Mojotone British 50 watt 1976 2204 model

[davidallenherrick]

Hi everyone I have a diagram from Mojotone British 50W 1976 2204 Rev 3 03/31/2025 On V2 it's showing no connection to pin 1 only a 330k resistor from pin 1 to pin 6 and a bus wire from pin 1 to pin 7 and a wire from pin 6 to a 50+50 can cap on the board is this correct??? I'm getting a voltage reading of 76v on pin 1 I've been told it should be 190v to 215v.....anyone familiar with this build and should I be following the diagram to the T or is it just for reference?? Also is this 330k resistor wrong? should it be removed from pin 1 and 6 and replaced with a 100k resistor from pin 1 to B+ instead of jumping 1 to 6 only keeping the bus from 1 to 7....thanks for any help.

[sluckey]

All the wiring is correct but the 330K should be 100K. So, just replace the 330K with a 100K. Don't change anything else!

[davidallenherrick]

All the wiring is correct but the 330K should be 100K. So, just replace the 330K with a 100K. Don't change anything else!

Thank you much. My mistake though. The diagram they sent me was correct a 100k resistor. The diagram from the web sight listed a 330k resistor. And I was referencing that diagram for diagnostics. My problem is 76 v @pin 1and 7 v2, 79v pin 8, 307v pin 6 and v2 pin 3 =0.58v with a bad hum from the amp when I turn gain and volume up and I know the gain pot has two bad spots so I'm replacing that. Any Ideas or path to go from here?? I also have 1.05v on pin 3 V1 and 213v on pin 6, 1.91v pin 8, 160v pin 1. Power tubes pin 5 -31v on both pin 3,4 401v, pin 6 403v on both

[sluckey]

All your voltages are low as compared to the mojo schematic. Look at the attached schematic. Post the measured B+ voltages at points XA, XB, XC, XD, and XE. Also measure the resistance of the three 10K dropping resistors on the B+ rail.

[Stevem]

This is not a 2204, it's a 800.
With a 2204 V2 is that monotone's V3.

While your working the bugs out I would adjust pin 5 up to -40 or as high as the pot will allow it to go.

[davidallenherrick]

Just a clarification on the diagram from Mojotone I rechecked and I apologize I feel like an idiot. The one I built is the 1976 model. The 76 model shows a 330k resistor from pin 1 to pin 6. On the 1978 model it shows the 100k resistor. I've changed the bias setting to -41 volts it was -31 and that changed all the voltages. This is just an update. I'm still taking readings and then I'll present them to you. I will say a quick check on pin 1 V2 went up to only 85v after raising the bias. Is -41 to high? I thought -31 to -35 was the norm but the schematic does say -41. Anyways I have work to do, thanks everyone for your help.

[davidallenherrick]

I'm still trying to iron some more things out, and have been finding flaws in my build. I've corrected those. Now V2 voltages aren't looking very good either. Here's a list of my voltages. 
V1 
PINS 
1-185.1
2-0
3-1.139
4-0
5-0
6-239
7-0
8-2.09
9-0
V2
PINS
1-89.3
2-0
3-0.640
4-0
5-0
6-341
7-89
8-92
9-0
V3
PINS
1-248
2-27
3-43
4-0
5-0
6-246
7-28
8-43
9-0
XA-450
XB-450
XC448
XD379
XE-449
XF IS (-41)
Mains AC voltage from Variac is123vac
Believe it or not the amp was fine for like 4 weeks but I did notice a crackle and or low scratchy sound at about 1 and at 7. So I know it needs a new gain pot. I missed 2 center turrets on soldering and I was missing a bus bar connecting  the 2 1N5408 Diodes that stuff is all fixed now. Now I'm getting a hum like when you put your finger on the tip of the guitar cord. I have a 8ohm dummy load on the output connected to an isolation transformer and a variac set to 123 v ac. Just can't figure out why my voltages are so low. Originally I had my bias set to -31 then I noticed the schematic said -41 so I did that and it changed the voltages it made them higher but not enough I'm stumped. Pretty sure a lot of the hum will go away when I change the gain pot. The biggest issue is getting the voltages corrected. I built the 1976 version with the 330k resistor at V2 pin 1 and 6 instead of the 100k like in the 1978 version. I've been told those resistors shouldn't be there at all and that's the problem.(AI) It wants me to connect a 100k resistor to pin 1 of V2 and run that to a B+ rail. Please help

[sluckey]

Thanks for setting me straight Stevem. I saw 2204 and automatically jumped to JCM-800 2204 without realizing there also was a JMP 2204.

