Welcome! Need support, you got it. Or share your ideas and experiences.
23.30.223.225
OK, first the history of the ST-70. I've had it for almost 30 years, it was a dumpster refuge...
Since I've had it I've replaced the power transformer (shorted), pulled the quad cap and selenium rectifier and replaced with Joe's board, and recently re-capped the driver board. Also, as I was rebuilding another ST-70, I put new production Mullards (Russian) EL-34's in it.
During the recap, I found the solder joint on one of the coupling caps bad, which I would account for drifting bias voltage on that channel. Also, it started blowing fuses after warm up, found that the 5AR4 would arc internally just before the fuse blew. Put a new Sovtek in, have not had the problem in several months.
So..... to the thing that almost scared the .... out of me. Last night I forgot to turn the amp off, so it was on for say like 6 or 7 hours on idle. Nothing bad happened. This afternoon, I turned it on and was listening to it, again, no problems. Got a phone call, so turned the volume down. After the call, kind of forgot about it, but then heard a rising 60 hz hum, glanced over to the amp and it appears that one tube (right channel next to transformer) is red plating. Immediately hit the power switch. Since I have the cage on my amp, could not be sure it was only the one tube, but I'm thinking it was. After everything cooled, looked around, saw nothing burned. Re-seated the tubes, powered up, everything looks good. Needed slight adjustment to bias, but nothing more than I would account to normal drift.
So - first all don't want to think of what would've happened if I hadn't been there, especially since it had run all night by itself the day before
- assuming it was just the one tube, and the tube is working now, am I interpreting all the websites that it may have been as simple as a bad tube socket connection? As the tubes are only a few months (2-3) old, is it possible I have tube that's going?
- anything I should look at ?
- I have an original (phenolic) PC-3 board, could this be contributory?
Thanks for the groups learned opinions.
Follow Ups:
Ok, may have answered my own questions..... (or may have generated more questions!)
Looked closely at the PCB, found the land had pretty much lifted and possibly started to melt attaching to the coupling cap. (see pics, sorry for bad picture). So, either this is the problem, or I have multiple problems. You can see that the board has seen better days. It's sort of coated with waxy coating that I suspect is the board going bad. And there are several other lands that have started to lift.
So, fortunately I have a spare ST-70 to use, but now have to decide what to do with this one. The spare has new PC-3A (7199) driver board. Do I just get another (I do obviously have good 7199's), give up and get a PC-3B (6GH8's) or something else. Joe Curcio's? VTA's?
And I was just starting to like this amp (again!).
Back to the red plating, opinions, did I find the issue, or are there more?
Check the warranty for the new tubes. Some places give up to 6 months warranty for a bad tube. Also, you should add a 470 Ohm, 1 w resistor in series with each screen grid. That may prevent future red plating caused by screen shorts. I did it and haven't had a tube go red since a few years ago.
I replaced the four power tube and rectifier tube with Japanese ceramic octal sockets in case loose pins are the issue.
The wax has been there since the board was made. The problem is, as you noticed, the pads and traces are not reliable. If they have lifted or have been damaged in different places, then you need to either make repairs or replace the PCB. There are a number of good choices. IMO, the original 7199 driver still makes a fine-sounding amplifier.
If the tube red-plated, it might now possibly be damaged, i.e., gassy, even though it is still working.
Yes, it could be the socket. Try cleaning and retensioning contacts #5, though it might be necessary to change the sockets (a lot of work, which is why I suggest cleaning and retensioning, first). I suggest an interdental brush of appropriate size with 99% isopropyl.
It could have been the tube to begin with.
If you have new capacitors on the PCB, and if the solder connections are good, then I am not too inclined to think it is a problem on the PC board, though it still might be. For instance, if an etch is broken somewhere that results in an intermittency, though this is just speculating.
Have a read here .
Thanks Peter! I'm inclined to think I grabbed it quickly enough to save the tube. Literally less than 30 seconds from when I heard the hum and I killed the power. Weakened it, absolutely, but it's a new tube, so there should be some life to it.
The old driver board obviously is showing all of its (at least) 40 years. I think it needs replacement before I can look at anything else. (My spare has a new fiberglass reproductions, much nicer quality obviously)
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: