JCM 800 mods

[Lane Sparber]

Oh Lane...that was just cold man...stone cold

Maybe so, but all in good fun!

-Lane

 

[peterichardz]

If one of the amazing real techs on the Marshall Amp Forum would write a book on several useful simple tone mods it would sell!

 

[demonufo]

Little known fact: Demonufo IS the Tone Lizard. Sorry to out you in this format, bro, but the time has come for the truth to come out...J/K!





-Lane

SSSSSShhhhhhhhh. I thought I'd slipped in unnoticed.
If I was I think I'd have probably taken that page down by now.

Seriously though, there is a lot of good stuff on his site, but like many, opinion comes into play a lot. (Gawd knows what a site by myself would be like, but I suspect a whole lot worse)
This page is one of the lowlights of his.
He does pop up on some of the forums. He even had the decency to use Tone-Lizard as a username on Plexi Palace.

To be fair, stuff like this got ME started (fell really ill a few years back and had nothing better to do than apply my avionics knowledge, despite being terrified of HT (Had a real bad jolt off a transformer many years back when leaning over to switch the mains off at the plug)). I soon discovered what was wrong with the advice on that page though.

 

[NewReligion]

Awesome thread guys. Thanks for spelling out your opinions about some of the somewhat popular "known" mods and tweaks. This really has helped me as I have just began to understand some of the circuitry.

Much appreciated, David

 

[Jimmy Bondy]

I'm going to mod my 2204 (4104 actually) cause it has too much treble. I allways keep the presence and treble pot set to 0. On this website:

Tales From The Tone Lounge; The Ultimate JCM800?

I found that there are two treble peaking circuits, wich I am going to mod. I've already done the bright cap mod, any other mods you guys recommend??

Mods I'm planning to do:

#1 and #3 from the tales from the tone lounge website

Mods I have already done: Bright cab mod (mod #2 from tales from the tone lounge)

Hi!
Mods i have done to my 2204:

#1 take out 470K/470pf to only a 220k carbon resistor

#2 switch it out

#3 snip out 470pf

#4 piggyback a second 10K resistor so you got 5K, thats enough

last mod: there is a 820k resistor on 2Va bypass it with a 0.68uf cap

 

[Lespaulnmarshall]

So.. the mods I was going to do all suck, so... Wich mods do sound good? And wich parts do I need for em'? I don't want more gain. And I want to get rid of that anoying brightness.

How's the middle pot mod? (25k to 50k)

I still can't flip the board BTW, the wire from the tranny is too short I think, there ain't no filter cap under the board.

This is what I am doing to order to try out all of the mods you guys suggested + the middle pot mod:

SILVER MICA Cap, 22 PF @ 500 V
0.55EUR
SILVER MICA Cap, 150 PF @ 500 V
0.60EUR
SILVER MICA Cap, 500 PF @ 500V
0.95EUR
Resistor Carbon 7.5 KOhm
0.30EUR
SILVER MICA Cap, 250 PF @ 500 V
0.95EUR
MALLORY 150'S Cap, .68 µF @ 100 V
1.55EUR
2X Resistor Metal Film 6.8 KOhm
0.60EUR
Resistor Metal Film 8.2 KOhm
0.30EUR
_______________________________________

5.80EUR

I think it's gonna' be a lot of fun to do thers mods, I just love soldering...

 

[Lespaulnmarshall]

I´ll bump this thread one more time...

 

[TwinACStacks]

My favorite mod is a .68uf cap across R9 820R. Makes the Mids sound Plexi-ish.

TWIN

 

[demonufo]

Just try them carefully one at a time and play it lots to see if you can live with it before going to the next. If you have ANY reservations about any mod you have just done, return it to stock before going onto the next, otherwise you'll end up lost.

Oh yeah, I forgot, I do use one more mod in my amp, and that's the 50K mid pot. Very useful.

 

[demonufo]

My favorite mod is a .68uf cap across R9 820R. Makes the Mids sound Plexi-ish.

TWIN

This is fairly popular 2203/2204 mod. Another is placing a cathode bypass cap on V1b instead (the 10K resistor).

Another thought to maybe reduce the brightness a little - the 4104 has an extra capacitor in the tone circuit that may be improved on it's removal...

