Oops! I hope I didn't kill it. Tube amp guys, have a look.

I can't see that you're going to do any more damage by putting in a new fuse, old tubes, and firing it up - just flip the switch on and off to start with. If no popped fuse, switch it on until the tubes glow and see what happens.

If it works like it did before the tube switch, you're good to go until you can have someone work on it/ put in new tubes.

If the fuse pops, or it still doesn't make sound, then the problem is deeper seated, and someone with knowledge needs to work on it.
 
crazydoc said:
I can't see that you're going to do any more damage by putting in a new fuse, old tubes, and firing it up - just flip the switch on and off to start with. If no popped fuse, switch it on until the tubes glow and see what happens.

If it works like it did before the tube switch, you're good to go until you can have someone work on it/ put in new tubes.

If the fuse pops, or it still doesn't make sound, then the problem is deeper seated, and someone with knowledge needs to work on it.
Click to expand...

UPDATE:

I did just that - replaced the old tube set and fired her up. Got a different result this time.

The old finals had a consistent glow across the board but did not look nuclear like the JJ's did. I noodled around for about 15 minutes then left it to idle for a while with the volume down.

30 minutes later I checked in and the 3A slo-blow mains fuse had blown. (Previously, with the new tube set, it was the smaller 1 amp fuse that went out.)

I need to mention that I have been running the amp a bit differently than I had in recent years. This amp had been fitted with a master volume by an American tech, apparently upon its original import. In years past I ran the amp's volume off the master volume. (It's always been difficult to get this right since the master volume is very tricky and can have very bad tonal effects if it's down too low, and has a lot of interaction with the drive and volume controls.) In these recent circumstances I have been running the master volume wide open with the main volume control well down.

Anyway, I did a little head-scratching. I'm not sure why it occurred to me to do this but as a sanity check I thought I should check the impedance of my 2 X 12 cab versus the impedance setting on the head. (It's been a while since I built the cab so I couldn't remember.) Sure enough the cab is a 4-ohm cab (two Celestion G12K-85, 8 ohms each, paralleled) and the head was set for an 8-ohm load. So I've been punishing the thing with overcurrent. Great. My gear had been sitting in a friend's basement for years but I have no reason to think anyone had been messing with it - I think I just got lucky for a time.

I'm going to try again, replacing the mains fuse, with the impedance set correctly on the head, with the old tubes in. If that works out I'll try the new tube set again to see if the impedance setting had any effect on that situation.
 
that little impedence mismatch did NOT cause your problem... and the fact that the primary fuse blew this time does NOT bode well... you did use a slowblow fuse in that other holder did you??? that should have been a quick-blow... you deffinately need to see a tech now...
 
dementedchord said:
that little impedence mismatch did NOT cause your problem... and the fact that the primary fuse blew this time does NOT bode well... you did use a slowblow fuse in that other holder did you??? that should have been a quick-blow... you deffinately need to see a tech now...
Click to expand...

The 1-amp fuse is a quick-blow, the 3-amp is a slo-blow.
 
good for you... sometimes guys miss that one and end up roaching the whole thing because of it... still think it's time to see the doctor though... incedently a real good tech wouldnt necessarily need a schemo though it's best to have one... you see the bias circuit is rarely hard to detect... and the phase invertor/driver section most use the same type (called a long tailed pair)...
 
You don't think the impedance issue is really a player, then, Demented?

If it isn't, then it's time for the doctor. I just have to find one.
 
Zaphod B said:
You don't think the impedance issue is really a player, then, Demented?

If it isn't, then it's time for the doctor. I just have to find one.
Click to expand...

I'd say not. Tube amps can typically handle from X2 down to /2 in output impedance without a prob.
 
ggunn said:
I'd say not. Tube amps can typically handle from X2 down to /2 in output impedance without a prob.
Click to expand...
Right, that's what I'd always thought, too. Guess I'm just grasping at straws.

