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

Zaphod B

Zaphod B

Raccoons-Be-Gone, Inc.
I decided to re-tube my Matamp GT 100 since one of the old tubes had become micophonic.

This amp uses 2 12Ax7 premap tubes and 4 EL34 finals. I ordered Sovtek 12AX7's (I was looking for an earlier breakup and this is what was recommended) and a matched quad of JJ EL34s. I ordered the tubes from Antique Electronic Supply and the salesguy seems knowledgeable.

Anyhow, old tubes out, new tubes in. I have no idea if this amp is supposed to require re-biasing with new tubes but the last time I did this with non-matched garden-variety Sylvania tubes the amp sounded great and ran fine.

Hooked up the speaker cab, plugged in the head, powered up and let it warm up. Plugged in the Les Paul and the amp was noticeably louder. The tubes took a good five minutes to warm properly and clean up. Everything sounded good.

I noticed that all 4 of the final tubes were lit up like a fucking barbeque grill. I don't recall this phenomenon from the last set, but I may not have ever really looked. The head was generating a lot of heat but it usually does.

I decided to let it idle for a while to see what would happen. I turned the preamp drive all the way down, as well as the bass boost and overall volume control. The final tubes were staying well lit up, and it kind of looked like a blast furnace back there.

It took 15 minutes for the pop. No smoke or fire. The finals were still lit equally but the amp was dead silent (none of the normal low-level hum) and produced no musical output. I uplugged it and let it cool, and checked the two fuses. The mains fuse was OK but the other 1-amp quick-blow fuse had blown.

I hope I haven't really fucked it up.

I plan on replacing the blown fuse and putting the old tubes back in to see if there is any life left. If it works as it did before, I will first try the new preamp tubes with the old power tubes, then the old preamp tubes with the new power tubes to see if I can isolate it a bit.

I think I'll be visiting Mr. Amp Repairman in any case.

Hold me. :o
 
I'm sure you haven't fucked it up. That's what fuses are there for.

I'm not positive on this, but seems logical to me - The fuse could have blown because the amp wasn't properly biased for the ratings of the new tubes. Maybe?
 
Guitarer said:
I'm sure you haven't fucked it up. That's what fuses are there for.

I'm not positive on this, but seems logical to me - The fuse could have blown because the amp wasn't properly biased for the ratings of the new tubes. Maybe?
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That's certainly one possibility. I don't know what the bias voltages are supposed to be, or even where the test points are.
 
any time you replace power tubes, you should have the amp rebiased. biasing will set/reset the amount of voltage going to the tubes. in this case, i'd suspect that it was biased too hot.

it's good that the fuse popped, that's what they're supposed to do.

don't turn it back on, and take it to your amp man to get checked out. i bet it just needs to be rebiased.

EL34's have a LOT of voltage going through them, and they *are* going to look like that, at least to some extent.

bottom line, just go get it checked out.

cheers,
wade
 
mrface2112 said:
any time you replace power tubes, you should have the amp rebiased. biasing will set/reset the amount of voltage going to the tubes. in this case, i'd suspect that it was biased too hot.

it's good that the fuse popped, that's what they're supposed to do.

don't turn it back on, and take it to your amp man to get checked out. i bet it just needs to be rebiased.

EL34's have a LOT of voltage going through them, and they *are* going to look like that, at least to some extent.

bottom line, just go get it checked out.

cheers,
wade
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Good advice, Wade. I have a call in to the amp guy that my preferred guitar repair shop uses.
 
Sounds like the bias was wrong and the tubes "red plated". There is a really good chance that the tubes are fried.
 
Shit, the local amp guy needs a schematic and I sure don't have one, and there is none to be found on the 'Net that I can see.

There's a shop in Spokane, Washington that specializes in these things but I shudder to think what it would cost to ship the thing there and back. :eek: I hesitate to call the guy and ask him for free help. Maybe he'd sell me a schematic.
 
Zaphod B said:
Shit, the local amp guy needs a schematic and I sure don't have one, and there is none to be found on the 'Net that I can see.

There's a shop in Spokane, Washington that specializes in these things but I shudder to think what it would cost to ship the thing there and back. :eek: I hesitate to call the guy and ask him for free help. Maybe he'd sell me a schematic.
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Did you try calling Antique Electric Supply back? I've not had any personal dealings with them, but I've always heard good things. They may be able to give you the correct bias and/or get you a schematic. From what I've seen they seem to have a good collection of old schematics. I've not heard of the amp that you have, but it's very possible that it is a clone of some standard circuit. They may be able to cross reference it and get you the standard schematic for that circuit.
 
do you ever get to Houston? (closer than Spokane, anyway) Power Audio Service would be good to call -- ask for Hoss or Pete -- PM me for the number, if that works.

Matamp amps are fairly new British made amps?
 
Can't hurt to ask,.. what can he do??? say no.......

