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Rectifier tube keeps blowing

I have a cheap Chinese tube amp. It uses a 274B rectifier tube from Valve Art. Once during listening, the rectifier tube blew and so did the fuse. I replaced the fuse and replaced the rectifier with another 274B from Valve Art. Then, days later, I turned the amp on and same thing happened at power up. Fuse blew, rectifier went. I changed the fuse again and added my third (and last) rectifier tube. Every time I turn the amp on now, I take a deep breath and turn my head. What the hell is going on here? I think a rectifier turns AC to DC (dangerous). If the rectifier goes, than damage could occur to the transformers,tubes or the whole amp could take a dump. I just put some great KT77's in here and I don't want anything bad to happen. Is it the Valve Art tube? Is it shoddy workmanship in a cheap Chinese amp? Help.
devilboy
atmasphere's avatar
atmasphere
11,846 posts
 
Devilboy, try starting up the amp with no output tubes in the sockets. If the rectifier blows, your filter caps are shot. Its a hellava way to find bad caps IMO, but you would know for sure.

The fact that it did this before and after power tube replacement points to the filter caps, assuming that the power tubes were biased OK. Is it also true that the rectifier would blow in fairly short order (seconds rather than minutes)?
devilboy OP
1,176 posts
 
Atmasphere: The first time the rec. went, it happened after 30 mins of use. The second time, it happened two seconds after power up. The amp is going back, but thanks for getting this back on track Atma.
trelja
3,223 posts
 
Yes, a good suggestion, Ralph.

Along that train of thought, you can also start the amp up without the rectifier tube plugged in. That lets you see where the bias is without (hopefully) the amp going down, and get it right if it's way out of whack, or at least turn it way down. I would actually do this before firing up the amp with the rectifier in and the output tubes out.

Of course, this presumes the amp is fixed bias, and though we've not been told this (if I didn't miss it), have kind of been working under that presumption. A cathode bias circuit would open something else for him to think about.

There's a way to post photos here, Heyraz, but I've long since forgotten how. It used to be semi-common here back in the day. If no one comes to the rescue with that, I'll send something offline.
atmasphere's avatar
atmasphere
11,846 posts
 
Sounds like a bad filter capacitor to me.
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