First time for everything...

I got my first Stereo 70 in 1977 and have owned a variety of tube amps since. I have had two rectifier failures in 38 years, but until yesterday had never had an obvious issue with an output tube. Last night I powered up my MkIIIs (cold) and heard some thumps and saw flashes from one 6550 just as the bias came up. I'd estimate maybe three seconds between flashes. I powered the amp down and reseated the output tubes. Powered up and nothing but sweet music. Bias measured fine. No microphonics or sensitivity to tapping.

Tonight (24 hours later) I powered the amps up and the same tube flashed again. This time I powered down and swapped the positions of the two output tubes. Powered up and sweet music again.

So far, after power cycling the amps run fine. I guess I get to run one trial between 24 hour cooling off periods. What tube behaviors do you associate with pentode failure? I suppose internal/visible arcing could be due to B+ overvoltage or bias error or a short circuit in an OPT. I wouldn't expect those to go away by power cycling--but anything is possible. I haven't taken the amps to the bench yet. I'll do that over the weekend.

less hope it a ezey fix...........poor pin to socket contact .....look at this post if you have not...Genalex Gold Lion Bias Adjustment.......
Like you I run MK3s.....like these amps a lot....all stock 50 years old in my case...wih the old 6AN8..never lose tubes ...but do run all my tube amps an preamps on Varic,s......keep the B+ at about 480-90 for the best sound out of the MK3s....well to me

one mod I have done to all my tube amps is Bob Carvers.... DC restorer setup.....give more power output an just sounds better to me....other can say what thay well....but it cost vary little.....an is well worth the time....in 40 years I have had a lot of tube amps an done a lot of mods an up greads.... this  DC restorer  mod is the best thing I have ever done to any tube amps.....I just set this DC restorer setup on a jadis defy 7 I retube for a guy.....night an day ....for the better ....he can beleave it the same amp....cost less $5.....
you have questions e-mail Bob carver.....it may take time....but he well always get back to you...........
Bob carver is a great guy....an he winds his own... power an output transfourmer ....real tube lover
good luck

http://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=187358




thanks for the site Bob.............................

Interesting about the diode mod.  Had to look that one up.  Prevents the output tube from being driven to cutoff by the negative peaks of a large audio signal.  Here's a schematic page with the mod hand drawn.

http://www.vacuum-tube.eu/images/hk_citation2_mods/hk_citation2_mod2.jpg

Thanks for posting the H/K setup....... these same res ....an diodes can be used with any 450-500v B+ tube amp..........why would you not do this mod?....2ea diodes one res...pre ch........even if there are more output tubes in parallel......
Bob like the tube 6AL5 diode..$3.ea....it work will an sound better than SS.... Bob says it smoother sounding.....an well last for 20 years in this setup..need one pre ch................
but the SS diodes are ezy an sound great ..




thanks for the site Bob..........