Marshall 2204 Voltage Readings, are they off?

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Rocksy
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Marshall 2204 Voltage Readings, are they off?

Post by Rocksy » Wed Jun 27, 2018 10:56 am

Hey guys, this is my first post here!

I've been the owner of a Marshall JCM800 2204 (built in 1982) for about two years. Last week the amp blew an output tube, so I went through the amp looking for a possible culprit. I couldn't find any resistors that looked damaged or had shorted etc, so I put in a new output tube and everything seems to be working alright for now. While I was in the amp I decided to check the voltages at the tube sockets, but when I compared them to voltage charts I've seen for a 2204, my voltages seem off especially at the pre-amp tubes.

Any idea why my voltage readings are off? Is it possible I'm metering them incorrectly?

V1-1: 257v
V1-3: 2.8v
V1-6: 190v
V1-7: 0
V1-8: 1.7v

V2-1: 158v
V2-3: 1.1v
V2-6: 300v
V2-7: 159v
V2-8: 160v

V3-1: 131v
V3-3: 29v
V3-6: 179v
V3-7: 17v
V3-8: 29v

V4-3: 398v
V4-4: 393v
V4-5: -34v
V4-6: 398v

V5-3: 398v
V5-4: 391v
V5-5: -34v
V5-6: 394v

Heaters all measure about 3.3vAC

My amp is an "older" 2204 with the Lower Voltage PT's. The amp came stock with 6550's, but now runs EL34's. I put a single 47k resistor in parallel with the 47k resistor in the bias section to get the bias in range for EL34's, no other components in the amp have been changed.
Last edited by Rocksy on Tue Jul 10, 2018 8:51 pm, edited 1 time in total.

Rocksy
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Re: Marshall 2204 Voltage Readings, are they off?

Post by Rocksy » Wed Jul 04, 2018 4:36 pm

I went through the amp over the weekend, component by component, and found two resistors that were off.
One was a 100k by the first pre-amp tube (that was reading 150k), the other was an 82k going to the phase inverter tube (that was reading about 450k).

All the resistors in the amp are stock, so this was unexpected.
I replaced the resistors that were off, and will have to re-measure the voltages.
Last edited by Rocksy on Wed Jul 11, 2018 3:51 pm, edited 1 time in total.

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neikeel
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Re: Marshall 2204 Voltage Readings, are they off?

Post by neikeel » Wed Jul 04, 2018 5:49 pm

Sorry I missed this first post.
Welcome to the forum!
The high reading plate resistor in V1 would give a bit more gain and compression (some people use 120k or 150k plates for that purpose.
The 82k reading 450k will really have screwed the PI working properly.
Please let us know how you get on.
Neil

Rocksy
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Re: Marshall 2204 Voltage Readings, are they off?

Post by Rocksy » Tue Jul 10, 2018 8:01 pm

Hi neikeel, thanks for the warm welcome!

Here are the new voltage readings after replacing the bad resistors.

V1-1: 253v
V1-3: 2.8v
V1-6: 214v
V1-7: 0
V1-8: 1.9v

V2-1: 156v
V2-3: 1.1v
V2-6: 294v
V2-7: 155v
V2-8: 157v

V3-1: 228v
V3-3: 34.5v
V3-6: 220v
V3-7: 22.4v
V3-8: 34.5v

V4-3: 396v
V4-4: 392v
V4-5: -32.5v
V4-6: 397v

V5-3: 396v
V5-4: 391v
V5-5: -32.4v
V5-6: 397v

(I adjusted the bias slightly before taking these readings, from 36mA to 37mA)

As you can see, the voltages at the phase inverter (V3) are now up a lot, almost 100V in one spot!
The amp is sounding much better now, thicker and crunchier. Those resistors were probably bad when I purchased the amp about two years ago, so it's really nice to finally hear the amp sounding the way it's supposed to. I spent countless hours messing with the amp, trying a bunch of mods, and I always felt that something was off, that the amp didn't sound as good as an older JCM800 should. So I'm really happy now.

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