 33-60mV across 1ohm. Thanks a lot once more