My modified RCA BA2C has a mystery tube that went bad. There are no markings

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GreenFog_Studios

Active member
Joined
Nov 23, 2010
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27
Location
Miami, Fl
Ten years ago I bought my first RCA vintage tube pre amp and fell in love with them and I have gotten into a lot of the old RCA stuff. I bought it at Future Music in L.A. off Sunset. It used to belong to Mark Sampson of Matchless amplifiers and had ben modded. If you are looking at the pre amp with the volume knob in front, the middle tube closest to you which in a stock version is the 1620 on the right, has been replaced with an unmarked tube that is the width of the 6X5GT and about as tall as a 6L6. The tube has no markings and no longer lights up so I can't test it, measure the voltages and ID the mystery tube.

  Where the hole for the wire for the 1620 that has been swapped, there is a 5 way rotary pot which changes the character of the pre amp similar to a vari-tone guitar circuit and near the volume knob is a tone sweep pot.

  The BA2C sounded great until the tube started to die and now it doesn't pass signal. So I'm pretty sure that the issue is the tube. That being said does anyone know if the eq mods are related to the mystery tube and more importantly any ideas as to what the mystery tube is?
 
Absolutely no one other than the original modifier is going to know anything about this.  Get out your meter and reverse engineer the schematic, then sort out what socket configuration it is, and what tubes fit that configuration. 
 
pics?  top and a good close up of underside of the mystery socket would help.

filaments rewired to 7-8 now?
 
Thanks for all of the info. Hopefully we can figure this out. I have to admit I'm not experienced enough stuff to reverse engineer the mods from the original schematic. I don't even know what the original tester values would be let alone what they should look like with certain mods. I studied the wiring underneath some more trying rto figure out what was original and what looked older and stock. It seems that almost everything has been replaced when it was rebuilt and modded.

The 5 way knob has a bunch of silver capacitor looking things which are soldered to the pins of the socket with the mystery tube. The chicken knob on top goes to a white ceramic resistor I think that connects to to the 1620 tube socket.

I'm starting to think that the onlyoption is to have the BA2C restored to stock. It's ashamed to as the eq rolloff and tone sweeping was really nice for recording. It made a difference but wasn't so drastic that it sounded like it was recorded through too much eq.

The pic in this post is of the mods done underneath close up
 

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Pretty major rework.  Looks like an EL34, which is a bit frightening.  That would kill that output transformer, unless it's severely current limited, or it's a cathode follower and the output is already bad.  They added a choke in the power supply section.   Pretty wacky.   This must have been done long long ago, before anyone started paying much for these.   In regards to what to do, it'll be worth far more if returned to original.   That may not matter to you.  
 
The value of this pre amp to me was a great tool and a great sound on my recordings. I have worked with BA2C's that are stock and they sound great too. I figure that the only way to get this one back to work is to restore it. I am mainly an engineer and and a novice tech.

Even if I have to accept my secret weapon as gone it would be nice to have it restored any idea of how much it would cost to be restored?
 
I wouldn't imagine more than 2-3 hours time to figure out what's going on and sort out what the tube is.  Same if you wanted to convert it back to stock.    It's always tough to say without an initial exam.  I've worked on these, as has Lassoharp, if you want to send it off for study and rehab.  You could also draw out the schematic yourself, and then show us that.  The stock schematic can be found online, printed, and used as a starting point. 
 
I'm that good with schematics as it's been a while since I took my audio wiring classes. Alan contacted me about sending it to him for restoration. He said he'd look at the circuit too see what had been done. we are now talking about doing it in January. He said said he through in a db pad for line use and maybe build a second to match. I'm getting really excited about being able to use it again and maybe having a stereo pair.
 
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