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73 Marshall rebuild...I did it!!!!!!!

3/13/2000 9:19 PM
kevin Hamilton	73 Marshall rebuild...I did it!!!!!!! First, a BIG thank you to everyone who answered all my Marshall questions. I could not have done it without your input....from the Hoffman board, to the fried OP tranny, and finally all the historic questions about the different eras of JTM, Lead and Superlead amps. Here's the story and the parts that came together in building this amp. First, I traded for this amp (73 Marshall SL) and it was in great condition. The problem was that it was way too bright and the distortion just didn't sound right. It was harsh, brittle, just plain bad. I decided to get a Hoffman board so I could experiment with different values to maybe get the tone I was looking for. The board was great and was not too difficult to install. I fired up the amp, rebiased, and again was greeted by the same crappy tone. Now depression was setting in. Anyway, I rechecked the board connections and they were fine. But then I noticed something was out of place....the negative feedback wire was attached to the 4 ohm tap(This is how the amp came to me. You don't need to move the tap when installing the board). I moved it to the 8 ohm tap and got some terrible crackling noises. Same thing on the 16 ohm tap as well. Then I disconnected the NF altogether and tried each speaker output tap. The 8 and 16 ohm produced sound, but not the 4 ohm. Well, after a bunch of questions and tests, it was determined the OP was crapped out. I ordered a Mercury Magnetics OP transformer from the Plexi Palace. Well, it showed up this morning and I installed it. This one thing solved ALL the problems! This fucking amp has balls, crunch, sweet chocolatey distortion, and tone for days. Do you know how it feels to have a shit eating grin on your face that you just can't get rid of? OK. Now I tell you how it's all set up. First, if you are into strictly vintage parts, you may want to stop here..... The resistors are all Halco 1 watt audiofiles, except for along the power supply rail. Tubes are Telefunken EC83's and Svetlana EL34's biased at 34ma with 514V on the plates. I have a mixture of caps... for the bass channel I'm using a 220uf Black gate graphite cap with a 825ohm resistor. The bright channel has a Solen Fast Cap(French) .68uf cap with another 825ohm resistor(Not a 2.7k/.68). The second preamp tube currently has a 1k resistor with a Solen .68uf cap. The preamp coupling caps are both .02 (no .0022), the eq resistor is 33k with treble at 250pf instead of 500pf. Right now, but subject to change, there is a 500pf cap on the 470mix resistor and a 500pf cap on the bright channel. Believe it or not, the amp isn't too trebly. The phase inverter has .1 coupling caps and 100k resistors, instead of .02 and 220k. Also, the preamp coupling, EQ, and phase inverter caps are all paper-in-oil Sequa brand from Angela Instruments. They were around $3 a piece. I am also using a .68 solen cap on the presence knob with a 4.7k resistor. To my ear the presence is not as harsh as the .01uf cap. Finally, I pull out one of the 4 speaker jacks and put a SPDT swith in it's place. In one position, the amp runs like a pre 68 plexi with pins 3 and 8 of the first preamp tube jumpered and sharing the same 825ohm/220uf Blackgate cap. In the other position its like a 68-70 amp were pins 3 and 8 have their own cap and resistor. The pre 68 position is so smooth and creamy, but there's also some serious distortion going on. The 68-70 position has more gain, presence, and bite, but not that trebly ear piercing 70's SL tone. I know mods are sometimes frowned on, but this is easily reversable AND you get close to 2 different Marshall tones. About the only things I might change is the 500pf cap's on the bright volume knob and mix resistor, and I might remove the .68 cap on the second preamp tube (pin 3) Again, thanks for all the input. I not a techy so this project would have been impossible without your knowledge and experience. Kevin

3/13/2000 9:19 PM

kevin Hamilton

73 Marshall rebuild...I did it!!!!!!!

First, a BIG thank you to everyone who answered all my Marshall questions. I could not have done it without your input....from the Hoffman board, to the fried OP tranny, and finally all the historic questions about the different eras of JTM, Lead and Superlead amps.

Here's the story and the parts that came together in building this amp.

First, I traded for this amp (73 Marshall SL) and it was in great condition. The problem was that it was way too bright and the distortion just didn't sound right. It was harsh, brittle, just plain bad.

