D
dailey
New member
Strat players with Marshall’s who love Hendrix, Clapton, Blackmore & Yngwie will enjoy this amplifier design. The idea behind this design was to re-create the “recorded tone” of a dimed Marshall plexi that is using a treble booster or preamp “gain box” in the front end, close miked, run through a mixer & out-board gear applied in the final mix down. All this at a stage volume level suitable for clubs without a rack full of out-board gear & isolated speaker boxes. Not an easy task you say? Well, as it turns out the solution was much simpler than you would think. I’ll try to explain the design a section at a time.
The Preamp Part A:
A common path for all us tweakers is to build every multiple gain stage preamp we see a schematic for. Play through it, look at the wave form and decide if it’s a keeper. The same is true for boosters & gain boxes used by our favored players. Yet without the total package ( right guitar, speaker boxes, mikes & studio gear etc… ) we fall a little short of the target but come close enough to continue to pursue it relentlessly. Adding snubber caps to make it stable, slowly building gain through multiple stages, different flavors of cap values, filters & voltages at each stage, shunting high’s to ground at various stages while carefully adding treble peakers at voltage dividers & various other places in the circuit only to end up with something that sounds like you threw a blanket over the speaker cab. And then we start allover, from the beginning, again! Sound familiar? Yah I know! While running sims & studying graphs of tube gain stages and various pedals used by my favored players ( Dallas fuzzface & Rangemaster, Hornby Skewes Treble booster & DOD 250 preamp ) that although not exact, the frequency response curve & amounts of gain available are very similar too common design single and dual tube gain stages. Not really breaking news to any of us but, those facts made me dig into another issue, the love/ hate relationship of effects loops.
The Preamp/ effects loop Part B:
Running sims again made me realize that instead of trying to “stick” this circuit in between a good sounding preamp & power amp combo. The effects loop circuit itself could and should be a part of the final stages of the preamp gain structure design. Looking at the over all guitar sound of our hero’s as a whole, plus knowing a little about how they were recorded and when effects where applied to the mix made me look at how channels of mixers where designed and how or if it could be done with one triode in a guitar amplifier circuit. After all, a mixer does exactly what we want the loop to do right? The resulting circuit you see in the schematic is not only a mixing circuit but a final gain stage of the preamp design. The send & return level pots can be turned all the way up or down without changing the preamp signal at all because it “is” part of the preamp and not an add-on circuit. Parallel by design it is meant to use outboard studio grade gear to enhance the guitar signal like a studio mix but, with the addition of a SPDT switch could be made series as well. Almost every major amplifier manufacturing company today is trying to give the amp buyer these result with built-in effects and modeling technology. I personally prefer to use the outboard gear of my choice to enhance my guitar sound live and in the studio.
The Preamp/ CF & tone stack Part C:
The amplifier would not sound like a Marshall without the common values applied so no changes here.
The Preamp/ conclusion:
1st gain stage standard Marshall plexi circuit with .022uf coupling cap 1meg gain pot with .005uf bypass cap to 470k voltage divider with .001uf bypass cap instead of 470p to put a little upper mid bump in the signal, then off to the stock CF/ tone stack via the relay position.
2nd gain stage off 1st stage .022uf coupling cap via relay position, to 470k/ .003uf cap high mid filter pairing, to 1meg gain pot to 68k grid leak resistor. Standard Marshall values apply again except for the plate resistor is 150k. This stage is voiced similar to the Hornby Skewes treble booster that Blackmore used in his signal chain. From here it’s off to the stock CF/ tone stack stage via the Blackmore/ YJM switch position or to the third gain stage.
3rd gain stage off 2nd stage .022uf coupling cap to 470k voltage divider. 100k plate with 10k cathode resistor. .022uf coupling cap to stock CF/ tone stack via the Blackmore/ YJM switch position. Similar to how the DOD 250 preamp is designed, this stage is a gain multiplier of sorts. Plenty of “useable” gain provided here for the YJM tone & feel.
4th gain stage is a unity gain stage provided by the parallel out & in circuit before the stock PI. I have a hard time calling this circuit a loop since the return circuit could insert almost any type of line level signal into the amplifier “without” affecting the signal from the previous 3 stages.
NOTE: level controls for plexi mode & Blackmore/ YJM mode are provided to balance the volume levels between the 3 modes!
The power amp:
Standard Marshall phase inverter values are applied however, the presence pot is a 5k with a 47k tail to the 4ohm tap. The idea within the small changes you’ll see throughout the power amp are too emulate an almost “dimed feel” to the amplifier.
