JTM-45 Mod Suggestions

[Ed_Chambley]

Hey Guys,
I have built a JTM-45 clone (Mojotone Clone) using EL-34's and have all 12Ax7's RCA Black Plates.  I am using a Weber Copper cap for the rectifier.  The only mod is a Dual 500 pot as a PPMIV.  The amp has a great sound using my Les Paul and really creates that old Clapton sound, I would like to add some additional switchable (if possible) mods to brighten up the amp to move more in the direction of a JCM-800 ( I built one of these as well so I am not looking to turn the 45 into an 800, just add some variety in the tone). 

The JTM-45 is a very bass heavy amp which lends itself to that early Marshall tone, I would like to be able to get more versatility in tone as currently the Bass can only be run around 2 or lower and with a single coil TELECASTER it becomes very boomy and muddy, which I don't understand.  I would think the Humbucker would be a larger problem in this area.  The amp is incredible in single not solos, but chords is a different story.  Hit a low E chord and the speakers just fart away and distort unless I use an eq pedal.  The cabinet is a custom 2 x12 with a Weber Blue Dog and Silver bell AlniCO, 50 watts at 8 ohms.

I have attached the layout I followed except I placed one of the filter caps below the turrent board for less noise and shorter wire use.

[g-man]

Deleted

[LooseChange]

The Marshall classic cathode cap is a .68uf.
Did you include the bright cap on the volume control?

[Ed_Chambley]

I built the same kit a few years ago but I use 6L6's and tube rectifier. These suggestions may be obvious to you, but I changed the V1 cathode cap from 250u to 25u to tame the bass a bit. You could go even lower. I also jumper the normal and bright channels and adjust the individual channel volumes to balance it out more. That was enough for me but I'm sure you could lower some coupling cap values too.

I have used both tube rectifier and the weber copper cap.  Did not really hear any difference, just more negative bias with the tube.

[Ed_Chambley]

The Marshall classic cathode cap is a .68uf.
Did you include the bright cap on the volume control?

So check out a .68uf cap along with the 820k resistor instead of the 250uf.  I did indeed put in a cap on the treble volume.  Actually I made a clip-in to audition them.  I tried many different values of silver mica caps.  Actually ended up with a 222k sozo poly cap for now.  I did not hear lots of differences until I got above 330p in the silver mica, then it begun to get harsh.  It was actually ok with a 500p.  Seems like the amp has a muffled sound.  So the first e-cap is a good place to experiment?

[Ed_Chambley]

The Marshall classic cathode cap is a .68uf.
Did you include the bright cap on the volume control?

I have seen the use of the .68uf and 2.7k resistor tied to a 330uf and 820k resistor for v1a and v1b cathode the wired to pin 3 and 8 of v1.  I cannot remember the Marshall amp where this is used.  I cannot recall seeing a .68uf by itself as a cathode cap.  Could you explain as I am uncertain using a single .68uf.

[sluckey]

You're thinking of the JMP 1987 (Plexi).

http://www.el34world.com/charts/Schematics/files/marshall/jmp_lead_50w_1987.pdf

Very easy to make your V1 circuit look just like the Plexi. Then get a short patch cord to jump the normal and bright channels together. Very versatile. Probably all you would ever want to do.

[Ed_Chambley]

You're thinking of the JMP 1987 (Plexi).

http://www.el34world.com/charts/Schematics/files/marshall/jmp_lead_50w_1987.pdf

Very easy to make your V1 circuit look just like the Plexi. Then get a short patch cord to jump the normal and bright channels together. Very versatile. Probably all you would ever want to do.

Yep, that is what I have seen.  Now the type of components and hook up.  On one end they all go to ground?  Remove the e-cap which is 250/25 and replace it with the 320 at what voltage?  What voltage should the .68u be and should it be an e-cap?  Do they all have a common connection going to pin 3 and 8 of v1, or should they be separated with the 320 cap and resistor going to 3 and the .68u and resistor going to pin 8 on v1.  Sorry for the noobie question, but I remember reading somewhere where it refers to this as v1a and v1b.

[sluckey]

...should they be separated with the 320 cap and resistor going to 3 and the .68u and resistor going to pin 8 on v1.

Yes. Cut the jumper between pins 3 and 8. Both caps can be rated as low as 25V. The .68µF is not an electrolytic.

One other thing... See the 0.022µF cap connected to pin 6? Replace that cap with a 0.0022µF.

Look at this layout to see exactly how to do this...

http://www.ceriatone.com/images/layoutPic/marshallLayout/Plexi50LeadCeriatone.jpg

but I remember reading somewhere where it refers to this as v1a and v1b.

Yes. V1A is pins 1, 2, 3. V1B is pins 6, 7, 8.

[Ed_Chambley]

...should they be separated with the 320 cap and resistor going to 3 and the .68u and resistor going to pin 8 on v1.

Yes. Cut the jumper between pins 3 and 8. Both caps can be rated as low as 25V. The .68µF is not an electrolytic.

I just made a cap order a couple of days ago and did not know to order the 320uf.    What difference, if any will I hear by leaving the 250 in place and just adding the .68uf?  I hate ordering just a couple of parts, it seems like the shipper gets all the money.

[sluckey]

Probably none.

[thermion]

+1 on probably none.

