Fixed bias using PT with no CT ?

[dude]

I want to change my cathode biased, 6V6 18 watt lite to fixed bias.

The PT I'm using has no CT, I'm using a bridge R and getting 300VDC for a B+ 

Is it possible to use this PT and change to fixed biased?

I can't find any info as most schematics I've found have a CT or cathode bias .

A schematic of the PT wiring would be useful, the bridge R is producing about 305vdc B+, where would I tap the bias supply from?

Thanks in advance,
al

[sluckey]

Look at the bottom of page five in this pdf...

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[kagliostro]

I was wondering if back biasing is possible with Full wave bridge rectify

http://www.aikenamps.com/index.php/what-is-back-biasing



assuming the negative of the bridge as a virtual CT



Franco

[dude]

Look at the bottom of page five in this pdf...

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

Thanks, I see on page five the full wave bridge with a bias tap, for a Marshall JCM900 and some Ampegs.

I guess I'm looking for a negative voltage in the range of a Black face Fender Deluxe.

Franco, I did check out Aiken's  back biasing but I thought the the neg. bias voltage was too at -9 to -13, I guess  I could change some values to increase that.

thanks,
al

[sluckey]

I guess I'm looking for a negative voltage in the range of a Black face Fender Deluxe.

The circuit I pointed you to will do that. It's a stable and reliable circuit. Use the exact component values.

I would not bother with 'back bias' because the negative voltage will be dependant on the B+ current and will probably fluctuate between idle and full tilt.

Franco, your filter caps are drawn reverse polarity.

[kagliostro]

your filter caps are drawn reverse polarity

Thanks Steve

I apoligize for the mistake

Now I revised the schematic and the cap are connected in the right way (positive to ground)

---

I didn't post the link with the intention you follow that way of achieve the negative voltage, it was more as for connoissence

Franco

[dude]

Steve,

In your scrape book, pg 5 at bottom what is the .047 cap due? And the 56K to ground? These are on the ac leg before doide. (Marshall JCM900)

On Doug's Fender bias pg. he has the basic same layout but has no caps or R before the diode...?

Can't learn unless I ask, thanks for posting.
al

[sluckey]

In your scrape book, pg 5 at bottom what is the .047 cap due? And the 56K to ground?

You must have these two components for this circuit to work. You wanted to know how to get a negative bias from a PT with no CT on the HT winding that is feeding a bridge rectifier. Well, you must use this EXACT circuit.

On Doug's Fender bias pg. he has the basic same layout but has no caps or R before the diode...?

You talking about this page?

     http://el34world.com/charts/Biascircuits.htm

There is nothing on that page that deals with getting negative bias voltage from a PT with no CT that feeds a bridge rectifier. If this is not the page you are talking about, please post a link to that page.

[dude]

It was the layout, schematic at the bottom of that pg.

Basically the same thing. Doesn't show were the bias supply comes from but I assume it's from a bias tap on the PT since it's a Fender schematic and not the ground of a  bridge Rectifier.

Just trying to understand technically what the .047 cap and that 56K actually do in this circuit.

I assume I can change this "fixed" bias to adjustable by adding a pod like in Doug's drawing at the bottom of that page. I think he wires the pod like a variable resistor....? Or another suggestion, I'd want it adjustable.

 http://el34world.com/charts/Biascircuits.htm  drawing at bottom of pg.

thanks,
al

[Willabe]

Doesn't show were the bias supply comes from but I assume it's from a bias tap on the PT since it's a Fender schematic and not the ground of a  bridge Rectifier.

No, it's taken from off of 1 side of the rectifier tubes AC. Look at link below;

http://el34world.com/charts/Schematics/files/fender/Fender_Princeton-AA964-schematic.pdf

Just trying to understand technically what the .047 cap and that 56K actually do in this circuit.

That's not a .047 cap, it's a 47uF cap for filtering the ripple caused by rectifying the +AC to -DC. (Notice the SS rectifier and cap are backwards to make the bias voltage negative.)

I don't see a 56K anywhere in any of the bias circuits on the link you gave. But there's 2 R's and a pot to form a voltage divider, pots to make it adjustable.   


                   Brad     

[dude]

Look at this link of Steve's scrapbook, pg 5 at the bottom, bias of a Marshall JCM900 with the bridge and no CT



http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


See the .047uf small non-electrolytic cap and the 56K to ground...? The small cap is in series and the 56k grounded before the diode

al

[Willabe]

Ok, I see it now.

