Does anyone know why there's a trim pot on my bias supply?

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kiyiko

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The only thing my manual says about bias adjustment is a whole section on why Mesa doesn't put bias adjustments in their amplifiers.

I've got a Road King II that sounds like ****, a manual that explains why this amp has no bias adjustments, and a god **** 200k trim pot on the BIAS SUPPLY http://i.imgur.com/ob2PnL8.jpg

Can anyone explain this one?

http://i.imgur.com/UB9OLdG.png

If it's a bias adjustment, why's it not in the manual. If it's not bias adjustment, what's it do?

I figure I'd ask you guys before I call Mesa
 
kiyiko said:
The only thing my manual says about bias adjustment is a whole section on why Mesa doesn't put bias adjustments in their amplifiers.

I've got a Road King II that sounds like sh!t, a manual that explains why this amp has no bias adjustments, and a god **** 200k trim pot on the BIAS SUPPLY http://i.imgur.com/ob2PnL8.jpg

Can anyone explain this one?

http://i.imgur.com/UB9OLdG.png

If it's a bias adjustment, why's it not in the manual. If it's not bias adjustment, what's it do?

I figure I'd ask you guys before I call Mesa
The 200K trimpot is for the EL34's only, and is shown on the schematic. A quick circuit trace would have shown you that if you are at all familiar with bias supplies.

Once it is properly set at the factory, there should be no need to fiddle with it, hence why there is no mention of it in the manual.

Why are you so appalled that you found a trimpot? ("God **** 200K trimpot").

What seems to be the issue with the amp? "Sounds like sh!t" is not very specific.

Dom
 
I'm surprised that there's a trim pot because they they have a whole chapter on why the amp doesn't have one. I've spent years thinking there's no way the amp would need bias adjustment because of what the manual told me, and I cant find any information online about how to bias it because "mesa amps have a fixed bias".

I bought this used and abused so I'm sure if the previous owner could screw up this trim pot, they probably did. There's plenty of other stuff "wrong" inside of it.

How does it sound like ****? Note decay on the distortion is extremely fizzy and crackly and just sounds appalling. Might be crossover distortion, and I've already spent hundreds replacing all the tubes to try to fix the problem, only to have it still clearly sound terrible. But if that's a bias adjustment, then perhaps the gorilla that owned this amp before me just turned it all the way down and my tubes are biased cold.

Anyways, no, I don't understand bias circuits and schematics are Greek to me. I'm just so frustrated that I have the most expensive production amp on the shelf and it's barely musical sounding right now.

How do I go about making sure this trim pot is set correctly, or will I need to pay a tech to do it?
 
The trimpot is only for the EL-34's. It should be set so there is -38V on pin 5 of the EL-34 sockets.

Did you use Mesa branded power tubes?

Remember, there is almost 500V stored in those filter caps, be careful.

Dom
 
The 6l6s are all matching mesa, but the EL34 came with another brand in the amp. I wouldn't be surprised if the previous owner screwed with this trim pot while they were screwing with the rest of the inside of it.

Came with bodged transistors, melted wires, melted relay casings, and a half unsoldered foot switch din.

I sure hope the bias is cold cold cold, cuz if this ain't my problem then I'm out of ideas.
 
kiyiko said:
I've already spent hundreds replacing all the tubes to try to fix the problem, only to have it still clearly sound terrible............ no, I don't understand bias circuits and schematics are Greek to me

kiyiko said:
The 6l6s are all matching mesa, but the EL34 came with another brand in the amp. I wouldn't be surprised if the previous owner screwed with this trim pot while they were screwing with the rest of the inside of it.

Came with bodged transistors, melted wires, melted relay casings, and a half unsoldered foot switch din.

I sure hope the bias is cold cold cold, cuz if this ain't my problem then I'm out of ideas.
I was referring to your replacements, did you use all Mesa branded tubes, preamp and power amp?

How are you checking the bias? Do you own a good Multi-meter?

What is a 'bodged' transistor? Never heard that one before.

It sounds like you are not familiar with tube amp circuits, and if changing all the tubes has not fixed your issues I recommend you take the amp to a tech. There is enough voltage stored in the power filter caps (even when unplugged) to stop your heart if you should get shocked.

Dom
 
None of the preamp tubes are mesa. I installed the toan kit from Doug's tubes with all the boxes checked. http://www.dougstubes.com/guitar-amp-tube-kits/preamp-tone-kits/preamp-tonekit-12ax7-6.html

I say bodged transistor because it was replaced poorly and they even screwed up the trace on it. It's part of the solo circuit tho so it's not related. http://i.imgur.com/QytKkZT.jpg

I have no idea why it was replaced, I think another transistor this is connected to might have been replaced too. I'd like to know if it's some kind of mod, or if the previous owner exploded it somehow...

I measured the bias with a multimeter and it was - 34.8, so i brought it up (down?) to - 38

I can't tell if it sounds better or not. When I plug my guitar into the power amp it's crystal clear, and when I plug my preamp into my recording interface, I hate the way it sounds.

At this point I can't tell if my amp has any problems or if I just hate the distortion that dual rectifier preamps produce. There isn't a mesa dealer in the state, so i cant go play a brand new amp to tell if it sounds better to my ears. It's just so fizzy
 
With all due respect I suggest you take it to a competent tech.

It seems like you may be in over your head - in addition - a good tech will be able to tell you if it sounds the way a Mesa should
 
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