50K trimpot wiring in JCM800

[SleepLess]

Hi there!
I finished my build two days ago and haven't been able to hear it so far. I highly suspect a bad Piher 50K trimpot wiring. I have tried several positions and can't seem to find how to install it properly. My B+ is 450V and I have a -39V negative voltage at pins 5 with the pot fully counterclockwise. I get no voltage reading across the 1Ω/1w resistors at pins 8/1 to ground... So I suspect the pot to be wrongly installed. I have checked about everything else.

Here's the layout I used:


Thanks for your help!

[shooter]

I have a -39V negative voltage at pins 5 with the pot fully counterclockwise. I get no voltage reading across the 1Ω/1w resistors at pins 8/1 to ground.

with power off and tubes pulled OHM from the "tube side" pin 8 to ground for each tube.

[sluckey]

-39v on pins 5 should allow plenty of current through the tube and should result in many millivolts dropped across that 1Ω resistor. I suspect some other problem. What are the voltages on all pins of the output tubes?

Does the voltage on pin 5 change as you turn the bias pot?

PS... Here's a better quality layout...

[SleepLess]

Hi!
Hold on, I have compared the layout I used with another one from SDM's and mine is missing the connection between pins 6 of the EL34s and the cable that goes from pin 6 to the board and the choke. I'll add that one now and see if it corrects the rest. I hope the trimpot will be set the way it should...
Thanks!

[sluckey]

missing the connection between pins 6 of the EL34s and the cable that goes from pin 6 to the board and the choke.

No screen voltage. That would certainly kill the current frow thru your 1Ω resistor.

[SleepLess]

OK, that set the voltage issues, now the tubes receive the proper voltages and I was able to set the bias. But I still got no sound at all coming through... Crap!
I can't believe the Watts Audio layout missed those very important connections!

[sluckey]

It's very easy to make mistakes on a drawing. I rarely get all the details right in any of my drawings on the first try. Sometimes errors are found and reported by others much later.

The layout I posted was a later revision 03/31/2014. I picked it up from Watts website just yesterday. Your "washed out" layout is the same drawing but with an earlier revision on 01/16/2014. Where and when did you find that old layout?

[SleepLess]

I don't remember when I found it. I picked it up last summer...
Need to find why I don't get any sound now... 

[shooter]

why I don't get any sound now... 

Keep it simple, look at input jacks, speaker jacks and wires to and fro.  Full set of tube voltage readings is also helpful.  Think of it as an Easter egg hunt

[sluckey]

Look carefully at the input jacks. The 2204 uses some tricky wiring to provide the hi and low switching. Very easy to mess that up.

Tube voltage readings provide clues. They would have quickly revealed your missing wire on the output tube screens.

Divide and conquer. Turn the master pot all the way up. Now touch the wiper of the master pot with your finger (may help to use a nail or similar skinny metallic probe). If you can hear a buzz in the speaker then the PI and output stage are probably working OK. If so, look to the preamp for problems.

[SleepLess]

Got it! I had no wire going from V2 pin 6 to the board! Jeez this amp sounds great! I am even surprised by the clean sound you can get out of it. It sounds great through my Marshall 4X12 replica cab!
Ill post some pictures of the build when the sun comes back up, it's raining now!
Thanks for your help!

[sluckey]

Got it! I had no wire going from V2 pin 6 to the board!

Ha! Another error on that outdated layout. There may be more.