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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

Replies:

Rebel420 First off, congrats!! I think I"d m... -- 3/13/2000 9:33 PM
Robert Hi Kevin,I am glad ... -- 3/20/2000 4:32 AM