Problem with the tube amps...

nooshinjohn

Tonight, my left channel started popping the fuses on the input transformer. I have been using 6a 250v slo-blo fuses since the amps were moved into the new audio room, which has a 20 amp circuit. The amps have been plugged directly into power , and after the fuse popped, I moved the amp onto my power conditioner.

After ten to 15 seconds, another fuse blew. I watched the output tubes get an eerie Emo-blue glow to them before they went dark. I popped the bottom off of the amp and found no sign of blown components or other damage, and I do not want to risk causing any. Without a variac setup, I am unable to run any testing, so I am dead in the water for now. I am afraid to power them up again until they get checked out.

Just wondering what you guys think could be the cause... bad input tube(s) maybe???

I will likely need to send them to Bob and have him sort through it, but it sucks that now is the time for them to act up... is it possible that the amps do not like seeing a 20 amp service?

Wardsweb

Yes, it's all part of the tube experience. Like Zero said, pull the tubes or send it out. If you want to try, you can pull all the tubes and see if the fuse blows. If it blows with all the tubes out, you have an internal issue. If it doesn't blow, add the rectifier and try again. If it blows, try a different rectifier tube. Too bad, you live so far away. I could put it on the bench and troubleshoot for you, but Bob is closer than I.

nooshinjohn

I swapped the front end (12ax7, 12at7, 6al5) from the good amp to the problem child and got the same result. Ten to fifteen seconds and the fuse pops. I did not think an output tube would cause problems with the input side of things, but I will swap those tonight when I come home with new fuses.

I don,t know why but my gut suspects the input transformer...

@Wardsweb... I am grateful for the offer, and might just ship it your way depending on what Bob has on his plate.

Wardsweb

Is your amp tube or solid state rectified?

mhardy6647

Depending on the bias mode of the amplifier, a tired output tube may not hold bias; current could soar in the output section & cause fuses to pop.

Typical causes of instant fuse popping in vacuum tube amps (more or less in my suspected order of likelihood) -- shorted power supply filter capacitor(s), shorted rectifier(s), shorted primary or secondary winding in the power transformer, bad coupling capacitors, weak output tube(s). Input section issues not likely to cause enough current draw to pop fuses... but you never know, I guess. If it is a fixed bias amp, the bias supply could be the source of the problem, too.

As noted above -- pulling all tubes (including the rectifier if this amp uses a vacuum tube HV rectifier) and plugging in/powering on could reveal a short in the power transformer (i.e., if the fuse still pops). If the fuse didn't pop under that scenario, reinstalling the HV rectifier tube can reveal whether it might be the tube or the filter caps (i.e., if now the fuse pops) . A rectifier tube short can be easily diagnosed with a simple emissions-type tube tester (or a tube swap).

The nice thing about vacuum tube amplifiers vs. soiled state is that the former are really easy to troubleshoot! A DMM or VTVM, a Variac, and a Kill-a-Watt (and/or a proper ammeter/power meter) are extremely handy tools of the trade. ... and a box of fuses, too, of course ;- )

nooshinjohn

I did as instructed, and am no longer blowing fuses with the input tubes in place. I guess it would be a near certainty then, that my problem resides within an output tube, perhaps a pair of them.

Should I be looking at replacing all six in that amp, and storing the remaining good tubes as backups? I am also going to use the downtime to make a couple minor changes inside, like replacing the inrush caps and updating a couple diodes that are somewhat undervalued. By then my bias tools should be here and I can readjust the bias accordingly....

Plugged in the power tubes two by two, starting with the front two. They held, so I went with the two immediately behind them, and the fuse blew as scheduled. I pulled that pair and went with the last pair that fit behind the input tubes and they also held up well.

One of the failed pair was very warm to the touch while the other was cold as ice. I think I found the bad tube... I just ordered a new pair. I will get the other pair tested and keep the good one for backup.

nooshinjohn

DSkip wrote: »

Biasing tools just now coming in? Have you not biased your amps routinely?

The amps are auto-biasing. once set, they do not need attention until something like this happens....

drumminman

Glad you're getting it sorted John. I hate it when audio stuff doesn't work.

mhardy6647

Cold as ice is a problem... unless of course your test music was Foreigner.

nooshinjohn

mhardy6647 wrote: »

Cold as ice is a problem... unless of course your test music was Foreigner.

They don't get too hot in just 15 seconds
I think the hot tube is the good one but they both will be tested.

nooshinjohn

Just to update... all is well with the amps once again. A bad output tube was the culprit. They are now back to sounding like the glorious beasts they are.

I was finally able to get some time with the Pass Xono last nite, and with the amps back up to health, it was a magical experience.

Wardsweb

I'm glad you got it figured out. Tune time