Shorted tubes hurt my baby. Can anyone help?

[mThomasDutch]

Hi there. I just had one or more brand new el34s short out in my RM100. 2 of the tube fuses blew and I shut it off immediately. Now after replacing the fuses with like fuses and some known good 6l6s I have some problems. The tubes bias up fine, but after the amp gets good and warm a very slighly audible oscillation begins that seems to be in the 400 to 500 hz range. There is also very little volume as I increase the master volume. The amp is audible, but not much louder than speaking voice. Any ideas?

 

[Julia]

I wonder if one of the tubes took out or damaged a screen grid? If that's the case it tech time.

 

[mThomasDutch]

That would actually be good news. I'm fearing it's the OT. Had this happen in another amp (using a ^%$ EL-34 btw) and it was the OT. That amp would also play, but with very little volume. Never heard the oscillation thing before though.

 

[Julia]

I have heard oscillation when all the tubes were bad in the quad. I know you replaced the bad pair but chances are the entire quad was bad because it was from the same lot. Pull the other two EL34s and put another pair of 6L6s in it.

I've enough failures with EL34s I won't go near them. If I want the EL34 sound I'll use my modeler.

 

[kc2eeb]

Take a very close look at the tube sockets. Check for any sign of carbon traces from a bad pin connection. Look around the pc board for any sign of shorting. Look at the blade connectors for the plates. (Blue and Brown)
Use a magnifier. Make sure the sockets are tight on the tube pins.
Take the tubes out and check the screen grid resistors.(1k 5watt)
The fusing system works quite well in the amp so before assuming it's
the OT I'd thoroughly troubleshoot the PA board.
If you think you've found the problem, first power it up without the
tubes.

 

[mThomasDutch]

Good idea. I'll try that. It must be nice to be able to charge good money for tubes and then have a "use at your own risk" customer service policy.

 

[mThomasDutch]

I noticed when I pulled those fuses that they are all 400ma fastblow fuses. The manual states that the fuses should be 250ma fastblow. Has the recommendation on fuses changed, or did the previous owner put in fuses with too high a value? It seems like I saw a post on these forums once where someone "quoted" someone from Randall as saying that the 250ma was a bit too low.

 

[kc2eeb]

.400ma fast blow fuses are OK. Those fuses are on each heater of the
output tubes. Should a tube short, they blow in the case that a heater shorts to ground or the plate or more likely the screen grid shorts to the heater. I don't know how experienced you are with troubleshooting
vacuum tube amps, but it's best to be methodical and avoid a "shotgun approach" and change parts until it works. Contact customer service
1-800-877-6863 and get a schematic to work from.

 

[mThomasDutch]

.400ma fast blow fuses are OK. Those fuses are on each heater of the
output tubes. Should a tube short, they blow in the case that a heater shorts to ground or the plate or more likely the screen grid shorts to the heater. I don't know how experienced you are with troubleshooting
vacuum tube amps, but it's best to be methodical and avoid a "shotgun approach" and change parts until it works. Contact customer service
1-800-877-6863 and get a schematic to work from.

Thanks k2eeb. Unfortunately my troubleshooting skills are better than I want them to be - meaning I'm spending too much time working on my amps when I'd rather be playing them! I'm gonna check those resistors this weekend. I wonder about those fuse values because I have an H&K triamp that has the same fusing setup. H&K actually came out and said the 200ma values were too small and did away with the fuses altogether in later model years because of the problems they had with the small fuses blowing. I actually replaced all of mine with 500ma and they don't pop, but I fear that I'm protecting my $.10 fuses with my $200.00 transformer. I got some more of those 400ma ones on the way that I'm gonna put them in both amps.

 

[kc2eeb]

Well, to tell a story;
I just ordered a new power amp circuit board for my RM50.
It seems the plate connection to the pc board(blue wire) shorted to the board and
toasted it. I had had a very intermittant noise that sounded like high voltage arching that I could never trace down. Never blew a fuse, and then, disappeared. I did find an missed solder joint on on of the screen grid resistors that I fixed and a loose ground on the DC heater supply.
Finally, this past Saturday, it arched enough to blow the mains fuse.
It ate up about a 1/4 inch square of the copper ground plane.
I'll try to post pictures of the repair as I do it.

