Strange Voltages on JCM 800

[Wesmont]

Upon completion of my 2204 build I am getting some strange voltages.... My bias supply voltages are quite high, along the lines of - 160 VDC when all the way cranked. My pin 3 readings seem fairly normal with no Power tubes installed, around 450 VDC, but with tubes in place and biased, I'm only getting around 370 VDC. I am using a Mojo JCM 800 transformer set 360-0-360 PT. This PT is quite large, and it is getting pretty hot after an hour or so of low volume playing. Something seems amiss, even though the amp sounds pretty good.

[Wesmont]

BTW, I used the layout here for the 50 watt master volume on the Metroamp Wiki. Followed it to the letter I believe.

[Cabbagebreeze]

I don't want be master of the obvious, but there is a wiring error somewhere. Use a process of elimination to find it. There's no way someone out here will be able to pinpoint the problem with the info you've given

[Wesmont]

Well, that was/is what I want to know. Considering the PT I am using, are these voltages out of the norm? I thought they were, but am not sure. I am not looking for an answer for any wiring issues, I would just like to know if people think there is definetly a problem with these voltages. The amp sounds good BTW, I have been playing it all morning.

[demonufo]

BTW, I used the layout here for the 50 watt master volume on the Metroamp Wiki. Followed it to the letter I believe.

That's interesting. There are some wires missing altogether from that layout IIRC. Like two of the tonestack wires for instance. And the NFB wire at the back of the chassis. There are also a couple of other improvements that can be made. Also not on the layout, there should be a jumper from top left terminal of LO input socket to top right terminal of HI input socket.

That was a layout that Dan (BillyBatz) made just before he disappeared altogether, and never got round to 'repairing'.

[Wesmont]

I did make those changes myself. The wires that lead to nowhere were a dead giveaway that something was off. I also looked at some other layouts. Really they all seem very similar, except most do have the treble pot wired up.....

[Wesmont]

I guess I should have said i followed what was there to a T. I did make some additions, the same ones you mentioned.

[Wesmont]

Problem solved. I had the wrong bias pot. I was sent and installed a 500k pot instead of a 50k. I didnt notice until last night. I replaced the pot and not only did the bias voltages fall in line, so did the plate voltages. That huge pot must have been putting too much draw on my PT?? I dunno, it seems fine now.