Help! Tube preamp troubleshooting

Hi guys,

Just finished putting together a two-channel tube preamp based on Hampton's schematic. I'm getting way too much AC signal hitting the first grid (should be .48V, but it is 7.98!). So, the input is overdriven and as I turn up the volume (stepped attenuator) it quickly distorts and gets nasty.

The strange thing is that the output peaks at -15db (should be 1.24V, but reads around 41v).

Any thoughts? I'm pulling my hair out.

Here are some videos of the problem and schematic (330 ohm grid stoppers added).

Video 1: https://youtu.be/1iKeQzl_pdQ
Video 2: https://youtu.be/4H5U2fxRUKE
Video 3 (vocal sample, phantom power is fixed): https://youtu.be/NLa-8qjbzFs

measure signal into primary of transformer

measure signal on secondary of transformer

get a highliter pen and trace your chassis against the schematic, one wire at a time,

> too much AC signal hitting the first grid

So it can't be the amplifier. Sounds like the test operator is putting in like 1 Volt at the MIC input instead of a mike-level. (Or 8V at the Guitar jack?)

Testing the AC voltage levels at the first grid could be operator error, but it still doesn't explain the characteristics. And BOTH channels have the same distortion/low output issue.

Would having very high heater voltage do this? It's currently at 75v. Could I have fried something?

> output peaks at -15db (should be 1.24V, but reads around 41v).
> very high heater voltage do this? It's currently at 75v.

This all makes no sense. Either your meter is sick, you have a missing ground, or you are not measuring what you think you are.