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.

 

[beezer4]

Hi all,
Have a few updates/questions:

• Updated schematic below
• All the caps were bad or out of spec, so I replaced them.
• Noise is much better but there is still some 60hz hum. Do you think adding an additional PS filter stage would help? Could it be all the non twisted 6.3v wiring?
• Low freq is dropping off around 100hz. Any suggestions on how I might get some of that back?
• Added a 600 ohm resistor on the output XLR.
• I'm also debating adding a negative feedback control, similar to my Gates SA70 builds.

Thanks!

 

[soulsonic]

The .1uF coupling cap is too small with that 100K gain control. If you had used a 500K to match with the original values closer, .1uF would have been sufficient, but not so much now. So either higher value for gain control or higher value for that cap.
You might also consider making the .5uF screen bypass cap larger.

You can get away with the filament wires not being twisted, but it requires that you be very strict with keeping them absolutely neatly parallel to each other. If it's possible to tidy them up, it might help. Lifting the filament center tap from ground and replacing it with a "hum balance" pot might also help.

 

[MidnightArrakis]

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!View attachment 146710

[Just look for some thoughts or ideas from others] -- I just noticed while going through my RCA Receiving Tube manual that the 6AQ5 cathode (K) and its Grid-3 (G3) are internally connected together, which your schematic symbol doesn't show. Will this arrangement affect what it is that you want to do and how your schematic circuit diagram is designed and drawn? In addition, Grid-1 (G1) is also connected to 2 pins, Pin-1 and Pin-7. Just wondering.....

>> From your schematic shown above:




>> From the -- 1973 RCA Receiving Tube -- manual:

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[emrr]

The .1uF coupling cap is too small with that 100K gain control. If you had used a 500K to match with the original values closer, .1uF would have been sufficient, but not so much now. So either higher value for gain control or higher value for that cap.
You might also consider making the .5uF screen bypass cap larger.

It absolutely is not too small; this thing ain't gonna be recording the lows of pipe organs. Plenty of vintage things I've measured and posted here with 0.1/100K that go plenty flat enough. -3dB is 16Hz, that doesn't appreciably show up at 100Hz. Sure, you can go bigger until it motorboats and decide how far you want to take it. Low frequency drop here will predominately be the transformers; do they show 'flat' in testing with matched sources? I bet not. Adding more feedback can tailor the output transformer but not the input, to the benefit/detriment of response/breakup quality.

We have 20k now listed on the input pri, that's new info.

Is there still treble drop?

Try a hum pot instead of resistors or try an external DC filament supply to see what changes first.

Bummer on the caps. If they were truly shot and the treble is back, tells you ESR was sky high.

 

[beezer4]

@MidnightArrakis I did know about cathode (K) and its Grid-3 (G3) being tied. My bad on not updating the tube template. I didn't realize G1 was available at both pins, I'll go back and double check connections there.

@emrr - The input transformers test flat at 20hz and down 5db at 15k and roll off into 20k. I didn't test the output transformers out of circuit, so that could be causing the low end of roll off. Tests show the same high end roll-off (input transformer) and now the low end roll off as well. I'll test the output transformers.

Input transformers are 20k:200k
Output transformers are 5k (45ma DC) : 20k or 600 - I have them wired for 600 with a 600 ohm load resistor.
Both spec for 300 - 4000 CY +-.5db

I know I'm expecting more than they are rated for. They sound cool as is, was just hoping to sneak a bit more low end. I'm guessing I'll find it's transformer related.

Thanks as always for the input!

 

[MidnightArrakis]

>> The story so far:



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