I popped a fuse in my ST70 last night. I didn't have a spare so I went out at lunch and picked some up. I came home, put in a fuse and the unit powers up, at least the tube glow looks normal, but I'm not getting a reading across the bias points and of course no sound. I'm using a VTA board.

Has anybody ever encountered this or have an idea how to trouble shoot the problem?

Thanks,
Bugs

What voltages are you seeing on the B+ power supply?

Guys, thanks for your quick replies. I still don't know enough about electronics to be dangerous, but I know that electronics can be dangerous if you don't what you are poking around at.

So, question, When these pins are checked, is it with the tubes inserted and checked from the bottom or do you pull the tubes, turn on the amp and check from the top of the sockets?

Thanks.

A couple of caveats when you follow Bob's good advice:

Completely invert the amp. Do not run the tubes sideways. When the tubes get hot the elements inside can sag if they're sideways, and you just might find a tube shorting out or arcing over, compounding your problems.

When taking voltage measurements: I recommend that if you are not accustomed to doing this, use an alligator clip on the ground lead of the meter, and firmly clip it to the chassis. Poke around with the meter probe with one hand if you wish...but keep the other hand in a pocket.

If you get proper voltage on Pin 8 of the rectifier tube socket, then check for voltage on the other side of the filter choke (C-354; the smaller transformer-looking thing bolted to the underside of the chassis). If the second section of the quad capacitor dead-shorted, it may have burned up the choke winding. I have seen this happen before.

Get back to us with what you find.

Thanks for the good advice. I'll remember it the next time I need to go poking around on the inside of the amp. I tried the easiest fix first. I had an extra rectifier sitting around and put it in and the voltage came up across the bias just fine.

I've been trying to figure out what happened to blow the rectifier and the fuse. I'd been playing around with bi-amping which I was very happy with thanks to both of your advice. The other amp I was using was a Hafler 9270 that sounds very similar (to my tin ear) to the ST70.

I had shut the system down to check the cables or some such thing and probably started it back up with out much delay. Usually I let the tube amp cool down more between restarts. I heard that odd sound tubes make when they are arcing and imediately shut the system back down. The arcing I saw was coming from one of the power tubes. I did not see arcing in the rectifier, but it all happened pretty quick.

To try to make a long story shorter, I also blew a fuse in the Hafler SS amp. I replaced that fuse this morning too, but I have a dead channel. I thought I'd check the DC offset on it when I get a little more time.

I also put in a different quad of power tubes when I started back up. The tube that was arcing was a Genalex KT66 and right now I don't want to know if it's bad.

I'm not sure how one event could be related to the other?

I'm running biamped right now with the ST70 and a backup Hafler DH220 and everything sounds fine (Except that I just realized I'm playing some Hendrix too loud and I live in an apartment).

Bugs wrote:
I've been trying to figure out what happened to blow the rectifier and the fuse. I'd been playing around with bi-amping...I had shut the system down to check the cables or some such thing and probably started it back up with out much delay.

That might have done it if the GZ34 rectifier was still hot, so its "normal" time-delay wasn't happening. Most of the time we are not aware of this unless a non-delay type rectifier such as a 5U4 is being used; but turning on the AC onto hot tubes can cause a large turn-on surge. Might have been enough to push the rectifier into failure. The KT66 that arced over probably won't ever be the same, either.

That happened to me, on a Mark II. One arc was all it took to kill a Mullard EL34. It arced every time it was turned on, after that. It had seen ten years' service but was still working just fine.

It happens...that's all I can say.

Oh, I forgot to ask: you DON'T still have the original Dyna quad filter capacitor in there, do you? AKA "time bomb."

One further note: If you saw a tube arcing, it would have helped had you told us in the first post. That was a related part of the failure...it wasn't just a fuse blowing for no reason. The more information, the better.

The quad cap was upgraded along with the other power supply caps when I upgraded the board to a VTA. I used all of Roy's recommended upgrades.

I tried the KT66s last night and luckly the arced tube runs fine and doesn't hint at any problems to come.

Yeah, I guess tube arcing is a noteworthy observation . I think I was in denial, I'd rather blow a fuse than a tube.

Do you think the power surge through the ST70 could have caused the problems with the other amp? They were both running off of the same surge protector, but the protector didnt blow.