I received 7 tubes from an eBay guy with a perfect feedback who mostly sells tubes.
All are 12AT7. In my Mac 275 (a modern version, 2009 YOM) the tubes are weak, I bought it used, so they don't sound that good.
So. I get the tubes, all claimed to test close to NOS.
I test all 7 on my Hickok 600A.
5 do indeed test fine, 2 test weak, on the English scale - Replace, one is truly superweak, under a 1000 uMho (minimum 4000 for a new).
OK. So I put 3 of them in the amp, having forgotten to put the 4th one (there are 4 AT7). It plays fine for a few minutes, then I turn it off to put the 4th tube in, turn it on again, wait a bit, and hear a motorboat from the channel. Then one of the KT88 glows red. I turn it off, replace the tube with the 5th one (the last that tested goid). Turn on, everything fine. Play it for 2 minutes. Then - a really LOUD POP, and I see a red glowing KT88 in the other channel. Take all the AT7 tubes out, replace the original Mac labelled tubes back. Turn on....not working. The loud pop was blown fuse. Replace the fuse, then it works. With all original tubes, worked for an hour...no problem.
Now, don't know what to make of it. The seller suspects I am trying to stiff him, I guess, but....the tubes that are trying to destroy the amp test fine and I believe the guy did test them in an amp circuit as he said he did. Maybe the tube in Mac is stressed more? There are DC coupled curcuits there, if the bias goes to crap because of a tube, who knows what might happen. The driver tubes work as Cathoe followers driving the output tubes DC coupled.
Before you move on, have you put little 10 ohm 1/4 watt resistors on the cathodes of the OP tubes? This is a great way to measure current on the OP. It won't be applicable to all amps. AND it could save your OP transformers next time one of you finals goes "red".
Russ
No I have not.
To be truthful, this is a factory Mac, a modern one at that (2009 YOM) and the last thing I wanna do at this point is to put this 90+ lbs pig iron ingot on my bench and start digging. It ain't broke right now, I am looking to listen to it soon.
But I do want to call McIntosh and ask them what in their opinion could cause this and what to look for.
I even know what they are gonna say: buy McIntosh labelled tubes.
But I will ask anyway.
In the future if I feel like it (my chances of lifting a 100lbs thing with ageing do not increase
) I might do that.
This story has the second part to it.
I was playing my Nakamichi ZX-9 when it all happened.
When old tubes went back in, I heard only one channel working, so the crap was scared out of me.
I though "there go my Klipsches".
Switched the channels....no, Klipsches worked.
Then I swapped the output tubes...nope, same thing. The crap loudly complained about being scared, I literally had to cheer it up. He thought it was the output transformer.
And then....I noticed that just one channel of my Nak was having the indicator running. The other was dead.
I decided to play with the cable....no, the Nak was outputting just the right channel. Plus, there was a problem with one of the RCA outs GND. It wasn't there.
So today I put my Nak on the bench, opened it up....The Devil would break a leg inside it.
But first things first - the GND.
The Nak's 4 RCA jacks have their GND coming to them from a shiny metal plate to which they are fastened by staple-like brackets, and then the plate gets the GND from the cables coming from inside, by wire-wrap. three connectors were OK, one had intermittent or no GND. One bracket got just a little loose from reinserting the cables, but it was not noticed as the other channels provided the GND. But when I tested one jack at a time I discovered this.
I simply soldered the brackets to the plate - both were solderable.
One down, one to go.
Put the Nak to the stand, play a tape.....for a few seconds the other channel appeared, and then went away.
So, before I dug into it, I though - what if it is the head azimuth?
So I took a blank cassette and performed azimuth alignment.
After that all my cassettes played fine, I played several songs from various tapes, all was OK.
So what I think happened was, due to the voltage surge, the uProcessor screwed the azimuth data. What threw me off, one channel played OK, so I thought it was the signal path of the other channel. Good thing I tried the simple theories first!
Now I filed the case with eBay; I am not accusing the guy of anything but two of his tubes that he promised would test as NOS were simply weak, one was 1/4 value and less than 2/3 into "replace" on the English scale with the second triode in it testing as new. I photographed it and sent to eBay.
Not an awful lot of money but I think these tubes are iffy. Even if they work in fully AC-coupled circuit (which is likely how he tested it) it does not mean they work in mine.
