# What makes an amps voice? PA into guitar amp question



## Lincoln (Jun 2, 2008)

I've got a Garnet Rebel 90 PA head that's been laying around for way too long. I've tried using it as a guitar amp but the tone of it is very dark & muddy. No life or spark at all. Even as a bass amp it sounds too "dark". I've been playing with amps all my life, done the standard 5F1 and 5E3 builds but I will admit I do not understand what sets the voice of an amp. Anybody help me out here with some words of wisdom? Only thing I've noticed while looking at the amp is a whole mess of resistors on the inputs that I'm not used to seeing.

I really want to turn this PA into a guitar amp that will get used. It's got beautiful big transformers and so many other things going for it.......and it's just wasting away.


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## keto (May 23, 2006)

google 'amplifier tone stack'. Look at the Duncan tone stack calculator, which gives several different styles (Big Muff, Marshall, etc) and you can just plug in values and look at the frequency responses. That plus the input/output caps make up most of your frequency response.

It's just a few resistor + capacitor values. Pretty quickly you'll be able to separate out the tone stack out of any amplifier schematic, if you really want to, and modify it. Do you have a schematic for your Rebel?


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## Wild Bill (May 3, 2006)

Lincoln said:


> I've got a Garnet Rebel 90 PA head that's been laying around for way too long. I've tried using it as a guitar amp but the tone of it is very dark & muddy. No life or spark at all. Even as a bass amp it sounds too "dark". I've been playing with amps all my life, done the standard 5F1 and 5E3 builds but I will admit I do not understand what sets the voice of an amp. Anybody help me out here with some words of wisdom? Only thing I've noticed while looking at the amp is a whole mess of resistors on the inputs that I'm not used to seeing.
> 
> I really want to turn this PA into a guitar amp that will get used. It's got beautiful big transformers and so many other things going for it.......and it's just wasting away.


The tone stack is indeed part of it but there's MUCH more! A PA amp is designed to be clean, clear and distortion-free. It is not supposed to ever "over-load". The preamp circuitry in particular is designed to actually fight any chance of the amplifier doing such things.

A guitar amp circuit is quite different. It is SUPPOSED to have some distortion! Keep in mind that some forms of distortion are pleasing to the ear with a guitar and others aren't. Plus, we expect to hear boosts or cuts in certain portions of the audio spectrum, in different areas specifically involving guitar. 

Even more, a guitar preamp circuit will compress a signal, making it sound thicker and sweeter, on a ramp as it is overdriven that goes a long way before the effect becomes noticeable and an even longer way before it goes "over the top" and sounds unpleasantly distorted.

So I don't think that changing the tone stack will get you where you want to be, except possibly for bass guitar, where most like it "clean and snappy".

I've done over many such PA amps. Normally, I gut out everything but the tube sockets and the transformers. After all, the filter caps are likely old and overdue for replacement. The resistors and capacitors are just as old and wired totally wrong for guitar. So I use new parts and replicate a classic circuit, usually an old non-master volume Marshall PLexi type, since that's the sound I personally love! Especially with P90 pickups and the amp cranked to 11!

Wild Bill/Busen Amps


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## washburned (Oct 13, 2006)

Try this article out. Converting Integrated/PA Tube Amps into Guitar Amps.


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## Lincoln (Jun 2, 2008)

keto said:


> google 'amplifier tone stack'. Look at the Duncan tone stack calculator, which gives several different styles (Big Muff, Marshall, etc) and you can just plug in values and look at the frequency responses. That plus the input/output caps make up most of your frequency response.
> 
> It's just a few resistor + capacitor values. Pretty quickly you'll be able to separate out the tone stack out of any amplifier schematic, if you really want to, and modify it. Do you have a schematic for your Rebel?


Yeah, I've got pretty much a full set of Garnet schematics and I've compared the rebel to the rebel PA but there is so much going on in the pre-amp stages/input of the PA that I get lost. I't's beyond my understanding.