PPIMV:
Of course the LAR/MAR style master volume is a great & proven choice for this amplifier. The vintage/ modern Marshall uses a post PIMV as well as many others today and for club gigs this is a must. VVR or power scaling is yet another option.
Bias:
The bias supply is a standard bias circuit tied to the PPIMV in the usual way so no surprises here.
Power supply:
The filtering and HV dropping string are not standard Marshall but are of the design of Ken Fishers famous Express lead amplifier. Having built a few clones of these you find out very quickly they distort from the power amp first. To emulate the dimed plexi feel and sound at gig volume levels the Express circuit design works better than any other circuit.
Transformers:
I have built this same amplifier circuit using Fender Bassman transformers with 6L6’s, TrainWreck Clone transformer’s as well as Marshall clone transformer’s with EL34s. And the overall performance of this amplifier design was outstanding. The schematic shows the standard JCM 800 Drake transformers but don’t feel limited to using only those.
Credits go to:
Nothing about this amplifier is of my design, it is a combination of tried & true simple circuits that play well with each other:
Preamp & power amp credits go to Marshall, they are just split up by relays & SW’s.
Effects loop/ mixer credits go to early 70’s Fender PA head & KOC’s BFX design.
PPIMV credits go to Larry & Mark @ the metroforum.
Power supply & filtering credits go to Ken Fisher & his Express design.
Layout design:
The best chassis I’ve used is from a Laney donor with 5 triode & 4 octal punched holes already in it. Short leads as always & keep the gain & level pots close to the input jack. Use V2 for the loop and hang the circuit components off the socket, jacks & pots for it. Everything else on the board will fall into place like a normal Marshall layout. I have built this in Twreck chassis, Bassman chassis & 18”x8”x2-1/2”. But you have to stagger the extra preamp holes & it makes the leads a little longer off the board!
Conclusion:
Years ago I disliked multi-tube preamp designs, but today its common and needed in order to have your cake & eat it too. Fender has never been shy about adding more valves for a specific function as far back as the early 60’s. This is a good sounding amplifier with simple circuits & mod friendly. I hope to see some great improvements on this circuit from the brilliant designers that read this material on various forums. I’m a player first, builder second & designer is way down on the list!
The Clips:
I used a stock 2007 strat with maple fretboard & HS3’s in the neck & bridge positions.
1983 Marshall slant cab with G12H 30’s.
ART multi-verb in the loop.
SM 57 close miked.
1973 Moog Taurus pedals.
Fostex VF-16 Digital recorder.
Audacity for MP3 converter.
https://soundclick.com/ddailey
The Preamp Part A:
A common path for all us tweakers is to build every multiple gain stage preamp we see a schematic for. Play through it, look at the wave form and decide if it’s a keeper. The same is true for boosters & gain boxes used by our favored players. Yet without the total package ( right guitar, speaker boxes, mikes & studio gear etc… ) we fall a little short of the target but come close enough to continue to pursue it relentlessly. Adding snubber caps to make it stable, slowly building gain through multiple stages, different flavors of cap values, filters & voltages at each stage, shunting high’s to ground at various stages while carefully adding treble peakers at voltage dividers & various other places in the circuit only to end up with something that sounds like you threw a blanket over the speaker cab. And then we start allover, from the beginning, again! Sound familiar? Yah I know! While running sims & studying graphs of tube gain stages and various pedals used by my favored players ( Dallas fuzzface & Rangemaster, Hornby Skewes Treble booster & DOD 250 preamp ) that although not exact, the frequency response curve & amounts of gain available are very similar too common design single and dual tube gain stages. Not really breaking news to any of us but, those facts made me dig into another issue, the love/ hate relationship of effects loops.
The Preamp/ effects loop Part B:
Running sims again made me realize that instead of trying to “stick” this circuit in between a good sounding preamp & power amp combo. The effects loop circuit itself could and should be a part of the final stages of the preamp gain structure design. Looking at the over all guitar sound of our hero’s as a whole, plus knowing a little about how they were recorded and when effects where applied to the mix made me look at how channels of mixers where designed and how or if it could be done with one triode in a guitar amplifier circuit. After all, a mixer does exactly what we want the loop to do right? The resulting circuit you see in the schematic is not only a mixing circuit but a final gain stage of the preamp design. The send & return level pots can be turned all the way up or down without changing the preamp signal at all because it “is” part of the preamp and not an add-on circuit. Parallel by design it is meant to use outboard studio grade gear to enhance the guitar signal like a studio mix but, with the addition of a SPDT switch could be made series as well. Almost every major amplifier manufacturing company today is trying to give the amp buyer these result with built-in effects and modeling technology. I personally prefer to use the outboard gear of my choice to enhance my guitar sound live and in the studio.