 

[marvin]

This is fairly popular 2203/2204 mod. Another is placing a cathode bypass cap on V1b instead (the 10K resistor).

Another thought to maybe reduce the brightness a little - the 4104 has an extra capacitor in the tone circuit that may be improved on it's removal...

I have a 4104 that I had done the T-Liz mods 1,2 &3. After reading this thread I put it back to stock except for your suggestion about using the 250u cap. in place of the stock cap. Wow, it's really brite!!

Please tell me more about the extra cap. in the 4104.

P.S. Does the "plexi mod" work on a 73 100W Superlead as well?
TIA,
Marvin

 

[demonufo]

Please tell me more about the extra cap. in the 4104.

P.S. Does the "plexi mod" work on a 73 100W Superlead as well?
TIA,
Marvin

On a later 4104 (horizontal inputs) this cap is right next to the middle pot at the front of the board. On an earlier version it's actually on the mid pot itself. Easy to remove. This is the same with all the 2204/2203 style combo's, not just the 4104.

Plexi mod may or may not help on a 73 1959, as the earlier circuits will already have v2a cathode resistor bypassed with a .68 cap. They removed these around '72 (or maybe later) but you still see some later amps with them. If your 820ohm v2a cathode resistor isn't paralleled with a .68 cap, then you can by all means try this.

 

[TwinACStacks]

Just try them carefully one at a time and play it lots to see if you can live with it before going to the next. If you have ANY reservations about any mod you have just done, return it to stock before going onto the next, otherwise you'll end up lost.

Oh yeah, I forgot, I do use one more mod in my amp, and that's the 50K mid pot. Very useful.

Demon I tried the 50K Mid pot but I ended up changing it back. I found it affected the Range of the Bass and Treble controls a Tad to much for My personal tastes. I did however Change the Presence pot out to 4.7K with a .68 Cap. I really like what it did for the Presence.

TWIN

 

[TwinACStacks]

THIS is what I actually did to the Head. Some of the mods are non open source and as Joey has asked me not to reveal their Values I Have XXX'd these out:

(RETURN ANY MODS TO STOCK VALUES)

REPLACE FILTER CAPS with new Caps ****DONE Used CE 50/50
Tighten Grounding ring/cap bolts ****DONE
REPLACE All Potentiometers with new ****DONE
Replace 22K PRESENCE pot with a 4k7L pot ****DONE
Replace C17 .1uf for presence control with .68 Sozo ****DONE
Move R3 .68 input resistor from Board to V1 socket and install with shielded Coax to input jack
And replace wire from PreAmp Volume wiper to V1 with shielded Coax ****DONE 1/2W Carbon Film and RG174 Coax
Replace Both input Cliff Jacks ****DONE
Drop Voltage to preamp Replace 10K voltage drop resistor (R29) with XXk 2 Watt Metal Film****DONE
Replace Bias Caps C19 & 20 10 uf with Sprague Atoms****DONE
Change Bias Resistors R26 & 27 56K and 15K****DONE Carbon Film 1/2W
REPLACE ALL DIODES WITH NEW****DONE
Replace Bias splitter resistors R24&25 with new 220K, Also Grid Output 5k6 R31&32****DONE Carbon Film ½ W
Change R22 /100K NFB Resistor to XXk****DONE 1/2W Carbon film
Change C3, C4 and C7 to .0022 to tighten Amp. (Changing C4 (Away) to.0022 will increase low mids) .****DONE Sozo Vintage Caps .0022uf ALSO NEED to install 1M resistor across Preamp pot for C4 Mod ****DONE 1/2W 1 Meg. Carbon Film
Put .68uf Sozo Cap across R9 820R resistor ****DONE
REPLACE C8, C10 with 470pf HF Silver Mica!!!! C2 with 100pf Silver Mica, ALSO C18 with 47pf silver Mica!!!! ****DONE
RE-BIAS AMP (32-36ma) Plate Voltage is 471V
CHANGE ALL SIGNAL CAPS WITH SOZO VINTAGE****DONE
Cap orientations towards tube side of chassis:

C1 .68 AWAY D C16 .022 Towards D
C3 .0022 Towards D C17 .68 AWAY D
C4 .0022 AWAY D
C7 .0022 Towards D
C9 .022 AWAY D ****.68 AWAY Across R9---DONE
C11 .022 Towards D
C12 .022 Towards D
C14 .022 Towards D
C15 .1 AWAY D

