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Vintage threads from the first ten years

Re: See KG's 'Tone ruminations' thread below

7/2/1999 1:23 PM
FJR	Re: See KG's 'Tone ruminations' thread below If there is a "brite-cap" across the 470ohm resistor goin' over to V22... you can remove that...I did & it fattened things-uo...Frank

7/3/1999 4:53 AM
Bruce	Re: "Marshall tone mods" OK Bill... Ya know what? I think this is a good example of the classic difference of opinions between electronic engineers and musicians engineering electronics. Although I'm sure with enough tweaking you could get this amp to sound great... 800v on those EL34 tubes is very high and you're going to have a HARD time getting it to sound "plexi good" (subjective) no matter what the Svetlana "gurus" say about the voltage. I'm not that worried about the 800vdc being way too high for saftey either, but, IMHO, it is WAY too high to get a good warm tone. Lowering the screen voltage to compensate for overly high plate voltage is a good way to really chill down the tone of the tubes too. So I think that is backwards "tech"ing. What plate load Zed are you using with these tubes now? Try using NO cathode bypass caps and if you feel the pre amp gain is too high. Assuming the amp is built as posted... Take the .68uF cap off the cathode of the triode ahead of the CF triode and use a .001uF cap across the 470K contouring resistors on the stage before. Try using a 250pF to 1000pF cap across the plate load resistor on one of the preamp stages to roll some of the highs to ground too. Never use the 5000pF bright cap on the volume control unless you can run the amp at near 50% to 75% volume levels. Try a 110pF-150pF instead. There are other things to do but you'll need to mess with them one by one. Bruce

7/3/1999 4:53 AM

Bruce

Re: "Marshall tone mods"

OK Bill...

Ya know what?

I think this is a good example of the classic difference of opinions between electronic engineers and musicians engineering electronics.

Although I'm sure with enough tweaking you could get this amp to sound great... 800v on those EL34 tubes is very high and you're going to have a HARD time getting it to sound "plexi good" (subjective) no matter what the Svetlana "gurus" say about the voltage.

I'm not that worried about the 800vdc being way too high for saftey either, but, IMHO, it is WAY too high to get a good warm tone.

Lowering the screen voltage to compensate for overly high plate voltage is a good way to really chill down the tone of the tubes too. So I think that is backwards "tech"ing.

What plate load Zed are you using with these tubes now?

Try using NO cathode bypass caps and if you feel the pre amp gain is too high.

Assuming the amp is built as posted...

Take the .68uF cap off the cathode of the triode ahead of the CF triode and use a .001uF cap across the 470K contouring resistors on the stage before.

Try using a 250pF to 1000pF cap across the plate load resistor on one of the preamp stages to roll some of the highs to ground too.

Never use the 5000pF bright cap on the volume control unless you can run the amp at near 50% to 75% volume levels.

Try a 110pF-150pF instead.

There are other things to do but you'll need to mess with them one by one.

Bruce

7/5/1999 3:51 PM
Trace	Bill; I think Bruce made a very good point about the voltage. It's going to be hard to make the amp sound "warm" as Bruce suggested. Typically speaking the ton will have alot of highend and more so the "harsh" kind. As Dave suggested Bill, I wish you could have searched the archives because there's been ALOT of threads on this amp in the past year alone. I would try some of the suggestions that Bruce gave you and see if that helps you out. I think that's a pretty good place to start. Trace

7/5/1999 3:51 PM

Trace

Bill;

I think Bruce made a very good point about the voltage. It's going to be hard to make the amp sound "warm" as Bruce suggested. Typically speaking the ton will have alot of highend and more so the "harsh" kind.

As Dave suggested Bill, I wish you could have searched the archives because there's been ALOT of threads on this amp in the past year alone. I would try some of the suggestions that Bruce gave you and see if that helps you out. I think that's a pretty good place to start.

Trace

7/6/1999 0:40 AM
Bill Costello	Bruce, you're absolutely right! It's just that my goal is to get the most out of what I have, at minimal cost. I'm not a professional musician - what musical background I have was a short time when I was younger as a roadie/soundman for a "B" circuit band. I have had a lot of experience as a "techie" and have built a lot of tube transmitting equipment. My family is my first budget priority and besides, I enjoy modifying old stuff. If I had wanted a true Marshall amp I would have used this "donated" boat anchor in the fist place! What you and others have and are doing for me is giveing me extremely valuable insights into how the theory applies to the desired sound. Of course with an 11k plate load and these high voltages I can never achieve the output trannie saturation of a true Marshall. I don't expect to! What I AM finding out is how good this amp can be made to sound, with in the parameters with which I have to work! So far I'm very pleased with the "punch" and sustain. I had one of my old professional buddies try the amp and his first comment was "Sounds like you re-invented the Marshall"! The preamp obviously is the larger part of the tone. The advice I'm getting now will help me understand what exactly is going on and thus suggest useful approaches towards fine-tuning the sound. When I'm finished I've no doubt the verdict will be "it's like a Marshall only different!" Your suggestions as to preamp mods should improve the warmth and bottom end somewhat - we'll only know how much after some more soldering. I feel that the voyage itself is invaluable to my education - much more than if I had simply built a Marshall amp "Heathkit" style. I seem to be in uncharted territory. At the end, I will not be at all disappointed. As I said, the amp already sounds great to my ears and my own local "guitar god". It looks (if I may be so pretentious) beautiful! I'm getting the loan of a digital camera in the next little while and perhaps I'll submit something to the homebrew corner - with a .WAV file for everybody's consideration! Best of all - it's already a TERRIBLE amp for country!

