D120 II volume dropout and tube red plating

Amps & Cabs Deliverance
1

I have been having some short intermittent volume drops with my D120. I bought it new in 2022 and it still has the original tubes. I usually play with master volume around 9 o’clock. I recently noticed the volume dropout was happening at shorter intervals and looked in rear to find one of the power tubes red plating.

My amp came with KT88’s labeled with Ip = 72. I understand 6550 is now the recommended replacement. I haven’t taken any measurements in the amp yet but will likely swap the current output tubes for 6550’s. Are there any notes or procedures specific to biasing D120 that I should know about?

2

Johnny,

I would recommend Sovtek 6550WE in this case.
You probably don’t need a bias adjustment, but if you have the skill and tools to do it, I’ll be happy to advise.

Dave

3

OK, thanks for the response.

I ordered the JJ 6550 based on what I read of your preference:

I took a look at the amp earlier… didn’t see any dedicated test point. I measured plate voltage of 440v. Is there a point where I can easily measure the center tap voltage?

John

4

Nevermind… I think I got it sorted out

5

Hey @johnnyswing

Glad to hear it! What did you do in the end?

Regards

Dan

6

I installed the 6550’s and biased just under 70%
the assumed max plate dissipation of 35W. Amp is working as expected so far. Sounds good

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