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 1 response by fatparrot

My understanding of tubes in relationship to your problems: 1) As tubes get older, you will have to increase the bias voltage/amperage. I always turn DOWN the bias setting before installing new tubes(should need less bias for new tubes). I am using Atma-sphere M-60 Mk 2.2 monoblocks. I noticed that bias amperage will increase SLIGHTLY after about 1/2 to 3/4 hours of use. 2) I just had a weird and frustrating problem with a pair of 12AU7's (Telefunkens, no less!) After a few months, I was getting weird noises from the left channel, and the DC offset light kept illuminating on my Atma-sphere MP-3 preamp. Not a bad tube, but in working on some of the components inside the pre-amp, I must have bumped a tube, moving the pins slightly out of alignment in the socket holes. Also, your pins on the 12AT7's might be either slightly oxidized or UNDERSIZED for the socket holes. Clean the pins with Caig De-oxit, then use their G5x preservative. Use the G5x very sparingly on the pins, and YOU MUST USE THE G5x (extended temp.), NOT THE REGULAR G5!! You can check the pin sockets, bending slightly inward any that seem to have a larger gap. BE SURE TO LEAVE YOUR AMP/PREAMP OFF FOR AWHILE (I usually leave it UNPLUGGED overnight!) DO NOT TOUCH ANYTHING ELSE THAT IS METAL while working on the pin sockets!!! Using a small jewelers screw driver is your best bet. Also, there are many variants of ALL tubes, usually followed by a letter or letters. These can refer to the MAX. voltages in the tubes sections, and can vary! Check with the manufacturer, or Andy at Vintage Tube Services for specific applications:

http://www.vintagetubeservices.com/ (Tel # 616-454-3467)

Stay away from the other big name tube guru! The N.O.S 6AS7 power triode that one of his "helpers" recommended WOULD HAVE FAILED IMMEDIATELY! This "chowder-head" said that GAS7G's (I'm not sure of the last letter) were a direct replacement for GAS7's....NOT!!!! Andy is a tube whiz, the "real deal". As for buying on ebay, sometimes a bargain can turn out to be really expensive. You probably have a better chance pulling the handle of a slot machine!