the real RCA ba71?
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hitchhiker
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T1 is NTE 103a and the other 5 are NTE 102
hitchhiker
Well-known member
I just used those standard aprox 2"x6" plug in bread boards and placed
the transistors close where I could and it came out very quiet.
I did up a second one and it sounds identical.
Lance
the transistors close where I could and it came out very quiet.
I did up a second one and it sounds identical.
Lance
For the record:
BA-31A and BA-71A are the same 6 transistor all germanium amp circuit, if still stock. RCA pushed very hard to sell replacement 4 transistor circuit boards when they did the revision in 1961, so very few 6 transistor circuits are still out there. You will find many 'A' versions with the replacement 4 transistor boards. I think the 'A' circuit was only sold for about a year.
The 'A' versions two extra transistors are simply an extra paralleled output stage, which was done away with when the later silicon design proved capable of same performance with only one output stage. This is somewhat similar in concept to having a push-pull pair of 2A3 tubes being equal in performance to four 45 tubes in push-pull-parallel mode.
BA-31 has PSU, BA-71 does not.
BA-31/71 B and C are basically the same 4 transistor circuit replacing all but the central driver transistor with silicon.
The 'B' version uses a 2N2270 input transistor.
The 'C' version goes to a 2N3391A input transistor for lower noise, though in practice I've heard no noticeable difference in original parts comparisons. I have put new 2N3391A's in many 2N2270 positions and eliminated a grainy noise/distortion. See later post here: the real RCA ba71?
Then comes the BA-72A revision, which re-arranged the package layout and changed one circuit value - the output cap goes from the previous 100 mfd to 1500 mfd, so there's your low end difference. Keep in mind that cap values in those days were to a degree dictated by size, and 100 mfd fit comfortably, and then some years later 1500 suddenly fit. I usually rebuild these with 2200 mfd and a bypass film cap for better high's.
The BA-41A is a BA-72A with separate PSU in one package like the BA-31.
The BA-72A also has a slightly higher frequency spec printed on the transformers, which I suspect are really the same as the previous. You tend to see Kenyon rather than UTC on the BA-72A's, though I have seen UTC's on them as well. Strangely, every BA-41A package I've personally seen has had Kenyon's, while loose BA-72A's from consoles have more often had UTC's. Probably a coincidence in my experience.
I don't think I've ever encountered Kenyon's on the 31's or 71's.
These amps have an input overload level of -22 dbm, which was something of an unofficial standard for about a decade. If you mic up a rock band without input pads you will likely overload them, or any other vintage pre for that matter. Wiring for 600 ohm input will of course give you a few more db of input headroom (about 3-4 db I recall) over the 150 ohm winding, while wiring for 37.5 ohms will give even less. The +6 db strap also lowers the input headroom by another 6 db.
Veering slightly off-topic, keep in mind that these have a bridging load on the secondary, so the actual input impedance is roughly 14.66 times higher than the rated 150 or 600 ohms, or roughly 2200 ohms and 8800 ohms. This is a vast simplification of what's really going on here, but does inform the basic electrical situation. There are other interactions here that are beyond my comprehension (some rocket scientist please step in here; I've yet to see a discourse addressing AC impedance issues with mic transformers). Even with a 1 to 1 transformer (say, for mic splitting purposes) you will get a different sound with 150:150 tapping versus 600:600 tapping when connected to a standard 150 mic input due to the varying AC impedance load of the 1:1 transformer. The DC impedance difference of these winding options will usually look something like:
150 ohm tapping = 20 ohms DCR
600 ohm tapping = 60 ohms DCR
This is obviously different from strapping a 20 or 60 ohm resistor across the mic and preampo combo.
The input transformer secondary value is quite on purpose. I don't go for this crap POV that companies the size of RCA in olden days fudged things in their desire to use up old stock of transformers. RCA was huge industry, far larger than anyone existing today in audio, and maybe larger than all of todays audio companies combined. They would just shit-can or surplus old iron and redesign for their new products, maybe set it aside for service work and replacement orders. Looking back at their amps all the way to 1930 you find constant and countless revisions of transformers. Many in cases where they could easily get away with using the same iron in the next product. During the mid 1930's you find amps that were made for only 3 years that have 3 different sets of transformer revisions, all in the quest for best operation.
The 6 transistor 'A' versions have less low end (smaller output caps) and a slightly less linear sound than the 4 transistor versions. They are a little smearier/browner. They sound a little more in the direction of lower feedback tube circuits, though that's a ridiculously general comparison. I keep some around (with larger output caps) for some guitar tracking, as the difference is worthwhile. I use the 4 transistor version for drums all the time, and bass pretty frequently. I acquired these some years after having worked on a Neve with 33114 and 33115 modules, and have to say that they remind me of that sound more than any other amp I've used, and to me sound more like Neve than pieces like the Vintech clone.