 

[JKD]

Well, to tell a story;
I just ordered a new power amp circuit board for my RM50.
It seems the plate connection to the pc board(blue wire) shorted to the board and
toasted it. I had had a very intermittant noise that sounded like high voltage arching that I could never trace down. Never blew a fuse, and then, disappeared. I did find an missed solder joint on on of the screen grid resistors that I fixed and a loose ground on the DC heater supply.
Finally, this past Saturday, it arched enough to blow the mains fuse.
It ate up about a 1/4 inch square of the copper ground plane.
I'll try to post pictures of the repair as I do it.

Wow..similar happened to my RM50 a little while back although I never heard any arc'ing noise or evidence until the amp blew and stopped making noise...never even had to change a fuse.

My amp lost the plating in two places...one around the HV line also the screen grid nearest to it....there's pictures on here somewhere....

http://mtsforum.grailtone.com/viewtopic.php?t=5681

 

[kc2eeb]

Did you cut away the burnt board and direct wire the HV lead to the socket
and cut the trace?

 

[kc2eeb]

Yep! That's exactly what happened to mine!!

 

[JKD]

Did you cut away the burnt board and direct wire the HV lead to the socket
and cut the trace?

Nope....I took Randall up on their offer of a new board. It took a few weeks for them to find a working board and it has some kind of wire-wrap fix under neath it but at least it had the screen grid resistors correctly soldered in.

Although oddly enough, the bias section again had an ungodly messy hand rework on it :-/

 

[kc2eeb]

You know, "this is getting old" to coin a phrase. When I get the board
I will coat the pertinent traces with corona dope. My RT2/50 had to go back. I bought it in '02 or '03. I opened it up and the PA board had been reworked. The HV traces had been cut on the board and the leads
directly connected to the tube sockets. When it was reassembled, they
missed a connection. Never said what it was.
I hope this fixes it. What really irks me is that it is such a good, versatile
amp with great tone. You can turn it into anything you want.
I'm not one of the people who say "if it's not hand wired on a turret board
by Leo's ghost" I don't want it, but, what we see in the pictures is just
BAD DESIGN.

 

[kc2eeb]

JKD
My best guess is they hard wired any HV voltage path and then cut the
traces on the pc board to avoid arching. I'll post what I find when I remove the board from my RM50. I might repair the one in the amp and keep the new board I bought as a back up.
Any further pictures you could post would be most appreciated.

 

[mThomasDutch]

Soooooo, after an hour or so of troubleshooting this problem, we ruled out grid resistors and the OT. Only after applying a sine at the input and checking for AC on the grids of the power tubes were we able to trace the problem back. It was really bizarre - something I've never seen before. It was... wait for it... a shorted 12AX7 in the phase inverter position. It was likely that this tube shorting cause the power tubes to haul arse toward meltdown and blow the fuses. She seems to be fine now. One bullet dodged. Thank all of you for your advice.

 

[kc2eeb]

Exactly where I was going next. I play my amp with all the master
volumes full up. With 6V6s (JJ) with an idle bias of 19 and 21ma, even thought on my Longhin Lo460 tube matcher they are within .75 ma each
to 460 volts on the plates, when playing full out, V5 draws 100 to 115
ma while V4 draws near 200 ma and starts to red plate. I've seen this happen when the screen grid resistors start to fail towards open or
a grid resistor problem.
Glad you found the problem. Will let you know what I find.

 

[mThomasDutch]

Best of luck. I've been thinking of trying 6v6s in mine. I've got a pair of jjs, but I'm not crazy about running a pair of tubes in a quad amp. I've also been curious about running kt88s in the rm100. I think in that case I would just run the pair. I'll have to search around a bit and see if anyone else has had luck with that setup.