Amp Tech Thread / Ask a tech Q

[LogicalFrank]

Yeah, if you have an eight ohm tube amp, a four ohm load should be OK.  The way it works is a tube amp puts out about the same amount of power no matter what load you plug into it. So if you have too high a load, it is sort of like shoving ten pounds of shit in a five pound bag. Too low and it is like shoving five pounds of shit into a ten pound bag so it works but you're still using the wrong bag... Having no cab attached is like shoving infinity pounds of shit into a sheet of plywood. So that's my colorful simile of the day even if it's not strictly true in a technical sense.

It varies a lot by design and manufacturer though. Some Fenders you can abuse them w/ all manner of wrong impedance, low and high, and they might burn through tubes a bit faster but they'll live. Marshalls, on the other hand, are pretty notorious for being picky. For myself, I never mismatch if absolutely necessary (using a backup head at a show, &c.).

Solid state heads will put out as much power as they can dependent on load. If you plug a hundred watt amp that wants to see an eight ohm load into a four ohm load, it will try to output more than a hundred watts worth of signal and then it will overheat, maybe blow a fuse, maybe let the smoke out.

[Hemisaurus]

Never do it (unless the manufacturer says otherwise).

There are some designs that will tolerate a mismatch quite happily, some Peavey tube amps, I forget which say any impedance above 4 ohms, others will go POP quite happily. Unless you can predict which it is, or pay the damage if you are wrong, don't do it.

[VOLVO)))]

+1 on never mismatching.

[ROWDYBEER]

70's Ampeg v4 (the one with the distortion channel 1 clean channel 2)

Was sounding like shit. I play it a lot tubes had about a year of abuse so I auditioned all new tubes (pre and power).

Sounded amazing. Like a new amp. Real tight. Very good range. No mud. Very clear even with a lot of dirt.

Play a show and have a practice everything's great.

Then all of a sudden its back to sounding like shit. Very loose and muddy. Not clear. No low mid or high definition. Just loud vomit.

All tubes seem uniform in color and are in there sockets tight. I havent changed anything as far as my set up. Same guitars, same pedals, etc. And its back to the same way it sounded before new tubes.

Any thoughts. So frustrating and sad to sound like shit when you know the potential of your set up. I mean it sounded so fucking good with new tubes I need to capture that sound again. Im chasing the dragon.

[Hemisaurus]

This is either a take it to a tech, or be prepared to do a bit of work on it situation. You've done the obvious stuff? None of the tubes are red-plating?

You basically need to check if the amp is eating the tubes, you'll want to check the bias, preferably indivdually on each power tube, and also if you can, narrow down the area which is giving you the problem, pre-tubes or power tubes (swap pre-tubes one at a time, but power tubes in sets). If it's a problem in the preamp tubes, it's either you got a bad tube, a fault in the amp, or maybe it got knocked while the tubes were hot and one of the tubes is now bad.

If it's in the power section, and the bias is all A-OK, it could (hopefully) just be a bad set of tubes, happens all the time, I re-did Freightshaking Bob's amp recently, he bought a quad set of EL-34 GrooveTubes from GC, they crapped out after a couple of practices, on the day of a show, thankfully we'd kept the old tubes, and it sounded good with the old power tubes and the new pre-tubes in, so we ran with that and he exchanged the tubes later at GC. So I'd hope it's just dodgy tubes, rule that out first if you can.

If you eat a third set of power tubes, and the bias is still OK, I'd want to soak test the amp, run it hard for a good long time, whilst monitoring the current through those power tubes.

Of course this is all from no knowledge whatsoever of your amp, I'd run a full set of voltage checks on it, check the ripple on the supplies to see if your resevoir caps are going bad, etc.. etc. as well.

[ROWDYBEER]

Thanks hemi. I brought it to my tech. Hopefully he can get to the bottom of it.

[Hemisaurus]

Thanks hemi. I brought it to my tech. Hopefully he can get to the bottom of it.

Let us know what he finds, if you can, ta

[Wesogkan]

Anyone have a Matamp GT120 they've mapped the schematic on? I found a pic of one line, but none of the values are readable...

