RCA BA11 / GE 4BA1 Racking Project

[RicharD]

I have a friend who has enlisted my services to rack up a pair of old tube preamps.  The GE 4BA1 is in the foreground and the RCA BA11 is in the background.  Also shown is a 4 part schematic.  The RCA BA11 (000 numbers) and the GE 4BA1 (200 numbers) are combination copy / reverse engineered diagrams.  The RCA circuit was spot on but the GE circuit was somewhat different form the Collins 356a schematic I was told was the same.  The power supply (100 numbers) is something I came up with yesterday as is the incomplete interconnect (300 numbers) schematic.

so....

Besides making these thangs work, I don't want to destroy their collectors values.  The BA11 was easy to recap whereas the electrolytics were built into an octal plug.  No such luck with the GE box and I have not tackled this task yet.  I plan to stick with a can cap and I might actually build some modern radial caps into an old can.

I think it would be nice for these units to have a variable gain.  This seems easy enough to do.  I can change R5 to a 1M pot in the RCA circuit and change R204 to a 2M pot in the GE circuit.  There are existing holes in the noses of both of these chassis so I can cable out through these w/o having to make any serious alterations. Anybody see any problems with this plan or is there a better option.

The power supply is based upon available transformers and PRR's comment (from another forum) that V202 is running at it's bleeding edge limit @ 300V.  The B+ supply voltages shown are copied from their respective schematics.  I don't see any problem shaving a tad off and running at about 265V.  I am also thinking about adding a rectifier and regulator to the filament supply to have DC filaments.  This seems like a good idea.  One concern is phantom power across the input transformers.  It should be fine but I get paranoid when fooling around with irreplaceable vintage transformers. 

While thinking about the input transformers, which impedance connection would be considered universally acceptable in this day and age?  The owner is planning on using them with ribbon mics.  I don't know which make -n- model.

My last quandary is metering.  The BA11 has taps and a switch to meter current from cathodes of the tubes.  This is the "old school" quiet way of attaching a meter directly w/o adding meter noise by tapping into the signal path.  I could simply do the same thang to the 4BA1 circuit.  I'm considering building an opamp buffer circuit to insert between the outputs and the meters.  This put negligible load on the outputs and adds no noticeable noise.  The benefit as I see it is to know the actual output level.  Of course I also need to consider the owners budget for this project.

I greatly appreciate any feedback whereas these are vintage components and they are not mine.  I kinda only get one chance to get this right so the 6 P's are exceeding important.  (Prior planning prevents piss poor performance.)

Thanks!
-Richard






 

 

[PRR]

> quiet way of attaching a meter directly w/o adding meter noise by tapping into the signal path.

That does not meter signal. (Note V2's tap is below a 40uFd cap.)

It meters tube DC current. The master power supply had a DC meter. Touch each tube's switch, see if the meter comes to the "OK" mark.

Note that V1 is metered at 2K and V2 is metered at 150r. Evidently V1 runs at 150/2000 or 0.075 the current of V2. The 6V6 in the main program output might have 33r meter taps.

You log the readings daily/weekly. If they run 1.05 1.04 1.06 1.04 0.99 0.8 0.6 you say "oh-oh!" and replace the failing tube.

That's when you have 50+ tubes and 24/7 commercial reliability. For small studio, ignore the diagnostic aids.

Pot between IT and first grid hurts noise figure. Do it anyway; but for lowest hiss keep the pot full-up.

BA11's gain can be shimmed a small amount with the 100K NFB resistor. Open for max gain. 50K will reduce gain but you get into stability issues.

Original power supply was much cleaner than C-R-C. You may be OK because of the large filtering in each preamp.

36VAC seems hardly-enuff to support 48V regulated.