Boogie Bias Mod - Help!

[EFrost]

Hey all. I decided to open up my Boogie Rectoverb the other day (head version) and suddenly felt the urge to add a pot and resistor in place of the single bias resistor. I've looked high and low, but haven't been able to find any schematics or websites that detail this procedure. So I beseech ye brilliant members of TGP to assist me in my quest - to find the Holy Resistor!

I've got a couple of snapshots Here (http://photobucket.com/albums/d148/boxedlight/?sc=1) of the guts, hopefully they are large and clear enough! I can take more pictures if necessary. Thanks!


Eric

[Blue Strat]

Hey all. I decided to open up my Boogie Rectoverb the other day (head version) and suddenly felt the urge to add a pot and resistor in place of the single bias resistor. I've looked high and low, but haven't been able to find any schematics or websites that detail this procedure. So I beseech ye brilliant members of TGP to assist me in my quest - to find the Holy Resistor!

I've got a couple of snapshots Here (http://photobucket.com/albums/d148/boxedlight/?sc=1) of the guts, hopefully they are large and clear enough! I can take more pictures if necessary. Thanks!


Eric


Thanks for reminding me why I don't work on Mesas ;)

[EFrost]

Thanks for reminding me why I don't work on Mesas ;)

Hahaha damn! That's pretty much why I was afriad to ask... Why can't Randall Smith just put a bias adjust knob in :horse

[Blue Strat]

Hahaha damn! That's pretty much why I was afriad to ask... Why can't Randall Smith just put a bias adjust knob in :horse

So he can sell more tubes:)

In reality, any tube dealer familiar with the Mesa bias range can sell you power tubes that will bias up fine.

[EFrost]

So he can sell more tubes:)

In reality, any tube dealer familiar with the Mesa bias range can sell you power tubes that will bias up fine.

I have seen that offered from almost all the major tube vendors, but it's just not the same as biasing it to taste or on the fly or whatever. I have a whole bunch of tubes, but I don't think any are Mesa spec'd, and I'd hate to have them all go to waste.:BITCH

[Blue Strat]

I have seen that offered from almost all the major tube vendors, but it's just not the same as biasing it to taste or on the fly or whatever. I have a whole bunch of tubes, but I don't think any are Mesa spec'd, and I'd hate to have them all go to waste.:BITCH

Since you'll have to measure bias current anyway, get a bias probe (or use whichever method you prefer) and check the tubes you have with the amp "as is". Some may work out for you.

[VintageJon]

What Mike said...

In addition I find there is often not enough negative voltage available to really make an adjustment- especially EL84 "designs". I usually get these right by adding a cathode resistor, thus making them "mixed-bias". (A phrase I coined for a combination of Cathode-Bias and Fixed-Bias.)

In EL34 and 6L6 examples you can usually find a pair or quartet that work if you spec it all out, assuming you have a large tube inventory with high plate currents. Fortunately I have the stock, knowledge, equipment to do this. A Mesa Consumer is usually nor so lucky.

For the Consumer, it's just a way to sell Mesa tubes. They are usually Sovtek but testing on the Mesa-marked examples has shown very close matching.

Don't get me started on Mesa! Everyone eventually buys at least one Mesa, but likely not another! Perhaps too much said...

-Jon

[brad347]

What Mike said...

In addition I find there is often not enough negative voltage available to really make an adjustment- especially EL84 "designs". I usually get these right by adding a cathode resistor, thus making them "mixed-bias". (A phrase I coined for a combination of Cathode-Bias and Fixed-Bias.)

In EL34 and 6L6 examples you can usually find a pair or quartet that work if you spec it all out, assuming you have a large tube inventory with high plate currents. Fortunately I have the stock, knowledge, equipment to do this. A Mesa Consumer is usually nor so lucky.

For the Consumer, it's just a way to sell Mesa tubes. They are usually Sovtek but testing on the Mesa-marked examples has shown very close matching.

Don't get me started on Mesa! Everyone eventually buys at least one Mesa, but likely not another! Perhaps too much said...

