Bias modification to my Mesa Boogie

[beasleybodyshop]

After swapping out the fixed bias resistor in an old Ampeg I was working on, it got me interested in converting my Mesa Express 5:50 from a typical fixed bias resistor to an adjustable trim pot. As many of you know, mesa doesn't like you monkeying around with bias adjustments, and you have to buy their matched tubes...eh... not my cup of tea any more.

I have seen somewhere on the web where someone has done this mod, but did a whole bunch of other things (B+ checker, freq snubber, etc) that i don't really care to do.

My question is this: In the attached schematic below, would i remove R55 and R54 from the bias circuit, and install a 50KL bias pot across where R54 was? Or do i only remove one of the resistors?

[Phil_S]

Looks to me like the easiest thing to do is replace R93 (1.8K) with a 1KB pot and a ~1.5K resistor in series.

The schematic says the 5:50 doesn't have R54.

What is EXPRT? It looks like a resistor in parallel to R55. You'd have to know what both are together to put the pot+R on the ground side of the divider.

[beasleybodyshop]

Phil, I imagine XPRT is something that is only on export models?

[xtian]

No, it's obviously for people who are EXPRT at setting the bias, duh.

[jestaudio]

Might be worth posting the same on Grail tone Boogie board, one of the guys on their regularly does the bias mod on Boogies, gave me the schemes for ,my Dc5 and Dc3

[Phil_S]

Phil, I imagine XPRT is something that is only on export models?

Ummm...yes. What I meant to ask, is it on the board, is it really a resistor, and if so what is its value! Have you looked in there yet? Better yet, post a picture.

[TUBEDUDE]

Better get a waiver from mr. Smith to avoid a lawsuit! ;=)

[beasleybodyshop]

Picture attached. There was only the 22K resistor. I took it out and swapped in a 50K linear pot. Fires the amp up and adjuating the pot definitely swings the negative voltage, but occasionally as I turn the pot I see a little spark pop up near where the solder lugs meet the body. I'm using an Alpha pot...whatever the standard wattage is for their guitar sized ones (1/4w?)

[beasleybodyshop]

Picture

[vibratoking]

Is R93, 15k resistor installed? It looks like it is not there? The silkscreen is obscured by the white wire you installed. I would install it if it is not already and possibly adjust its value. I would also suggest a fixed resistor from the bottom side of the pot to ground instead of connecting the pot directly to ground. The fixed resistor will limit the bias from getting too positive and overbiasing your tubes if the pot shorted out. What is the range of bias as you have it now?

I would also twist those wires to the pot for some noise immunity, but that is a small issue.

[beasleybodyshop]

Is R93, 15k resistor installed? It looks like it is not there? The silkscreen is obscured by the white wire you installed. I would install it if it is not already and possibly adjust its value. I would also suggest a fixed resistor from the bottom side of the pot to ground instead of connecting the pot directly to ground. The fixed resistor will limit the bias from getting too positive and overbiasing your tubes if the pot shorted out. What is the range of bias as you have it now?

I would also twist those wires to the pot for some noise immunity, but that is a small issue.

R93 is there, i just cant find it lol. The only resistor I removed was the 22K

What size resistor should i use on the other lug? 27K?

And what would cause the sparking? Cheap pot? 1/4W too small?

[Phil_S]

The general idea is the bracket the 22K value.

According the way I read the 5:50 schematic, R93 should be 1.8K. It really doesn't matter if it is 15K, as that just changes the math. What you have is 1.8K - cap - 22k - ground.

IMHO, your pot value is too large. It is also a good idea to use a tail resistor to ground in series with the pot. If the pot is set at 0, you still have some bias reference. So it is the combination of the pot and a resistor that should replace R55.

So, what you've got, assuming R93 is 1.8K, is a 1.8:22 voltage divider. You are going to vary the ground side of the divider. Pick an amount. Let's say you need it to swing between 27K and 15K. If your minimum is 15K, then that is the value of the tail resistor to ground. Put that in series with a 10KB or 15KB pot. This should provide an ample range of voltage on the bias supply.

The spark is bothering me. I think you have a problem with that. Get a trim pot, which is the proper thing to do that job. You can buy a small bagful of them on eBait for cheap. You'll just have to wait for delivery. Like this: http://www.ebay.com/itm/10x-RM065-Carbo ... 565a252a7c (make sure you choose the value you want). One end goes into the board. Tie the wiper to the other outer lug. Attach the tail resistor to the pot and the board.

There is very little current flowing on the bias circuit. Those 0.1W trimpots I left a link for are more than adequate.

[beasleybodyshop]

The general idea is the bracket the 22K value.

According the way I read the 5:50 schematic, R93 should be 1.8K. It really doesn't matter if it is 15K, as that just changes the math. What you have is 1.8K - cap - 22k - ground.

IMHO, your pot value is too large. It is also a good idea to use a tail resistor to ground in series with the pot. If the pot is set at 0, you still have some bias reference. So it is the combination of the pot and a resistor that should replace R55.

So, what you've got, assuming R93 is 1.8K, is a 1.8:22 voltage divider. You are going to vary the ground side of the divider. Pick an amount. Let's say you need it to swing between 27K and 15K. If your minimum is 15K, then that is the value of the tail resistor to ground. Put that in series with a 10KB or 15KB pot. This should provide an ample range of voltage on the bias supply.

The spark is bothering me. I think you have a problem with that. Get a trim pot, which is the proper thing to do that job. You can buy a small bagful of them on eBait for cheap. You'll just have to wait for delivery. Like this: http://www.ebay.com/itm/10x-RM065-Carbo ... 565a252a7c (make sure you choose the value you want). One end goes into the board. Tie the wiper to the other outer lug. Attach the tail resistor to the pot and the board.

There is very little current flowing on the bias circuit. Those 0.1W trimpots I left a link for are more than adequate.

thanks phil. I have a few fender 10Kl bias pots, going to sub one of those in with 15K to ground on the outer lug.

[beasleybodyshop]

Quick question - if I need more negative voltage range, do i increase or decrease the resistor on the bias pot to ground?

[Phil_S]

You've got a voltage divider. The higher the value on the ground side, the more the "drain" is stopped up, pushing more voltage up the line to the tubes. More = less, as the voltage is negative, if you get my drift. Moving closer to 0 Ohms on the ground side moves the bias voltage closer to 0 (hotter).