ALSO if MODS thin out bass too much it can be added back in without tubbiness by inserting a 220-330K resistor across a XXXXuf cap in SERIES with the XXk NFB resistor R 22 ****DONE using Sozo XXXX uf and 220K carbon film resistor (Sozo oriented towards switch)

TWIN

 

[NewReligion]

THIS is what I actually did to the Head. Some of the mods are non open source and as Joey has asked me not to reveal their Values I Have XXX'd these out:

(RETURN ANY MODS TO STOCK VALUES)

REPLACE FILTER CAPS with new Caps ****DONE Used CE 50/50
Tighten Grounding ring/cap bolts ****DONE
REPLACE All Potentiometers with new ****DONE
Replace 22K PRESENCE pot with a 4k7L pot ****DONE
Replace C17 .1uf for presence control with .68 Sozo ****DONE
Move R3 .68 input resistor from Board to V1 socket and install with shielded Coax to input jack
And replace wire from PreAmp Volume wiper to V1 with shielded Coax ****DONE 1/2W Carbon Film and RG174 Coax
Replace Both input Cliff Jacks ****DONE
Drop Voltage to preamp Replace 10K voltage drop resistor (R29) with XXk 2 Watt Metal Film****DONE
Replace Bias Caps C19 & 20 10 uf with Sprague Atoms****DONE
Change Bias Resistors R26 & 27 56K and 15K****DONE Carbon Film 1/2W
REPLACE ALL DIODES WITH NEW****DONE
Replace Bias splitter resistors R24&25 with new 220K, Also Grid Output 5k6 R31&32****DONE Carbon Film ½ W
Change R22 /100K NFB Resistor to XXk****DONE 1/2W Carbon film
Change C3, C4 and C7 to .0022 to tighten Amp. (Changing C4 (Away) to.0022 will increase low mids) .****DONE Sozo Vintage Caps .0022uf ALSO NEED to install 1M resistor across Preamp pot for C4 Mod ****DONE 1/2W Carbon Film
Put .68uf Sozo Cap across R9 820R resistor ****DONE
REPLACE C8, C10 with 470pf HF Silver Mica!!!! C2 with 100pf Silver Mica, ALSO C18 with 47pf silver Mica!!!! ****DONE
RE-BIAS AMP (32-36ma) Plate Voltage is 471V
CHANGE ALL SIGNAL CAPS WITH SOZO VINTAGE****DONE
Cap orientations towards tube side of chassis:

C1 .68 AWAY D C16 .022 Towards D
C3 .0022 Towards D C17 .68 AWAY D
C4 .0022 AWAY D
C7 .0022 Towards D
C9 .022 AWAY D ****.68 AWAY Across R9---DONE
C11 .022 Towards D
C12 .022 Towards D
C14 .022 Towards D
C15 .1 AWAY D

ALSO if MODS thin out bass too much it can be added back in without tubbiness by inserting a 220-330K resistor across a XXXXuf cap in SERIES with the XXk NFB resistor R 22 ****DONE using Sozo XXXX uf and 220K carbon film resistor (Sozo oriented towards switch)

TWIN

Very Nice Twin. I am familiar with some of these mods. Joey was kind enough to turn my 2203X into a Scary MoFo!

David

 

[TwinACStacks]

Yep David, He put up with my stupid questions and Got my 2204 Scary good as well. His Guidance is Greatly appreciated.

I really think lowering the Voltage to these 800's preamps REALLY helps them a lot.

TWIN

 

[Lespaulnmarshall]

I judt love this thread. I still have one question though, I still can not flip the PCB, I have removed the pots, and the inputs, and I still can't flip it. And there ain't no preamp filter cap under the PCB.

 

[Strateuphoria]

(((ppimv)))

 

[demonufo]

I judt love this thread. I still have one question though, I still can not flip the PCB, I have removed the pots, and the inputs, and I still can't flip it. And there ain't no preamp filter cap under the PCB.

I think we need to see some pictures.

 

[TwinACStacks]

I'm not familiar with the combo but could there be some kind of a mounting/chassis standoff bolt under the PCB somewhere? Look for some brass mounting nuts flush with the PCB Board.

TWIN