Mods to a Peavey Mace for Steel Guitar

I bought a Peavey Mace to specifically use with my Lap and Pedal steel guitars.  I need it to be clean, clean and loud. with 6x 6L6s in this thing, it seemed like a great choice.  I plan to make a few mods to it in order to get it cleaner if I can.

The tube side of things, I'm pretty good on. Were going to change the screen resistors to 470r parts, change  IR9 to a pot to make the bias adjustable. Increase the value of the bulk filter caps for the power tube side to 220uf. Replace the power tubes and rebias.

The Solid State side, while fairly simple is still a little black boxy to me.
On both, I will replace the tone stack caps and general coupling caps with better quality parts.
All Electrolytics will be replaced.

In the effects channel, is the function of CR6 and 7 for clipping purposes? If so, can I remove them (and R62) to make it cleaner?
What is the function of the parts like CR5, CR10 and CR11?
Any other places I could mod to clean it up?

Link to schematic
https://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-Mace-Deuce-VT-Schematic.pdf

this book right here>

I have been outside the castle walls for almost 20 years and I was never in those specific trenches but IIRC steel players liked clean wide-range power with lots of headroom... The Peavey keyboard amps were popular (like KB300?) with steel players because they were full frequency response and used real speakers, not guitar speakers that have their own (distorted) sound.

YMMV

JR

heh... I have that book. Its a great book and I know Gerald personally. He's an interesting person and about as Texas as they come.

This amp has a Solid State preamp but a tube power amp.
I'm essentially doubling the Power tube power supply. That should stiffen it up nicely. While I was in the power supply, I also replaced all the diodes. I used 1N5399 for the Power tube side. It was 1N4007s.
For the low voltage side, I used 1N4007s. They were 1N4003.
I also replaced the rectifier for the Bias supply. It was a 1N4003, its now a 1N4007.
For most of the Power supply caps, I used higher rated parts.
I replaced the 220k resisters in both totem poles. 2 of them were reading largely out of spec.
I also replaced all the snubbing caps with newer parts. The old ceramics were starting to crumble.

So with just about every part on the power supply board replaced, Im ready to move on to the preamp board, which is where most of my questions lie.

Mylithra said:

In the effects channel, is the function of CR6 and 7 for clipping purposes? If so, can I remove them (and R62) to make it cleaner?
What is the function of the parts like CR5, CR10 and CR11?
Any other places I could mod to clean it up?

Link to schematic
https://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-Mace-Deuce-VT-Schematic.pdf

Click to expand...

Mylithra said:

While I was in the power supply, I also replaced all the diodes. I used 1N5399 for the Power tube side. It was 1N4007s.
For the low voltage side, I used 1N4007s. They were 1N4003.
I also replaced the rectifier for the Bias supply. It was a 1N4003, its now a 1N4007.

Click to expand...

Sorry to be blunt, but this just shows you have a deep un-understanding of what it takes to "improve" an existing design.
Like putting a 390cfm Holley on a 2-liter Toyota.
These mods to the power supply would make sense only if you changed the PT.

Mylithra said:

In the effects channel, is the function of CR6 and 7 for clipping purposes?

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Yes.

If so, can I remove them (and R62) to make it cleaner?

Click to expand...

That would not be different than turning the Colour know fully CCW.

What is the function of the parts like CR5, CR10 and CR11?

Click to expand...

CR5 and CR10 are attached to the control input of OTA's (operational transconductance amp). My understanding of the circuit is that it creates a non-switchable expander... Seems weird to me.
CR11 & 12 are there to prevent reverse-voltage on the control pin of the switching IC's.

abbey road d enfer said:

Sorry to be blunt, but this just shows you have a deep un-understanding of what it takes to "improve" an existing design.
Like putting a 390cfm Holley on a 2-liter Toyota.
These mods to the power supply would make sense only if you changed the PT.

Click to expand...

I'm not expecting to draw more current than what the amp already does. I'm just using these particular parts as its what I had on hand and are over-speced for the job. I don't expect these to change the performance of the amp. I expect these to perform the same.

I do expect the larger filter caps to stiffen the amp a touch more than it already was when it was younger.
This amp is nearly 40 years old. (1981 was nearly 40 years ago?!?) so I expect all the chemical components to be at least less functional if not dry by now.

As to the solid state side, I will confess, I dont know enough about it to make an improvement one way or the other.  And I don't think I'm smarter than the Peavey Engineer who designed this thing, but they designed this thing with electric guitar in mind, not steel.  I have a stack of different guitar amps and tried them all but they all have 1 thing in common, they break up way too early for a steel.  A lot of that is by design these days. I kind of expect the same thing out of this amp.

