All these wires and voltages,so little knowledge

[danman]

The low channel is vastly different and caught me by surprise the first time I used it. That said, it should still be fairly loud with both volumes dimed. Since it does pass signal, I'm thinking the wiring is correct but there may be a bad ground, bad solder joint or a stray wire touching something and grounding the signal. Be sure the 10k cathode resistor for that channel has a solid grounding point. The fact that the pre volume pot isn't working would also point to a ground issue or cold solder joint. I have had joints at pots that looked beautiful but were not making a solid electrical connection. Took me awhile to track that problem down. Poking around with a wooden chopstick with the amp on and volumes about halfway up will help to pinpoint any loose joints.

[Mrcads]

Here are the voltages
V1
pin1- 251
pin2- input jack
pin3- 2.78
pin4/5- 3.21
pin6- 210
pin7- volume
pin8- 1.96
pin9- 3.21

V2
pin1- 171
pin2 -0
pin3- 1.08
pin4/5- 3.22
pin6- 291
pin7- 171
pin8- 172
pin9- 3.22

V3
pin1-215
pin2- 23
pin3- 36
pin4/5- 3.24
pin6- 217
pin7- 23.5
pin8- 36
pin9- 3.24

V4
pin2- 3.24
pin3- 452
pin4- 451
pin5- negative 43.75
pin6- 453
pin7- 3.24

V5
pin2- 3.24
pin3- 450
pin4- 450
pin5- negative 43.6
pin6- 452
pin7- 3.24

I hope I got them right. I omitted the decimal in most places.

Thanx

[danman]

Nothing in your voltages look to out of the ordinary. Every amp is a little different and the tubes used will also change voltages slightly from one to the other. I never asked if you tried reseating the v1 tube and/or tried using another one? Sometimes it's the simple things that cause the most problem. Another thought I had was to check the shielded cable for an internal short. It is very easy to melt the insulation when soldering and cause the signal to ground out. If your meter has a "beep" function, touch the center conductor with one lead and the shield with the other meter lead. If the meter beeps, you have an internal short. You may need to lift it from the volume pot to check it though as it may give a false beep since the pot is grounded.

[Mrcads]

Does it matter to which grid the input/volume goes? (2/7 vs 7/2)

[danman]

If you compare your layout versus the original Marshall, yours has the two channels flipped at v1. This has been done before and would not affect the amps operation as long as you followed the layout properly. When using the hi input, signal flows through one half of v1, gets amplified there and then it is sent back through the low input where it travels through the other half of the v1 tube. It is cascading the stage by using the second half of v1 to amplify the signal more and since this second half uses a 10k cathode resistor (cold clipping stage), it adds a different type of distortion to the sound. Plugging into the low input bypasses the first amplification stage so the sound is much weaker.

If the low side of v1 wasn't wired properly or amplifying signal, you would have a weak high channel or no sound. The fact that the high sounds correct means that everything along the path is functioning properly. I am confused as to why the volume pot is not working properly. This would normally mean a bad ground joint or cold solder joint at the pot lugs. I would reflow everything around the pot including all ground points. Also check the leaf switch inside of the low jack to be sure it opens fully when inserting a cable. You could take a small pick and gently lift the leaf contact while plugged in to see if the amp makes any sound. If these switches do not open fully, the signal will ground out.

[Mrcads]

Well...all it took was walking away from it for a while.
The problem was: the wires going to the plates on V1 were connected as shown in the Watts layout. I switched those wires around and all the problems went away:)
Thank you for all the help!

[danman]

Great! I was worried that the reversed layout may not match the board that you purchased. Glad you got the issue resolved

[Mrcads]

One more question: why are both leads on the presence cap grounded on this amp?

[danman]

It's normal. The cap and resistor affect certain frequencies and send them to ground. This is the newer style presence circuit that employs a 25k pot with the 5k resistor. The old style circuit used a 5k pot, but since the circuit allowed a small bit of DC voltage on the pot, they were always scratchy sounding when adjusting them. I believe the newer circuit was developed to eliminate this issue. Some say they sound slightly different but I cannot confirm that personally.

[neikeel]

It's normal. The cap and resistor affect certain frequencies and send them to ground. This is the newer style presence circuit that employs a 25k pot with the 5k resistor. The old style circuit used a 5k pot, but since the circuit allowed a small bit of DC voltage on the pot, they were always scratchy sounding when adjusting them. I believe the newer circuit was developed to eliminate this issue. Some say they sound slightly different but I cannot confirm that personally.

I think you get a slightly wider sweep with the 25k/4k7 combo

[danman]

I may have to experiment with the newer circuit a bit in my 1987 build to see if I prefer it.