That type of B+ take off is coupled from the high voltage B+ with a cap. Just a different way to do it.

The 56K is bleeding off/knocking down some of the high B+ voltage before the SS rectifier. (Just a guess, they might have put it in to make it a little easer on the SS rectifier?)


                    Brad       

[sluckey]

Doug's circuit is a generic bias circuit that's meant for a PT that has a center tap. IT WILL NOT WORK WITH A PT WITH NO CT THAT FEEDS A FWB.

Here's the circuit from my scrapbook. It always had a bias adjust pot. Maybe you missed it? If you want to derive negative bias from a PT without a CT that makes B+ with a FWB then you must use this circuit. It's a simple circuit. The .047µF cap and 56K resistor between the PT and the diode are required.

[sluckey]

The 56K is bleeding off/knocking down some of the high B+ voltage before the SS rectifier.

There is no high B+ before the diode. The cap ensures that.

[Willabe]

The 56K is bleeding off/knocking down some of the high B+ voltage before the SS rectifier.

There is no high B+ before the diode. The cap ensures that.

My mistake. I meant high AC voltage. (B+ is DCv     )


                   Brad 

[Willabe]

Doug's circuit is a generic bias circuit that's meant for a PT that has a center tap. IT WILL NOT WORK WITH A PT WITH NO CT THAT FEEDS A FWB.

Sluckey's correct, I'm wrong, sorry for the confusion. It has to be capacitively coupled because your PT has no CT and is using a SS FWB.

(It was my mistake. I thought dude was talking about a CT PT when he referenced Doug's link.)


                    Brad 

[dude]

Doug's circuit is a generic bias circuit that's meant for a PT that has a center tap. IT WILL NOT WORK WITH A PT WITH NO CT THAT FEEDS A FWB.

Here's the circuit from my scrapbook. It always had a bias adjust pot. Maybe you missed it? If you want to derive negative bias from a PT without a CT that makes B+ with a FWB then you must use this circuit. It's a simple circuit. The .047µF cap and 56K resistor between the PT and the diode are required.

Yeah, I see it. When an arrow is drawn threw a pod does that mean that pot is wired like a variable resistor?

In your schematic the pot is wired with only two tabs on the pot, so it's like a variable resistor, correct?

Sorry for my ignorance.,
al

[sluckey]

Yes, it's a variable resistor. Only need to connect the wiper and one end of the pot. I prefer to jump the wiper to one outside lug just for a safety net in case the wiper fails. However, in this circuit if the wiper does open then the bias voltage would simply go to some max value and just bias the output tubes colder, rather than go to zero volts and melt them.

I really need to redraw that pot. The arrowhead is really hard to see.

[dude]

Quick question

The two E caps between the 15K divider R for the neg bias , I have two 47uf @100v but most use 10uf, I don't see any harm in using the 47uf's @100v. Fine...?

A quick explanation to determine what valve is best here would help me in the future builds.

Thanks in advance,
al

BTW, Steve I sincerely appreciate your time to explain the reasoning behind your help, rather then just have it work I can understand "why".
Thanks again. 

[sluckey]

I prefer 47 over 10 because of the additional filtering.

[HotBluePlates]

See the .047uf small non-electrolytic cap and the 56K to ground...?

That type of B+ take off is coupled from the high voltage B+ with a cap. ... The 56K is bleeding off/knocking down some of the high B+ voltage before the SS rectifier. (Just a guess, they might have put it in to make it a little easer on the SS rectifier?)

The 0.047uF and 56kΩ are an a.c. voltage divider. You could make such a divider with 2 series caps instead of a cap & a resistor, or with 2 resistors.

The PT secondary has 60Hz a.c. The reactance of the cap at 60Hz is:
1/(2*Π*60Hz*0.000000047F) = ~56kΩ.

So the cap and resistor divide the raw secondary a.c. voltage by half. If you started with a 320vac secondary, you will have 160vac input to the bias rectifier. As you have something more like 215vac, you'll start with about 107vac to rectify for bias voltage.

Assuming you don't get enough bias voltage when you're done, you can raise the value of the resistor to get more raw a.c. input (or raise the value of the cap, which lowers its reactance at 60Hz, but 0.047uF @ 600v is probably the largest convenient value for you).

The two E caps between the 15K divider R for the neg bias , I have two 47uf @100v but most use 10uf, I don't see any harm in using the 47uf's @100v. Fine...? 

If I'm guessing correctly at your PT secondary voltage, then the peak a.c. input to the rectifier is 107vac * 1.414 = 151v peak. I'd be looking to have at least 150v rated bias caps.

