JTM 45 Groove Tubes KT66 trouble

[dariez75]

Hi guys,
I,ve build a jtm 45 using metro amp manual, I've follow every single details and it work fine! I using tung sol 12ax7 and Kt 66. Two day ago I've found in my local shop three oldest duet of groove tube s KT66. I've bought a duet for 50 euros. Just in the night I've try to install the GT in my jtm45: trouble! the tubes sound oscillating and the light into start flashing: I swap the tube with the tong sol and a duet of GT El34 and the amp sounds fine. Yesterday the shop change the gt duet with another GT KT66. When I was at home I've try to install the newest tubes and the sound was the same of the other GT KT66.
Please help me: Is my amp or the tube? I thank is my amp but where is the mistake? My jtm 45 in PPIMV

[martin manning]

Did you reset the bias for the KT66?

[dariez75]

Yeah, did. It sounds the same even with the trim pot all clockwise (minimum). I've installed the gt kt66 in mydumble clone from ceriatone kit and they are good. Ceriatone had different bias feed...

[martin manning]

Where do you see light flashing? Inside the tubes? If so they may be oscillating. What resistors do you have on pins 5 (control grid) and 4 (screen grid)?

[dariez75]

Yeah flash is inside the tube. I have 427 ohm 5W screen gird restor but I don't have installed a 5.6K grid stopper resistors...

[dariez75]

sorry 470 ohm at pin 4 and no grid stopper at fin 5...

[martin manning]

Perhaps the 6550's are just a bit more susceptible to oscillation than the EL34's. I would look over the wiring carefully to be sure the grid wires are routed correctly, and also look at the nfb wire to be sure it is routed correctly. By routed correctly I mean not near something that could create a feedback loop. Move them around with a chopstick to see if there is any effect. If it all looks good, try adding a grid stopper on pin 5's

[dariez75]

Thank Martin, now I found 2 5.6K resistors and I try to use as output grid stopper.

[dariez75]

Martin you are great!!! I've installed a 5.6k grid stopper and no more oscillating noise or flashes into the tube. Despite the tung sol GT kt66 shows a softly blue cloud. I've rebias but a strange issue I observed: turning fully clockwise the bias trim pot (minimum) the tubes shows a little red plate (core)... And the bias value is 50 mv (measured at pin 1 after 1 ohm resistor linking pin 1 and whereas the ideal bias value for 405 Volt in pin 3 is 45.67 mv...

[maxwedge]

You're running those KT66s to hot! If you can't get the bias you need then you need to change the bias voltage dropping resistor. It's the one connected to the diode. I have a 220k in mine and have no problem getting the correct bias. You should be in the high 30's to low 40's ma

I see that you have 2 threads on this.

[dariez75]

yeah a did two different topics in order to explain better the trouble

[Structo]

I would not worry about the blue cloud in the tube, it is normal for certain brands and designations.

[dariez75]

Ok, I believed it was a tube failure bias, now I'm fine.