Random red plate on a stock Dynaco ST-70 amp

boeingpilot wrote:A little confused as most postings with issues are for VTA ST70s....

Running my stock ST70 (was gone through about 5 years ago with new tubes, caps, bias supply, new reproduction 7199 driver board) and heard a slight click, buzz and slow increase in noise.  Saw through the cage that one of the tubes on the right channel starting to glow, shut the amp down. Amp was on for about 30 minutes.

After everything cooled down visually inspected, couldn't find anything.  Checked all the tubes on my Jackson tube tester.  All check good except for 6U8A (run though adapter to 7199) on that channel.  One half reads low, but servicable, other half good.

Reassembled, and powered up. No issues.  After 1/2 hour bias settled down and was set to 1.45v (15.6 resistors still in place).  

Fast forward to tonight.  Ran the amp for about 3 hours and while listening started to hear the tell-tale hum, looked over, saw same tube start to glow.  Shut it down.  

Decided to pull the pair on that side and put in some older good AEGs I have had around.  No problem, but then 15 min in now the other tube on the channel starts to glow.  Mmmmm.. Reseated, ran for another 1 hour, no problem, could be that the tube sockets are dirty.

Questions -

Best way to clean and retension sockets?
Could the 15.6 be thermally bad?  Should I replace?
Anything else I should look at?
Could the fact that the 6U8A is looking marginal be contributing?

Thanks in advance

Scott

peterh wrote:
Get a new set of EL34, preferable a matched quad.
The 15.6 and 6U8 has no part of this event.
If parts have been changed, check or reflow all solder connections.

boeingpilot wrote:

peterh wrote:
Get a new set of EL34, preferable a matched quad.
The 15.6 and 6U8 has no part of this event.
If parts have been changed, check or reflow all solder connections.

Ok, guess I'm really confused.....    What would changing out the tubes do, other than risk red plating a new set?  And when I say red plate, my guess no greater than 30 secs on one tube.  I did swap that channel (had two JJ EL34s) to 2 known good AEG EL34s.

The problem I've had has only affected the right channel.   No parts have been changed in 5 years since the amp was rebuilt.  Looking at the comments on the VTA board ST70's that have had red plate issues, it appears that the 10 ohm between 1+8 to gnd is always a suspect.  Wouldn't the 15.6 on stock ST70 be the same suspect?  Plus, looks like this may not have been the first time.... one of the channels has a replacement resistor (actually two in parallel) replacing the 15.6 (have to take it apart to see if its this channel or the other one).  This was done before I got the amp.  Seeing as when I checked the resistance it checked, I wasn't inclined to replace it.  However, if either that or the original 15.6 is drifting after the amp warms up.....

Since I have at least 45 year old tube sockets, I'm still wondering if the issue is related as well.  Again, can someone perhaps point me to a good tutorial on best way to clean and retension these?    Of course, I'm running the amp as I type, and no issues.  I won't leave it unattended for now, but curious to see if / when the problem reoccurs.  

PeterCapo wrote:While the problem could have more then one cause, contact #5 in the socket that red-plated should be cleaned and re-tensioned.

Clean tube socket contact from the top side of the chassis with 99% isopropyl applied and scrubbed-in with an interdental brush of suitable size.  Try Googling "interdental brush."  Place a cloth directly under the tube socket to absorb any alcohol runoff.

While the socket contact is still wet, blow it out immediately with compressed air.  Repeat cleaning if contact is still dirty.

If you look down into the socket from the top side of the chassis, you should see that the contacts are shaped like the letter "C"  Use a small flat blade screwdriver, or a small awl, and carefully compress the contact.  Don't press on the side of the contact directly opposite the split in the "C"  Press on the top or the bottom of the "C"  Don't close the contact too much, or you might not be able to get the tube back in.

You might also try re-flowing the solder on pin 5, underneath.  Do this by either applying some solder flux and then heating it with the soldering iron or by applying some fresh solder.

The connection for the bias voltage could be compromised farther back.  Try tracing back from pin 5 to the 1KΩ, to the PC board, to the 270KΩ, and either E6 or E21 depending on which one is on the side where the tube red-plated.  Please consult schematic and pictorial wiring diagrams https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf  From either E6 or E21, trace the circuit back to the bias supply.  Look for cold solder, frayed or loose connections, etc.

If the tubes red-plated, they are likely damaged and should be replaced.

Another possibility is the coupling capacitor on the PC board that couples to the socket that red-plated.  When you had your amp serviced, were the parts on the PC board replaced, including the large coupling capacitors?

Can't quite get a fix on the situation with your bias resistor(s) on the side that red-plated?  Can you say more or post a photo?