Dept of Commerce CA-1588 Preamp Questions

[beezer4]

Hey all,
I snagged a pair of these Department of Commerce preamps the CA-1588 Dual Channel Amplifier.

I attached a crude drawing of the schematic below and pictures. I'm hoping to turn this into a proper mic preamp/saturation device. The transformers test flat at 20 hz and down 5db at 15k and roll off into 20k, so I know it won't have full frequency response.

I was looking for any suggestions on tweaking the circuit. I'm planning to replace the power supply caps and possibly setup better filtering.

Just look for some thoughts or ideas from others.

Thanks!

 

[dmp]

Cool Gudeman caps.
Other than the obvious components missing (one of the transformers )
The 5.1nF caps are smaller than you'd want, they form a HPF with the 470k well into the audible range. I don't see where those are in the picture but you'd want them to be larger, ~100nF
Bumping up the filtering caps when you replace them, possibly add another pi stage - it's probably insufficient as drawn, consider cathode bypass caps.
Figuring out what gain it is currently designed to give and what gain you want, may help inform further changes.

 

[emrr]

I wouldn’t touch any of those caps if they work. That’s a lot of the character. They generally don’t go bad.

The filament needs CT grounded resistances or a hum pot. Don’t blame filter caps for anything until that’s done.

Sure, parallel some more across the 0.0051. 0.047 total is sufficient, any more may make it motorboat.

A very small cathode bypass cap can give a treble boost. Look at Collins 6Q. Bigger cathode bypass will raise gain and lower distortion, probably not what you want.

The NFB is doing very little. That’s another option.

Lose the 620 input load

200K input sec, that’s gonna naturally have treble rolloff and the 200k gain pot doesn’t help. Consider making V2 grid 470k a 500k gain pot instead, or better yet 100k with the coupling cap changed to 0.1.

 

[heatwalk]

If we are looking for more creative options and not something straightforward like cathode caps for more gain - You could try a triode/pentode “morph” control on V1.

If you are happy with the gain, instead of a full frequency cathode bypasses, you could try a cap and variable resistor in parallel with a cathode resistor? Gets you some HF shelving boost options

 

[beezer4]

Thanks all!
This is super helpful. I'll play around with some of the suggestions and see what I like sound and gain wise.

@dmp I just pulled that input transformer out for testing, so I got both.

The second unit I have is a little different with a solid state rectifier instead of the 5Y3. I believe the circuit is the same, but I still need to double check.

 

[heatwalk]

@emrr why are 100k volume pots common to mic pres vs higher values in guitar amps?

 

[emrr]

The higher value the voltage divider in a grid, the more treble losses with attenuation. Guitar doesn’t care so much.