2. amps-preamps
  3. 1052070447

Ebay Tube Blues

This might be a lesson of woes or perhaps just buyers remorse.

I bought 2 EL34 (EI Big Bottle) replacements and a set of 4 12at7ws (Phillips) tubes for a VTL ST-85. I bought them from sellers who had a resonable seller history, but as far as I could see, they weren't commerical sellers. Also, these tubes are common so I wasn't worried about conterfeits. Both of the sellers stated they were "new" tubes.

I installed boths sets of tubes according to VTL's instructions of muting the preamp and letting the tubes warm up for 10 minutes before I bias the EL34s.

Thats when I ran into the following problems. The 12at7s had two problems. One was so microphonic I had to replace it immediately. It had a nasty hum with the muted preamp. The other side had an intermident snap-crackle-pop. After that happened a few times out came the new 12at7s and back in went my old 5 year old ones.

As for the EL34s, I haven't been able to bias either of the new ones up to the factory suggested 300mA specification. I can get close (approx. 240 in one and 255 in the other), but thast's with the pox turned all the way up. The old EL34s bias just right in those sockets.

My question is: Are these issues just part of a breaking in sequence for the tubes or is there something wrong here. I thought I would ask here before going back to the sellers with these problems.

Right now, I think I have some bad 12at7s and it's time to return them. I not sure about the EL34s. Do you think they'll allow more current through once they have run for a while?

Any thoughts would be welcome,
Rick
rrick

Showing 3 responses by rrick

rrick OP
23 posts
 
I can attest to the sometimes fragile world of tubes. The EL34 pair were a replacement and backup for a recently bought used set (yep, from Ebay too) where I burnt out a single bulb. In that case, I wasn't careful about the bias levels and I can accept that failure. Each of the 3 original ELs bias up to 400mA real easy, where the new pair max out at 240mA and 295mA. This doesn't sound like new tubes to me and they supposedly tested as new.

As for 12au7ws, they are a direct replacement for the current ones. Both sets are made by Phillips with the same specification. That's why I thought these would be a no brainer. As to the big bad hum (it was almost scary), is this a case of microphonics, or is this a case of bad tubes?

I do plan to have these tubes tested next week at the local electronics swap meet. If any of you are from the San Francisco Bay Area and know of a shop (preferably in the East Bay) with a tube tester, please add its address to this posting.
rrick OP
23 posts
 

I have biased the original EL34 tubes of the exact same type in the same locations and the originals bias just fine. The original tubes can handle 400mA, no matter what socket they sit in. That's why I suspect these new ones have problems. If this is not a good assumption, let me know.

I suppose the connections of the EL34 replacements could be the problem, but I have cleaned them and they appear reasonaly clean. Just to be completely sure, I plan to get the pin cleaner and then try again. But, I'm not too hopeful.

Thanks for the tip about the JAN-Phillips. Its been 5 years since I bought the amp, and since they are pretty cheap, I thought I would just start with new tubes all around.
rrick OP
23 posts
 
Thanks for the info.

This gives me a good understanding of the issues here and will let talk intelligently with the people I bought these from.
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