JCM900 Rebuild into JCM800

[Mr_bibbles]

Hello,

I am finally looking to complete a fun project now that I have some extra time on my hands! I have an old JCM900 that had a small fire on the board years ago, and it's become so unreliable I thought a fun project would be to re-purpose the chassis / transformers / etc into a 2203 or similar JCM800. I plan to but a turret board and wire it up as a point to point using the existing 3x 12ax7 sockets / 4x el34 sockets, but there are a couple issues;

#1. While the output transformer seems like an exact match, the power transformer has on the 900 is lacking a 100v Tap the 800 uses for bias, and instead has a 20v tap for IC power. The 900 taps the bias voltage off the HV secondary... can I use the 900 transformer, and tap the bias voltage off the secondary HV in the JCM800 design? Any pitfalls to this approach?

#2. Does anyone have recommendations for improving the Preamp-Out from the 2203 design? This is high priority for my personal amp, and I know the vintage designs are a bit lacking in fidelity. I'm considering the Mojo-tone SS effects loop to keep things tidy,though I really only NEED a preamp out, not a return.

#3. I'd like to re-purpose the extra chassis holes for resonance control, and a mid-contour control. This part seems very straightforward, .0047 / 1 Meg for resonance, and replace "mid" resistor with a variable pot

Other than that, I know it will be a lot of elbow grease... but I've always wanted to do this, and then tell people it's a "JCM 9000" Thank you in advance!

900/800Schematics

https://www.drtube.com/schematics/marshall/jcm800pr.gif

https://www.drtube.com/schematics/ma...d0192-iss7.pdf

TRANSFORMERS
https://www.tubesandmore.com/sites/d...s/p-t290hx.pdf

https://www.mojotone.com/British-900...er-Transformer

[The Dude]

[QUOTE=Mr_bibbles;n985756......can I use the 900 transformer, and tap the bias voltage off the secondary HV in the JCM800 design?......[/QUOTE]

Yes, you can do this without issue. There are many Marshall amps you can copy the circuit from.

[Mr_bibbles]

Yes, you can do this without issue. There are many Marshall amps you can copy the circuit from.

Great! Thanks Dude, I'll probably just use the 900's bias design, and inject it into the 2203 power section, should be a snap!

#3 I think is straightforward enough, but how do you feel about those old-school preamp-out circuits? Good? Crap? I specifically want it to feed a wet/wet pedal that will be "killdry" wet.

[The Dude]

What amp do you intend to feed after the pedal?

[Mr_bibbles]

What amp do you intend to feed after the pedal?

Was trying to avoid making this about my gear hehe, but probably a good place to start;

It's a really compact Wet/ Dry/ Wet setup, but with only one amplifier; Guitar --> Amp ---> Preamp out ---> Source Audio Collider (STEREO FX pedal, kills the dry, wet only) ---> Cab Simulator ---> FOH mix.

Soundguy gets Dry signal from the cab, then feeds L/R wet through the PA, mixes them together to taste... So there won't be very much processing, BUT, there wont be much dry signal, just the "wet" remnants.

[The Dude]

If you're using a cabinet emulator, that pre-out circuit should be just fine. That's why I asked. The direct out can be a little high end/gritty/too hi-fi sounding for my tastes without a cab simulator.

[Mr_bibbles]

If you're using a cabinet emulator, that pre-out circuit should be just fine. That's why I asked. The direct out can be a little high end/gritty/too hi-fi sounding for my tastes without a cab simulator.

Great! So now that I know transformers will work, I tore everything out except the sockets / transformers / Filter caps. I will come back and update this as time goes by for anyone else looking to do the project. The main tasks will be install a turret board in the relativley tight space (less than 2 inch width remaining) and mount some new caps! I appreciate your time Dude, I mainly wanted to see if it was a fool's errand.

[Mr_bibbles]

OK! Very closed to finished, been getting an hour in here or there over the last couple months when work was slower.. Been using this Triode Electronics diagram as a reference but am realizing now that the JCM900 PT has no center tap!

I realize this is not an issue since we're already using full wave bridge rectifier, but been reading on Valve Wizard and other sites but can't make sense of WHY they're using a center tap here when it's already a full wave bridge rectifier... are they splitting and balancing voltage between the two filter caps in series? Would ignoring the CT and flipping the switch leave me with "doubled" voltage compared to the schematic since it's referenced to half a wind?

Can someone suggest an elegant workaround to having a no CT in this circuit? Or is it negligible and I can Ignore it?

[loudthud]

Just add a pair of resistors across the series connected filter caps like you see in a Fender amp. Something like 220K 2W should work.

