Marshall JMP 2204 v1 voltages

bigjav

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Hello everyone!

I had a problem with one of the plate resistors at v1 of my JMP 2204 from 1977, which I think I solved. It seemed that the 100k resistor had gone open since there was no voltage reading on one half of this tube (yes, I changed tubes before doing this, and also measured without the tube installed). Unfortunately, I had not taken any readings before the failure occurred, so I do not know what the original values are. After installing a new 100k, I am getting 245vDC on one site of the tube and 204vDC on the other.

Would someone please, share with me the voltages you are getting at your JMP 2204's v1. I found some schematics indicating 100vDC which I doubt are correct

Thanks a lot!
 

Pete Farrington

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So that’s the first, non cascaded version?

Has it had the bias supply mod?
If you don’t know, would you mind checking the VDC on pin5 of either output valve, whilst in standby?
Unmodded will be below -1V, modded will be roughly -35V.
 
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bigjav

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Thanks! Well, actually it is the factory cascaded version.
The chart I just looked at on The Amp Garage showed 253 and 211. Yours at 245 and 204 sound close enough for government work.
would you be so kind as to share that chart, please? I can not find a legitimate one.
 

william vogel

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Here
JCM 800 2204 Voltage Chart
JCM800 2204 CF 120V
TubeTypePin1Pin2Pin3Pin4Pin5Pin6Pin7Pin8Pin9
V112AX7253 2.5 211 1.8
V212AX7180 0.9 294180181
V312AX7225 39 223 39
V4EL3435mV 411406-36410 35mV
V5EL3435mV 411406-36410 35mV
HT311
HT311
OT CT400
 

South Park

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Not all power transformers put out the same voltage . And testing with no load what matters is the plate has power
 

william vogel

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It seems unusual for there to be a higher voltage at the output valve anodes than at the OT CT?
I suppose the mains voltage may have decreased in between the readings being taken.
It’s not my chart. I simply got it from the amp garage. He asked for a comparison and I googled the chart. It could be invalid but the amp garage is generally a good source of information.
 

bigjav

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Here
JCM 800 2204 Voltage Chart
JCM800 2204 CF120V
TubeTypePin1Pin2Pin3Pin4Pin5Pin6Pin7Pin8Pin9
V112AX72532.52111.8
V212AX71800.9294180181
V312AX72253922339
V4EL3435mV411406-3641035mV
V5EL3435mV411406-3641035mV
HT311
HT311
OT CT400
Thanks for sharing! And thanks to the moderators for moving this post to the right place (I just realised they did this).

The DC voltage at my power tubes (ElL34) is 380v. Does that mean that the correct dcv at V1 should be lower than the above
Curiuosly, the readings on my v1, v2 and v3 are more or less the same as those in the shared chart here, but with 380vdc on theel34s.
 

jgab

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Here is my voltage chart for my three Canadian 2204 amps. I did a comparison to some 1981 Marshall specs I found online, and to get some baseline info in case I need to do some work on these units.

Your V1 voltages are really close to mine, so I would say you are in good shape.

BTW - I might have my A and B sections mixed up on my tubes, but you should get the point...



voltages pic.JPG
 

jgab

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Mark's amp only has one 10K dropper after the screen (before the PI) and my amps have two 10k droppers. People need to know what their amp/s have before they can compare to a schematic. I noticed that some 2204 amps have one and some have two 10K droppers before the PI.

You need to know how many droppers you have if you are referencing schematics. Big differences in preamp voltages because of this...
 

XTRXTR

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Wow I guess it really depends on the actual transformers in the amp and the wall voltage, 120/240. Who was working when winding the transformer. 🧐

Edit: yes how many 10k droppers were used as well
 

XTRXTR

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@jgab I am adding a 4th dropper in my 2204 for the FX loop I'm adding and my voltages are really high. 486 on the power plates.
 

Pete Farrington

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Mark's amp only has one 10K dropper after the screen (before the PI) and my amps have two 10k droppers
I think that may be a 6550 - EL34 thing. 6550 need a greater magnitude of bias voltage, and hence a greater signal level at their g1 to overcome that bias. Thereby achieving g1-k=0V on signal peaks, which generally defines the max power output for AB1 amps.
Higher VDC at the LTP HT node facilitates higher max signal output.
Note that with JTM45, whose KT66 require even higher bias voltage, the dropper is typically even lower, 8k2.

Having a lot more signal available from the LTP than is needed to provide full power output can lead to blocking distortion when overdriven. So it’s a good idea to get the dropper value correct for the valve type in use.
Too high and the LTP will start to round off the waveform before the output valves have reached Vg1-k=0, and the amp won’t be able to achieve its max clean power output.
Too low and there will be an increased tendency to blocking distortion when the power amp is heavily overdriven.
 
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bigjav

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Thanks a lot for all comments!
I was looking for specific readings you might share because, as I said, the info I am able to find on the net is quite controversial and misleading for me.

For instance, I find this chart https://el34world.com/charts/Schematics/files/marshall/Marshall_ampchart.gif,
taken from this tread https://el34world.com/Forum/index.php?topic=22218.0.
100vdc on v1 on MK2 50watter...! How is that possible?
In theory, it has to be correct because it represents the lower voltage at the PT screens (390v, same as mine), but it is quite a lot of difference. So, that would mean that we are all far away at the v1 readings..
 

Pete Farrington

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I find this chart https://el34world.com/charts/Schematics/files/marshall/Marshall_ampchart.gif
100vdc on v1 on MK2 50watter...! How is that possible?
Without knowing which specific model it applies to, that chart is of limited utility.
Note that the other 50W chart there has rather different V1 voltages.
I don’t think that ‘ampchart’ is an official Marshall thing, rather it seems most likely to have been cobbled together from various technical info documents.
Have you got an actual issue with your amp, is it working and sounding ok?
 

bigjav

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You made a valuable point, Pete., and thanks for asking!
The amp had much more gain on the high input, far more than the average 2204. Now the gain dropped, and I do not know if to normal levels or below average, because I got used to how gainy it was. I remember the gain pot doing almost nothing past 5 up to 10, it was already gainy.
Could it be from that plate resistor drifting in value, and going bad over time? Now the gain pot reacts in a expected way, but I have a feeling that gain could be a little bit higher. May be I am wrong and it is just a subjective impression, that is why I wanted to make sure that voltages are at least correct.
 

Pete Farrington

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The amp had much more gain on the high input, far more than the average 2204. Now the gain dropped, and I do not know if to normal levels or below average, because I got used to how gainy it was. I remember the gain pot doing almost nothing past 5 up to 10, it was already gainy.
Could it be from that plate resistor drifting in value, and going bad over time?
Yes, that’s possible.
Maybe try a higher value anode load resistor if you want a couple of dB more gain, eg 470k.


I remember the gain pot doing almost nothing past 5 up to 10, it was already gainy
Due to the big 1nF bright cap, that’s to be expected. The tone should get a bit fatter as the gain is turned above 5 though.
 

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