The Preamp/ CF & tone stack Part C:
The amplifier would not sound like a Marshall without the common values applied so no changes here.
The Preamp/ conclusion:
1st gain stage standard Marshall plexi circuit with .022uf coupling cap 1meg gain pot with .005uf bypass cap to 470k voltage divider with .001uf bypass cap instead of 470p to put a little upper mid bump in the signal, then off to the stock CF/ tone stack via the relay position.
2nd gain stage off 1st stage .022uf coupling cap via relay position, to 470k/ .003uf cap high mid filter pairing, to 1meg gain pot to 68k grid leak resistor. Standard Marshall values apply again except for the plate resistor is 150k. This stage is voiced similar to the Hornby Skewes treble booster that Blackmore used in his signal chain. From here it’s off to the stock CF/ tone stack stage via the Blackmore/ YJM switch position or to the third gain stage.
3rd gain stage off 2nd stage .022uf coupling cap to 470k voltage divider. 100k plate with 10k cathode resistor. .022uf coupling cap to stock CF/ tone stack via the Blackmore/ YJM switch position. Similar to how the DOD 250 preamp is designed, this stage is a gain multiplier of sorts. Plenty of “useable” gain provided here for the YJM tone & feel.
4th gain stage is a unity gain stage provided by the parallel out & in circuit before the stock PI. I have a hard time calling this circuit a loop since the return circuit could insert almost any type of line level signal into the amplifier “without” affecting the signal from the previous 3 stages.
NOTE: level controls for plexi mode & Blackmore/ YJM mode are provided to balance the volume levels between the 3 modes!
The power amp:
Standard Marshall phase inverter values are applied however, the presence pot is a 5k with a 47k tail to the 4ohm tap. The idea within the small changes you’ll see throughout the power amp are too emulate an almost “dimed feel” to the amplifier.
PPIMV:
Of course the LAR/MAR style master volume is a great & proven choice for this amplifier. The vintage/ modern Marshall uses a post PIMV as well as many others today and for club gigs this is a must. VVR or power scaling is yet another option.
Bias:
The bias supply is a standard bias circuit tied to the PPIMV in the usual way so no surprises here.
Power supply:
The filtering and HV dropping string are not standard Marshall but are of the design of Ken Fishers famous Express lead amplifier. Having built a few clones of these you find out very quickly they distort from the power amp first. To emulate the dimed plexi feel and sound at gig volume levels the Express circuit design works better than any other circuit.
Transformers:
I have built this same amplifier circuit using Fender Bassman transformers with 6L6’s, TrainWreck Clone transformer’s as well as Marshall clone transformer’s with EL34s. And the overall performance of this amplifier design was outstanding. The schematic shows the standard JCM 800 Drake transformers but don’t feel limited to using only those.
Credits go to:
Nothing about this amplifier is of my design, it is a combination of tried & true simple circuits that play well with each other:
Preamp & power amp credits go to Marshall, they are just split up by relays & SW’s.
Effects loop/ mixer credits go to early 70’s Fender PA head & KOC’s BFX design.
PPIMV credits go to Larry & Mark @ the metroforum.
Power supply & filtering credits go to Ken Fisher & his Express design.
Layout design:
The best chassis I’ve used is from a Laney donor with 5 triode & 4 octal punched holes already in it. Short leads as always & keep the gain & level pots close to the input jack. Use V2 for the loop and hang the circuit components off the socket, jacks & pots for it. Everything else on the board will fall into place like a normal Marshall layout. I have built this in Twreck chassis, Bassman chassis & 18”x8”x2-1/2”. But you have to stagger the extra preamp holes & it makes the leads a little longer off the board!
Conclusion:
Years ago I disliked multi-tube preamp designs, but today its common and needed in order to have your cake & eat it too. Fender has never been shy about adding more valves for a specific function as far back as the early 60’s. This is a good sounding amplifier with simple circuits & mod friendly. I hope to see some great improvements on this circuit from the brilliant designers that read this material on various forums. I’m a player first, builder second & designer is way down on the list!
The Clips:
I used a stock 2007 strat with maple fretboard & HS3’s in the neck & bridge positions.
1983 Marshall slant cab with G12H 30’s.
ART multi-verb in the loop.
SM 57 close miked.
1973 Moog Taurus pedals.
Fostex VF-16 Digital recorder.
Audacity for MP3 converter.
https://soundclick.com/ddailey