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## Lincoln (Jun 2, 2008)

Wild Bill said:


> The tone stack is indeed part of it but there's MUCH more! A PA amp is designed to be clean, clear and distortion-free. It is not supposed to ever "over-load". The preamp circuitry in particular is designed to actually fight any chance of the amplifier doing such things.
> 
> A guitar amp circuit is quite different. It is SUPPOSED to have some distortion! Keep in mind that some forms of distortion are pleasing to the ear with a guitar and others aren't. Plus, we expect to hear boosts or cuts in certain portions of the audio spectrum, in different areas specifically involving guitar.
> 
> ...


That explains a whole lot of things perfectly. Thank you. So basically I need to gut this thing, build in a 5F4 circut board or make it a bassman. Starting over is something I can do.
Is there a marshal that can use 6L6's?


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## Lincoln (Jun 2, 2008)

washburned said:


> Try this article out. Converting Integrated/PA Tube Amps into Guitar Amps.


good read man, thank you.


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## Wild Bill (May 3, 2006)

Lincoln said:


> That explains a whole lot of things perfectly. Thank you. So basically I need to gut this thing, build in a 5F4 circut board or make it a bassman. Starting over is something I can do.
> Is there a marshal that can use 6L6's?


The first Marshall used 5881's, which is an industrial number for a 6L6. Go to schematicheaven.com and look up a Bluesbreaker schematic.

Wild Bill/Busen Amps


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## washburned (Oct 13, 2006)

Wild Bill said:


> The first Marshall used 5881's, which is an industrial number for a 6L6. Go to schematicheaven.com and look up a Bluesbreaker schematic.
> 
> Wild Bill/Busen Amps


What he said!


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## jb welder (Sep 14, 2010)

Lincoln said:


> the tone of it is very dark & muddy. No life or spark at all. Even as a bass amp it sounds too "dark".


 Not trying to discourage you if you really want to gut it & rebuild, but it sounds like this amp has a problem. Many people now run bass direct into PA amps, you should at least be able to get a good bass sound out of it if it is working correctly. And even with guitar, "no life or spark at all"? It's still a tube amp, it has a master volume doesn't it? With the input volume cranked and the master at a low setting, can you not get a decent distorted tone? Once again, not trying to discourage you, but would hate for you to rebuild it because it had something wrong with it. Maybe if it was working right you would like the tone, or at least be close. I just find it difficult to believe you can't get anywhere near a decent guitar sound out of a tube PA head that is working right.


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## nonreverb (Sep 19, 2006)

Actually, the 5881 was VERY short lived in the Marshall amps. They were replaced by the venerable KT66 in short order (prior to the final conversion to EL34). It is a superior tube in that application IMO...and one of my all-time favourite tubes 
Having said that, you should be able to use any of the schematics from that era of production. I believe the schematics for the Bluesbreaker show KT66's as the power tubes.


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## Wild Bill (May 3, 2006)

nonreverb said:


> Actually, the 5881 was VERY short lived in the Marshall amps. They were replaced by the venerable KT66 in short order (prior to the final conversion to EL34). It is a superior tube in that application IMO...and one of my all-time favourite tubes
> Having said that, you should be able to use any of the schematics from that era of production. I believe the schematics for the Bluesbreaker show KT66's as the power tubes.


Quite right about the 5881, NR! Still, the changes in tubes had nothing to do with a search for tone. It was simply all about money!

At the time Marshall had just started selling its line of amplifiers, the audio industry entered a "panic" for 6L6 tubes, under any number in that family. Prices soared and lead times stretched out to months. Marshall had to go looking for an alternative and the EL-34 (on this side of the Atlantic we called them 6CA7s) seemed a good choice. Same pinout, so it would plop into the same socket. Similar specs, so no need to change anything but the bias voltage. It was an almost completely painless alternative.

There was one important difference however that did not at first become apparent but guys like Pete Townsend and Jimmy Page noticed immediately. The EL-34 only needed a bit less than 2/3 of the driving voltage for full output than did the 6L6. This meant that the preamp designed for 6L6s was able to drive EL-34s into creamy, power tube distortion much more easily!

