OT? troubleshooting amp

[guitarhacknoise]

hey people,
as of yesterday, I am down to zero amplifiers!
I'm taking all three in later today.
anyway, here is the deal,
mid 80's jcm 800 50w superlead, side by side inputs.
one year ago I had new power tubes put in and biased by a rep. service tech.
sounded killer, but I only got about 5 hours of playing time out of it before it just started popping the 1/2 amp fuse immediately on power up.Since it was not the best amp for what I was doing at the time AND the fact I could'nt find the receipt for the tube/service warranty and even though it is a small shop I guess rules are rules and I need to accept that, so anyway, the amp has been sitting in the closet. and now the warranty is up and for other reasons have decided not to use the guy anymore

So this morning I figure i'll start the trouble shooting process as I await the appointment with a NEW tech.

o.k.
new fuse, power up, pop. o.k.

pulled the tubes,
2 x 6550 for the P.T.(not originally assigned to THIS amp but some were and these are what were in it when we first met so thats what I've stuck with)
3 x 12ax7 for the pre-amp.

new fuse, power up. the fuse lives

thinking I would hear the fuse go, I just turn the amp of and replace the tubes one at a time turning the power off between each addition,

well needless to say I DID'NT hear the fuse pop, but it had.

so I smack myself and start the process all over, this time visually checking the fuse with each tube,  
The fuse lives and I play some guitar for about 20 minutes, sounds killer! big smiles.
BUT, way back in my nugget I know that something aint right.

I am about to do it all again, I want the fuse to pop, I want it to be fixed and be reliable again, help?
-matthias

[R.G.]

Another non-reader of GEO.

You want to read "The Tube Amp Debugging Page" at GEO:
http://geofex.com/ampdbug/ampdebug.htm

Well, OK, maybe you don't want to, but you should.  

It is highly likely that the pin(s) on the grid(s) of the output tube(s) were not making good contact with the sockets, and so they went unbiased and popped the fuses. You semi-corrected that by pulling and reinserting the tubes, scraping off some of the gunk that kept them from making contact.

It will happen again when the crud builds up again unless you clean and re-tension the socket pins. But that will be a while and be random.

[AL]

Could be any number of things. Check the Tube Amp Debugging article at GEO. I'm still a little miffed that your "tech" wouldn't take a look at it. I realize the rules thing but if you did that after only 5 hours and he still wouldn't look at it!!! He should have had a record at his shop of the repair. Time for a new tech - that sounds kind of shady to me.

AL

[vanhansen]

Hey guitarhacknoise, where in Texas are you?  I know a real good tech in my area.  If you're in the DFW Metroplex, I'll PM you the name of the place.

[guitarhacknoise]

R.G.
I'm sorry,  :oops:
Fact is I started on this first thing this morning, and was going mostly from memory.
Even though my retension is crap, I know that dirty pins were/are a possibility,
But I also know my luck.
It's conditioned me to beleive that It has to be something more major/expensive than that.
So far though it does seem to be exactly that, dirty pins. she's sitting in the corner as we speak, humming away, happy to be alive!

On a side note, the mighty bassman that has mismatched tubes and has never been serviced on my watch, yet always was there for ya with that sweet bassman sound, also died........................ :oops:

I do read your stuff  R.G. I really do!
thing is, I have much more desire than actual smarts.
reassurance is what I need.

[guitarhacknoise]

Vanhansen, I'm smack dab in the middle of austin. I have been referred to this new guy way down south, so I'm going down there later today. There is someone here in town that I would rather have work on it but............................Elusive & exclusive

AL, yeah, well when a hippie sticks to the rules................something is up..........The real clincher came when he tried to sell me one of aron's 3pdt's to me for $20.00 plus tax!   :shock:
Edit: I would like to clarify the fact that I am also to blame for waiting/procrastinating, and it's not like he would'nt look at it ,it's just that I could'nt pay for it with out a warranty, especially after being $200 in the hole from the initial repair.

[vanhansen]

Dang.  I don't know any techs down there.  I took mine to Earhart Music in the Ft Worth area.  He does great work.  I never heard my Marshall sound so good.  Good luck.