-Jon

I've never even bought one... I happen to think most of them (newer ones anyhow) don't sound so hot...

definitely too much said!

[VintageJon]

Brad- good decision.Sir!

The last good Mesa was'nt a Mesa at all but a Boogie MkII. (Fortunately I have an early Boogie Princeton Reverb.)
From this point onward it's a long and tangled tale of Lawyers and I won't go any further without first-hand knowledge.
I have discussed a couple of design problems with Mesa Engineering and I can't report anything that gives me hope...

I had a Mesa-EL84-something-or-other that I converted to Mixed-Bias and put up for sale. It sold over a year ago and I haven'y heard any problem.
since. It was OK sounding for a Mesa, but not a keeper for me.

Latest Mesa production follows Bessent's 3rd Law which states that "The amount of trouble you will have with an amp is directly related to the number of knobs on it. The more knobs, the more trouble and less satisfaction you will have with it."

-Jon Bessent

[brad347]

Jon, it may sound silly, but I judge an amp mostly by

a) looks

and

b) number of knobs.


The cooler looking the amp is and the fewer knobs it has the more likely i am to be happy with it. Seriously.

[VintageJon]

Brad,
You are follwing Bessent's 3rd Law!

My favorites have between one and three knobs.

May The Tone Be WIth You,
Jon

[scottywompas]

I'm a MarkIV owner and a fifteen year boogie user. I've never had a problem with the mesa tubes or the amps. I realize everyone's need for customization and the quest for the perfect tone but mesas are designed without a bias pot for a reason. If that's what you need move on to something else

[brad347]

reason being: so they can sell more tubes! :D

[EFrost]

Ka-Ching!:BEER

[EFrost]

If anyone is still interested, I found the bias resistors. They are the 220k ohmers in the red circle: http://i34.photobucket.com/albums/d148/boxedlight/untitled.jpg

Sorry for the cruddy quality but it's the best I can do. Anyways, I verified that those are in fact the bias resistors. Randall Smith does a neat trick with 4 bars of color on the resistors. I don't know if what he has is common, but the third and fourth bars multiply together to give you the actual value.

If anybody else owns a Recto and is interested in this mod, it's fairly simple (more simple than I imagined) with a decent soldering gun (25 watt Weller from Home Depot will work). If one doesn't like the mod, and doesn't snip the original resistors, it can be returned stock in no time.

Eric

[VintageJon]

EForst- THANKS for send though I'll have to check as 220K pair are likely isolation/voltage-divider rather than the bias-limit resistor...

Mike- looking at pics I wonder why I mess with it too! It looks just like I recall, yet I go forth as some poor fool, with big sad puppy-eyes and a sad story, has bought it and I wanna save his ass if I can!
One day I'm gonna come to my senses and stop going into the money-hole...

May The Tone Be With Ya'll,
Jon

[trdlasvegas]

Randall Smith does a neat trick with 4 bars of color on the resistors. I don't know if what he has is common, but the third and fourth bars multiply together to give you the actual value.

Eric

Randall doesn't do any trick with the colors. Those are standard Vishay 1% Metal Film resistors available from Mouser. You read the color code just any other 1 or 2 % resistor. The first three bands translate into numbers, the 4 band is the multiplier, and the fifth band is the tolerance.

The ones you are refering to are:

red - 2
red - 2
brown - 1
orange - 1000
brown - 1%

221000 ohms or 221K ohms

-Tony

[EFrost]

Randall doesn't do any trick with the colors. Those are standard Vishay 1% Metal Film resistors available from Mouser. You read the color code just any other 1 or 2 % resistor. The first three bands translate into numbers, the 4 band is the multiplier, and the fifth band is the tolerance.

The ones you are refering to are:

red - 2
red - 2
brown - 1
orange - 1000
brown - 1%

221000 ohms or 221K ohms

-Tony


Blast! Foiled again... So after all that I haven't found the bias resistors?