That tone was the Marshall "grind"! With the humbuckers popular in those days Marshall now had a distinctive "snarl" that set its amps apart from its American competition! Over here, although Fender never embraced the EL-34/6CA7, preferring to stay with its established tone already the standard in its marketplace, Pete Traynor enthusiastically switched to the new tubes! Not only did Traynor Amps save money from a lower cost but shipping delays disappeared, enabling them to get their amps out to their distributors. Besides, Traynors with 6CA7s sounded much better to rock and roll ears!

If that shortage of 6L6s had never happened, rock and roll might have evolved quite differently.

Wild Bill/Busen Amps


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## mhammer (Nov 30, 2007)

Following a special issue of POLYPHONY on modding guitar gear, I adapted a blackface Tremolux head I owned at the time, to go from 6L6s to 6CA7s. I also added a post phase-splitter master volume and a tonestack-lift switch (just disconnect the Middle pot from ground) to provide a solo boost. Kickass head. Sorry I sold it. If you ever run into it, let me know.


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## mhammer (Nov 30, 2007)

back to the amp in question....

I suggest you turn your attention to the input. I don't know which particular Rebel 90 you are working with, but one of the schematics I have for it shows a 100k resistor to ground on the input. I contrast that with the 1M resistor you are likely to find in the identical position on guitar amps. Keep in mind that the PA amplifier anticipates seeing a 600ohm dynamic mic on its input, which 100k is very high in relation to. The gutar, on the other hand, typically presents a input load somewhere in the neighbourhood of 10x higher than a dynamic mic, in which case the terminating resistor on the preamp input needs to be higher.

Stated simply, the dullness you note may well be simply a case of the input being too loaded down. The way to put that to the test in a noninvasive way is to stick some kind of buffer between the guitar and the amp input so that the amp sees an input source with an impedance somewhere not far off from what a voice mic would be.


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## nonreverb (Sep 19, 2006)

Wild Bill said:


> Quite right about the 5881, NR! Still, the changes in tubes had nothing to do with a search for tone. It was simply all about money!
> 
> At the time Marshall had just started selling its line of amplifiers, the audio industry entered a "panic" for 6L6 tubes, under any number in that family. Prices soared and lead times stretched out to months. Marshall had to go looking for an alternative and the EL-34 (on this side of the Atlantic we called them 6CA7s) seemed a good choice. Same pinout, so it would plop into the same socket. Similar specs, so no need to change anything but the bias voltage. It was an almost completely painless alternative.
> 
> ...


Needless to say, cost was always a factor however, the GEC KT66 was and still is a superior tube. Probably expensive at the time which explains why it didn't last long during early Marshall production.
The irony Bill is that all the "6CA7" tubes in Traynor amps were in fact rebranded Mullard EL34's. So were the preamp tubes. I don't know if lower cost was the only answer as all those tubes were shipped from England. 
The same goes with Hammond Organ co. All their tubes, with very few exceptions, were Mullard. I believe they used them because of pricing versus quality.


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## Wild Bill (May 3, 2006)

nonreverb said:


> Needless to say, cost was always a factor however, the GEC KT66 was and still is a superior tube. Probably expensive at the time which explains why it didn't last long during early Marshall production.
> The irony Bill is that all the "6CA7" tubes in Traynor amps were in fact rebranded Mullard EL34's. So were the preamp tubes. I don't know if lower cost was the only answer as all those tubes were shipped from England.
> The same goes with Hammond Organ co. All their tubes, with very few exceptions, were Mullard. I believe they used them because of pricing versus quality.


Well, as I said, we're talking the SAME tube, regardless of where it was made! Europeans have their own numbering system. That's why they call a 12AX7 an ECC83.

