M-Pre partial success, some troubleshooting to go

[talking_glowstick]

Hi, I've recently built up a PSU-PRE and an M-Pre.  I have partial success, but I'm having two issues and I've run out of things to check/know to troubleshoot.

1) In each channel, I get a good quality sound (clean in one, distortion in the other).  However, the EQ doesn't work in either channel.  The knobs make no difference as if the tone stack is completely defeated.

2)  Also in each channel if I turn the volume up above 1 o'clock then it goes from normal-sounding to suddenly heavily clipped, fuzzy, lower volume. Playing harder makes it worse and tends to make it cut out entirely.

Of note:  
This is a fully 'stock' 800 build, 12V heaters.
I've carefully triple-checked all my wiring.
I gently scratched between all joints, and then used alcohol and a plastic brush to clear any possible solder bridges.
I can power it up without tubes, through a PLSS with a 60W lamp and read 371 at the plate input, -12 at the heater input.
Changing tubes does not affect anything.

Does anyone with more experience have any suggestions about what else to check?

[K O'Connor]

Hi Guys

Note that all the resistors in our kits are 1% tolerance (E96 values), which means they have three number bands, then the multiplier, then tolerance band. It is easy to confuse the values if you are only used to 5% resistors (E24).

Also note that for the 800 wiring there is a cap position left open (C3,5). Adding a cap here greatly increases the gain from the stock 800 level. The schematic has a note to this effect.

Regarding the EQ: No effect suggests the ground is missing.

With the tubes in place and Volume and Gain at zero, measure the plate voltages for each tube. This is pin-1 and 6 of each tube. These voltages should be 50-100V lower than the voltage at the related supply node.

Since you are getting sound, it is likely that the heaters are wired correctly: Twisted wire pair to 12V wire block; NO LINK in 6V block. And actually getting 12V from the PSU.

Note that when soldering any double-sided PCB, there should be solder wetting around component leads on both the top and bottom. Ideally the plated hole provides conduction between top and bottom traces but this should not be counted on.