[VintageJon]

As I said earlier- it's hell to find and likely won't be enough bias voltage to do you any real good anyway. My "mixed-bias" mod is still recommended...

-Jon

[Mr. SD-1]

I have a couple of questions relevant to this topic...

1) What is the Mesa spec bias current for a 6L6 in a Dual Recto head?
2) How does the Bold/Spongy switch affect the bias voltage and current?

[EFrost]

I have a couple of questions relevant to this topic...

1) What is the Mesa spec bias current for a 6L6 in a Dual Recto head?
2) How does the Bold/Spongy switch affect the bias voltage and current?


Sorrry but I cannot answer that - I have a Single Rec Head and Singles don't have the Bold/Spongy switch. But I think it acts like a variac and reduces the voltage in the amp? That's a stab in the dark, hopefully someone with a DR will chime in.

BTW I kept at it and this time I'm pretty sure I found the bias resistor. Replaced it with a pot, and there is an audible difference in tone from cold to hot. If anybody is interested I can provide a pic of the *so far* right resistor.


Eric

[amper]

Bias spec for a Rect-O-Verb should be -48V. The bias resistors are indeed 220K on the schematic...but I don't own a ROV, so I can't tell ou *which* 220K resistors in the chassis. FWIW, the bias supply is spec'd at -49V before the resistors.

[EFrost]

Bias spec for a Rect-O-Verb should be -48V. The bias resistors are indeed 220K on the schematic...but I don't own a ROV, so I can't tell ou *which* 220K resistors in the chassis. FWIW, the bias supply is spec'd at -49V before the resistors.

Interesting. I was looking at some DR schematics for reference, and they also cited the 220k resistors as bias resistors, but it seems that the 220k resistors are isolation/voltage-dividers like VintageJon said. There's a 22k resistor right towards the rear of the small filter cap that I replaced with a trimpot. Adjusting the trim pot induced effects of bias adjustment - brittle sounds with higher bias settings, and glowing tubes with less bias setting.


Eric

[amper]

Interesting. I was looking at some DR schematics for reference, and they also cited the 220k resistors as bias resistors, but it seems that the 220k resistors are isolation/voltage-dividers like VintageJon said. There's a 22k resistor right towards the rear of the small filter cap that I replaced with a trimpot. Adjusting the trim pot induced effects of bias adjustment - brittle sounds with higher bias settings, and glowing tubes with less bias setting.


Eric

The Dual Recto schem I have is awfully blurred, but it appears that there are two 220K resistors in series with four (what looks like) 1.5K resistors. The 1.5K's are tied to the input grids of each tube, and the 220K's are attached to the bias supply, like in the ROV. Bias supply voltage appears to be -51V for 6L6 setting and -39V for EL34. Don't take this as gospel, though, as I said my schem is *very* blurry, and I don't own a Recto.

[trdlasvegas]

I have a couple of questions relevant to this topic...

1) What is the Mesa spec bias current for a 6L6 in a Dual Recto head?
2) How does the Bold/Spongy switch affect the bias voltage and current?

Mesa's spec for biasing a Dual Recto is -52V (6L6), -39V (EL34) at the bias supply.

The Bold/Spongy switch selects different Primary input windings on the Power transformer. I believe all the internal DC readings including the bias voltages given above are taken in the "Bold" position.

Finnaly the easiest place to install a bias pot on a Dual Recto is to remove the 22k resistor that is in parrallel with the 82k resistor in the bias supply and install a pot there. Somtimes the 22k resistor may not be stuffed on the PCB as they use this to set the fixed bias at the factory. In which case you just solder a pot into it's position.

-Tony

[slhguitar]

If you are replacign said 22k resistor with a pot, what rating of pot should you use? I have never done any mods to an amp, but am interested in this if it will afford me enough bias range to use different recto tubes with various 6L6s....

[Blue Strat]

If you are replacign said 22k resistor with a pot, what rating of pot should you use? I have never done any mods to an amp, but am interested in this if it will afford me enough bias range to use different recto tubes with various 6L6s....