47uF is fine. It should suck a little more current through the rectifier at first (but no where near the allowable current for a 1N4007), take a little longer to fully charge than a smaller cap, and take a little longer to discharge than a smaller cap. None of these seem like serious considerations in your amp.

[dude]

Thanks Hot Blues Plate, good stuff to know.

My B+ is low at 305 vdc, I don't know the math at this point to know if the 100v bias caps are high enough...? but hopefull with this low B+ they are...?

Also:

Isn't there a mistake in this layout in the bias circuit?

The two bias filter caps are marked wrong, the neg should be the positive ....?

There backwards, correct? 

I know I have a different circuit then this layout which has a CT tap. I followed your schematic bu twas lookin gat this to learn and think there is a mistake. The caps would explode the way it's wires in this layout of Marks.

al

[sluckey]

There backwards, correct?

correct

[HotBluePlates]

Isn't there a mistake in this layout in the bias circuit?

The two bias filter caps are marked wrong, the neg should be the positive ....?

You sure you want to convert to fixed bias? 

I'm only kidding. First, the 0.047uF cap is connected directly to the PT secondary; it does not see any d.c., only the a.c. voltage of your PT secondary. Be sure to attach it where a secondary wire attaches, not at the d.c. output of your bridge.

So what is the a.c. voltage of your PT? I have guessed it to be about 215vac based on the numbers you gave for rectified B+. The resistor and cap knock that down by half, or 107vac = 151v peak. The rectifier and filter will knock this down some more and your bias voltage won't be nearly as high, but it's good form to rate filter caps for all the d.c. they could see under any circumstance.

The bias is a negative voltage. The diode is backwards of the diodes in your B+ rectifier (the "line" in the diode goes toward the most-positive voltage), so where the B+ rectifier outputs a positive voltage, the bias rectifier outputs a negative voltage. Therefore, the bias filter caps have the + terminals connected to ground, and the - terminals connected to the rest of the bias circuit.

The caps would explode the way it's wires in this layout of Marks.

There are a lot of errors related to bias in the layout you attached. If nothing else, it is a completely different bias circuit and not what Sluckey drew.

[dude]

 

There are a lot of errors related to bias in the layout you attached. If nothing else, it is a completely different bias circuit and not what Sluckey drew.

Yeah, I know the circuits are different, I used a basic 18 watt super lite schematic that I had but changed it to 6V6s. I used some caps and resistor values from Mark's 6V6 plexi. The amp sounds nice but a little too buttery and brown tone-ish to me. I wanted to get a little more bite and clarity and thought fixed bias might be nice since a lot of my small amps are cathode biased.

Each leg of the ac is 125v to ground and together they are 252v. The B+ right off the FWB is 311vdc, I have a 150 ohm sag R so I get 305 from there.

This amp was a VC 20 Crate, way too much gain, I did a lot of things to get it to sound better (all from basically you) but it was useless. I took your advice and gutted the thing and put this 18 watt Super Lite w/6V6s and got close to plexi valves.

I have a schematic of an Allen amp I build that has one a single filter on the bias 100uf/100v, his PT is 340-0-340, so I figured 100v on the bias cap and I'm good to go...?

I appreciate learning all I can, thanks for sharing.

al

[HotBluePlates]

... together they are 252v. The B+ right off the FWB is 311vdc ...

Following the previous explanation as a model, 252vac/2 = 126vac. Peak voltage of the 126vac input to the bias rectifier is 126vac * 1.414 = 178v peak.

I have a schematic of an Allen amp I build that has one a single filter on the bias 100uf/100v, his PT is 340-0-340, so I figured 100v on the bias cap and I'm good to go...?

And the amp didn't have a bias tap? That would be a shock, as Allen builds mostly Fender-style amps which have a 50vac bias tap.

50vac * 1.414 = 70.7v peak. A 100v bias filter cap is appropriate, because it's larger than any likely d.c. that could get into that circuit.

In light of this, caps in your circuit should be as high as 200v, but I'm betting even 150v caps will work. Their lifespan will be reduced compared to 200v rated caps, though none of them will probably see 178vdc for more than an instant.

[dude]

Now I see the picture.

Yes, Allen's amps have a 50v bias tap, not 125v like i have because of the FWB and no CT .

So in reality I need a 200v cap there?

You said: "So the cap and resistor divide the raw secondary a.c. voltage by half. If you started with a 320vac secondary, you will have 160vac input to the bias rectifier. As you have something more like 215vac, you'll start with about 107vac to rectify for bias voltage."

