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Got a pair of "matched" EL34 tubes from a reputable online dealer.
Install the tubes in my Marshall JTM45 clone. Go to adjust bias and the tubes are 9ma apart. Shut the amp down. Swap the tubes/sockets, fire amp back up and the bias problem follows the tubes. "Hot" tube is still "hot" in the other socket, and "cold" tube is still "cold". Same 9ma difference. My conclusion is that the tubes are not matched. End of story.
Am I missing anything?
Follow Ups:
You might find my post regarding "tube problems" in the FAQ to be useful, check it out at the link below. Item #6 in my post details why a responsible dealer will try to verify what is occurring before replacing a tube which is a hassle for all involved. So saying that the dealer has determined that "you're the idiot" may be a bit of a harsh conclusion to jump to, don't you think?
Sometime tubes drift further apart as they age yes - but sometimes they will drift closer as well. Both scenarios can occur, but we rarely have reason to even notice if tubes have gotten closer matched so it often goes unnoticed.
Also - BE SURE the tubes and the amp are fully warmed when you measure current. I say 1/2 hour is the minimum - an hour is even better.
A last point - is the tube matched for plate current or cathode current? Screen current varies from tube to tube just like other parameters vary. If the dealer matches for cathode current (like I do and I STRONGLY recommend!) and you are checking plate current only (or vice-versa) then some error is highly likely. Again, item #6 in my post deals with this.
You may indeed find a need to get a better matched pair. But there are a lot of factors that could influence what you are seeing. So I think the dealer seems to be trying to do right by you - at least from what you posted.
Disclaimer - I am not the dealer who sold these tubes, I have no connection with the purchaser as far as I know either.
Hi Jim.
I wont post the email, and I won't even if I end up stuck, but yeah, right now I am pretty much the "idiot". It's understandable because the dealer doesn't know me from Adam. That said and as mentioned, the dealer is working with me so I have no gripe at this point. That wasn't the point of my post. Again, just making sure I'm not missing anything.
Duly noted regarding the rest of your post.
Thanks
I know this isn't any help but maybe it will help in the future.
Find and stick with some one you trust. Or has been recommended by several posters in a tube forum. I personally deal with Jim McShane. Years of good advice and never once tried to over sell me. He always steered me in the right direction.
In the long run Jim has actually saved me money by avoiding bad choices in tubes and stereos. Also I never had an issue with any tube he sold. If there was a problem he has always been of great help (usually it was something I did anyway but let's not tell Jim that eh)
charles
That's good advice. I usually buy new production tubes from one of two dealers. Neither is Jim, but I know his reputation. The rep. of this dealer is good among the guitar guys but I've never bought from him before. My normal dealers do not sell the Svetlana EL34's which I wanted to try out. Again, he's working with me and it will probably work out ok.
Matching is only for the conditions under which the tubes are tested.
Were they "matched" for both Gm and current draw?
Matching is usually considered within 10%. If they were matched for a target of 90 mA current draw, then a 9 mA difference may be considered matched.
The # of hours on a tube will also have an effect.
Did the tubes come marked with the test conditions and results?
If it is a reputable dealer, I would let them know and see if something can be worked out for an exchange.
Thanks for the reply.
GM is the transconductance, right? So yes, they are matched for GM and current draw. According to his numbers, the tubes tested within 10% for GM. Current draw was listed as just under a 1ma difference.
Already contacted the dealer. He asked that I swap sockets to see if the difference followed the tubes. I had already told him that it did in my original message to him. He missed it. Waiting to hear back.
Ask for replacement. IME, "unmatched" tubes can become even further apart with use. You paid for "match" --- and maybe a lot of $$$ for NOS EL34 tubes. Get your money's worth.
If you are in the 40-45mA per tube range, should be less than 3mA difference, to be matched. IMHO.
Good luck!
I agree, and I am working through it with the dealer. Of course, to the dealer I'm the idiot until proven otherwise. I can deal with that if it gets me to a resolution.
In the meantime, I'm posting here to make sure I didn't miss anything that could be related to my gear (or my being an idiot). They aren't NOS but I still paid for a matched pair. I have a pair of Valve Art (Chinese) EL34's that were dead on when I first put them in and they are only 2ma apart after 30 or 40 hours. JJ's were about 1.5ma apart when I first got them and after probably close to 100 hours were about 3.5ma apart.
At any rate, based on the above, and common logic that if it stays with the tube when I swap sockets, it must be the tube, I don't think I'm missing anything. But I wanted to check here with folks who are smarter than I am.
Thanks
If you've bought "matched" tubes before and they match-up using your amp and measuring devices --- I'd say the pair you have now are not matched.
You did the correct things to test the tubes. Get another pair.
JOC... what brand and era manufactured are these tubes?
They're current production Svetlana.Since the dealers rep. is pretty good, I'm guessing he may have dropped one of the tubes in the wrong box. He doesn't label the tube bases, just the boxes.
Thanks.
Edits: 07/05/16
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