Yes, that is a 1.5 inch knob. I'm not sure what you mean by pointer. What's pictured is all there is to the knob. Did you mean a metal pointer as found on some vintage knobs? I've never seen a metal pointer on an RCA knob of that era. The earlier ones had them but that is cosmetically a totally different knob type.
RCA Ba2A Pre amp qestions & answers
- Thread starter gary o
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Help Support GroupDIY Audio Forum:
I mean little "nose" or "pointer" at the bottom of the knob, it has white infill. I'm not sure if diameter of knob with this pointer is 1,5 inch or a little more. Letterbeacon told me from memory that diameter with this pointer is a little more than 1,5 inch, probably 40mm. Those 2mm make a difference because i would like to make proper scale for knob. Thanks!
letterbeacon
Well-known member
Thanks for reminding me to measure!
The diameter of the round bit (i.e. not including the pointer) measures 38.4mm (1.5"). Measuring from the tip of the pointer to the other side is 41.2mm.
The diameter of the round bit (i.e. not including the pointer) measures 38.4mm (1.5"). Measuring from the tip of the pointer to the other side is 41.2mm.
buildafriend
Well-known member
emrr said:
Wait, my brain just did something logical.. (thats rare).
I have low gain from my DI signal and from my mic signal. Maybe my output transformer choice sucks!? maybe thats why I have low gain.. RRRG JUST WORK.
Hi guys,
i got back to BA-2 and measured some important voltages. First grid shorted and pot wide open, output trafo is Cinemag CM-9661 that takes up to 10mA unbal. current (50:50 version), sounds fine. Voltages seem quite a bit off compared to others, except for HT:
HT=263V
Anode 1 108V, cathode 1,79V
Anode 2 262V, cathode 5,67V
Also measured AC at reg. HT where it is 1mV, heaters have 50mV which seems quite high compared to previous builds.
This is compared to voltages i found here, they might be taken by old voltmeter which causes this difference:
HT 248VDC/ 5,6mA
V1 plate 73VDC cathode 2.4VDC
V2 plate 232VDC cathode 8.8VDC
I'm using EH EF86, looking at anode voltages it seems like they take a little less current than they should. Should i do something about it, or go on and close the lid? I built quite a few tube preamps where voltages were 10% off at most, often closer to reference.
i got back to BA-2 and measured some important voltages. First grid shorted and pot wide open, output trafo is Cinemag CM-9661 that takes up to 10mA unbal. current (50:50 version), sounds fine. Voltages seem quite a bit off compared to others, except for HT:
HT=263V
Anode 1 108V, cathode 1,79V
Anode 2 262V, cathode 5,67V
Also measured AC at reg. HT where it is 1mV, heaters have 50mV which seems quite high compared to previous builds.
This is compared to voltages i found here, they might be taken by old voltmeter which causes this difference:
HT 248VDC/ 5,6mA
V1 plate 73VDC cathode 2.4VDC
V2 plate 232VDC cathode 8.8VDC
I'm using EH EF86, looking at anode voltages it seems like they take a little less current than they should. Should i do something about it, or go on and close the lid? I built quite a few tube preamps where voltages were 10% off at most, often closer to reference.
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The curves for EF-86 triode mode are probably a little different than 6J7. If that extra bit of biasing difference is something worth pursuing then maybe. If you're driving 10K or > inputs only you probably have enough current there. You can always compare headroom with 4ma biasing and see if it's worth changing.
I'd be interested in seeing freq response in general for the Cinemag OT if you have a set up for it. It sounded pretty good on paper as I recall but no idea what the actual inductance figures were.
Yeah, EF86 and 6J7 are a bit different. I will probably leave first stage as it is and on second gradually make cathode resistor higher by 500R. Output stage needs good drive because it will often go into UA176 which has 600ohm input. Direct coupling is new to me, so i hope this plan is ok for the start. If anyone thinks i could do differently without changing tubes please let me know.
Those 50mVAC is still a mystery to me, it could be one of diodes from bridge, or maybe to low voltage into reg. (there is another 1,5v higher tap to try). I will scope it tomorrow unloaded because i know previous regs had 1mV this way...
Don't have a setup to measure inductance of 9661, if you tell me what you would like to know, my toroid winder could probably measure it. It was quite a revelation when i made A/B tests with good big vs small core trafos, thanks for that!
Those 50mVAC is still a mystery to me, it could be one of diodes from bridge, or maybe to low voltage into reg. (there is another 1,5v higher tap to try). I will scope it tomorrow unloaded because i know previous regs had 1mV this way...