I decided to get a Hoffman board so I could experiment with different values to maybe get the tone I was looking for. The board was great and was not too difficult to install. I fired up the amp, rebiased, and again was greeted by the same crappy tone. Now depression was setting in.

Anyway, I rechecked the board connections and they were fine. But then I noticed something was out of place....the negative feedback wire was attached to the 4 ohm tap(This is how the amp came to me. You don't need to move the tap when installing the board). I moved it to the 8 ohm tap and got some terrible crackling noises. Same thing on the 16 ohm tap as well. Then I disconnected the NF altogether and tried each speaker output tap. The 8 and 16 ohm produced sound, but not the 4 ohm. Well, after a bunch of questions and tests, it was determined the OP was crapped out. I ordered a Mercury Magnetics OP transformer from the Plexi Palace. Well, it showed up this morning and I installed it.

This one thing solved ALL the problems! This fucking amp has balls, crunch, sweet chocolatey distortion, and tone for days. Do you know how it feels to have a shit eating grin on your face that you just can't get rid of?

OK. Now I tell you how it's all set up. First, if you are into strictly vintage parts, you may want to stop here.....

The resistors are all Halco 1 watt audiofiles, except for along the power supply rail.

Tubes are Telefunken EC83's and Svetlana EL34's biased at 34ma with 514V on the plates.

I have a mixture of caps... for the bass channel I'm using a 220uf Black gate graphite cap with a 825ohm resistor. The bright channel has a Solen Fast Cap(French) .68uf cap with another 825ohm resistor(Not a 2.7k/.68). The second preamp tube currently has a 1k resistor with a Solen .68uf cap.

The preamp coupling caps are both .02 (no .0022), the eq resistor is 33k with treble at 250pf instead of 500pf.

Right now, but subject to change, there is a 500pf cap on the 470mix resistor and a 500pf cap on the bright channel. Believe it or not, the amp isn't too trebly.

The phase inverter has .1 coupling caps and 100k resistors, instead of .02 and 220k.

Also, the preamp coupling, EQ, and phase inverter caps are all paper-in-oil Sequa brand from Angela Instruments. They were around $3 a piece.

I am also using a .68 solen cap on the presence knob with a 4.7k resistor. To my ear the presence is not as harsh as the .01uf cap.

Finally, I pull out one of the 4 speaker jacks and put a SPDT swith in it's place. In one position, the amp runs like a pre 68 plexi with pins 3 and 8 of the first preamp tube jumpered and sharing the same 825ohm/220uf Blackgate cap. In the other position its like a 68-70 amp were pins 3 and 8 have their own cap and resistor. The pre 68 position is so smooth and creamy, but there's also some serious distortion going on. The 68-70 position has more gain, presence, and bite, but not that trebly ear piercing 70's SL tone. I know mods are sometimes frowned on, but this is easily reversable AND you get close to 2 different Marshall tones.

About the only things I might change is the 500pf cap's on the bright volume knob and mix resistor, and I might remove the .68 cap on the second preamp tube (pin 3)

Again, thanks for all the input. I not a techy so this project would have been impossible without your knowledge and experience.

Kevin

3/13/2000 9:33 PM

Rebel420

First off, congrats!! I think I"d marry my '73 if I could *laughs*

quote:
"I decided to get a Hoffman board so I could experiment with different values to maybe get the tone I was looking for. "

See, you go to point to point wiring, and you give the tone gremlins a place to live!

quote:
"But then I noticed something was out of place....the negative feedback wire was attached to the 4 ohm tap(This is how the amp came to me. You don't need to move the tap when installing the board)."

If I remember correctly, the 6550 powered units needed the feedback on the 4ohm tap... ironically, it was the one thing that wasnt done when my '73 came to me switched over to el34's.

quote:
" I know mods are sometimes frowned on, but this is easily reversable AND you get close to 2 different Marshall tones."

I think it's the mods that render an amp into a noisy, unreliable piece of crap, and then gets sold to the next victim the mods that get frowned upon. I look at it tihs way, I'm willing ot do any mods on my amps, only because Im not goin to sell them etc... but for smeone else, i suggest eveyrthing to make all the mods easily reversible.

