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Mike Sommer
A Hundred Dozen
Joined: 05 May 2008
Posts: 1222
Location: Boss Angeles
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 Posted: Thu May 08, 2008 7:48 pm Post subject: Retro Magic Box- RCA BA-31B |
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Here we see my newly racked RCA BA-31b completly rebuilt with vintage new old stock germanium transistors, Spraige caps and a few mods: 20 db pad, +6db boost switch on the amp face, output gain, turbo charged, bucket seats and a Muntz 4track audio player- sweet!!!!
This tiger really sounds good, it has a nice little growl in it from the Germanium transistors, yum!
I'm going to play with the output capasitor to give it a little more low end, but the "orange drop" bypass caps that I added really open up the top end - scrumptious!
The BA-31A replaced the legendary RCA BC-2B tube pre in the early 1960-61. For the most part these were in every major recording & broadcasting studio in America until the 1970's.
This unit sounds very similar to a Neve 33114 pre... closer then the Vintech clone.
I can get a nice Vintage sound out of this pre; Ribbons sound fantastic, the EV-RE15, Sennheiser MD 421, Gefell UMT 70s, Neumann's and Studio Projects C1 all sound yummy. I haven't found a mic that does not work well on this amp. As a matter of fact, I believe this amp improves the sound of the TLM103, which I don't care for at all.
My next project will be an RCA BA-43, this amp has a ridiculous amount of head room perfect for the RCA 44's and 77DX or any vintage ribbon mic.
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Yoda117
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Mike Sommer
A Hundred Dozen
Joined: 05 May 2008
Posts: 1222
Location: Boss Angeles
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| Posted: Thu May 08, 2008 11:13 pm Post subject: |
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| Yoda117 wrote: |
any issues with the tertiary feedback winding in the output transformer? |
This amp is golden. There is usually very few problems with these units that would go for whole series (BA-31, BA-41, BA-71, BA-72).
New caps, and transistors replaced a few resistors just because and she was a runner.
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Acoustics are counter-intuitive. If one thing is certain about acoustics, it is that if anything seems obvious it is probably wrong. |
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Yoda117
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todd ellis
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Joined: 02 Jan 2007
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Location: little egypt
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| Posted: Fri May 09, 2008 10:33 am Post subject: |
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welcome to the board, mike. now greg has someone to play with.
what the crap did you two just say?!?
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Mike Sommer
A Hundred Dozen
Joined: 05 May 2008
Posts: 1222
Location: Boss Angeles
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| Posted: Fri May 09, 2008 11:17 am Post subject: |
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| Greg wrote: | | those buggers (output xfrmrs) are a PITA to repair. |
Translated:
Those F___ing output Transformers are very troublesome to fix.
I would say they are impossible to fix Greg and that's the magic of these amps nobody makes iron like that anymore. Everything can be replaced but the transformers, or at least not without major $$$. If you're haveing problem it could be the taping of the output xfrmr 150 ohm and 6oo ohm, My out put is 600 ohm with a 1k pot, I'd like a 600/600 stepped Daven, but as it is I hardly use the out put gain.
The out xfrmr will oscillate without a load resistance, but I've never had that problem myself. A 20db pad on the front end would be considered a load, but so as long as you have a mic plugged in it should be fine.
But I've never come across one that had a bad xfrmr, they are very reliable. Though the power xfrmr can be the weak link for this amp.
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Acoustics are counter-intuitive. If one thing is certain about acoustics, it is that if anything seems obvious it is probably wrong. |
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Yoda117
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Joined: 20 Dec 2006
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Location: Philadelphia, Pennsylvania
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| Posted: Fri May 09, 2008 11:41 am Post subject: |
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| Mike Sommer wrote: | | that's the magic of these amps nobody makes iron like that anymore. |
No Mike, I went another route but I'm very familiar with the preamps (used to use the console pres... very nice).
It's expensive to deal with the tranformers, but there are still folks making stuff like that, you just have to look.
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Mike Sommer
A Hundred Dozen
Joined: 05 May 2008
Posts: 1222
Location: Boss Angeles
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| Posted: Fri May 09, 2008 11:51 am Post subject: |
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What source did you find for the ins and out xfrmr?
Curious minds would like to know.
You might find something close on paper as far as the sound the color is off.
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Jeffrey Kafer
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Joined: 09 Dec 2006
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Location: Location, Location!
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Yoda117
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| Posted: Fri May 09, 2008 12:04 pm Post subject: |
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Jeff... what did I tell you about taking Photos of Philip when he's sleeping?
At least include Jazz...
To answer Mike's question, there are a few homebodies that do work here and there. The guys at Mercenary might have the capabilities (I know they can do a lot of that work, but not sure what their experience with RCA outboard gear is).
My personal vote would be to Johnathan Little at Little Labs. He's big on using techiques and proven tech from the older stuff and incorporating it into newer designs. His current Op-Amps and transformers are, frankly, out of this world.
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glittlefield
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Joined: 08 Mar 2006
Posts: 2039
Location: Round Rock, TX
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| Posted: Fri May 09, 2008 1:39 pm Post subject: |
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Looks great, Mike. Personally, I enjoy watching discussions like this where audio gear is described like old cars. 
(steps around Jeffrey to look under the hood a little more closely...)
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