[pdf64]

You‘re using the mains transformer and HT caps of the 900, so the obvious way forward is to use the 900’s rectifier and reservoir, HT and bias supply arrangements.

In the 2203, all the CT of the HT winding is doing is balancing the voltage across the reservoir cap series pair.
But for your ‘2100 to 2203’ conversion, it seems pointless and somewhat a waste of resources to series connect HT caps that each have a sufficient (500V) rating to be used directly.

If you’re retaining / using the standby function, you should verify that the bias supply charges up whilst in standby.
If it doesn’t, the output valves will all pass very high, potentially damaging cathode current for a few seconds after standby is flipped with hot valves. For EL34, the (average) cathode current limiting value is 150mA.

Initial power up should be via a light bulb limiter, no valves fitted. Check all valve pins will have the necessary voltage levels on them, in standby and operate modes.

[loudthud]

The JCM900 power supply clearly has all the kinks worked out to work with the JCM900 power transformer. Pdf64 makes a good point about the bias supply working when the amp is in Standby mode. This is critical ! Follow the Standby switch details like the JCM900 uses.

IT SHOULD BE NOTED BY ALL that the capacitor coupled bias supply such as the JCM900 uses (and slight variations that use a large value resistor in place of the 47nF cap) doesn't charge up very fast. Those 10uF caps are critical to how fast the supply comes up when power is applied. Don't increase their value !

[g1]

Those 10uF caps are critical to how fast the supply comes up when power is applied. Don't increase their value !

Also recommend a minimum voltage rating of 630V for C15, the 47nF feeder cap for the bias circuit. When the 900's first came out, they had 250V caps there and we saw several catastrophic failures when the cap in that position opened. From what I recall it seemed related to standby voltage.

[Mr_bibbles]

Thanks all, I'll try and bullet point this to keep it clear

- For clarity's sake this is a 4100 --> 2203, not 2100 ---> 2300

You‘re using the mains transformer and HT caps of the 900, so the obvious way forward is to use the 900’s rectifier and reservoir, HT and bias supply arrangements.

- I think I understand that in the 800 design they're using two caps in series balanced by the CT to reduce strain on either cap, but that's not necessary here. HOWEVER... I already have the extra caps installed and boarded... so adding the two resistors as an artificial center tap as suggested by loudthud seems like the best path? I assume some of the 800 magic is the 12 filters (vs the 4 of the 900) and this amp is definitely going to be played at motorhead-esque volumes where that might matter in terms of stiffness and sag... or am I deluding myself into a trap of more = better?

- HUGE thanks to PDF64 for the tip about the bias- I have it tapped directly off the rectifier ala the 800, so I do not think it will be charging bias, since the supply is downstream of the switch. I'll definitely have to rewire the bias feed to come from the "Off" lugs of the switch or something so it charges as soon as mains are applied, and I'll test the crap out of it.

- G1- I had a 600v Orange drop in that position, is this toeing the line? I can swap up to 1000v if needed, but with the AC voltage coming out it seems like 600 should be OK unless I'm missing something about inrush or peak draw or something like that

TLDR Questions:

1. I have the series caps installed already and wired up like the 800 schem, should I apply the artificial CT via 2x 220k Resistors as a somewhat wasteful, but elegant fix? Or drop it down to 4x 500V filters instead of 12. Would this change the sag / stiffness of the amp?

2. Is 600v enough for C15? Or should Is 630 a true minimum... seems like 600 should be OK from th

[g1]

The 4100 bias feed comes off the transformer winding before any standby so that is the best place to take it from. 600V for C15 is fine.
The resistors at the filter caps offer balancing but should not be confused with 'artificial center tap'.
For what you want from the amp, I thinkt the 'more' scenario for the filter caps is preferable.
Take heed of Loudthud's warning about the bias filter caps values (post #11).

[Mr_bibbles]

The 4100 bias feed comes off the transformer winding before any standby so that is the best place to take it from. 600V for C15 is fine.
The resistors at the filter caps offer balancing but should not be confused with 'artificial center tap'.
For what you want from the amp, I thinkt the 'more' scenario for the filter caps is preferable.
Take heed of Loudthud's warning about the bias filter caps values (post #11).

Thanks as always G1, you're the man. I figured more filters can never be worse when you're in the motorhead range, although they can certainly be.... "overkill". And yea I'll probably install a lug to tap the AC off of so it can run to the switch and the Bias feed- right now the first termination would be the "off" lugs of the switch but that just seems... strange.

I'll see if I can get some photos of the finished product and some clips for y'all. Still want to monkey around with installing a few bells and whistles, but should be done very shortly.

THANKS!