HT fuse blowing

[Wizard1183]

My 1969 Laney Supergroup LA100BL is blowing HT fuses. I replaced the tubes recently with Tung Sols and it ran fine for about 30 min? Then quit. Put another fuse, 2 min blows. Spoke to local (who doesn’t get back to me to bring amp in to have a look) and he told me add a new fuse, pull all tubes and install 2 middle EL34s, then 2 outside, then left to right each 12AX7 to see which section blows. Well I did and at idle amp stays on. Minute I throw a jack into the input, it blows the fuse. I never noticed any red plating. I do notice 2 sockets seems to have loose pins when tubes aren’t in them. I can shake the RS 5W 470 resistors or try to tension the tube pins it’ll move the resistor on 2 of the EL34s. I can shake 2 of them. The solder joints are fine. It’s the pins themselves that are quite loose. However when the tubes are installed the tubes aren’t loose that I can tell? I had a tech look at it last Yr I believe and it didn’t last long after he worked on it. It blew fuse and I never brought it back or found anyone else to work on it. Just wondering what possibilities could it be? Does it require another bias due to installing new tubes? They’re supposedly quad matched. But I know tubes these days aren’t superb sometimes.

[studiodunn]

I would swap the PI out next. I just had a bad PI tube causing me the same situation.

[pompeiisneaks]

That sounds tricky. It could also be one of the sockets has gone bad, and it's pushing a little extra current through above idle that's causing some kind of arcing etc. Look anywhere you can at the sockets and see if you see any melted if non ceramic or black areas if ceramic near the pins?

If not check the plate current and see if it's running hot, maybe the tubes may not quite get to redplating before the fuse blows if they're really pulling a ton of current on all 4.

~Phil

[Orson Maxwell]

Well, if it is the fuse, it should be the power stage. Ideally, you need a tech to monitor both plat/cathode current on both tubes and their bias while you're playing. probably someone with a scope or at least a couple of voltmeters. If I saw the correct schematic, the bias circuit is not adjustable and doesn't look bad, but it doesn't hurt to check if that cap is shot.

[Wizard1183]

Correct. No bias adjustment. I’ll bring it in to a truck and tell him what’s going on. I looked at the sockets and no melting or black around them. I appreciate the advice

[tubeswell]

Nearest schematic I can find on a quick search is http://el34world.com/charts/Schematics/ ... _a100h.pdf and that is a fixed bias output stage. I'd check the bias supply circuit is functional - esp the filter caps* in the bias supply. You need to have a steady -ve DC voltage on each of the output tube grid pins before you plug any output tubes in.

* In fact, if none of the electrolytic caps have ever been replaced, all the filter caps will be well overdue for it.

Also check and re-tension your output tube socket pin clamps. (with the amp off, unplugged and the filter caps discharged)