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Post by zpilot on Nov 16, 2016 18:48:19 GMT -7
I just had an experience I thought I'd relate. I just bought a new MAZ 8 and in an effort to get maximum clean headroom I decided to pull an NOS RCA 5751 from my stock and try it in V1. The amp hummed horribly. It had worked fine with the stock JJ 12AX7. I had another NOS RCA 5751 in my stock so I tried it and the amp sounded great; exactly what I wanted. Now, what to do with the bad tube.
I had purchased both at the same time several years ago (probably 2010) from KCA NOS Tubes so it was definitely past any consideration for a warranty. I contacted Mike at KCA and related what had happened and that, considering the time, I certainly wasn't wanting a full replacement or refund. I asked if I could get any kind of discount on a replacement I had ordered the day before which was part of a larger order. After some back-and-forth about cleaning pins and such he offered to send me an ANOS RCA 5751. That was more than I expected.
I don't test NOS tubes that I stock when I receive them because I buy from sources that claim THEY do. Almost all of my NOS tubes have come from KCA. I have been buying from Mike for almost 20 years and this is the FIRST issue I've had and he resolved it more than fairly. He recommended that I test the tubes when I receive them. I will start doing this even though I assume they have been tested and pay a premium price for that. All in all I'm happy. This is just a heads up.
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Post by perryr on Nov 17, 2016 23:50:44 GMT -7
The best test is in the circuit. I've seen tubes have some hum in v1 and still measure good and work ok in another position. It could just be one side of a tube, and in some amps only one side of the tube is used. The best way to test a 12ax7 type is put it in v1 of an Antidote, Remedy or 4 input Marshall where each side of the tube feeds a seperate channel, then crank the gain independently on each side. Its a pain! Lots of retailers have some form of listening apparatus where they can listen to each side of the tube (I use a maxi-preamp for this), but truly the best test is in the position where its gonna be used. And if it doesn't work good there, it doesn't necessarily mean its bad (may work fine somewhere else). I would say a majority of 12ax7's have some degree of microphonics, and a little micro can sometimes really sing.. so I guess what I'm saying is there is some subjective nature to the beast.. depending on where the tube is placed.
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Post by zpilot on Nov 18, 2016 8:33:09 GMT -7
Well, I don't have any of those amps and none of the ones I do have are like that. My tube tester is an old Triplett that tests at 150 volts on the plates which is higher than most testers and fairly close to what it would see in an amp. It also has a headphone output so I can check for microphonics. I think I'll just send the bad tube back to KCA because it REALLY hummed and I doubt I could use it anywhere except for a tremolo. I already have plenty of used tubes for that. I just dislike having to take the time to do all that testing. Such is life with vacuum tubes.
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Post by helmi on Nov 19, 2016 4:45:41 GMT -7
your very lucky he gave you a replacement after 6 years. I would have told you to go pound salt and scolded you for not trying it when you got it! (lol, just kidding)
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Post by zpilot on Nov 19, 2016 19:24:38 GMT -7
Yes, I do feel fortunate. He offered it without me asking. The tubes I ordered came yesterday along with the replacement which was an ANOS RCA currently listed on his site for $35.00. All the tubes tested fine.
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