Here's a response comparison between the 'A' and the 'B' type, both having 2200 mfd output caps rather than the stock values:
http://farm4.static.flickr.com/3174/2606251950_c103fe86cf_o.png
I also wrote a bit on the history of the BA-33/43/73 program amps over here:
https://groupdiy.com/threads/r-c-a-ba-43-program-amps.15372/#post-181244
BA-31A and BA-71A are the same 6 transistor all germanium amp circuit, if still stock. RCA pushed very hard to sell replacement 4 transistor circuit boards when they did the revision in 1961, so very few 6 transistor circuits are still out there. You will find many 'A' versions with the replacement 4 transistor boards. I think the 'A' circuit was only sold for about a year.
The 'A' versions two extra transistors are simply an extra paralleled output stage, which was done away with when the later silicon design proved capable of same performance with only one output stage. This is somewhat similar in concept to having a push-pull pair of 2A3 tubes being equal in performance to four 45 tubes in push-pull-parallel mode.
BA-31 has PSU, BA-71 does not.
BA-31/71 B and C are basically the same 4 transistor circuit replacing all but the central driver transistor with silicon.
The 'B' version uses a 2N2270 input transistor.
The 'C' version goes to a 2N3391A input transistor for lower noise, though in practice I've heard no noticeable difference in original parts comparisons. I have put new 2N3391A's in many 2N2270 positions and eliminated a grainy noise/distortion. See later post here: the real RCA ba71?
Then comes the BA-72A revision, which re-arranged the package layout and changed one circuit value - the output cap goes from the previous 100 mfd to 1500 mfd, so there's your low end difference. Keep in mind that cap values in those days were to a degree dictated by size, and 100 mfd fit comfortably, and then some years later 1500 suddenly fit. I usually rebuild these with 2200 mfd and a bypass film cap for better high's.
The BA-41A is a BA-72A with separate PSU in one package like the BA-31.
The BA-72A also has a slightly higher frequency spec printed on the transformers, which I suspect are really the same as the previous. You tend to see Kenyon rather than UTC on the BA-72A's, though I have seen UTC's on them as well. Strangely, every BA-41A package I've personally seen has had Kenyon's, while loose BA-72A's from consoles have more often had UTC's. Probably a coincidence in my experience.
I don't think I've ever encountered Kenyon's on the 31's or 71's.
These amps have an input overload level of -22 dbm, which was something of an unofficial standard for about a decade. If you mic up a rock band without input pads you will likely overload them, or any other vintage pre for that matter. Wiring for 600 ohm input will of course give you a few more db of input headroom (about 3-4 db I recall) over the 150 ohm winding, while wiring for 37.5 ohms will give even less. The +6 db strap also lowers the input headroom by another 6 db.
Veering slightly off-topic, keep in mind that these have a bridging load on the secondary, so the actual input impedance is roughly 14.66 times higher than the rated 150 or 600 ohms, or roughly 2200 ohms and 8800 ohms. This is a vast simplification of what's really going on here, but does inform the basic electrical situation. There are other interactions here that are beyond my comprehension (some rocket scientist please step in here; I've yet to see a discourse addressing AC impedance issues with mic transformers). Even with a 1 to 1 transformer (say, for mic splitting purposes) you will get a different sound with 150:150 tapping versus 600:600 tapping when connected to a standard 150 mic input due to the varying AC impedance load of the 1:1 transformer. The DC impedance difference of these winding options will usually look something like:
150 ohm tapping = 20 ohms DCR
600 ohm tapping = 60 ohms DCR
This is obviously different from strapping a 20 or 60 ohm resistor across the mic and preampo combo.
The input transformer secondary value is quite on purpose. I don't go for this crap POV that companies the size of RCA in olden days fudged things in their desire to use up old stock of transformers. RCA was huge industry, far larger than anyone existing today in audio, and maybe larger than all of todays audio companies combined. They would just shit-can or surplus old iron and redesign for their new products, maybe set it aside for service work and replacement orders. Looking back at their amps all the way to 1930 you find constant and countless revisions of transformers. Many in cases where they could easily get away with using the same iron in the next product. During the mid 1930's you find amps that were made for only 3 years that have 3 different sets of transformer revisions, all in the quest for best operation.
The 6 transistor 'A' versions have less low end (smaller output caps) and a slightly less linear sound than the 4 transistor versions. They are a little smearier/browner. They sound a little more in the direction of lower feedback tube circuits, though that's a ridiculously general comparison. I keep some around (with larger output caps) for some guitar tracking, as the difference is worthwhile. I use the 4 transistor version for drums all the time, and bass pretty frequently. I acquired these some years after having worked on a Neve with 33114 and 33115 modules, and have to say that they remind me of that sound more than any other amp I've used, and to me sound more like Neve than pieces like the Vintech clone.
Here's a response comparison between the 'A' and the 'B' type, both having 2200 mfd output caps rather than the stock values:
http://farm4.static.flickr.com/3174/2606251950_c103fe86cf_o.png
I also wrote a bit on the history of the BA-33/43/73 program amps over here:
https://groupdiy.com/threads/r-c-a-ba-43-program-amps.15372/#post-181244
Last edited:
[quote author="emrr"]
I use the 4 transistor version for drums all the time, and bass pretty frequently. I acquired these some years after having worked on a Neve with 33114 and 33115 modules, and have to say that they remind me of that sound more than any other amp I've used, and to me sound more like Neve than pieces like the Vintech clone.[/quote]
Thanks for this info. Quick Q, are you talking about the BA71A or BA72A being similar to the 33114? If the 72A, do yours have Kenyon or UTC transformers?