Well, I am done troubleshooting for today. It's the bourbon time!
A well deserved bourbon it sounds like. 8) While I didn't understand parts of what you explained
I'm glad it worked.
The aftermath:
I tried to reason with seller, having been very polite and non-insulting. Eventually had to file a case with eBay.
He offered me some little money back, I refused - I cannot use the whole batch.
So he started insulting me, suggesting I am trying to return him some other tubes (to which I asked him if he would not recognize his own) and today he started directly calling me name.
here are some fine examples:
1. "now u get 0 from me for being a a-hole! u are lying about the hole thing because u tried to scam about your equipment getting blown up and now u are trying to turn it around on my tubes being bad. don't contact me any more. shove em!!!"
2. "STOP HARASSING ME!"
3. This one is in response of me telling him that he is being insulting:
""U ARE INSULTING ME! B.S. ARTIST!""
So, the guy's name is "jamesl708".
the very first response of his was very crass, and then he in fact apologized (I simply inquired about his tubes as to what "RCA 12AT7'S LOT OF 7 BLACK PLATES TEST STONG TO NOS TESTED IN AN AMP CIRCUIT" meant, and he just got pretty rude replying that I could be less paranoid as I should look at his feedback). So it should've alerted me but heck, I saw tubes that test close to NOS for a decent price and went for it.
I guess buying tubes on eBay for HiFi equipment like a Mac might not be a good idea after all.
Right now i am talking to Jim McShane, a reputable guy recommended on Audiokarma who is on the Tubeasylum site.
Hopefully this time it will be good, though his tubes are not cheap. 12AT7 / 4024 Mullard NOS is about $40 apiece.
Yea there's some real mutts in the world and sometimes you end up trying to deal with them.
I tried to reason with the guy...
Well, eBay is dealing with it, they will announce their decision in 48hrs; if I lose - well, not a biggie, though I'd rather not; If they side with me, OK, it's pay for 1.5 out of 4 tubes I need to buy now since these do not work.
Good luck, Mike. I hope eBay rules in your favor.
I've seen the name Jim McShane mentioned on AudioKarma from time to time...and never in a negative connotation.
Funny...before the vintage audio amp craze hit, one could find 12AT7s, 12AX7s, etc. all day long for peanuts. They were often found with bushels of 1960s TV tubes. Who knew that they would suddenly become rare, precious, beautiful...and valuable.
Sort of like Catalin radios. I recall seeing a butterscotch FADA (not the little bullet, another cabinet style) at a roadside yard sale in the early 1980s for $10. I passed, thinking that such a radio would never be valuable. What did I know...
Yeah...to paraphrase a Russian card game adage, If I knew what's in the bank, I'd live in Beverly Hills.
Or if I knew were I fall, I'd throw some hay there.
But being in need of the tubes, short of buying a lab grade tester, I will have to buy guaranteed toobs from other folks.
Well....the guy crossed the line, sent me an email using my PayPal email directly with threats (u busted, I am gonna get you banned etc).
I reported him to eBay, they agreed he stepped over the line.
Just so you know, the guys name is James Latimer.
He is pretty.... curmudgeonly, if you ask me
This is the first time this is happening to me in my 15 years of dealing with people on eBay. I did have some resolutions (very few) before but this is the first time it got so nasty on the seller's side.
So eBay decided in my favor. Gave me the shipping label.
The guy decided to send me a message.
New message from: jamesl708 (541Purple Star)
hey a**hole! u better send me back the exact tubes i sent u. and if your s**t blew up that's on your dumb a**!
So....I was patient long enough, so I answered.Here it is.
Yes. You are getting your exact tubes back. And then you can shove them to your you know what. Pins first. P!ease.
Have a pleasant time with it.
Have a wonderful Thanksgiving. Try not to choke. Too much gall is bad for you.
With deepest respect and best wishes.
I wonder if he responds or....
(Sorry for quoting the foul language)
VERY,,VERY,,Nicely Said,,,,I really think ,,you won Here,,,Cheers
He responded with some mixed grrrr hrrrrr p!us some mixed expletives and flying saliva (I am sure he had to wipe his monitor after that).
I decided one answer is good enough.
Poor guy.
...good grief...yes, Mike, you answered well. Probably better than what I would have written to the shmendrik.