I apologize for my ignorance. I finally saw the mojo schematic for the 1976 2204 and there is indeed a 330K on the V2 socket. Here's the original Marshall schematic with voltages. Your voltages ***BEFORE*** you changed the bias voltage were very close to the voltages on the original schematic. I don't see anything wrong with your voltages. I've attached the original schematic.

If you still want to replace 330K with a 100K to make it look more like the 1978 version, then simply replace the resistor on the socket. Nothing else needs to be changed.

[davidallenherrick]

Thank you for your reply. I'll change my bias back to -31v unless You think it should be higher. I was thinking between -31 to -35. I wanted to mention I put a multimeter on my gain Potentiometer Set it to millivolts connected negative lead to ground lug POS to center lug Reading was -0.266 Does this indicate DC leakage If I switch the leads it goes +0.266 And is this why I'm getting a hum buzzing when I turn the volume and gain up. I had the amplifier powered on stand by On with gain potentiometer at full blast and got this reading. was not sure if this mattered but I figured I needed to have the power on in order to get the DC leakage. You will have to forgive my lack of terminology I'm still learning and have a long way to go. This is my first build so Please be patient with me. I don't know which capacitors I need to check for this DC leakage if that's the case. it's the only thing I can think of that would give me a buzz or hum it's not so bad when volume is turned down only when I get up to around 5 on up with gain maxed. I understand hiss when everything is on full that I know is normal What I am experiencing is a audible buzz hum like when you put your finger on the tip of a guitar chord when it's plugged into your input jack. Thanks For any help so far everybody has been great. I certainly have learned valuable lessons here.

[Helmholtz]

The plate resistor value of V2A has no influence on power tube bias.
Power tube grid bias should not be measured at the grids (because the meter will pull down the voltage depending on meter input resistance).
Rather it should be measured at point XF, i.e. between the 220k grid leaks.
With EL34s I'd start around -40V (-31V certainly isn't enough).
But what really matters is power tube idle current.
The grid bias voltage should be adjusted for an idle cathode current between 35mA and 40mA.

To measure idle currents best add 1R current sense resistors at the power tube cathodes.

[Stevem]

Just to make sure you know what’s going on here maybe if I explain it this way it will help you more.

That negative voltage on pin 5 sets up how much idle current the output tubes draw from the power supply.

The lesser you make the negative bias voltage the more idle current those tubes will draw, this in turn will lower the overall B+ voltage the amp has to run on.

So an adjustment of -31 will idle the outputs hotter since they will draw more current then having -40 volts .

Now you can’t idle too low because crossover distortion will start to creep in and that sounds like crap.

Also when the output stage is idling too cold the amp will have no punch.

With a plate voltage on pin 3 of about 440 to 450 volts idling each tube at 31 to 34 ma should have the amp sounding good and you will have a normal life expectancy out of those 34s.

[Stevem]

Just to make sure you know what’s going on here maybe if I explain it this way it will help you more.

That negative voltage on pin 5 sets up how much idle current the output tubes draw from the power supply.

This is just idle current, the current will rise up higher when there’s actual audio being pumped into the output tubes.

The lesser you make the negative bias voltage the more idle current those tubes will draw, this in turn will lower the overall B+ voltage the amp has to run on.

So an adjustment of -31 will idle the outputs hotter since they will draw more current then having -40 volts .

Now you can’t idle too low because crossover distortion will start to creep in and that sounds like crap.

Also when the output stage is idling too cold the amp will have no punch.

With a plate voltage on pin 3 of about 440 to 450 volts idling each tube at 31 to 34 ma should have the amp sounding good and you will have a normal life expectancy out of those 34s.

[Helmholtz]

..idling each tube at 31 to 34 ma...

The range I suggested above is for cathode current of which 15% is screen current.
Plate idle current alone should not be below 30mA IME.

[Roe]

All your voltages are low as compared to the mojo schematic. Look at the attached schematic. Post the measured B+ voltages at points XA, XB, XC, XD, and XE. Also measure the resistance of the three 10K dropping resistors on the B+ rail.

move the bias supply before (not after) the standby switch! This will save tubes and transformer

[Stevem]

Here’s some idle current to plate voltage guides .

60% will give far better tube life then the 10% difference between 60 and 70% suggest.