Question about KT66's

white room · Post by **white room** » Thu Jul 07, 2005 4:44 pm

When I go to change over from EL34 to KT66 tubes, am I going to have to change anything such as the 1 ohm resistor between pins 1 and 8 that I currently have on my power tube sockets or anything else?

Flames1950 · Post by **Flames1950** » Fri Jul 08, 2005 8:10 am

The one ohm resistor doesn't change. Be ready with some bigger values for the bias resistor though (the one in-line with the bias pot.) I had to go from a 47K to a 100K in my 50 watt (I think those were the values but it's been a while -- I just remember I needed a bigger resistor for sure!)

white room · Post by **white room** » Fri Jul 08, 2005 8:51 am

Flames,

I remember you saying something about that awhile back so I have a 100k 2W MO ready for the occasion if needed.

I was looking at the schematic symbol for the KT66 over at http://tdsl.duncanamps.com/ and I was comparing it to the EL34; so I was just curious as to whether anything had to change as far as the tube socket was concerned ( jumpers needed or not) or if I could just plug them in, set the bias and go. I was curious about pins 8 and 1 on both tubes where pin 1 goes to the surpressor grid ( I think ) on the EL34 but pin 1 goes to nothing and pin 8 (cathode) ties to what I assume is the supressor grid on the KT66 internaly. Am I looking at that wrong?

Maybe I'm just being overly cautious.

Flames1950 · Post by **Flames1950** » Fri Jul 08, 2005 10:55 am

The EL34 has an internal connection between the cathode and that suppressor grid that the KT66 doesn't have. The socket is wired fine.
It's the reverse that's often the problem, Fender and others use that pin 1 for the input grid resistor since the KT66/6L6 tubes have nothing using that pin. They mount the resistor from pin one to pin five and you don't do that with the EL34-wired socket.
No socket mods needed.

white room · Post by **white room** » Fri Jul 08, 2005 11:07 am

Thanks

NY Chief · Post by **NY Chief** » Fri Jul 08, 2005 11:16 am

Flames1950 wrote:The one ohm resistor doesn't change. Be ready with some bigger values for the bias resistor though (the one in-line with the bias pot.) I had to go from a 47K to a 100K in my 50 watt (I think those were the values but it's been a while -- I just remember I needed a bigger resistor for sure!)

Does this apply to JTM45 as well? I have a pair of KT66's that are going into my JTM45 RI.

rjgtr · Post by **rjgtr** » Fri Jul 08, 2005 1:13 pm

You shoud be able to just drop the KT-66s in your JTM-45 RI and rebias. The KT-66 is part of the 6L6 family.