JCM800 2204 Build

Get support and show off your MetroAmp 50 Watt kit builds.

Moderator: VelvetGeorge

Post Reply
Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

JCM800 2204 Build

Post by Brad H » Wed Jul 25, 2018 2:46 pm

Need some help fellas! Just tested my completed chassis on a JCM era 2204 build (6550 tubes). Below are my voltages, pretty high in the pre-amp:
V1
Pin 1: 362
Pin 3: 3.87
Pin 4/5: 3.37
Pin 6: 295
Pin 8: 2.89
Pin 9: 3.35

V2
Pin 1: 244
Pin 3: 1.46
Pin 4/5: 3.37
Pin 6: 418
Pin 8: 246
Pin 9: 3.35

V3
Pin 1: 300
Pin 3: 57.9
Pin 4/5: 3.34
Pin 6: 301
Pin 8: 58
Pin 9: 3.36

Schematics from original Marshall’s show a decoupler/dropping resistor of 10k2w x 2 in 2203’s...but only 1 resistor in 2204’s. I went with 1. Output tubes seem to check good, bias is right where it needs to be. B+ is sitting at about 478-480. Heaters are a little high, but not bad...these voltages are with all tubes installed. With no tubes my heaters are about 3.53.

I’m thinking I need to add the second 10k2w dropping resistor like the 2203 and see if that’ll tame the pre-amp voltages. Don’t want a Plexi sounding pre-amp, but I don’t want to eat tubes either.

What do you guys think?

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Wed Jul 25, 2018 3:16 pm

Forgot to mention, VAC in from wall is 122.5.

danman
Senior Member
Posts: 765
Joined: Tue Sep 10, 2013 9:09 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by danman » Wed Jul 25, 2018 7:47 pm

Marshall employed several dropping resistor combos over the years depending on which PT was used and where the B+ ended up. I would try the extra 10k and then adjust the value to fine tune things later. I have used various values (5k-15k) for the second resistor to fine tune preamp voltages.

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Wed Jul 25, 2018 8:04 pm

**Update**. I made some changes:

1. Added another 10k2w resistor to the to make a total of 2 in the decoupling/voltage drop like a 2203.

2. Replaced the 10k2w resistor between the pre-amp filter cap and the PI voltage dividers, with a 27k2w.

3. Removed changed out the PI voltage splitter resistors (82k & 100k 1w) with 100k & 150k 1w.

Here’s the new readings:

V1
Pin 1: 273
Pin 3: 2.92
Pin 4/5: 3.37
Pin 6: 245
Pin 8: 2.41
Pin 9: 3.35

V2
Pin 1: 204
Pin 3: 1.2
Pin 4/5: 3.37
Pin 6: 346
Pin 8: 206
Pin 9: 3.35

V3
Pin 1: 274
Pin 3: 51.8
Pin 4/5: 3.34
Pin 6: 261
Pin 8: 51.8
Pin 9: 3.36

What you guys think? Should I tweak some more? I’m pretty happy with the voltages at this point...not crazy about v2 pin 6, but with the 100k resistor on that socket, it drops before pin 1. I’ve seen schematics with voltages that day 346 is dead on...not sure if that’s right.

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Wed Jul 25, 2018 8:26 pm

Wondering if I should plus up the other PI voltage divider to 150 like I did the other, since I’m running a 12ax7 with balanced triodes in v3? Should I try and match pin 1 & 6 as close as I can in v3?

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Thu Jul 26, 2018 5:11 pm

Some more tweaking in the preamp. And I ran parallel resistors in the split that runs to v3p6. Perfect! But I still don’t like the voltage to v2p6...too high?

Here’s the new readings:

V1
Pin 1: 265
Pin 3: 2.98
Pin 4/5: 3.37
Pin 6: 246
Pin 8: 2.42
Pin 9: 3.35

V2
Pin 1: 204
Pin 3: 1.2
Pin 4/5: 3.37
Pin 6: 349
Pin 8: 206
Pin 9: 3.35

V3
Pin 1: 251
Pin 3: 48.6
Pin 4/5: 3.34
Pin 6: 252
Pin 8: 48.6
Pin 9: 3.36

Is that v2p6 running too hot? My B+ is 478-480. Just worried since max plate voltage for a 12ax7 is 300.

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Thu Jul 26, 2018 6:06 pm

Correction: parallel resistors to v3p1. Now both pin 1 & 6 are running dead even in PI socket.

danman
Senior Member
Posts: 765
Joined: Tue Sep 10, 2013 9:09 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by danman » Thu Jul 26, 2018 8:05 pm

V2 voltage is pretty high on that triode because of your higher than normal B+ voltage. Just be sure to run a solid tube in that spot. Marshall's have been known to beat that tube to death because of the higher voltage so not every tube does well in the cathode folllower position.

You mention a voltage divider to the PI. I believe you are referring to the 82k/100k plate load resistors. There will be a slight voltage difference between 1 and 6 because the circuit is set up with a slight imbalance. It was designed this way intentionally and is part of the sound of the amp. Nothing in the circuit and surrounding parts are perfect because of resistor and capacitor tolerances, not to mention slight imbalances in the OT windings so I would not worry too much about achieving a perfect balance between the two halves of the PI.

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Thu Jul 26, 2018 10:03 pm

Yes, I meant PI plate resistors. Thanks for the info, hope I don’t burn too many tubes on that socket. Was hoping to get that down to about 295.

danman
Senior Member
Posts: 765
Joined: Tue Sep 10, 2013 9:09 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by danman » Fri Jul 27, 2018 8:59 pm

Your B+ is a touch high for the 50 watt circuit but there are options available for lowering it. You could focus on the preamp only but raising the dropper values will lower the entire preamp. I kinda like lower voltages in the preamp as it makes the amp a bit warmer and browner sounding.

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Sat Jul 28, 2018 7:40 am

Had to order more resistors, I’m running low and I needed a few odd ball sizes. When they get here, since the PI is where I want it, I think I’m going to raise the dropper that feeds v2 by about 6k and see where it takes me, but then I’ll have to lower the 10k that feeds v1 to get those voltages back up.

Brad H
New Member
Posts: 35
Joined: Tue Jun 20, 2017 7:02 pm
Just the numbers in order: 13492

Re: JCM800 2204 Build

Post by Brad H » Sat Jul 28, 2018 8:56 am

Going for an aggressive heavy tone, not “browned” too much. Just don’t want to be replacing v2 every three months in order to get the aggressiveness I want.

Post Reply