Shipping from England probably wasn't all that dear. After all, tubes are rather light in weight and in those days were likely shipped by cargo ships, not air freight. I don't know if American manufacturers would have been seeing much volume in the 6CA7 when up until the time in question the overwhelming volume would have been in 6L6 types. Using a 6CA7 in a guitar amp was not only a brand new thing but guitar amps period were a low volume application for a tube manufacturer. The big money was in hifi and industrial applications. Remember, you have to look at the money before the world changed, not after! :food-smiley-004:

I have mentioned many times before that I worked for a few years at the end of the 80's for the Canadian Westinghouse Tube Division and learned much of how that world had worked from the old folks at the desks beside me. Like all manufacturers, they had needed a minimum volume of committed orders before it was worth it to set up the machines to run a certain tube type. If they had a smaller requirement it was standard practice to just buy from a competitor and rebrand the tubes under their own name. During the 50's there was no NAFTA or even Auto Pact. British products were quite competitive and very common. Lots of my friends had British Minis, Austins and the like for their first car, up until the mid*70's. So Mullard tubes would have been quite common, along with Valvo and Telefunken from Europe and Germany. Westinghouse tended to avoid buying Valvo 12AX7s as they had a tendency to go microphonic, if memory serves.

So my experience makes me believe that the major factor WAS price, NR! You see, in those days tubes were the standard active part used in electronics. The volumes made were huge! Production runs were typically in the tens if not hundreds of thousands. The quality of a brand was taken as a given. Either your tube met the industry data sheet specs extremely closely and the quality was acceptably high or NO ONE WOULD BUY FROM YOU! A tube manufacturer needed those high volume customers. If he ticked them off, he was stricken from the buyers' list, which meant he almost certainly would go out of business.

So a Mullard EL-34 and a Sylvania 6CA7 would BOTH have been quality tubes! The only real way to compete would have been on price. And Britain was hungry for trade! The war had almost bankrupted her. They would have been willing to sell at VERY tight margins!

Another reason why we would have seen so many rebranded Mullards in Traynors would have been deliveries. As I related, the whole reason guitar amps started using them in the first place was due to a scarcity of 6L6s. Britain likely had extra capacity due to not having as much market share as the Americans. They could crank up EL-34 production while their competition was busy with tubes for TVs or whatever.

Last, I've never found Mullards to be exceptional in quality. Certainly, they were every bit as good as RCA or other American manufacturers but like the Valvo's, they too had a tendency to go microphonic. 

Unless you mean quality as regards to tone. I can't debate with you on that because I simply do not believe that different brands of tubes sound different at all. There is no sound inside a tube, just electricity. No maple or rosewood grids to affect the tone!:sport-smiley-002:

Perhaps this is a good place to end my post!

Wild Bill/ Busen Amps


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## Scottone (Feb 10, 2006)

Bill is right as usual. I ended up doing the same thing on some PA amps that i rebuilt. One exception was a Masco MA50. On that one, I kept the original paraphase inverter because it sounded really good....everything else was changed though 



Wild Bill said:


> The tone stack is indeed part of it but there's MUCH more! A PA amp is designed to be clean, clear and distortion-free. It is not supposed to ever "over-load". The preamp circuitry in particular is designed to actually fight any chance of the amplifier doing such things.
> 
> A guitar amp circuit is quite different. It is SUPPOSED to have some distortion! Keep in mind that some forms of distortion are pleasing to the ear with a guitar and others aren't. Plus, we expect to hear boosts or cuts in certain portions of the audio spectrum, in different areas specifically involving guitar.
> 
> ...


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## nonreverb (Sep 19, 2006)

For sure, tubes from all the manufacturers into the 80's were good in varying degrees. There are some today that are decent as well and agreed, Mullards can be microphonic just like RCA's from the same era for that matter. As for tone, I agree brands are not a yardstick to measure by however, what I was alluding to was the difference between a KT66 and a 6L6 or 5881. They're different tubes with different characteristics...
Personally, I like the GEC KT66 and KT88 they're excellent sounding tubes albeit expensive. Heck, it's the KT88 that was used in the Marshall Major!! :food-smiley-004:



Wild Bill said:


> Well, as I said, we're talking the SAME tube, regardless of where it was made! Europeans have their own numbering system. That's why they call a 12AX7 an ECC83.
> 
> Shipping from England probably wasn't all that dear. After all, tubes are rather light in weight and in those days were likely shipped by cargo ships, not air freight. I don't know if American manufacturers would have been seeing much volume in the 6CA7 when up until the time in question the overwhelming volume would have been in 6L6 types. Using a 6CA7 in a guitar amp was not only a brand new thing but guitar amps period were a low volume application for a tube manufacturer. The big money was in hifi and industrial applications. Remember, you have to look at the money before the world changed, not after! :food-smiley-004:
> 
> ...


