Just installed new 7591 output tubes and while I was adjusting the bias, the amp made a low, but audible hum (amp itself, not through speakers).  From the FM tuner, I had sound before that and before starting to check the bias, but after the hum, sound was gone.  (I had turned off the tuner before testing the bias.)  When I tried to adjust the bias voltage, it would not change.

 What kind of amplifier?

 BTW, the amp is a Scott LK-72.

 did any of the magic escape? translation the blue smoke inside the parts. No bad smells as in fried transformers? Is this an older amp? I am sure that a serious tube guy will jump in here soon and give you a list of things to check out. Sorry I cannot offer more. Good luck and best regards Moray James.

 if you have a rectifier tube...have it tested....I usally variac amp up that needs bias adjust due to tube replacement to make sure the tubes get no time in an over bias'ed condition.  Once the bias is known to be below range then I bring it up to power.  

speakerfritz:

 if you have a rectifier tube...have it tested....I usally variac amp up that needs bias adjust due to tube replacement to make sure the tubes get no time in an over bias'ed condition.  Once the bias is known to be below range then I bring it up to power.  

 

Good advice.  The hum was probably from the power transformer in response to excessive current draw.  Is the filament of the rectifier tube still lighting and did you note any arcing within the tube when you first powered up the amp with the new tubes?

 

 There was no smoke, no arcing and the filament is not lighting up.

 supply related I would guess, I am not a tech guy so others must help not me. Good luck help is on the way I am sure. Best regards Moray James.

youngjj:

 There was no smoke, no arcing and the filament is not lighting up.

 

I assume that the other filaments are lighting (if not, of course check the fuse)?  If that's the case try a new rectifier, but as SF mentioned, use a variac to start out with a lower than usual line voltage (say 95-100) until you have an idea about what the bias is doing.  As I recall, the LK-72 does not have any way to adjust the bias as it was generated at a fixed value by the DC supply for the front end filament string, so I assume that you installed bias adjustment pots.  I'll be glad to talk you through the whole troubleshooting process if you send me an email.  Regards-- Maynard         

 

It blew the fuse. If you do not have fuses it requires a style "3 AG, 2 1/2 amp slow blow" buy at least 3. Try a new fuse. If the new fuse blows instantly then remove the 5AR4 rectifier and install a new fuse. If the fuse lives without the rectifier in place then get a new rectifier tube. If it blows again without the rectifier tube get ready to spend some money the power transformer is toast. If this amp has not been rebuilt by you with lots of research to figure out the proper approach or by a known good professional you should not be using it. Tube amps blow fuses for all kinds of reasons and many of them are not good... it usually takes a serious fault in the circuit to blow a fuse. 50+ year old tube gear has absolutely no business being plugged into the wall without a good quality overhaul.

 Amen to your good advice. Best regards Moray James.

There are lots of probally's, could have, may have, etc in this thread. if you need all you tubes tested pm me for my address and I'll test them for you. then at least you will know if it is or is not probally, maybe, perhaps, could be, the tubes.

 a tube tester still leaves plenty of those probably's, could have, may have, etc in place since they NEVER run the tubes in the same electrical environment as the amplifier they are plugged into... All a tube tester proves is the tube "may" not instantly short out and has a good "chance" of working in the amp...especially output tubes. It kills me when some arm chair person buy's a tube tester and starts acting like he knows WTF he's talking about.

NOSValves:

 a tube tester still leaves plenty of those probably's, could have, may have, etc in place since they NEVER run the tubes in the same electrical environment as the amplifier they are plugged into... All a tube tester proves is the tube "may" not instantly short out and has a good "chance" of working in the amp...especially output tubes. It kills me when some arm chair person buy's a tube tester and starts acting like he know WTF he's talking about.

WTF is right....offering to test the tubes eliminates a factor in the OP's analysis process ....offering to do it for free simply qualifies me as not being a forum troll looking for a restoration client.....not that anyone on this forum would ever do such a thing....

Yea but you have sold more gear and tubes on this forum directly then I ever have...in fact it has been many years since I sold anything on this forum like 10 maybe?... kind of the coffee calling the kettle black, hell I've seen you listing countless tubes (with false information about them) and countless amplifiers for sale on many occasions. you are just another profiteer hiding behind a hobbyist blanket. Guys like you are all over the internet... Half ass business men that don't have the balls to go into a legal business.

  Show me where I ever in the last maybe 6 to 8 years solicit anyone on this or any forum to send me work? And no I never PM people about it either. In fact if you go to my website right now I have shut down all new shipments because I have too much work!! I come here as a form of entertainment and nothing more. Again testing tubes does NOT insure anything. It helps but you are the one that came in here with the snide remarks.... I can diagnose problems with tube amps over the internet with my eyes closed and sleeping better then you could with my help.