Ebay Tube Blues

Rrick, as far as I know, the EL 34s need less than half a minute to pass maximum current, as you put it. Buying tubes on Ebay is always a bit of a gamble, but of course you can strike lucky. I've only bought from dealers whose feedback was good and who publish measurements with their offerings. Also I would not buy tubes from Ebay without a reliable tubetester, so you can check tubes on arrival and return those, which don't measure up or are doa. I've had very good experience with German sellers. I would say, that I've had to return about 3 % of the merchandise that I have bought through the years and never had trouble getting my money back in the case of duds. Cheers,

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!

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.

If you have a pot that adjusts a grid negative voltage, you should make it less- negative and than you'll be able to get larger bias voltage(current).
If you don't have it displayed than you should probably open the pannel or let tech do the work for you to adjust grid voltage.
VTL do not require matched tubes since bias is done individually for each tube(you might probably only need matched pairs for different channels)
As to input tubz, I wouldn't suggest using the bad ones since they can do a further damage to the rest of electronics.
I used a couple NOS ones but figured that stock JAN-Phillips used by VTL never goes bad and stays almost forever.

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.

I've generally had good luck buying tubes on eBay. If they've tested the tubes and have a lot of old feedback you can read, I don't think it is any more risky than buying from a tube dealer.

Rick,
Your assumption is not good since tubes are matched to each other. The negative grid offset voltage may vary in EL34 from -35V to -55V the more negative is grid voltage the less current flows through the testing socket.
The grid supply is adjustable in all VTL-s although I'm not sure if the pot is accsessible from outside just like the bias ones. In worst case you should bring it to tech and he/she will set the bias supply correctly for your new tubes. If you're near NY area than you can bring it to me and I'll take care of it in few minutes of waiting.

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.

To your more information:
Grid-bias pot can damage the circuit if not properly adjusted or turned to the extreams. In my vintage model MB100 it's accessible from the top pannel. I don't know what's in your case but I assume that this pot might be hidden on-purpose. I'm sure that with help of VTL(call or e-mail) you'll find out fast how to locate the master-bias pot and will be able to do a simple steps that(not in either VTL website or manuals) I'll share with you.

In your case(under-bias) which is quite normal, you simply set all tubes to max possible equal bias voltage and than adjust master-bias to the nominal manufacturer recommended level taking measurements from only one of tubes. Repeat measurements on the rest of tubes to ensure their equal bias voltage. Please note that under-bias cases are not distructive unless you have a largely exceeding negative grid voltage. Your bias-voltage readings will be somewhere bellow 100mV!!! You than have a few seconds to turn your amp off. Negative grid voltage is checked between pin5 of any tube socket and ground that should be in value ranges specified by manufacturer(find that out too) with amplifier on with no tubes. Readjust the bias voltage if neccessary to normal using master-bias pot.

If the case is opposite, i.e. you're having bias voltage too high at minimum pot setting you should turn off the amp immediately, remove the tubes turn the amp back on and measure between ground and pin5(grid) of one of the tube sockets. Increase the negative voltage(using master-bias pot) by 4...5V making it for instace instead of -45 to -50V and re-measure results with tubes and amp on. Turn the amp off before installing new tubes. If after number of measurements you end-up with negative grid-bias voltage too high than either the tube is probably bad or deffected or you've got added technical issues with unit that needs to be serviced.

As you might know that aging tubes tend to change their characteristics and even if you know you're setting bias pots at extreams in order to reach normal you'd better readjust the master-bias to have your local ones in the middle area for larger bias flexibility.