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## nonreverb (Sep 19, 2006)

Ha Mark!
I got talked into modding a BF Bassman with 6550's last year...it actually sounded pretty good!


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## Lincoln (Jun 2, 2008)

Wild Bill said:


> The first Marshall used 5881's, which is an industrial number for a 6L6. Go to schematicheaven.com and look up a Bluesbreaker schematic.
> 
> Wild Bill/Busen Amps


horribly neglected as a child, I've never played through a Marshall or even played/jambed with anymore who had Marshall. So I don't know what I'm missing out on other than rep & hype. However, you've got me thinking it's time to venture outside the box a bit. (is there more to life than Fender?) Do you think my Rebel 90 power & output tranformers (made for 2 x 6L6) would support a JTM45 build ok? Or should I be looking more at the 18 watt setup and run it with 6V6's? I don't need the extra power, 18 watts with bit of pedal boost will cover every situation I'll ever be in. what makes me happy is a basicly clean tone with that little ragged edge when it's pushed hard.


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## nonreverb (Sep 19, 2006)

The JTM45 would be the easier option as the plate impedance of the 6V6 is different plus the plate voltage from the stock power transformer would be a tad too high for 6V6's. 



Lincoln said:


> horribly neglected as a child, I've never played through a Marshall or even played/jambed with anymore who had Marshall. So I don't know what I'm missing out on other than rep & hype. However, you've got me thinking it's time to venture outside the box a bit. (is there more to life than Fender?) Do you think my Rebel 90 power & output tranformers (made for 2 x 6L6) would support a JTM45 build ok? Or should I be looking more at the 18 watt setup and run it with 6V6's? I don't need the extra power, 18 watts with bit of pedal boost will cover every situation I'll ever be in. what makes me happy is a basicly clean tone with that little ragged edge when it's pushed hard.


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## Lincoln (Jun 2, 2008)

keto said:


> google 'amplifier tone stack'. Look at the Duncan tone stack calculator, which gives several different styles (Big Muff, Marshall, etc) and you can just plug in values and look at the frequency responses. That plus the input/output caps make up most of your frequency response.
> 
> It's just a few resistor + capacitor values. Pretty quickly you'll be able to separate out the tone stack out of any amplifier schematic, if you really want to, and modify it. Do you have a schematic for your Rebel?





mhammer said:


> back to the amp in question....
> 
> I suggest you turn your attention to the input. I don't know which particular Rebel 90 you are working with, but one of the schematics I have for it shows a 100k resistor to ground on the input. I contrast that with the 1M resistor you are likely to find in the identical position on guitar amps. Keep in mind that the PA amplifier anticipates seeing a 600ohm dynamic mic on its input, which 100k is very high in relation to. The gutar, on the other hand, typically presents a input load somewhere in the neighbourhood of 10x higher than a dynamic mic, in which case the terminating resistor on the preamp input needs to be higher.
> 
> Stated simply, the dullness you note may well be simply a case of the input being too loaded down. The way to put that to the test in a noninvasive way is to stick some kind of buffer between the guitar and the amp input so that the amp sees an input source with an impedance somewhere not far off from what a voice mic would be.


Well guys, I've been busy sorting this thing out. I've been reading lots and playing with tone stack ideas on an old 5F1 circuit too.
Power output stages are identical. Power supply is the same. Everything starts going astray at the phaze inverter and the pre-amp. Inputs are totally out to lunch of course. I'm going to set things up like a Rebel 90 (maybe with stinger even) and give it a try. Wish me luck 

if I mess it up too bad, then I'll use the parts to make a bassman or bluesbreaker clone (plan B)


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## Lincoln (Jun 2, 2008)

Here's the before shot of the PA90
View attachment 2394


Close up of the input/tone section. Pretty busy
View attachment 2395


the more I get into it, the more I think it's been tampered with.


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