Guitar Gear Forum

Just measured all the voltages in my 2204 clone, JCM800 2204 i should mention, before i go ahead and wack in an FX loop.

I will say it is very lightly modded, changes made are:

V1a cathod resistor changed to 2K2
V1b cathod resistor is stock with 2K7/.68uf
V2a cathod resistor 820 ohm bypassed with .68uf cap
PI fizz cap increased to 100pf
Used a 32uf+32uf can cap for pre-amp filtering (although i have seen both 47+47 and 32+32, so i went with the lower)
Added depth knob control.

Other than that it is to Marshall spec.

Anyway most of the voltages look okay looking on the Marshall date sheet. My HT/B+ is a bit higher, i think, but then again i have 245VAC at the mains coming in so that might infulence it.

Anyway, the numbers:

V1

1) 105v
2) /
3) 4.49v
4) 2.9v
5) 2.9v
6) 117v
7) /
8) 5.23
9) 2.9

V2

1) 170v
2) /
3) 1.16v
4) 2.9v
5) 2.9v
6) 312v
7) 167.4
8) 174.6
9) 2.9

V3

1) 248v
2) /
3) 42.4v
4) 2.9v
5) 2.9v
6) 223v
7) /
8) 42.5
9) 2.9

V4

1) /
2) 2.9v
3) 487v
4) 486v
5) -48.3v
6) 488v
7) 2.9v
8) /

V5

1) /
2) 2.9v
3) 489v
4) 485v
5) -49.8v
6) 486v
7) 2.9v
8) /

What do you'll think?

Hi Liam,

I wouldn't say so. Esp V1 looks way off . . .

http://www.freeinfosociety.com/electron ... echart.gif

Hi Olaf,

You just be sick of my questions already!

Here's what I have for valuables, buy I also have another chart in the history of Marshall book that is different to this.

Voltage Chart

Pin 1 2 3 4 5 6 7 8 9
V1 296 ~0 3.3 H H 245 ~0 2.2 H
V2 182 ~0 1.30 H H 245 182 183 H
V3 253 44 45 H H 251 44 45 H
V4,5 0 H 468 464 -57 (468) H 0

I agree that V1 is way out on pins 1 & 6, the rest of it it's too far out. My HT is higher than the above chart. Also I'm running EL34s so I'd expect pin 5 of V4&5 to be around -48-50 or so.

I'm not sure why. I'm going to put it back to stock and re-measure.

What can be a likely cause of this?

Thanks
Liam

I'm not, Liam. I love amps and DIY

Cathodes of V1 are too high IMO. And reagarding your chart: 300V on V1's plate is probably before the plate resistor, thats why.

What would be helpful: measure the B+ line from the start (490V = OT middle) down to the last filter. And which valaues for the resistors do you have?
Like this: rectifier --> F1 (490V) --> Choke (486V) --> 2nd F (value) --> 20k (value) --> 3rd F (value) --> 10k --> 4th F (value).

Olaf

Edit: where are you located? UK? Ireland? And if you have xxx V from the wall, which setting do you have for your PT?

You can also follow this schem with volts . . . gives you some add. information.

http://mhuss.com/MyJCM/JCM800_2204.gif

Duesentrieb wrote:I'm not, Liam. I love amps and DIY

Cathodes of V1 are too high IMO. And reagarding your chart: 300V on V1's plate is probably before the plate resistor, thats why.

What would be helpful: measure the B+ line from the start (490V = OT middle) down to the last filter. And which valaues for the resistors do you have?
Like this: rectifier --> F1 (490V) --> Choke (486V) --> 2nd F (value) --> 20k (value) --> 3rd F (value) --> 10k --> 4th F (value).

Olaf

Edit: where are you located? UK? Ireland? And if you have xxx V from the wall, which setting do you have for your PT?

Thanks Olaf,

I will do some more measuring tomorrow and report back.

I am in England so mains voltage should be 230V, i actually measured 245V coming into my amp so its a tad high. Using the 240V tap from the PT

Liam

In the UK you should have 240V from the wall, no?

edit: just saw its 230V, so they dropped it, right?

Do you have a 230V setting on the PT?

Yeah that's right Olaf, mains voltage should be 230.

For the PT I have it wired to the 240v tap. They other taps are for 120v & 220v

I will get some more measurements later this evening after work.

Hi Olaf,

I have now put my amp pretty much back to stcock. Exceptions are 32+32 for pre-amp filtering and V1b cathode resistor is 1.8K. I have double checked everything and it look correct.

One thing though before i fire it up again and re-measure. I have seen the 47pF phase inverter fizz cap in two places:

1) Across V3a and V3b plate resistors (82k and 100k)

2) Across the bair of 220K bias feed resistors.

I had it in option two as this is what is on the Marshalll schem, but the link you provided it is in option one.

Which is correct?

Thanks

I think i have sorted this! What do you think Olaf?

Had a look at several other amps schem and they all have the fizz cap across the plate resistors so i went with that. Voltages are looking better to my eyes.

Mains voltage= 245v

Mains Filter Cap= 500v

Screens/PI Filter Cap= 498v

Phase Inverter HT= 416v

V1

1) 296v
2) /
3) 3.65v
4) 2.9v
5) 2.9v
6) 266v
7) /
8) 2.45
9) 2.9

V2

1) 196v
2) /
3) 1.33v
4) 2.9v
5) 2.9v
6) 360v
7) 194.5
8) 207
9) 2.9

V3

1) 270v
2) /
3) 44.2v
4) 2.9v
5) 2.9v
6) 244v
7) /
8) 44.2
9) 2.9

V4

1) /
2) 2.9v
3) 495v
4) 493v
5) -48.4v
6) 493v
7) 2.9v
8) /

V5

1) /
2) 2.9v
3) 495v
4) 493v
5) -48.2v
6) 493v
7) 2.9v
8) /

That looks alot closer to the schem you sent with the voltages on. With my slight higher HT i would expect overall higher voltages, which seems to be hapening. Much better than before though.

All thanks to that fizz cap in the wrong place. Damn you History of Marshall book and your incorrect schematic!

Aditional thought! I did have some NOS Mullard 12AX7s in the preamp, unfortunately i don't have a way to test the ones i have but i have used others and know they can be hit and miss. Maybe the ones i had in there wheren't that great. For the above voltages i had some EH 12AX7s.

I assume a bad 12AX7 will cause off voltages, is that correct Olaf?

Looks good now.

Yeah, fizz cap on the 82k/100k.

Of course a bad pramp tube will fuck the voltages inside, Liam.

That's great. Thanks so much for your help Olaf!