Cheers,
T
I use the 4 transistor version for drums all the time, and bass pretty frequently. I acquired these some years after having worked on a Neve with 33114 and 33115 modules, and have to say that they remind me of that sound more than any other amp I've used, and to me sound more like Neve than pieces like the Vintech clone.[/quote]
Thanks for this info. Quick Q, are you talking about the BA71A or BA72A being similar to the 33114? If the 72A, do yours have Kenyon or UTC transformers?
Cheers,
T
I'm referencing a memory of similar sounding equipment. We all know how trustworthy that is. Someone could surely put the two side by side and list a billion ways that I am wrong, but that was the impression I got when first hearing the RCA's, and those were the Neve's I had worked with to any significant degree. The first Vintech product doesn't sound like either one to me; it sounds more like some Audix units I had at one point. My personal units are all 71's with the modifications to meet the 72 spec (the 71A's having the larger output cap for flatter lows), and all have the UTC's.
Mine are hand wired stepped, if it's not obvious. No room for davens in this box.
Did you do any kind of input pad?
I may move the phantom to the back, and put in the +6 switch where the phantom lives on the front currently.
Did you do any kind of input pad?
I may move the phantom to the back, and put in the +6 switch where the phantom lives on the front currently.
lewilson
Well-known member
- Joined
- Aug 9, 2005
- Messages
- 218
No there 72a preamps. the ones I have with kenyons have 500uf at 50v output caps.I have 10 with kenyons and they all have the same cap set up. ive been working on two of them that are very noisy.Lots of crackling like a bad tube. I changed all caps with new spraugs and tried replacing some of the resistors aound the first gain stage. Still noisy. i think I have traced it down to the 2n404 transistor . Id like to find some nice ones to replace them with. I see the nte 128 crosses but I have had bad luck in the past with some of the nte stuff being noisy .
The 72's I've seen, Kenyon and UTC, all had 1500 mfd outputs. I did see one once that had something like a 25 mfd aftermarket cap on the output, and I assumed it was a purposeful low rolloff mod.
It's interesting to hear about a noise problem with these; in my experience they seem to either work well, or not at all. The only noisy ones I've ever seen were 71A six germanium types, and it was bad output caps causing crackling sounds. The times I've swapped transistors in these types have caused little observable difference. They are usually always very quiet in the first place. Not true with the higher gain program amps, where input transistors can make a difference.
Just some commentary; maybe not much help. Good luck fixing them.
It's interesting to hear about a noise problem with these; in my experience they seem to either work well, or not at all. The only noisy ones I've ever seen were 71A six germanium types, and it was bad output caps causing crackling sounds. The times I've swapped transistors in these types have caused little observable difference. They are usually always very quiet in the first place. Not true with the higher gain program amps, where input transistors can make a difference.
Just some commentary; maybe not much help. Good luck fixing them.
[quote author="kubi"]
The input Z must be higher than 600Ohm.
According to the posted schematic ( http://www.geocities.com/pepperplace/BA71.html?1097956825668 ) the transformer secondary's load is (R1||R2)||R22 which is (20k||20k)||22k = 6,875k. That gives 2,75k (=6,875k / 1.58**2) at the primary.
Someone to confirm?[/quote]
Sure it's higher than 600 ohms.
I addressed that in my big post. Unless I'm wrong, it's the 22K reflected back to the primary rating which comes to 2200 ohms for the 150 tap. I could be wrong. Don't see how R1 / R2 have anything to do with load reflection to primary. Maybe I'm wrong.
That is the BA-31A / 71A circuit. I just love how everyone ignores the letters at the end and throws their hands up in the air and says 'who knows' about the specifics of anything old.
The input Z must be higher than 600Ohm.
According to the posted schematic ( http://www.geocities.com/pepperplace/BA71.html?1097956825668 ) the transformer secondary's load is (R1||R2)||R22 which is (20k||20k)||22k = 6,875k. That gives 2,75k (=6,875k / 1.58**2) at the primary.
Someone to confirm?[/quote]
Sure it's higher than 600 ohms.
I addressed that in my big post. Unless I'm wrong, it's the 22K reflected back to the primary rating which comes to 2200 ohms for the 150 tap. I could be wrong. Don't see how R1 / R2 have anything to do with load reflection to primary. Maybe I'm wrong.
That is the BA-31A / 71A circuit. I just love how everyone ignores the letters at the end and throws their hands up in the air and says 'who knows' about the specifics of anything old.
[quote author="lewilson"]Yes, its wierd. If I let them run for a few hours they get Fairly quite, then the next day after setting off for a while they power up noisy again.[/quote]
Have you swapped transistors from good units?
Have you swapped transistors from good units?