I have 250vac, or 125v at one side of ac for bias voltage, much more than 50vac.

I don't have any caps at the moment that are 150v or even 200v.  I have larger voltages but they're much  too big to fit in this small crate chassis I converted.

Could I just use one E cap for a filter after the 15K? I have one 10uf @450...?

Or could I use the 10uf @450 then 15K then the 47@100v?

Both these caps are small.


 

[HotBluePlates]

Could I just use one E cap for a filter after the 15K? I have one 10uf @450...?

Or could I use the 10uf @450 then 15K then the 47@100v?

The 2 caps with a dropping resistor (15kΩ) between makes a better filter than a single cap.

You could use what you have. Or you could buy a handful of 47uF-100uF @ 100v caps. They're fairly small, and 2 could be placed in series for a 200v cap at either 23uF or 50uF.

If you honestly have everything except these caps to make a go of the circuit, maybe look at a local electronics place. 100v caps may not be common, but they're probably available. If you have to buy anything else, you might as well add a few of these. Hoffman has them for a buck or 2 a piece.

[sluckey]

You don't need a 200V cap. Just use your 100V caps. Both of these amps do...

     http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm900_21xx_25xx_100w.pdf

     http://el34world.com/charts/Schematics/files/ampeg/Ampeg_g15_gemini_ii.pdf

The .047 needs to be a high voltage class X cap.

[dude]

You don't need a 200V cap. Just use your 100V caps. Both of these amps do...

     http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm900_21xx_25xx_100w.pdf

     http://el34world.com/charts/Schematics/files/ampeg/Ampeg_g15_gemini_ii.pdf

The .047 needs to be a high voltage class X cap.

I have a Mallory 150 series (yellow) 630v .047, don't know what a (class x cap) is. I assume the 56K and 47K are 1/2 watt...?

The Marshall is solid state and a little hard to follow for me, but the Ampeg uses the same schematic you posted just a higher value bias range I assume for the more negative voltage for the 6L6s.  Also doesn't have the 15K divider and extra cap. I know the double filtering is better.

Doesn't the .047 cap block voltage... (ac and or dc ?)  I'm just a little confused on the .047 cap and the 56K to ground, that .047 gets hit with a 125vac before the diode that changes the voltage to dc, true?  And the 56K to ground before the diode bleeds ac voltage to ground...?

Hot Blues Plate's rating of 171v on the caps seems he means for an instant on start up, I assume. (extra safety)

I'll use the two 47uf@100v, just let me know if 1/2watt R's are Ok. 

I guess I can wire things up and take voltages to understand more. Knowing what to do and why are two different things, if I know why I won't have to ask questions repetitively, well hopefully

al

[sluckey]

Class X is just a rugged cap made to work on high AC voltage circuits, usually across the mains input. They have a high AC voltage rating.

The .047 cap in this circuit works exactly like a coupling cap between tube stages... It passes AC voltage and blocks DC voltages. The cap and 56K resistor work together as a voltage divider to provide a proper level of AC voltage to the diode rectifier.

The Marshall bias circuit is on page 2 of that pdf and can be a little hard to find. That's why I originally pointed you to my scrapbook. Here's a cutout of the Marshall circuit. Look familiar?

You have all the info you need to build this circuit. I would recommend leaving your cathode biased 6V6 circuit intact. Build this circuit and test it without actually connecting the bias output voltage to anything. Play with it. Get the range of the bias pot to swing from about -30 to -40 volts. When you are satisfied with the operation of the bias circuit then remove the ground from the junction of the 6V6 grid resistors and connect the bias output to the junction of the 6V6 grid resistors and short the 6V6 cathodes to ground.

[dude]

Yes, I see the bias circuit now in the Marshall 900, I was looking for the FWB but only saw the bridge for the SS preamp.

Yes, your idea makes a lot of sense wiring up and fooling with the fixed bias getting different voltages and see what resistor changes do..

Doing the change the way you suggested, I could even keep both cathode and fixed bias on a switch for a brown or black face tone.

This post has taught me a lot more on biasing then I "understood before", thanks. Even the math from HBP, but I'd have to read up on that a bit more.


That Mallory cap, 150 series is rated 630vdc/220ac, so I assume that it can handle up to 220vac and will work fine on 125vac. Probably should go to the 1000 vdc/260ac

Thanks,,
al

[sluckey]

Yes, I see the bias circuit now in the Marshall 900, I was looking for the FWB but only saw the bridge for the SS preamp.