[Hemisaurus]

You've seen that one too 

Best I'd suggest is, either talk to Electric Amp, or Matamp, or look at the Orange Field Guide and guess the rest.

Are you repairing or cloning?

[Wesogkan]

Hoping to clone...

[Hemisaurus]

Hoping to clone...

Well you can get the basic ideas from the orange field guide and maybe Webers 6O100.

You can see the difference, sort of, on the schematic. The original Orange has one rotary switch, the newer Matamps (and Electric I think) use two which switch in various RC combinations to monkey with the tone, the phase splitter and power sections are pretty standard.

What are you planning to use for power tubes and transformers?

Maybe someone would be willing to supply some detailed gutshots?

[Wesogkan]

I'll probably want el34's...not sure about the transformer yet, but a big hairy one for sure.

[Hemisaurus]

Well a quad of EL-34's you could get away with any 100W output transformer really, though some like more iron in their core (the orangefg guy wound a 50W tranny on a 100W core). You could even get the Weber Orange kit and just modify the input section, use the standard Baxendall tone stack (bass / treble), and replace the mid pot with a second rotary switch like the Matamp has. I don't know if it's cheaper to buy their kit or collect the parts, or scrounge up the parts used.

https://taweber.powweb.com/store/kits_60b.htm#6o100

They just use the same 100W tranny for all their 100W heads I think.

[TheWolf IsLoose]

Freakin' sweet...

[VOLVO)))]

Freakin' sweet...

Pics or it didn't happen.

[Hemisaurus]

Freakin' sweet...

Pics or it didn't happen.

Calm down, I doubt even John McNeece can wire up a tube amp in an afternoon, I think he was maybe just expressing excitement

[VOLVO)))]

Freakin' sweet...

Pics or it didn't happen.

Calm down, I doubt even John McNeece can wire up a tube amp in an afternoon, I think he was maybe just expressing excitement

ACTUALLY, HE CAN! haha, John is my amp tech in town. I'm going to Atlanta to see Loss tonight. hate me.

[Hemisaurus]

Nah, I just feel sorry for John, having to deal with your collection of Beta's

[VOLVO)))]

He hates them.

I love them so much, though. They rarely go down anymore.

[hayseed]

Not sure if this is the proper thread but you guy s seem to have a great deal of knowledge on these things. Sorry if i posted in the wrong spot.

I have a strange buzzing going on with my cab. I have an older Sunn Sorado cab that i recently dropped 2 EV SRO 15's into. Any way when i play it, i get a strange buzzing/farting sound from the top speaker that will increase in volume as you turn up the volume on the head. I took the back off and examined the speaker and found no holes. Tried it again and NO NOISE! Yesterday i plugged in and the noise is back.

Did i wire it up wrong? Are the speakers to big? Help me Hemi..you are my only hope!

BTW i am using a Sunn Coli 300.

[Hemisaurus]

It's probably a loose something, rather than a broken something. Old cabs are fun that way, not only do you have to make sure that the speakers are nice and tight in the cab (and hopefully you have some kind of gasket on the mating surfaces, if not use some weatherstripping?) you have to go round the cab and check that the panels are still nice and tight against each other. If you can get someone to play through the cab, whilst you go around pushing where it seems to rattle, 'til you find your buzz.

[VOLVO)))]

Loose mounting holes, loose corners, loose handles, glue joint gae up the ghost
... 90% chance it is something loose.

[hayseed]

I will check that out and get back to you guys. Thanks. This is a rear loaded cab, should i use some kind of gasket for the speakers?

[hayseed]

Problem identified! The center dust cover on the speaker came unglued and slide beneath the grill cloth and the speaker. My 5 yr old daughter was "helping" me and she noticed it rattling inside the grill cloth. What do i use to re glue it and will it hurt to play with it off?

[beardofcthulhu]

A dust cover is just a dust cover.  I had an Ampeg 215 that had NO covers.  Both coils were completely exposed, but it didn't affect the sound whatsoever.