Metropoulos Forum

I built a JTM50 style amp from Metro about 6 years ago. I gigged it heavily, no problems.
Its a lead spec 50w, tube rectified with el-34s.

I noticed one power tube was shorting internally. I thought it was a bad solder joint. After going through many solder joints, I replaced the tubes. The new tubes are RCA 6CA7s. I still have the same problem. With these tubes, I noticed the one in question is 4.5mA out of bias with the other.

Could this cause the problem? The inside of the tube is arcing at volumes above 7. Biased at around 32mA. At lower volumes, the amp sounds great.

I'm confused because the other tubes did the same thing. And I'm not a tech.

If the components and solder joints are fine, might just be a bad tube socket.

I bought some brown power tube sockets from mojo. They appear to be the same quality as the metro kit. Is there other brands I should look for?

JDW3 wrote:I bought some brown power tube sockets from mojo. They appear to be the same quality as the metro kit. Is there other brands I should look for?

If you are running high voltages and running into problems the ceramic ones avoid arcing although are more prone to being microphonic (tho' preamps mainly)

Are you saying that you already replaced the tube socket with one from Mojo or is that what you used in your original build? If you haven't replaced it yet then it would be wise to do what the guys suggested and purchase a new one. Don't worry about that one tube being 4.5ma different than the other. Anything up to a 6-7ma difference will be ok.