I agree with ocnor,.... except I think you were saved by the fuse...

I had an amp do that, turned out it WAS too hot on the plates, and required a higher value resistor to bring down the voltage a bit....

but that was with an EL84 based amp,..

Plate voltages of the bigger EL34 are similar and will be good at about 380-400, altho... if you do go in there yourself, BE VERY CAREFUL those kind of voltages can kill you.....test what you got at the socket with a vtvm. if it is over 400 volts,... your power transformer needs some resistance at that point.

a 6L6 runs at about 350-360 peak, and a 6CA7/EL34 is hotter than that.... but not by a lot, they typically run in the range stated above, between 370 and 400 volts. any hotter than that, and they will fry at anything approaching 450 volts. especially the new crap they make now,... just does not hold up under anything other than 'Normal' operating voltages.stress them just a bit and they give it up.

Take it to a tech, or if you have some knowledge, test the voltages at the socket, I'll lay odds it's a bad resistor, and yes you prob need new tubes again.
 
Topcat53 said:
Did you try calling Antique Electric Supply back? I've not had any personal dealings with them, but I've always heard good things. They may be able to give you the correct bias and/or get you a schematic. From what I've seen they seem to have a good collection of old schematics. I've not heard of the amp that you have, but it's very possible that it is a clone of some standard circuit. They may be able to cross reference it and get you the standard schematic for that circuit.
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That's a good idea, I'll give 'em a call.
 
antichef said:
do you ever get to Houston? (closer than Spokane, anyway) Power Audio Service would be good to call -- ask for Hoss or Pete -- PM me for the number, if that works.

Matamp amps are fairly new British made amps?
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Thanks for the Houston recommendation.

No, this is an amp that was made somewhere between the late '60s and mid-'70s. They did start manufacturing the Matamp name again with the old designs as a starting point but their circuitry is not the same.

JCH said:
Some of the old Orange amps are identical to the Matamps of the same period.
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Correct. All of the sources that I can find on the 'net are showing Orange amps whose physical chassis design differs considerably from my amp (even though the front panels look close) so I don't think it's safe to assume much about their circuitry. And schematics for old Oranges are damned few and far between as well.
 
I do plan on putting the old tube set back in and firing it up, unless you guys think that's a stupid idea. At least that might tell me if the problem is (or has become) more fundamental than bias voltages.
 
the odds of a "thermaled" tube still being good is about 50/50 so i hope you luck out there... it's really hard to repair over the net especially so without a schemo... i've seen lots of mis information in these type threads in general and this one as well... so lets see if we can clear some of it up... the HT voltage also historicly called the B+ is the primary operating voltage and is given as an operating range for the particular tube... an el34 (also known as 6ca7 and kt77)can operate up to @500v and according to the data sheets that i have (and are not particularly good for this tube)would require @-44v of bias...so if the B+ of your amp is less than 500v it will take less bias... the hard part is to figure out how much less and there's no easy answer.... so what is bias???? bias is a neg voltage applied to the input of the tube in order to turn it off... yep turn it off... so not enough bias and the tube thermaled (went cherry)... there are basicly 2 methods of setting bias... the first and oldest is to put a sine wave into the amp and look for crossover distortion... what this looks loke on an o'scope is a flattening out of the sine wave as it getts to the zero crossing point on the display... make sense????? the second is to use whats called a bias probe... this little jewel looks like a male and a female socket with wires coming off that goes to you multimeter and the current draw of the tube is read directly from there... you can build your own bias probe... i built mine... and ther's also a modification you can do to an amp to build it into the circuit... maybe i'll describe it sometime but do at your own risk... using the bias probe method of adjusting you need to know waht your B+v is and then you read from a list based on what the power of the amp is supposed to be indexed against the # of tubes...a 4tube 100 wt amp with a B+ of 500v for instance requires a bias current (note current)of 25 ma/tube... adjusting the bias is done by changing the resistence thats inlinefrom the bias circuit... on some amps it's done with a pot... on others you have to remove a resistor and dick around with it till you find the right value by substitution (often this can be modified as well to a pot)... so now you know why ya should take it to someone who knows something about them and/or do your homework before trying it yourself....
 
dementedchord said:
... so now you know why ya should take it to someone who knows something about them and/or do your homework before trying it yourself....
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Thanks, demented, great info. Not that I needed any more convincing, but you've got me doubly convinced.

I was not going to try to dick around with the bias myself. I just hope I can find someone in the D / FW area that knows how to deal with this thing.

I always have the option, if the thing still operates with the old tube set, of just selling it and using the money towards a different amp or another guitar.
 
Zaphod B said:
I ordered Sovtek 12AX7's (I was looking for an earlier breakup and this is what was recommended) and a matched quad of JJ EL34s.
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Were the JJ tubes EL34 or E34L? I've read that the E34L's can toast the screen grid resistors in some amps. I didn't think to ask you this before.
 
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