Hi everyone. Been running my rebuilt st70 for about 2 years now. Recently I've noticed one channel (right) fading after an hour or so of use. I suspect a capacitor but would like to know what other areas I should look at. I've already swapped output tubes, checked my preamp and used other interconnects. Would appreciate any ideas. Thanks
4 posters
1channel fading
grizzinca- Posts : 40
Join date : 2012-01-01
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
Hi,
Is this a STOCK or stock type driver board ?
Have you tried swapping DRIVER tubes ? Also - try cleaning the right channel driver tubes socket(s) with an electrical contact cleaner. If it is a tube, a slow fade like that is more common a driver tube problem than an output tube problem IF it is a tube problem.
Bob
Is this a STOCK or stock type driver board ?
Have you tried swapping DRIVER tubes ? Also - try cleaning the right channel driver tubes socket(s) with an electrical contact cleaner. If it is a tube, a slow fade like that is more common a driver tube problem than an output tube problem IF it is a tube problem.
Bob
grizzinca- Posts : 40
Join date : 2012-01-01
Thanks for the reply Bob. Haven't swapped the driver tubes, but have cleaned ALL tube sockets. I'll try this and see what happens. Thanks again. And the driver board is not stock, but VTA upgrade.
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
grizzinca wrote:Thanks for the reply Bob. Haven't swapped the driver tubes, but have cleaned ALL tube sockets. I'll try this and see what happens. Thanks again. And the driver board is not stock, but VTA upgrade.
If it is a VTA driver board, then either the center or the right driver tube could be the cause of the right channel fading. What you should do is buy ONE new 12AU7 (or 12AT7 if you have an older board) and substitute the tube into the center position and see if that helps. If that doesn't do it then try the new tube in the right position. Swapping the tubes you have in there is probably NOT the best way. If swapping in a new driver tube at the center and/or right positions doesn't help, then you have to look at the other components on the right side of the board.
Bob
grizzinca- Posts : 40
Join date : 2012-01-01
Thanks Bob.
sKiZo- Posts : 1530
Join date : 2013-04-01
Location : Michigan USA
Why stop at one? 
Swapping tubes can really change the entire personality of the amp, and you can get some real bargains in the 12AU7 family. RCA clear tops are very highly regarded and extremely affordable. IMHO, you can pay a whole lot more for a lot less.
Any working tube should be good for testing, but rule of thumb, it's good to eventually match the side drivers for the best results. Center can be a different brand, and even a different tube type, depending on your tastes. I'd pick up at least one complete spare set for "sonic emergencies" ... Don't forget to check with Bob to see what he's stocking right now.
Luck with it, and be sure to post back with your results ...
Swapping tubes can really change the entire personality of the amp, and you can get some real bargains in the 12AU7 family. RCA clear tops are very highly regarded and extremely affordable. IMHO, you can pay a whole lot more for a lot less.
Any working tube should be good for testing, but rule of thumb, it's good to eventually match the side drivers for the best results. Center can be a different brand, and even a different tube type, depending on your tastes. I'd pick up at least one complete spare set for "sonic emergencies" ... Don't forget to check with Bob to see what he's stocking right now.
Luck with it, and be sure to post back with your results ...
grizzinca- Posts : 40
Join date : 2012-01-01
Hi again. Got new tubes and swapped as instructed. Amp played nice for about 20 minutes then right channel quit. If I run the volume up the right channel comes back, then fades again. I'll put it to rest till I can get some expert advise as to where to look next. Thanks for helping.
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
sounds like a bad socket or a bad solder connection somewhere on the bad channel, easy fix with a solder iron!
grizzinca- Posts : 40
Join date : 2012-01-01
I'll take a look. Thanks
|
|
|