7/6/1999 0:40 AM

Bill Costello

Bruce, you're absolutely right! It's just that my goal is to get the most out of what I have, at minimal cost. I'm not a professional musician - what musical background I have was a short time when I was younger as a roadie/soundman for a "B" circuit band. I have had a lot of experience as a "techie" and have built a lot of tube transmitting equipment.

My family is my first budget priority and besides, I enjoy modifying old stuff. If I had wanted a true Marshall amp I would have used this "donated" boat anchor in the fist place!

What you and others have and are doing for me is giveing me extremely valuable insights into how the theory applies to the desired sound. Of course with an 11k plate load and these high voltages I can never achieve the output trannie saturation of a true Marshall. I don't expect to! What I AM finding out is how good this amp can be made to sound, with in the parameters with which I have to work!

So far I'm very pleased with the "punch" and sustain. I had one of my old professional buddies try the amp and his first comment was "Sounds like you re-invented the Marshall"! The preamp obviously is the larger part of the tone. The advice I'm getting now will help me understand what exactly is going on and thus suggest useful approaches towards fine-tuning the sound.

When I'm finished I've no doubt the verdict will be "it's like a Marshall only different!" Your suggestions as to preamp mods should improve the warmth and bottom end somewhat - we'll only know how much after some more soldering.

I feel that the voyage itself is invaluable to my education - much more than if I had simply built a Marshall amp "Heathkit" style. I seem to be in uncharted territory. At the end, I will not be at all disappointed. As I said, the amp already sounds great to my ears and my own local "guitar god". It looks (if I may be so pretentious) beautiful! I'm getting the loan of a digital camera in the next little while and perhaps I'll submit something to the homebrew corner - with a .WAV file for everybody's consideration!

Best of all - it's already a TERRIBLE amp for country!

7/6/1999 12:40 PM
Dave H.	In this pre-amp the gain in the mid/treb range is a lot more than the bass so by the time the signal gets to the tone stack there isn’t much LF for the bass pot to work with. Also the low value (33k) "slope" resistor in the tone stack reduces the range of the tone controls. Try changing it to 56k. There will be more attenuation through the stack but the tone controls will be more effective. To add more "bottom warmth" you really need to reduce the gain of the amp at mid/treb. The 470p cap on the treb pot lets the upper mids through which can make the amp sound "hard" rather than "warm", a 250p may be better. Try removing the 470p caps across the 470k resistors and the 1n bright cap on the vol pot (or change it to 47-100p). The 0.68u cathode bypass cap on the first stage will also give more mid/treb gain. Remove it or replace it with 25u for more gain at all frequencies. Try fitting a centre off min toggle switch so you can select between 0.68u, 25u and no cap (Steve A. made me say that). It’s great for fine tuning the voicing of your amp to match your guitar and speakers. These mods won’t cost much and are easy so if you do a mod and the amp ends up sounding good for country rip it out quick. Dave

7/6/1999 12:40 PM

Dave H.

In this pre-amp the gain in the mid/treb range is a lot more than the bass so by the time the signal gets to the tone stack there isn’t much LF for the bass pot to work with. Also the low value (33k) "slope" resistor in the tone stack reduces the range of the tone controls. Try changing it to 56k. There will be more attenuation through the stack but the tone controls will be more effective.

To add more "bottom warmth" you really need to reduce the gain of the amp at mid/treb. The 470p cap on the treb pot lets the upper mids through which can make the amp sound "hard" rather than "warm", a 250p may be better. Try removing the 470p caps across the 470k resistors and the 1n bright cap on the vol pot (or change it to 47-100p). The 0.68u cathode bypass cap on the first stage will also give more mid/treb gain. Remove it or replace it with 25u for more gain at all frequencies. Try fitting a centre off min toggle switch so you can select between 0.68u, 25u and no cap (Steve A. made me say that). It’s great for fine tuning the voicing of your amp to match your guitar and speakers.

These mods won’t cost much and are easy so if you do a mod and the amp ends up sounding good for country rip it out quick.

Dave

7/6/1999 12:49 PM
Dave H.	"How about increasing a coupling cap or two from .022 to .05 or so? All of them? Or just certain stages?" I missed that one. 0.022u into 470k should be big enough to not attenuate the LF of a guitar so increasing the value probably won’t make much difference.

7/6/1999 2:14 PM
Doc	The 800v/400v anode supply voltages and the 11k load resistance value are listed in the Amperex 6CA7/EL34 data sheet under the operating characteristics for a class B amplifier. Grid bias voltage is around -40v, and idle current about 25ma per tube. This amp may sound rather cold until you crank it. In order for a transformer with an 11k primary impedance to have decent bass response, the primary inductance needs to be huge. That means very large laminations and many primary turns. Many class B amps were designed for sound reinforcement (usually movie theaters) where 100hz was the lowest frequency needed at max power. Regular Marshall amp transformers are pretty large, and will easily pass low frequency large signals if the preamp passband is voiced accordingly. Does your output transformer seem about equivalent in physical size to a marshall plexi's?

7/6/1999 2:14 PM

Doc

The 800v/400v anode supply voltages and the 11k load resistance value are listed in the Amperex 6CA7/EL34 data sheet under the operating characteristics for a class B amplifier. Grid bias voltage is around -40v, and idle current about 25ma per tube. This amp may sound rather cold until you crank it.

In order for a transformer with an 11k primary impedance to have decent bass response, the primary inductance needs to be huge. That means very large laminations and many primary turns. Many class B amps were designed for sound reinforcement (usually movie theaters) where 100hz was the lowest frequency needed at max power. Regular Marshall amp transformers are pretty large, and will easily pass low frequency large signals if the preamp passband is voiced accordingly. Does your output transformer seem about equivalent in physical size to a marshall plexi's?

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