The 220Ks are bias feed resistors which just about every common guitar amp uses and have virtually nothing to do with setting the bias.

The 22K is likely the one. Don't just replace it with a pot, use a smaller resistor between the pot and ground so if the pot should short, the bias voltage won't go to zero (just lower) and cook your power tubes and possibly/ultimately your output transformer.

More negative bias/grid voltages mean cooler (lower idle current) running power tubes and vice versa. YOU CANNOT BIAS TUBES PROPERLY knowing the bias voltage alone. Each pair of tubes will draw a different amount of current with the same bias/grid voltage which is why we order matched power tubes. Get a bias probe or use some other method of measuring bias current.

Jon: How many "normal" amp jobs does it take to financially recover from each Mesa job;) ?

[trdlasvegas]

Don't just replace it with a pot, use a smaller resistor between the pot and ground so if the pot should short, the bias voltage won't go to zero (just lower) and cook your power tubes and possibly/ultimately your output transformer.

I think I'd worry more about a NOS tube shorting out before the bias pot shorts out.

-Tony

[Blue Strat]

I think I'd worry more about a NOS tube shorting out before the bias pot shorts out.

-Tony

Is that a shot?;) I think you missed :D

No one knows which will go first. Using a resistor in series with the pot is good engineering. Feel free to assume any risk you like with YOUR equipment.

[trdlasvegas]

I certainly agree that a resistor in series is the "correct" engineering way, but seriously just how many bias pots have you replaced because they shorted? The odds of a tube shorting out are way higher that the odds of a pot shorting out. :angel

-Tony

[Blue Strat]

I certainly agree that a resistor in series is the "correct" engineering way, but seriously just how many bias pots have you replaced because they shorted? The odds of a tube shorting out are way higher that the odds of a pot shorting out. :angel

-Tony

For the extra 2 cents and 3 seconds it takes, why not do it the right way?

[BK-Amps]

The 22K is likely the one. Don't just replace it with a pot, use a smaller resistor between the pot and ground so if the pot should short, the bias voltage won't go to zero (just lower) and cook your power tubes and possibly/ultimately your output transformer.

Jon: How many "normal" amp jobs does it take to financially recover from each Mesa job;) ?

If you just replace the 22k pot you will indeed place your amp at risk. This 22k resistor goes to ground. If you replace the 22k resistor with a pot and rotate the pot too far, you will be buying new tubes.

Secondly, you mess up the operation of the EL34 switch as this simply adds a second resistor in parallel to the 22k you had replaced.

The best place to put this pot, without cutting traces, is right after the reverse polarity diode (probably near a blue wire) and before the pi filter filtering the bias supply. There should be a resistor there already (don't know the value). More resistance here drops more voltage giving you less negative voltage, if that makes sense...

1/2 watt pot is okay. The bias supply doesn't draw much current.

BTW, I have no problem working on Mesa's. Although, they have been consistenly producing cheaper and cheaper quality boards over the last 10 years. They are quickly getting to the point where the copper cladding is so light, just holding an iron near it will cause a trace to lift off the board!

BK

[amper]

The 220Ks are bias feed resistors which just about every common guitar amp uses and have virtually nothing to do with setting the bias.

Thanks, Mike...I was looking at the wrong part of the schematic. The first thing I looked at was the power amp section, rather than the power supply. Now I can clearly see what you mean. Looks like an 82K resisitor in parallel with a 22K and a 10K that can be switched in or out for 6L6/EL34 operation. But again, my schematics are *really* blurry, so don't take my word for it.

[VintageJon]

Mike- you never really "recover" from a Mesa job. Between the hair-tearing, hours-spent, bad PR in the end, it's just a loosing battle. Reckon you are smart to avoid them, yet I keep trying to make people happy.
(I don't have enough sense to count the money- thus people consider me the Top-Tech in Austin. Maybe I'll get an economics lesson and change my ways... As long as I can make the ends-meet I don't worry too much. It's a Fools Paradise, but it's a Paradise!)

See ya in a couple of weeks-
Jon