HR DeVille 212 Reverb and power loss

[Teleman55]

A customer brought in this amp stating that lately at gigs around the 3rd set the reverb faded(had to turn it to max) and the power faded(He said that the tone got real sharp before fading).When I tested the tubes I found that one tube was 45 degrees hotter than the other,so I swapped the tubes-Tube A socket A to tube A socket B etc. The temp difference followed the tubes.The Owner had a set of new Tung Sol tubes that I installed and biased at 64mv.,a week later the same problem including temp difference.I tried some used tubes I have.Most exibited the same problems but I found a set of Groove tubes that worked fine and biased them at 64mv.No problems for two weeks,the owner was happy.I warned him that used tubes are not warranted and only serve as a testbed to prove that the Output tubes were at fault.So He installed new Ruby Output tubes and chinese preamp tubes.Within a couple of gigs the same problem occured and I can't get it to happen in the shop.Why did the Groove Tubes work through many gigs with out fault? Can 2 sets of different make tubes be faulty? Or what can cause this to happen?

[Enzo]

Did you swap them BACK and try a third time?

SOmetimes coincidence can fool you. I was recently working on something and I was toggling between two states - switching channels I think - and the amp had an intermittent problem where something happened to the overall sound. I convinced myself it was channel related since pretty much every time I switched to the one channel the symptom appeared.

The clue was after I stopped my toggling. The amp continued to cycle in and out of the symptom at about the same rate as I had been toggling. I hindsight, I tend to listen to the channel for about the same length of time each time befoer I feel I've heard what I need. Just happened to be the same amount of time between symptoms.

Point is that the correlation was irrelevant, my problem had nothing to do with one channel over the other. No I don't remember now just what it was, only that I felt foolish afterwards.

SOmetimes something like cracked solder on a tube socket pin can cause intermittants, and just the act of plugging tubes in and out can re-establish contact in the solder joint for a time. Might not have been the tubes. FOr example, if the screen grid loses voltage, the tube will run a lot cooler.

And that is a problem seen more often than elsewhere - The DeVilles cracking their solder on teh power tube sockets. Check that real close for little cracks in the solder and resolder them. The preamp tubes don't seem to suffer, but look them over.

When one tube is running hottier than another, check the voltages right at the tube sockets. See if anyone has lost the screen B+ or has a drifting bias.

[Teleman55]

Thanks Enzo-I am sure I must be missing something simple.I checked voltages at both sockets and found no problems.I often find solder joint related problems in modern amps,I am not complaining just stating how I have learned to narrow my search for problems.When the amp came back to me the Owner said he has also had a problem with the channel switching.I tested the footswitch and cable with another amp and they worked fine. I will check the jack,maybe the problems are related. I can relate to your tale Enzo.I sometimes get so focused or caught up in what I am doing that I miss simple steps and make ingenious discoveries only to find out that I was blindsided.It sure helps to have a place to share problems and Ideas.I find this often gains me a fresh perspective when I get stuck,while being guided by people with far more knowledge and experience.I will let you know how I make out-Kevin

[drewl]

Definitely reflow the socket connections. I do it to all of these amps that I come across.
Also clean and retension the sockets.
Some tube pins are larger than others which spreads the pins open, and if you install tubes with narrower pins they will be intermittant.

[Enzo]

Aside from powr issues, check the two dropping resistors that serve the zeners for low voltage rails +/-15VDC. They like to crack their solder. 5W cement resistors. Make sure the solder under them is OK, in fact, I prefer to replpace them and leave the leads long underneat and lay them along the traces for a ways, and sweat solder them length of it. That way it is harder for it to unsolder itself. Low voltage circuits that would be affected include chanel switching.