I wouldn't be surprised if more than one of these 30+-year-old capacitors have gone over. Someone's got to do a pretty thorough look, I would think, to return this thing to the reliable tank that it used to be.
 
i know almost nothing

about tube circuitry

but i do know ohm's law

and if you run the same voltage

through half the resistance (impedance)

you draw twice the current

which may cause components

to heat and fail:

so running 8 ohm output

into a 4 ohm load

may be pertinent here



additionally

Tubes which are conducting too much bias current (older tube-techs would say these are "underbiased" or "biased too hot") can cause blowing fuses, excessive power supply ripple and 120Hz hum, burned out rectifier tubes and could in the long run kill an output transformer or power transformer. They overtax in general everything in the electrical path from the AC power plug to the output transformer.
Click to expand...
http://www.geofex.com/ampdbug/biasing.htm

so maybe installing the new tubes

possibly biased "hotter"

has caused another component to fail



sounds like it's time

to see the amp doctor
 
I've emailed Matamp UK to see if they have any of the old schematics for sale. I'll follow up with the U.S. shop in Spokane for the same.

I don't see dumping a bunch of money into it, because even though it's a cool old amp, it's not particularly versatile by modern standards. If I have to ship it to the shop up in Washington state the shipping will cost at least $50 each way, plus time and parts. Those guys buy Matamps so I may just try to work a deal with them.

We'll see.
 
cannabis said:
i know almost nothing

about tube circuitry

but i do know ohm's law

and if you run the same voltage

through half the resistance (impedance)

you draw twice the current

which may cause components

to heat and fail:

so running 8 ohm output

into a 4 ohm load

may be pertinent here
Click to expand...

That's true for solid state amps

which put out the same voltage

no matter what.

Tube amps are different.

Burma Shave
 
arcaxis said:
Did a search for Matamp schematics and the closest I came was for an Orange amp which presumably is a close relative. Does your amp have the AC mains voltage selector and output impedance selector? And maybe the Slave, Return, and Send jacks?

May not be exactly what you have, but it's probably pretty close.

If you notice in the power supply the circle with the N.B. in it, next to it is a 100k pot that adjusts the bias voltage to the circled N.B. in the power amp section.

This is the link where I got the schematics
http://users.telenet.be/orangefg/orange_mods.htm#_schematics
Click to expand...
Thanks, arcaxis!

I can't tell offhand how close that schematic is. My amp does have an AC mains voltage selector and an output impedance selector (actually both of these are, for lack of a better term, jumpers that you plug into the appropriately marked inputs along a circular set of holes, rather than switches).

But that is a great starting point. :)
 
The overall circuit topology looks very similar.

Edit: The link that arcaxis gave has several versions of very similar power supplies, preamps, and output amp designs. I'm hoping that the guy at Matamp U.S. up in Spokane can identify which mine is - I've taken some photos of the inside of the chassis so maybe that will help.
 
ggunn said:
Burma Shave
Click to expand...


just what i was thinkin... saw the worlds fastest indian the other night and it had a burmashave reference in it...

how far are you from zaph??? i know texas is big but i cant recall much about where fort worth and austin are... is ressurection electronics still open??? the owner bob overton is a pretty good tech...
 
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dementedchord said:
just what i was thinkin... saw the worlds fastest indian the other night and it had a burmashave reference in it...

how far are you from zaph??? i know texas is big but i cant recall much about where fort worth and austin are... is ressurection electronics still open??? the owner bob overton is a pretty good tech...
Click to expand...

It's about a 4 hour drive away. Do you mean Guitar Resurrection? Yes, they are still going, but they moved to north Austin, and being a Bubba (south Austinite) I don't get in there nearly as much as I used to.
 
dementedchord said:
how far are you from zaph??? i know texas is big but i cant recall much about where fort worth and austin are... is ressurection electronics still open??? the owner bob overton is a pretty good tech...
Click to expand...
Yep, it's about a 4-hour drive from Ft Worth to the south end of Austin.

Thanks for the tip on Resurrection. I'd rather drive the thing three hours to get it fixed than ship the damned thing all the way to Washington State.
 
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