Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Sansteeth on April 11, 2022, 04:33:53 am
-
Hey folks,
been a while I haven't posted something in here, here's the thing, a while back a customer came in for a retubing + bias so I installed brand new EHX EL34 on his Ampeg VL-1002.
After one rehearsal, his mains fuse blew and brought it back to me, one of the tubes' getter had turned white, I managed to have another one back with the same measurements through my supplier for free.
Flash forward to now, the amp is back on my bench, mains fuse blew, another tubes' getter turned white. (he first said that the fuse blew because he used a speaker cable that he later found out was defectuous)
My question is, if a getter turns white is it always the matter of a bad tube (poor vacuum, cracked glass, etc...) or can the amp (plate or screen voltages, ultra sonic oscillation or no load on the speaker output) cause that?
I'm getting:
Vp: 482VDC
Vg2: 478VDC
In this economy, finding one matched tube is going to be next to impossible, so before I go for another full retube, I'd like to make sure there is nothing wrong amp-wise that keeps turning getters white and that it's just the matter of a bad set.
I'm curious to know what your knowledge / experience is on that particular subject?
Thanks a lot!
-
This is posted in the wrong board!
Moving
-
My advice is don’t treat modern EL34 as 25W tubes. In terms of reliability, they’re more like 20W tubes and should be biased accordingly if you don’t want your customers repeatedly coming back with blown output transformers IMHO. And you should definitely replace all the tubes with matched tubes - unless you’re doing individual bias for each tube (don’t just replace 1 tube in a pair or 2 tubes in a quad etc- that’s asking for trouble. ‘I took it home, plugged in my power brake and dimed it through my stack of quad boxes. The tubes were glowing red BUT it was sounding really awesome Man, then the fuse blew, so you must’ve sold me a dodgy tube’ ). YMMV
-
Hey thank you for your response!
(and thanks you to the moderator who moved my post to the right board :smiley:)
I had them biased at 70W of power dissipation, but you're making a fair point, that's still probably too hot for modern EL34.
I'm sorry if I wasn't clear in my first post: the first tube that turned white after one rehearsal was replaced by my supplier with another tube of the same make and the same Gm and Ip under the same test conditions (they do in-house matching). At this point the amp has probably seen 2 or 3 rehearsals and one show (thanks COVID...), I think it's pushing it a bit but considering the scarcity... I wouldn't do that on a set of tubes that had seen a 1 year of intensive use, that, I agree is asking for trouble.
‘I took it home, plugged in my power brake and dimed it through my stack of quad boxes. The tubes were glowing red BUT it was sounding really awesome Man, then the fuse blew, so you must’ve sold me a dodgy tube’
Thank god for now my customers have all been pretty reasonable people who don't pull off that kind of stuff
-
... the first tube that turned white after one rehearsal was replaced by my supplier with another tube of the same make and the same Gm and Ip under the same test conditions
Be aware that if the amp has been run hard, there could have been damage to the output tubes from red plating, and so even if the other tubes didn't lose their vacuum, they may still have suffered from melted/bent grid support rods which would throw out the Gm and Ip, so that even if you got a replacement tube matched to the previous set, it may not be matched with the others. You need to check the plate current in all the tubes.
-
Yes, I do check the bias (and therefore the Ip) anytime I'm changing the power tubes. If I am to believe my repair log, I was getting pretty consistent results on all 4 power tubes after changing the one faulty tube.
Just to be clear, I am not asking whether or not, after 6 months of use, I should try to find a brand new "matched" tube to complete the set. I agree that it makes sense to buy a whole brand new set at this point in time.
This particular amp has been running more or less 70W of dissipation on the previous set for years, it was given to me for a retubing and bias which I did with a set of brand new EHX EL34, at the same power dissipation, the getters of two tubes of the brand new set turned white in less than 6 months, I was wondering if it really just a matter of "bad" tubes (the previous EL34s (J/J's) held up fine until they died a natural death)
-
...I had them biased at 70W of power dissipation ...... 'The tubes were glowing red....'
So, 145mA per tube? Yeah, that would glow.
-
I think you are referring to Tubeswell fictionnal scenario where an hypothetical customer would abuse their amp to the point that it would fail and then blame it on a faulty tube. I suppose that's where you got the idea that the tubes were glowing red. I understand that's confusing but at no point did I mention tubes red-plating.
How long do you reckon a single EL34 would last at 70W of power dissipation at idle?
-
I am sorry I mis-read the thread.
I still wonder why the 25W tubes were(?) idling at 70W.