3/20/2000 4:32 AM
Robert	Hi Kevin, I am glad to hear things went well with the rebuild. I am hoping to have a shit eating grin on my face sometime soon. I just removed the "Drake" output transformer on my 50 watt plexi reissue. I am replacing it with one from Obsolete Electronics. I hope this makes the "big" difference for me. I also have the hoffman board which really makes it handy to experiment with component changes. The only changes I have made so far is replacing the .02 phase Inverter coupling caps to 0.1uf. I have not tried replaceing the 220k's. What does this resistor change to 100k do here? What tap did you eventually use for negative feedback? Now we get to ask you the questions !! Thanks, Rob

3/20/2000 4:32 AM

Robert

Hi Kevin,

I am glad to hear things went well with the rebuild.
I am hoping to have a shit eating grin on my face sometime soon. I just removed the "Drake" output transformer on my 50 watt plexi reissue. I am replacing it with one from Obsolete Electronics. I hope this makes the "big" difference for me. I also have the hoffman board which really makes it handy to experiment with component changes. The only changes I have made so far is replacing the .02 phase Inverter coupling caps to 0.1uf. I have not tried replaceing the 220k's. What does this resistor change to 100k do here? What tap did you eventually use for negative feedback?

Now we get to ask you the questions !!

Thanks,

Rob

3/20/2000 12:35 PM
kevin Hamilton	I was told that changing to 100k resistors made the high voltage easier on the power tubes and also make the tone "browner". I don't know exactly how, but since I had the amp apart, I figured what the hell. As for negative feedback, I have a choice. Let me explain what "extra things" I did to the amp to give me more tonal options. My 73 Superlead has 4 speaker output jacks in the back and also a polarity switch up front. When I redid the amp I rewired the AC power for a 3 prong plug and completely bypassed the polarity switch. What I use it for now is to select which size bias resistor and speaker tap I want to use. One position is 27k/16 ohm tap like the JTM45 and early plexis. The other position is 47k/8 ohm tap like later 68-70 Marshalls. I also replaced 2 of the 4 speaker outputs with two way switches. The first switch lets me either jumper or split the V1 cathodes. The second switch lets me either use the .68 cap on V2 cathode pin 3 or bypass it. All of these "mods" are completely reversable and there really is a difference in tone for each position. As far as tone differences with the NFB you definately hear it. In the 27k/16 ohm position the amp is cleaner, darker, and smoother. It's a very bluesy tone. The 47k/8 ohm tap is louder, rawer sounding, and has more gain. Now, if you were to try one, then unsolder it, and put in the other to try, you might not notice that much of a difference. With a switch you just turn the amps standby switch off, select the new position, turn the standby switch on, and play. The difference just jumps out at you. Same thing goes for the spilt/jumpered switch and the .68 in/out switch. Kevin

3/20/2000 12:35 PM

kevin Hamilton

I was told that changing to 100k resistors made the high voltage easier on the power tubes and also make the tone "browner". I don't know exactly how, but since I had the amp apart, I figured what the hell.

As for negative feedback, I have a choice. Let me explain what "extra things" I did to the amp to give me more tonal options.

My 73 Superlead has 4 speaker output jacks in the back and also a polarity switch up front. When I redid the amp I rewired the AC power for a 3 prong plug and completely bypassed the polarity switch. What I use it for now is to select which size bias resistor and speaker tap I want to use. One position is 27k/16 ohm tap like the JTM45 and early plexis. The other position is 47k/8 ohm tap like later 68-70 Marshalls.

I also replaced 2 of the 4 speaker outputs with two way switches. The first switch lets me either jumper or split the V1 cathodes. The second switch lets me either use the .68 cap on V2 cathode pin 3 or bypass it.

All of these "mods" are completely reversable and there really is a difference in tone for each position. As far as tone differences with the NFB you definately hear it. In the 27k/16 ohm position the amp is cleaner, darker, and smoother. It's a very bluesy tone. The 47k/8 ohm tap is louder, rawer sounding, and has more gain. Now, if you were to try one, then unsolder it, and put in the other to try, you might not notice that much of a difference. With a switch you just turn the amps standby switch off, select the new position, turn the standby switch on, and play. The difference just jumps out at you. Same thing goes for the spilt/jumpered switch and the .68 in/out switch.

Kevin

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