Don't have a setup to measure inductance of 9661, if you tell me what you would like to know, my toroid winder could probably measure it. It was quite a revelation when i made A/B tests with good big vs small core trafos, thanks for that!
Cinemag CM-9661A-L can't take those few mA of DC from output stage, no way! I wrote it can do 10mA because it was mentioned by a guy here who used to give very good information... With only a few mA it distorts pretty bad and also limits the output signal. Correct trafo is on the way, can't wait to play with it a little further. Calculating and understanding direct coupling gave me a little headache, but it feels so good to understand it properly.
I was pretty surprised at how warm EF86's get at this bias. Tried Tesla EF806's, RFT EF86s, Svetlana EF86, Philips EF86 and new production EH EF86. Beside Svetlana which had really low gain and barely glow, all of them get similarly warm. It was surprising to hear new production EH EF86's sounding as good as nos i mentioned, only RFT was a little more "alive", although it had quite higher gain than the others. I also tried to run EH as pentode with NFB, can't say it is different than nos. Two of them run for a few months wired as pentodes or triodes, they seem to be fine. Hope we finally have good new production EF86.
Datasheet for EF86 shows max Ik at 6mA, operating characteristics when wired as triode give 3,6mA at 400V, BA-2 biasing seems well below limiting values. Output stage has ~4mA which should be fine, i'm probably just used to very low current and heat at pentode wired input stages.
I was pretty surprised at how warm EF86's get at this bias. Tried Tesla EF806's, RFT EF86s, Svetlana EF86, Philips EF86 and new production EH EF86. Beside Svetlana which had really low gain and barely glow, all of them get similarly warm. It was surprising to hear new production EH EF86's sounding as good as nos i mentioned, only RFT was a little more "alive", although it had quite higher gain than the others. I also tried to run EH as pentode with NFB, can't say it is different than nos. Two of them run for a few months wired as pentodes or triodes, they seem to be fine. Hope we finally have good new production EF86.
Datasheet for EF86 shows max Ik at 6mA, operating characteristics when wired as triode give 3,6mA at 400V, BA-2 biasing seems well below limiting values. Output stage has ~4mA which should be fine, i'm probably just used to very low current and heat at pentode wired input stages.
I think that unit is designed for PP stages. I imagine it would distort. From David Geren:
"The CM-9661A-x is suitable for push-pull applications. The Altec 436 used the 16402, which used "L" laminations instead of E-I. Those laminations are "unobtanium" at this time because the tooling ended up at Tempel Steel who sold it for scrap a number of years ago. The CM-9661A-x has far more level capability with a turns ratio of 7:1. The turns ratio for the 16402 was 6.2:1. The 16402 laminations were 49% nickel. The CM-9661A-L uses a mix of 50% high-nickel and 50% steel laminations. They are $105.87 in quantities of 1-10 pieces.
Best regards,
David"
The CM-2092CF Steel Lams version sounded like their best bet for a suitable sub. I've never tried it but was sent some test data. From David Geren:
"Here is what it looks like with a 20K source impedance. Because you will be driving it from a significantly lower impedance, it will have better low frequency characteristics. It will have no problem taking 9mA on the primary. Nickel laminations are not used for butt-stacked transformers."
Source Z for V2 is probably around 12-13K.
"The CM-9661A-x is suitable for push-pull applications. The Altec 436 used the 16402, which used "L" laminations instead of E-I. Those laminations are "unobtanium" at this time because the tooling ended up at Tempel Steel who sold it for scrap a number of years ago. The CM-9661A-x has far more level capability with a turns ratio of 7:1. The turns ratio for the 16402 was 6.2:1. The 16402 laminations were 49% nickel. The CM-9661A-L uses a mix of 50% high-nickel and 50% steel laminations. They are $105.87 in quantities of 1-10 pieces.
Best regards,
David"
The CM-2092CF Steel Lams version sounded like their best bet for a suitable sub. I've never tried it but was sent some test data. From David Geren:
"Here is what it looks like with a 20K source impedance. Because you will be driving it from a significantly lower impedance, it will have better low frequency characteristics. It will have no problem taking 9mA on the primary. Nickel laminations are not used for butt-stacked transformers."
Source Z for V2 is probably around 12-13K.
I used CM-9661A-L only for R-C coupled preamps, it was very popular OT for Redd47 and similar. David Geren proposed it to me for 47 years ago, though about PP because of CT and core but never got any data until now.
Hope it will survive at this current until it will be replaced by CM-27101. I like this one a lot for R-C coupling too when ungapped, iirc David wrote it is a bit higher quality than CM-2092 when used in BA-2. Stock gapped 27101 has a stack of 4x4 Ni and Fe lams (hope i wrote this correctly, it is 4 Ni lams in one direction, 4 Fe in other, each end has 3x3) and is pretty cheap too. I really like how it sounds in this preamp with CMMI-10B...
