2 tube microphone preamps sharing a tube PSU?
« on: October 07, 2019, 01:32:15 PM »
I have an RCA BA-2C clone (minus the power supply) that I am looking to power from a tube PSU for another preamp
The PSU is a Gates SA-77 module (designed to run 1 or 2 SA-70 or SA-73 preamps).  I currently use the SA-77 to power the SA-70 that I bought it with. 
Both would run off of 250V B+ supply

Is there any reason not to simply wire both preamps to the SA-77 module?  I'd like to avoid modifying any of the circuitry but if its advisable to add another stage of power filtering I would consider it

Here's some info about the Gates power supply-
https://www.americanradiohistory.com/Archive-Catalogs/Gates-Harris/Gates-Speech-Input-Equipment-and-Price-List-1950.pdf
(page 29 gives the specs for the SA77 psu)
current ratings- 2A filament & 10ma plate



Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #1 on: October 07, 2019, 01:34:36 PM »
RCA BA-2C

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #2 on: October 07, 2019, 01:36:28 PM »
Gates SA-70

PRR

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #3 on: October 07, 2019, 06:22:12 PM »
Yes.

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #4 on: October 08, 2019, 10:43:09 AM »
Most all of the older valve desks used a common supply for the amps.  The transformer primary of your BA-2 amps which is used to feed DC to the anode will provide some isolation from channel to channel but I think a nominal series resistor shunt capacitor located on each BA-2 channel would be advisable.

Not exactly sure of the particular question  PRR's more succinct reply is answering 😄
The only explicit one I found was:
"Is there any reason not to simply wire both preamps to the SA-77 module?"

Regardless, I would always bow to his wisdom on this stuff.  👍

EmRR

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #5 on: October 08, 2019, 10:46:18 AM »
The PSU is in range for the BA-2, so run them together.  May require DC adjustment through resistor change, I haven't looked at that PSU schematic in awhile. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #6 on: October 08, 2019, 10:55:17 AM »
I bow to Doug's wisdom too, he's seen more of this stuff than I've had hot dinners.

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #7 on: October 08, 2019, 11:06:47 AM »
Thanks for the responses!

EmRR

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #8 on: October 08, 2019, 05:06:57 PM »
3.4mA for SA-70 at 250V.  BA-2 is?  Don't recall.  Maybe 5 mA, not much more. 

The SA-77 says 30mA at 250VDC continuous.  It can power NINE SA-70's no problem, just can't cover more than 2 for filament.   

Important thing with an original BA-2 is not having too much current through the output transformer:  if voltage looks really high then current will be higher too.   288V at the first filter condenser.  232V at the plate of V2.   Mated with the SA-77 you should be somewhat in the same ballpark, if not, then R1 and/or R2 in the SA-77 need to change to compensate.  They will probably need to be larger values to get the BA-2 in range. 

The SA-70 will be fine wherever it lands; no current in any transformers to worry about.  You won't go over on cap voltage either. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

PRR

Re: 2 tube microphone preamps sharing a tube PSU?
« Reply #9 on: October 09, 2019, 02:25:27 PM »
> BA-2 is?  Don't recall.  Maybe 5 mA, not much more. 

I cheated, perhaps unwisely. BA-2 has a 220 tap on V2 cathode resistor. I *assume* this is switched to the VU meter for monitoring tube health. Taking 0.7V on 220 I get 3mA in V2. V1 has huge plate resistor and will be <1mA.

2A heater ought to be enough for these tubes.

Yes, trim the drop resistors. For "serious" use we liked plenty of B+ for low THD when driving orchestra through a mile of cable. In today's world of small studios and near-perfect chips, you want "low" B+ to get some "color" without smoking your ADC. 200V may be fine.


 

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