2204 build voltages are off

[lgehrig4]

I started testing the build using a variac. Right now I have preamps installed, but not the power tubes.

At 120V there is 362V AC going to the diodes and the reading after the 5th diode (connected to the 220K resistor) is -51V DC. So far does this seem ok? If so, lets jump to the preamps.

I don't want to go past 60V on the variac because at 60 pins 1 of V1-V3 read as follows

V1 = 190
V2 = 210
V3 = 170

These are not far from final desired values and we are only at half power. I'm not sure where to troubleshoot.

I followed this layout and triple checked all of my connections.
https://www.joepopp.net/jcmtwins/jcmtwins003big.jpg

Voltage chart
https://mhuss.com/MyJCM/vchart.html

[sluckey]

Voltages will be high until you plug in the power tubes.

[lgehrig4]

Voltages will be high until you plug in the power tubes.

Thank you for letting me know! That makes me feel a little better

[lgehrig4]

Looking good now! The only questionable reading it V2 pin6 (336), which is approximately 80 higher than the chart. Should I be concerned about this?

V1 V2 V3
P1 285 189 251
P2 0 0 28
P3 2.96 1.1 46
P4 - - -
P5 - - -
P6 248 336 243
P7. 0 189 30
P8 2 190 46
P9 - - -

Heaters are 3.3

[Roe]

some 2204s had low voltages originally (around 370v)

[martin manning]

Looking good now! The only questionable reading it V2 pin6 (336), which is approximately 80 higher than the chart. Should I be concerned about this?

That appears to be an error in the voltage table you linked. It should be ~350, same as the B+4 node voltage.

[lgehrig4]

Looking good now! The only questionable reading it V2 pin6 (336), which is approximately 80 higher than the chart. Should I be concerned about this?

That appears to be an error in the voltage table you linked. It should be ~350, same as the B+4 node voltage.

Thank you for clearing that up!

[gldtp99]

Here is a schematic (w/voltages) of one 1983 Marshall 2204 (owned by mhuss)

These are typical voltages of a 1983-era 2204 amp --- This does not mean that EVERY 1983 2204 will match these voltages exactly --- this does not mean that EVERY Marshall 2204 ever made will match these voltages (my '79 Marshall 2104, combo version of a 2204 amp, has the lower voltages of the earlier 2204's)

https://mhuss.com/MyJCM/JCM800_2204.pdf

Pin 6 of V2 has a measured voltage of 348VDC in this schematic so the voltage you measured is nothing to worry about

Why are you going around measuring voltages without all the tubes installed ?

You won't get any accurate measurements, that mean anything, that way

2204-type amps can operate properly with a range of voltages --- all 2204's from Marshall did not have exactly the same voltages as each other

Tube Guitar Amps are not very consistent from one unit-to-another -- they are rather crude electronic devices and are not built with the tight tolerances of a NASA Deep Space Probe (tube amps don't need to be)

[lgehrig4]

Here is a schematic (w/voltages) of one 1983 Marshall 2204 (owned by mhuss)

These are typical voltages of a 1983-era 2204 amp --- This does not mean that EVERY 1983 2204 will match these voltages exactly --- this does not mean that EVERY Marshall 2204 ever made will match these voltages (my '79 Marshall 2104, combo version of a 2204 amp, has the lower voltages of the earlier 2204's)

https://mhuss.com/MyJCM/JCM800_2204.pdf

Pin 6 of V2 has a measured voltage of 348VDC in this schematic so the voltage you measured is nothing to worry about

Why are you going around measuring voltages without all the tubes installed ?

You won't get any accurate measurements, that mean anything, that way

2204-type amps can operate properly with a range of voltages --- all 2204's from Marshall did not have exactly the same voltages as each other

Tube Guitar Amps are not very consistent from one unit-to-another -- they are rather crude electronic devices and are not built with the tight tolerances of a NASA Deep Space Probe (tube amps don't need to be)

I'm still learning thats why That and one of the voltage charts I downloaded stated that the measurements were taken without the PT's installed. I didn't know that would effect the preamp readings. I expected the voltages will vary, but I felt 80v was ok excessive, especially considering all the charts I've seen were similar.

The amp sounds great BTW

[martin manning]

Why are you going around measuring voltages without all the tubes installed ?
You won't get any accurate measurements, that mean anything, that way

I think no-load voltage at the reservoir is a good piece of information when starting up a new build, and it's the first thing I look at, usually long before the build is completed. If the power supply is complete, with all of the other filters wired in, preamp voltages without tubes will tell you if you have a short or wiring error somewhere. Without tubes drawing current, there should be little voltage drop going down the power string if everything is correct. Checking the bias supply voltage and sweep before powering up with tubes installed is also a very good idea to avoid possible damage.