Thanks for sharing data for CM-2092CF, hope i will have a chance to properly test it side by side with gapped CM-27101.
I will report my results with EH EF86 because random samples work fine for quite some time.
Hope it will survive at this current until it will be replaced by CM-27101. I like this one a lot for R-C coupling too when ungapped, iirc David wrote it is a bit higher quality than CM-2092 when used in BA-2. Stock gapped 27101 has a stack of 4x4 Ni and Fe lams (hope i wrote this correctly, it is 4 Ni lams in one direction, 4 Fe in other, each end has 3x3) and is pretty cheap too. I really like how it sounds in this preamp with CMMI-10B...
Thanks for sharing data for CM-2092CF, hope i will have a chance to properly test it side by side with gapped CM-27101.
I will report my results with EH EF86 because random samples work fine for quite some time.
My3gger said:
Ahh . . . . cool. So do you happen to know if the 27101 is a new design? I recall David mentioning working on a gapped high ratio OT design using Ni. This was during discussions several years ago on what might work best for BA1/2 outputs. I never asked again to see if he'd come up with a new transformer but it sounded promising. How big does the core size look?
First time i heard about and bought ungapped 27101's in 2009. In 2012 we exchanged some mails with David, this is what he wrote:
"I have been considering re-working the CM-27101 which has a turns ratio of 7:1. It is not the highest performance design, but it is very good overall. Because you want a gapped transformer to take the dc current, it will have to be made with steel laminations. If you are interested, let me know and I will look into it further."
After my reply he wrote more, remarks about "butt stacked", larger transformer and more complex winding structure might be related to CMO-30/600 i asked about too:
"The new part number is CM-2092G. The "G" suffix means "gapped". We did not want to adopt something for "butt stacked"... that one will be a good choice, too. It is a larger transformer with a more complex winding structure for better bandwidth at the high end. The CM-27101 is great at high frequencies, but the CM-2092 is better.
In mid 2013 i bought first gapped 27101, it came with Ni and Fe lams stacked as i mentioned in last post. Core is EI48, lams look like a little less than 0,3mm thick.
Judging only by price and Ni/Fe lams stacking (at first it was supposed to contain only Fe), 27101 might be better transformer now. I think David confirmed my observation, can't say for sure because gmail search can't find it in so many mails we wrote. He is a very, very patient guy
I rather call now to make things easier.
"I have been considering re-working the CM-27101 which has a turns ratio of 7:1. It is not the highest performance design, but it is very good overall. Because you want a gapped transformer to take the dc current, it will have to be made with steel laminations. If you are interested, let me know and I will look into it further."
After my reply he wrote more, remarks about "butt stacked", larger transformer and more complex winding structure might be related to CMO-30/600 i asked about too:
"The new part number is CM-2092G. The "G" suffix means "gapped". We did not want to adopt something for "butt stacked"... that one will be a good choice, too. It is a larger transformer with a more complex winding structure for better bandwidth at the high end. The CM-27101 is great at high frequencies, but the CM-2092 is better.
In mid 2013 i bought first gapped 27101, it came with Ni and Fe lams stacked as i mentioned in last post. Core is EI48, lams look like a little less than 0,3mm thick.
Judging only by price and Ni/Fe lams stacking (at first it was supposed to contain only Fe), 27101 might be better transformer now. I think David confirmed my observation, can't say for sure because gmail search can't find it in so many mails we wrote. He is a very, very patient guy
I rather call now to make things easier.I just found an email from 2013 where David wrote that gapped 27101 is better transformer than 2092.
Compared to CM-9661, Sowter 9980 and some others, 27101 seems more balanced in the bottom. 9980 is quite similar at the top. I didn't pay much attention to Lundahl because they cost much more and i don't need all those taps.
Can't say about gapped because i didn't have a chance to really compare side by side. With CMMI-10B input in BA-2 it sounded just beautiful. Not very very clean like some cap coupled with nfb, and not distorted like others. It has a very nice color, i wouldn't change anything at the bottom or top, mids are good too. Ok, now i'm talking about the circuit with one OT transformer, but it really sounds very good and is not too colored or distorted.
Well, when i listened to BA-2 in this configurtion for the first time, i felt like a very happy guy with a white coat in some 50's US lab, with this preamp and Protools. Thanks to you guys
Compared to CM-9661, Sowter 9980 and some others, 27101 seems more balanced in the bottom. 9980 is quite similar at the top. I didn't pay much attention to Lundahl because they cost much more and i don't need all those taps.
