Sta-Level Vari-Mu Overdrive

[StarTrucker]

Built a Sta-Level and got it running with Abbey's and few other's help.

I have a problem where the amp distorts during compression especially on big hits - snare drums, bass slaps, or vocal plosives. The compression comes in smoothly with no thumps, but the signal itself is distorted.

I took pics on the scope of the compressor engaged and then in bypass and there are two different types of distortion on the grids of the 6v6. With the compressor engaged, there is a shelf in the side of the sine wave. At first I thought it was the compressor releasing or engaging, but I don't think that can be since the entire sine wave is squashed.

Pk-pk sine wave voltage on the grids of the 6v6 with the side chain bypassed = 37v
Pk-pk sine wave voltage on the grids of the 6v6 with the side chain engaged = 24v

here is the scope with the comp in bypass:



here it is engaged with faster-than-stock attack and release values:

this is with factory attack/release times, shelf moves up toward the peak of the wave and distorts the trough, making a sawtooth



The second kind of distortion is with the same input signal level but the side chain set to bypass (first pic). The amp itself distorts from the signal and shaves the top of the sine wave just like you would expect from an overdriven amp. Doesn't this mean that the amp will be distorted for a split second on every big hit before the attack kicks in?

Voltages in the amp are all dead-on to the schematic, except for 6v6 plates which only drop about 2-5 volts across the Xfrmr primary. Using stock values at the slowest times, with switches to speed things up.

Didn't include the single/double switch.

Thoughts from the council?

 

[gyraf]

To check vari-mu stage independently of sidechain, try removing sidechain connection and adding a variable negative voltage to the vari-mu grids directly.

This should tell you if you have significant distortion when gain-reducing

If you have, at say up to some 10-15dB reduction, check if your operating level is sane and if your 6386 is actually ok (or just good enough for this circuit, I would have liked to see anode/cathode balancing trimmes in here..)

Remember that recent production of those tubes are crap, and every single one of the NOS for sale has been tried and rejected more than once before..


/Jakob E.