JCM800 input question

[mort]

Is there a specific jack to use for the high input jack on a JCM800 50W 2204?

I just got my build up and running but I have only a trickle of signal when plugged into the high input, and when plugged into the low input it seems like I should have more volume, and the amp is way too clean.

Here's the schematic I'm using and there's an extra arrow where the switch leg from the low input connects to the sleeve of the high input. Is that another switch?? I'm using the standard switchcraft shunt jacks, as found in most Fenders. Should I be using a different jack for the high input?

I've doublechecked my wiring several times and believe it to be correct.

Here are some voltages and the schematic I worked from.

V1
1. 270
3. 3.05
6. 228.8
8. 2.13

v2
1. 165.5
3. 1.3
6. 314
7. 165
8. 167

v3
1. 140.8
2. 45.8
3. 81.8
6. 237.8
7. 50.5
8. 81.8

v4
3. 498
4. 494
5. -46.6
6. 497

v5
3. 498
4. 494
5. -46.5
6. 497

1 ohm resistors on cathodes have .025mA across them

[big_teee]

I've built variations of that amp a few times.
You probably don't have the input jacks wired correctly.
V1, V2 look good.
V3 voltages don't look right.
This is what I have on V3.
V3 12AX7 261V +DC 16V +DC 50V +DC 3.31 AC 3.31 AC 245V +DC 17V +DC 50V +DC
For V3 check the Presence control wiring, and check the resistor values, measure them with a meter.
Discharge the amp first.
Your Bias voltage looks high on V4, V5.
Mine is usually around -35 volts, but you will have to set that with current.
Check your jack wiring with this triodestore diagram.
I know it works, I've used it several times.
http://site.triodestore.com/JCM8002204Stock.pdf
GL,
T

[mort]

I figured it out.

I snipped a certain connection to test it and now have the high input working with full strength and the low input is dead. I'll have to get the correct jack.

[mort]

This is the 4th Marshall I've built but the first time I've needed to use a 4 lug jack. I've always wired them just like Fender inputs but this one is different.

[J M Fahey]

Is there a specific jack to use for the high input jack on a JCM800 50W 2204?

Yes, Cliff type "Marshall Jacks" , plastic and all contacts have switches.

I just got my build up and running but I have only a trickle of signal when plugged into the high input, and when plugged into the low input it seems like I should have more volume, and the amp is way too clean.

Here's the schematic I'm using and there's an extra arrow where the switch leg from the low input connects to the sleeve of the high input. Is that another switch??

Yes.
So if you're asking this you have clearly miswired it.

I'm using the standard switchcraft shunt jacks, as found in most Fenders. Should I be using a different jack for the high input?

Yes:


No:

I've doublechecked my wiring several times and believe it to be correct.

Sorry but you misunderstood input jack wiring, which also switches the first gain stage in/out the preamp chain.

[big_teee]

I use standard 502 cliff jacks.
https://tubedepot.com/t/diy-central/jacks-and-plugs

[mort]

Thanks gentlemen.

Another little question if I may... are 50W jcm800's typically brighter than say 50W Plexi's or JTM45's??

[Justin Thomas]

So, question. Is there a reason I couldn't use 2 Fender shorting jacks? Can't I run a jumper from the shorting contact of the Low Jack to the sleeve of the Hi jack? I accept that there may be "assumed" info in the schematic that I don't see or understand... I'd try it, but I don't have any JCM800-style amps laying around. How did Mesa do it - did they use Cliff jacks too? I'm thinking Boogie Mk1... I also know there's 4-contact Switchcraft jacks; could they work? I've got some pics of a Sovtek 100W 800 clone, and it uses Switchcraft-stule jacks. Sorry, I don't trust plastic-bodied parts with my soldering unskills. Not really, I just like Switchcraft. Metal threads are Yay!

Of course, who uses the Lo input on these, anyway? It's defeating the whole purpose of the amp in the first place!
Justin

[big_teee]

I never liked the 2204 that much in stock form.
You can tune Yours to suit. I always made mine halfway between marshall and bassman tones.
IMO the Cascade High input is the heart of the 2204 Amp.
GL,
T

[mort]

I usually build stock circuits. I don't get into alot of modifying but I'm curious what to do in order to tame some of that brightness.


Edit: nevermind. the guy that's getting it says he wants it bright. Thanks for the replies, fellas.

[big_teee]

I replace the 470k resistor on the low input with a 68k.
Try replacing the 500pf cap between v2, and the treble pot, by putting 1000pf there.
That should help on the brightness.
You can also try replacing the .022uf couplers at the phase inverter with .047uf's.
I would try that last.