Can't say about gapped because i didn't have a chance to really compare side by side. With CMMI-10B input in BA-2 it sounded just beautiful. Not very very clean like some cap coupled with nfb, and not distorted like others. It has a very nice color, i wouldn't change anything at the bottom or top, mids are good too. Ok, now i'm talking about the circuit with one OT transformer, but it really sounds very good and is not too colored or distorted.
Well, when i listened to BA-2 in this configurtion for the first time, i felt like a very happy guy with a white coat in some 50's US lab, with this preamp and Protools. Thanks to you guys
Don't have a setup to meassure L, i wrote DCR readings for 27101, not sure if it is in this thread. I will check later for this info.
Core has ~ 16,5mm. If right thing to do (27101 is in production), i can write exactly how it is stacked and how winding looks from outside. Can't comment about 2092 because i haven't seen it.
Core has ~ 16,5mm. If right thing to do (27101 is in production), i can write exactly how it is stacked and how winding looks from outside. Can't comment about 2092 because i haven't seen it.
mitsos
Well-known member
- Joined
- May 4, 2007
- Messages
- 2,886
Of course, one page back... sorry, should have looked better.
So it looks like a 5/8" core (same as API 2503 outputs). I can see how you'd get 30K using nickel/steel lams, but when you go just straight steel you lose a ton of inductance, and if you gap it, you lose even more. But if it sounds good, then it must be right...
So it looks like a 5/8" core (same as API 2503 outputs). I can see how you'd get 30K using nickel/steel lams, but when you go just straight steel you lose a ton of inductance, and if you gap it, you lose even more. But if it sounds good, then it must be right...
My3gger said:
Thought about this some more and should correct myself. Popular Redd47 has this nice edge at the top which stands out a little, so CM-9661A-x balances things nicely. This OT also has nice mid bass to it, so is very good choice for this preamp. I always used it with CMMI-7C input which takes a lot of signal and seems it is easier to work with it (avoiding clipping it, setting pad, etc). They are very nice pair and i want to check better their HiNi version too.
If i take in account suitability for Altec 436, primary tap and high level capability at more than 24dBu at 20Hz, it makes sense to have one.
buildafriend
Well-known member
Would CMMI-10C work on input? At a quick glimpse at it seems similar to the suggested CMMI-10PCA..I think? I'm not so amazing at reading transformer datasheets. I'm using the 10C..
CMMI-10C Datasheet:
http://cinemag.biz/mic_input/PDF/CMMI-10C.pdf
CMMI-10-PCA
http://cinemag.biz/mic_input/PDF/CMMI-10PCA.pdf
BA2A Schematic
http://www.preservationsound.com/wp-content/uploads/2010/10/ba2c.jpg
It's been over two years since I built this and ended with low output. I would like to think that I can go back over it and fix it after putting it away for so long and successfully building so many other projects.
My output transformer is the Cinemag 2092G which was also suggested earlier on ( I think page 16 )
Maybe I should update this post with voltages soon.. that and some transformer DCR to see how it compares to the others that seemed to have worked for most of you.
Power transformer is this guy:
269JX
http://www.hammondmfg.com/pdf/5c0026-27.pdf
AC heaters running at about about 7.4V (weird right?.. go easy on em jeeze)
Top of my power supply caps measure 366VDC, 325VDC, and 292VDC.
520Vac are hitting the 6x6 rectifier tube
plate voltage on V2 is 240VDC
plate voltage on V1 is 293VDC
CMMI-10C Datasheet:
http://cinemag.biz/mic_input/PDF/CMMI-10C.pdf
CMMI-10-PCA
http://cinemag.biz/mic_input/PDF/CMMI-10PCA.pdf
BA2A Schematic
http://www.preservationsound.com/wp-content/uploads/2010/10/ba2c.jpg
It's been over two years since I built this and ended with low output. I would like to think that I can go back over it and fix it after putting it away for so long and successfully building so many other projects.
My output transformer is the Cinemag 2092G which was also suggested earlier on ( I think page 16 )
Maybe I should update this post with voltages soon.. that and some transformer DCR to see how it compares to the others that seemed to have worked for most of you.
Power transformer is this guy:
269JX
http://www.hammondmfg.com/pdf/5c0026-27.pdf
AC heaters running at about about 7.4V (weird right?.. go easy on em jeeze)
Top of my power supply caps measure 366VDC, 325VDC, and 292VDC.
520Vac are hitting the 6x6 rectifier tube
plate voltage on V2 is 240VDC
plate voltage on V1 is 293VDC