The only guitar amps I've had that worked really well on Steel was a Twin Reverb and a Quilter solid state amp)
I do get the general gist of it though. TBH, I wouldn't be here asking if I knew 100% what I was doing with it already... part of the learning process.

Peavey loved those CMOS chips in a lot of the things they built in this time. It was part of their DDT compression which was in just about everything in that era. They used a lot of them in this amp. 

here is the Webb preamp just for kicks>

power section

Mylithra said:

Peavey loved those CMOS chips in a lot of the things they built in this time. It was part of their DDT compression which was in just about everything in that era. They used a lot of them in this amp.

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Huh? I am not aware of any CMOS in Peavey's DDT circuitry. The variable gain for limiting was performed using operational transconductance amplifiers. We used so many we had our own house part number (70407478). These parts were selected/guaranteed for low control voltage feedthrough, useful for gain changing without thumpy artifacts. 


JR

This was in the bottom of the chassis. I always thought OTAs were a form of CMOS. Are they not? I'm pretty certain the sticker is referring to that very part.

> not aware of any CMOS in Peavey's DDT circuitry.

FWIW, for some time Peavey used TL604 analog switches in guitar amps. This is technically P-MOS, not CMOS; but similar handling dangers.

Somehow the TL604s fail over the decades, and production was discontinued long ago. There are hacks; even those kits seem to be getting scarce.

Mylithra said:

This was in the bottom of the chassis. I always thought OTAs were a form of CMOS. Are they not?

Click to expand...

they are not, OTAs use conventional bipolar IC technology.

I'm pretty certain the sticker is referring to that very part.

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I am pretty certain it isn't... I am too lazy to study the schematic to find actual CMOS.

FWIW I used CMOS in a handful of designs while at Peavey and never stickered a chassis with that kind of warning.  In my experience early MOS was even more sensitive to ESD damage from handling, but over time we learned that even relatively robust bipolar can degrade from ESD hits and by last century all factory workers were outfitted with grounding straps when handling ICs and PCB assemblies.

Modern ICs, including bipolar, use extensive input pin protection clamps. One reason modern ICs are harder to kill... but not impossible.

JR

Thank you all for your help.  I have a much better idea of what happening here, what is doing what and where my knowledge deficiencies are.  Still have tons to learn.

Finally finished the work I had been doing on this. After a complete recap of all the electrolytics, I retubed the amp with new 6L6s. After doing all this work with it, I tried it and it was oscillating. Put the scope to it and saw the signal was clean all the way to the OTAs but the output here on both channels was doing it. I traced it back to a loose wire off the power supply.  Once I tightened that connector up, it stopped oscillating. After doing a little burn in a listen, its plenty loud and clean enough as-is.

I've been playing guitar for about 30 years now and I've played a lot of Peavey.
John, I'd be interested to hear about some of the work you did when you were with Peavey.
I was reading about the production of the T60 and a lot of the innovative work they were doing in that time.  Using the gun stock duplicating machines to do the necks on the T60s,  I've had my hands on quite a few T60s and they all felt the same.

Mylithra said:

Finally finished the work I had been doing on this. After a complete recap of all the electrolytics, I retubed the amp with new 6L6s. After doing all this work with it, I tried it and it was oscillating. Put the scope to it and saw the signal was clean all the way to the OTAs but the output here on both channels was doing it. I traced it back to a loose wire off the power supply.  Once I tightened that connector up, it stopped oscillating. After doing a little burn in a listen, its plenty loud and clean enough as-is.

Click to expand...

clean is good...

I've been playing guitar for about 30 years now and I've played a lot of Peavey.
John, I'd be interested to hear about some of the work you did when you were with Peavey.

Click to expand...

I was hired in 85 to work in the AMR recording products division. That lasted about 1 year before I was transferred into headquarters with more full line responsibility.

I was reading about the production of the T60 and a lot of the innovative work they were doing in that time.  Using the gun stock duplicating machines to do the necks on the T60s,  I've had my hands on quite a few T60s and they all felt the same.

Click to expand...

I worked there for 15 years but didn't have much to do with guitars, while I did drink beer with those guys. That part of the legend about using gun stock machine technology (German IIRC) is true, Hartley was an early adopter of using NC controlled wood shaping for consistent neck manufacturing.

I have many Peavey stories, most I should never repeat.  ;D There is an actual Peavey History book, written by Ken Achard (the first British distributor) years ago. I actually helped him with fact checking before his book release. The best way to look good in a book is to write your own history yourself.  8)

JR