 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
Well, after one week with the new Curcio PC3-R board, and absolutely no problems, I went to check bias after the initial setup last week..I can't get a reading on the left channel's biaset point. I just get a handful of milivolts instead of the 1.56 volts I should be getting. Opening the bias pot for the left channel all the way up makes for no difference -- the tubes never get red-hot.
Continuity checks from the biaset point back to the 15.6 ohm resistor shows everything's OK.
What could be causing this? The right channel biases OK, and the affected (Left) channel did too--last week, but not today.
The amp, believe it or not, sounds fine.
Any ideas, hints, thoughts?
Follow Ups:
(back to my real handle, dunno why I mangled my own name =o)Thanks to Joe and all who replied.
The shameful part is, I'll NEVER know what was wrong! In the process of removing the power tubes, I managed to launch one from the bad channel..it landed on my desk, made a nice *pop*
So to the music store I go, get another quad of Svets.. re-check every wire, every connection, every solder joint..all cool..stick new toobs in..bisases right up the first time.
Now I'll never know =o(
I'd say you do know what caused it. New tubes fixed the problem.Interesting that you said "another quad of Svets" I'm assuming that the last questionable tubes were Svetlana's as well.
My identical problem also involved Svetlana EL34's. People rave about them but I like Valve Art and EH better.
Now if I could only find two more double halo xf3 brown base Mullards.
John
New svets, as far as I can tell, with the old C-in-a-winged-circle Svetlana logo, in gold ink.The new quad is the same.
I prefer them sonically to the EH's... in fact, I found the EH's to be incapable of taking the full bias the Dynaco calls for. At approximately 1.45 volts, they'd start getting a dim, ruddy red and running extremely hot.
The Svets bias up at 1.56, and run at least half as cool.
But the damnest thing is, I"ve never seen this particular failure mode in any tube -- I've had gassers, I've had filament burnouts, I've had shorts, but I've never had a tube before that flat-out refused to bias -- and still worked.
Interesting to note, these new Svet 34's are double-getters. They look, and sound, similar to Mullard.
First - exchange the tubes from channel to channel to see fi the problem follows the tubes (this is probably not the problem but no harm in getting the tubes as the problem out of the way)Next - pull all four output tubes. Now check the voltages on the pins of the "good" channel and compare them aganist the voltages on the pins of the output tubes of the problem channel.
You should find one of two things on the problem channel a) the bias voltage on pin 5 is very negative (like negative 45 volts) on the problem chanel or b) the voltages on pins 3 & 4 are low or missing.
I think the problem will be (a) so lets focus there. Check the voltages on the _ends_ of the two bias adjust pots - both pots should be the same (about - 40VDC on one ends and - 28VDC on the other ends). If you get this now measure the voltage on the center pin of each bias pot _as you rotate the bias pot for that chanel_. The voltage should range close to the same voltages you measured on the pot ends. I suspect that you may find that the problem channel's voltage does not vary from the most negative voltage.
If you pass this test run it again but this time measure the voltage at pin 5 of the output tubes. You should get similiar results if the circuit is OK.
Same thing happened to me and it turned out to be an output tube. Even worse it would bias up correctly and five minutes later when it warmed up it, max bias I could achieve was also way down there. Some readings in the millivolts also.Mine also sounded good as well under this situation.
Good Luck,
John
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
