Metropoulos Forum

I am trying to get my JTM45/100 build finished up and am having bias trouble and tube trouble. I am using GEC KT66's and the problem is. As the amp warms up the bias increases on it own and the tubes in sockets 4 and 5 red plate. Valves 6 and 7 seem to be ok. I used the B. Batz layout and have check my work numerous times. The only mod I made was the bias resister from 56k to 47k and the addition of a 25k bias pot. Any help would be appreciated.

Post some clear pictures of your work.

What voltages do you get before you put the GECs in? Output socket please

How are you measuring bias?

Have you tried swapping tubes?

Are your OT primaries wired up correctly?

Have alook at these things and post back and we will get you up and running

Check everything that neikeel mentioned. Also measure your negative DC bias voltage at pin 5 of the sockets that are redplating. See if you are getting a stable negative voltage or not. You can do this with the power tubes removed. Another possibility is the OT primaries are reversed and the amp is oscillating which will start the redplating to set in.

Test with cheaper tubes first. Measure voltage at pins 3 and 4 and check bias

Got it figured out, Was some badly soldered connections. Thanks for everybody's help. I was thinking of changing the secondary's on my classic tone 40-18068 to the higher voltage. Any thoughts would be welcome.