Dynaco-Doctor Forum Welcome! Need support, you got it. Or share your ideas and experiences.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Dynaco-Doctor Forum ]

---

[ Alert Moderator ]

---

Thoughts on red-plating a tube?

Posted by boeingpilot (A) on May 21, 2015 at 10:15:16

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.

This post is made possible by the generous support of people like you and our sponsors:

Western Glow Tube Service

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---