The bridge is on the same page as the bias circuit. But it's even harder to find because it's not drawn like the 'diamond' we like to see. It is D2, D3, D4, and D5 and sits just above the PT.

Doing the change the way you suggested, I could even keep both cathode and fixed bias on a switch for a brown or black face tone.

That will give you an idea of the differences between the two modes. I did that on my November amp. I never could tell enough difference to bother with. Since then all the small amps I've done have been cathode bias just for simplicity.

Doing a Google search for "class x capacitor" will yield a good/simple understanding within the first couple hits.

[mister ed]

In Sluckeys' Ampeg/Marshall bias circuit, what is the value and voltage rating of the cap that spans the rectifier diodes... and is it absolutely necessary ?

[sluckey]

In Sluckeys' Ampeg/Marshall bias circuit, what is the value and voltage rating of the cap that spans the rectifier diodes... and is it absolutely necessary ?

That's the first B+ filter cap (sometimes called reservoir cap) and it is absolutely necessary. Value is typically 50 to 100µF and voltage rating greater than 1.414 times HT voltage.

[PRR]

It is not reasonable that a 6V6 with a 305V supply needs over 30V bias to shut-down. Allow for available-parts, maybe 45V at the second bias filter cap.

The first cap is on the high side of a 15K+10K+47K divider, so could be 57V.

There won't be any huge surges because all of this comes through a 220K resistor.

But allow 20% for "unknowns".

I can't see a need for more than 68V rating on the bias filter caps. Specifically: if he is anywhere near 68V, even 57V, even really 45V, at these caps, his 6V6s will be biased stone-cold, and he'll have to shim the bias down to avoid a hacking cough at low-low output.

And IMHO 6V6 at ~~300V with any sensible load do not need fixed bias. Fix-bias 6V6 is for OVER-volted amps, 350V and 400V supply, which need low idle current to avoid melting.

[sluckey]

There won't be any huge surges because all of this comes through a 220K resistor.

What 220K? The PT HT connects directly to the .047µF cap in this circuit.

[PRR]

Obviously I am totally confuse which plan is being discussed.

> the .047µF cap in this circuit.

Still a very significant impedance.

[mister ed]

In Sluckeys' Ampeg/Marshall bias circuit, what is the value and voltage rating of the cap that spans the rectifier diodes... and is it absolutely necessary ?

That's the first B+ filter cap (sometimes called reservoir cap) and it is absolutely necessary. Value is typically 50 to 100µF and voltage rating greater than 1.414 times HT voltage.

  I see it now.. I was confusing that cap with the one seen spanning the HV secondarys on a full wave bridge.. sometimes a .01 @1KV.. still, what is the function of this .01 cap ?

[dude]

Seems to work fine Slucky's way, used exact values from his scrapbook. Checked wiring a few times tested things and turned the power on and it biased right up fine.

Only problem is the 6V6's I have in there now are a little cold and I'm out of range on the pot.

I have two 1%, 1 ohm R's on 6V6 cathodes to ground and highest I get is 17 mA on one tube and 16.5 mA on the other. I'd like to get more range as I know these 6V6's are in the middle of a hard tube so others I have may be a problem. I'd like to get around 22mA/24mA as long as no red plating.

B+ now is 320vdc, was 305 with cathode biased  I like higher the B+

I assume I'd have to raise that 47K to ground off the variable resistor pot, can you give me a rough guess from this info on this post of the value I should try?

JUST WANT TO ADD THIS:  With the amp on and the volume up past halfway on the amp I get a signal to the guitar even with the guitar's volume down all the way...? Amp up 3/4's the way it's pretty loud with the guitar's volume down all the way (off). The guitar does not do this with other amps?

Thanks,   

[sluckey]

Decrease the value of the 47K bias range resistor to bias hotter. What value pot are you using?

[dude]

I had a small trim pot from a board  that Doug made years ago that I took out back then and replaced with a pot on the outside of the amp.

So this trim pot is a 50KL trim pot, turns about 3/4 of the way. I just put another set of 6V6s in and was able to get 21.5mA at the same bias setting. Sounds very good, seems like more headroom. I have 100K grid resistors not 220K to the the bias, pretty much like Mark Huss's little 6V6 plexi he made years ago.

So, maybe a 37K range R , I have a few?