[Tone Junkie]

If its overly bassy you do not want 320uf or 250 uf, 10uf to 4.7uf would be much better and definitely change the other cap and resister to the 2.7k and .68 that will get rid of  what your hearing i.
 That first cap lets everything through so low it gets you that farty boomy bass if you really want to clean it up you could go as low as 2.2uf with an 820ohm resister that would only let everything through at 82hrtz which is the resonance of your low E string on the guitar.
Thanks Bill

[Ed_Chambley]

If its overly bassy you do not want 320uf or 250 uf, 10uf to 4.7uf would be much better and definitely change the other cap and resister to the 2.7k and .68 that will get rid of  what your hearing i.
 That first cap lets everything through so low it gets you that farty boomy bass if you really want to clean it up you could go as low as 2.2uf with an 820ohm resister that would only let everything through at 82hrtz which is the resonance of your low E string on the guitar.
Thanks Bill

The idea of going as low as 2.2uf in place of the 250 which currently is there, does this cap need to be electrolytic?  I hace non-polarized in this value. 

What is the purpose of this being an e-cap? 

Also, you comment of the .68 and 2.7k.  These will be additions as the mojo layout jumpers pin 3 and 8 with the 250/25 and 820 resistor.  I will be adding the .68 and the 2.7 to pin 8 and cutting the jumper.  I have many parts to experiment with, I am just limited in the e-cap area, but I have a fairly good assortment, but I am certain I don't have a 2.2uf in an e-cap.  I don't recall ever seeing one.  I do have it in poly such as orange drop, mallory and SoZo.  Can these be used?  I don't see why not, but maybe someone can enlighten me.

[sluckey]

The idea of going as low as 2.2uf in place of the 250 which currently is there, does this cap need to be electrolytic?  I hace non-polarized in this value. 

What is the purpose of this being an e-cap?

It doesn't have to be electrolytic. Electrolytics are more common and widely available in that value. Cheaper too.

If you still intend to mod the input to Plexi, I'd leave the 250µF in place. This will give you a dark channel and a bright channel and by jumping the inputs you can get a wide range of tones. The dark channel sounds too bassy/muddy to me when used by itself, but it really adds some body to the chime of the bright channel when jumpered. You can always change the 250µF later if you're not happy with the Plexi voicing. I really believe you'll not be disappointed though.

Replacing the 250µF with a 2.2µF makes a lot of sense if you want a minimal mod, keeping the cathodes tied together. It will definitely cut down on the bass.

[Ed_Chambley]

The idea of going as low as 2.2uf in place of the 250 which currently is there, does this cap need to be electrolytic?  I hace non-polarized in this value. 

What is the purpose of this being an e-cap?

It doesn't have to be electrolytic. Electrolytics are more common and widely available in that value. Cheaper too.

If you still intend to mod the input to Plexi, I'd leave the 250µF in place. This will give you a dark channel and a bright channel and by jumping the inputs you can get a wide range of tones. The dark channel sounds too bassy/muddy to me when used by itself, but it really adds some body to the chime of the bright channel when jumpered. You can always change the 250µF later if you're not happy with the Plexi voicing. I really believe you'll not be disappointed though.

Replacing the 250µF with a 2.2µF makes a lot of sense if you want a minimal mod, keeping the cathodes tied together. It will definitely cut down on the bass.

I plan on doing the plexi mod first, my questions were just to make certain if I experiment I don't do anything wrong.  When I was building the board, I questioned jumpering pin 8 and 3.  I have not built but one Marshall based amp, sort of an 800 but I stole a lot  of the circuit from a Fargen ole 800 and it did not have the jumper.  Great sounding amp for modern tones, but definately not like a JTM 45 or a plexi.  Seems like the mods you suggested is what I am looking for as I will still be able to have that JTM 45 and add the versatility of cleaning it up some.  That is what I am after, but if it is still a little dark I just wanted to make sure what to change.  I have to tell you I am a little more than curious how this mod will sound.  Thanks for the guidance, I will report back when complete.

[Tone Junkie]

I really do like the sound of a jumpered plexi just like Sluckey said add a little of one balance with the other a good sounding amp from that standpoint.
Bill
I was just saying with an 820 resister you could go down as far as a 2.2uf and still have the full bandwidth, i think you will really enjoy the 2 channels once you split the cathode caps and then jumper them together with a cord

[Ed_Chambley]

Thanks for the suggestions of the Mods.  You guys are great.
Completed the plexi mod last night,  Wow, it really turned out nice.  Still have the JTM 45 as well as many different tones available.  I did ending up raising the 2k7 resistor value some.  Not much, buy experimenting with it I found it really change how the high end sounds.  It was a little harsh to me when I changed the .022 cap to .0022.  I ended up with 3k3 with the ,68k cap.  The doesn't seem like it change very much, but it does.  I was surprised.  Maybe someone could enlighten me as to why.

I installed in the back a SPST switch so I can wire in a channel jumper and have a couple of pieces of shielded cable soldered to it.  Grounded at the switch.  Before I go poking around in there, maybe someone could tell me where is the best place to wire this up would be?

[rock_mumbles]

I've just spent some time looking into this ... with small cathode resistors 680ohm to 1k ohm the cathode cap has less effect on the shape of the tone than with a larger cathode cap, also when tuning with the cathode cap you end up with a low frequency shelf, a rise through the half-boost frequency depending on the resistor(s) and cap, and then a high frequency shelf ... it looks kind of like this:
       ----
     /
---- 
with small value cathode caps in the 820 ohm range with a 0.68uf bypass cap the difference between the low frequency shelf and high frequency shelf is only about 4 dB

with a large value cathode cap like the 2.7k with a 0.68uf bypass cap the difference between the low frequency shelf and high frequency shelf is about 9 dB

A coupling cap has a different frequency response, it's a standard high pass filter that looks like this:
   ----
 /
/

So with a cold biased stage with large cathode resistor and correct sized cap you are able to control the low frequency content.  With a warm biased stage with small cathode resistors you need to tune the amp as far as frequency response with the coupling cap because the difference between the low frequency and high frequency response is smaller, the cathode bypass cap is important here for gain and harmonic content.