Is my rectifier tube bad?

I just finished my ST-120 this morning. Just before lunch I got the first 6550 biased with no problems following the initial startup instructions from Bob. I then shut the amp down to have lunch. Starting back up, I now have no bias and no B+. I still have 430 vac at the rectifier tube. Seems likely to be the rectifier tube but I wanted to run it by the experts here. I think I’ll switch to a Weber copper cap if that’s the issue, I do have the TDR.

Thanks for any help!

J.P. Mears
Dallas, OR—the OTHER Big D

Well, I was anxious to try the amp out so I bypassed the rectifier tube with a couple of 1N4007 diodes. Success! My amp biased up beautifully and fired up and sounds great! I have some modifications to do, so no pictures yet. I’m glad I opted for the TDR even though I planned to use a tube rectifier.

A quick conversation with Bob prompted me to look at my line voltage which is kind of high, almost 124 VAC. I will be picking up a variac. I’ve also ordered a new rectifier tube of a different brand and will do the diode backup mod on it.

Well done. 1n4007 diodes are your friend and every DIY tube audio fan should keep a supply on hand. Also, if you don't already have them, add a couple 22uF, 450V electrolytics to your tool kit. These are the easiest way to confirm/bypass a bad segment in a quad cap.

Thanks Bob! I should have it soon. In the meantime I’ll work on my mods. Thanks again for your help.

Thank God I purchase a Variac!
My in house voltage level varies as low as 117 and as high as 125 on a daily basis. Since the addition of the Variac it takes the guesswork out of the equation and knowing my m125's are operating safely.
A worthwhile investment.

I didn’t realize my voltage was so high. My variac is on the way. Mods continue, hopefully will wrap them up in a day or so.