That volume thing with the guitar is weird. When I turn the volume down on the guitar (off), there's still a little signal when I hit the strings but it's getting weaker. Sometimes it disappears and then comes back, scratchy noise going and coming. I think it's something in the wiring of the fixed bias (cold joint). One of the bias filters, the second one, the negative lead broke off at the cap (the only two I had). I soldered it back and it seemed to hold but it broke again putting it in, fixed it again. But once they break that close to the cap I think their done.

Your experience...?

Could this be giving me an intermittent filtering on the bias? Because it's gets better and then goes away only to come back scratchy and intermittent only when the amp is on high volume and on of course.

Any other idea what could cause a signal to come threw a guitar with the volume off, amp volume way up...?

Thanks so much, I'm really digging this amp but I'm out in the shop so I can't really put it through the cycle yet.

al

[Willabe]

It sounds like the volume pot on your guitar is either dirty or is on it's way out. Try a different guitar through the amp and see if the problem is still there.

The filter cap with the broken negative lead, I'd throw it out and put in a new cap.


                    Brad   

[dude]

It sounds like the volume pot on your guitar is either dirty or is on it's way out. Try a different guitar through the amp and see if the problem is still there.

The filter cap with the broken negative lead, I'd throw it out and put in a new cap.


                    Brad   

Thanks, it's fine on other amps. I need to check my bias circuit since that was the last change. Other amps, guitars work fine. That fixed-broken cap could be the culprit, I need to replace that before I jump to conclusions.

[sluckey]

I don't see how your guitar problem could be related to the fixed bias mod. But it should be a no brainer quick and easy task to return the amp to cathode bias.

[dude]

I don't see how your guitar problem could be related to the fixed bias mod. But it should be a no brainer quick and easy task to return the amp to cathode bias.

No way, this amp sounds fantastic, louder cleaner and just screams to turn up the volume o or gain up.

I'll put up with the low signal passing.

Thanks,
al

[sluckey]

I'm not talking about reversing the mod forever. Just long enough to see how your guitar behaves with cat bias.

[dude]

I'm not talking about reversing the mod forever. Just long enough to see how your guitar behaves with cat bias.

Things are better with known new guitar pots,  I think Willable is right, it's the guitar pots. They work fine in other amps but this one is sounding way better, much more touch senitive. I think maybe the guitar pots are old, I have vintage equipment.  The SG Special with Lindy Fralin P90's sings.

I think a lot of my gear needs new volume pots, most guitars are 30 or more years old. I'm just a player that digs that plain old vintage tone on a strat a little over driven when you  dig in.

I'll get it there, give me some time. I'll post a clip if it's accepted here....?

I'll change out the bias range R to get it right, hopefully. Then change the old guitar's pots, just a few...  See"Which way the wind Blows"!

Brad, I think you were right.

al

[dude]

I'm not talking about reversing the mod forever. Just long enough to see how your guitar behaves with cat bias.

Things are better with known new guitar pots,  I think Willable is right, it's the guitar pots. They work fine in other amps but this one is sounding way better, much more touch senitive. I think maybe the guitar pots are old, I have vintage equipment.  The SG Special with Lindy Fralin P90's sings.

I think a lot of my gear needs new volume pots, most guitars are 30 or more years old. I'm just a player that digs that plain old vintage tone on a strat a little over driven when you  dig in.

I'll get it there, give me some time. I'll post a clip if it's accepted here....?

I'll change out the bias range R to get it right, hopefully. Then change the old guitar's pots, just a few...  See"Which way the wind Blows"!

Brad, I think you were right.

al

After checking things out, it's not any of the guitar's volumes. It was an intermittent problem, most likely the second bias filter (I had broke the leg at the negative end of the cap, right at the point it attaches). I replaced the cap and no more scratchy volume, turning the volume down with amp on is now quite. I guess that connection I fixed was cold and somehow was giving a signal threw the guitar's  ground, maybe...?

Anyway,  I got what I wanted and even more, about 18v more B+, a much tighter tone with much more touch sensitivity. This mod to fixed bias was the best mod I think I ever did that make such a positive change to an amp.

I think I might change another amp from cathode bias to fixed. I have another 18 watt Marshall with EL84s that I changed the PI and grid resistors to get a cleaner tone, I might do fixed on that amp but it has a CT so things will be much easier.

Thanks everyone, and Sluckey you're the man.
al 

[2deaf]

the diode is reversed when compared to the diodes on the bridge, so the half wave voltage is negative only, (relative to the bridge).

I'm not seein' it.

the first cap assures the circuit starts with A/C only

I don't see an AC signal path.