Another BA-6A from scratch

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vinyvamos

Well-known member
Joined
Jul 15, 2014
Messages
109
Location
East Iceland
Hi folks! Long time listener, first time caller here :). After doing quite a bit of research, and going through Dave P's great thread named "A BA-6A from scratch", I have now started building my own one. As I'm not flush enough with cash to splash out on Sowters and whatnot I am making do with what I could get from good friends. I do have some Edcors for input and output but I wanna try the vintage ones that I have first. Here is the trafo list:

Input: 1940s Kenyon input trafo (no P/N or data). Tests out to be 1:15 Vratio which is too high. Secondaries in series give 116H with open primary at 1KHz. Might have to swap that out for an Edcor as it is a mic input trafo, and even though it is big it might saturate easily. I guess it would give the unit some mighty gain, but would require a low-z drive into it.

Interstage: FTR FT-2614. Stated 20Kct:20Kct on the top, and both windings read 88H at 100Hz. Here's hoping this is enough as I have read a lot from you guys regarding this inductance issue.

Output: Stancor A-3800. Stated 5K:500R with 4,8,16 and 250 ohm taps. 30 Watts so over spec'd a bit. Tests have brought doubt into it's LF handling though as the primary reads  only 3K6 ohm at 100Hz. Maybe this is not an issue, what do you think?
 

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Cheers Shabtek :).

Well I got the thing passing and compressing audio at the weekend. Had a headscratcher at first as I had fitted 6SJ7s rather than the 6J7s, to avoid sourcing top caps, screened grid wires etc, but had followed the pinouts on the original schematic. Oops! :p Rewired and then she was singing.

I have done no tests at all yet regarding gain, freq.response, or otherwise. Gonna get the rest of the build finished first. The meter switch is a patience tester! Got that all done this evening. Regarding trafos, I luckily landed a NoS UTC PA-19 output lump, which shows better behaviour in the LF region than the Stancor, and a higher P-P impedance, so I may swap out the Stancor for that after some tests. These both have 200 & 500 ohm secondary windings, as well as the usual 4-16 ohm speaker windings...

For the output attenuator I am going to try building one similar to the STA-Level. I have a huge wirewound 2K linear pot so if I put a 2K resistor across that I should get a log response, yes? Then the other resistors will need to be scaled for the impedance I'm after.

The filament voltage reference doesn't seem to matter regarding hum (responding to my query in the first post I made) . Although I have yet to measure it critically. But of course, the elevation of 21 volts is a good idea for healthy Vhk. For now I will keep the 21V strapped to V1 & V2 filament -v.

Here is a photo of the progress :)
 

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minor question, but what type of wire are you using internally?  I'm envious of your nice tidy bundles of wire turning at perfect 90 degree angles. Is it all solid core instead of stranded?
 
innerstage and OPT look ok, that Kenyon might suck donkey if it is like the one we hacked a while back,

1:15 will eat high end, saturation not a problem,  what is  DCR on Kenyon?

cool on the PCB caps!  ;D
 
minor question, but what type of wire are you using internally?  I'm envious of your nice tidy bundles of wire turning at perfect 90 degree angles. Is it all solid core instead of stranded?

Well I notice now in the photo that some are not so straight, so my OCD wasn't working so well :p. I have used solid core wire everywhere, except for the mains wiring and any wires going to switches and pots on the front panel, as the panel may be moved around during future servicing etc. Solid core wire is not a great idea for some projects though in my opinion. Especially guitar amps and things that get lugged around a lot.

innerstage and OPT look ok, that Kenyon might suck donkey if it is like the one we hacked a while back,

1:15 will eat high end, saturation not a problem,  what is  DCR on Kenyon?

cool on the PCB caps!

Yes the Kenyon has my suspicions alright after reading the thread where you tore one down. The DC-R of the secondaries in series is 3800 ohms and Inductance is 120H @ 100Hz.

Ha well spotted with the TOC caps :p. I simply used them as they look vintage and tested out perfect on my Protek LCR meter. I plan to start a rumour/myth that PCB caps sound better. Toxic snake oil for real!!!
 
wow good memory emmr!

check the leakage on that OPT, short the 8 ohm sec and read pri inductance across P-P leads,

what is the DCR?

do you have a signal generator? run 20 Hz thru the pri and measure AC  and voltage across winding so we ca figure pri ind.  down where it matters,

maybe run a 20 ohm resistor across the 8 ohm wind to drag more current,  or change output pad to  500 ohms,
 
CJ said:
wow good memory emmr!

check the leakage on that OPT, short the 8 ohm sec and read pri inductance across P-P leads,

what is the DCR?

do you have a signal generator? run 20 Hz thru the pri and measure AC  and voltage across winding so we ca figure pri ind.  down where it matters,

maybe run a 20 ohm resistor across the 8 ohm wind to drag more current,  or change output pad to  500 ohms,

Well this may be a decent Kenyon as I stripped two of them from a sorry looking 1940s mic pre which was originally built for National Broadcast use.

Ok so regarding the O/P iron, the dc-R of the primary is 282 ohms total. The inductance with secondaries open at 100Hz is 13H and leakage inductance is 53mH at 10KHz with all secondaries shorted . I am doing these readings with a fairly decent Protek 9216 digital LCR meter. Unfortunately 100Hz is as low as it will go. I do have a sig gen if we wanna measure L down at 20Hz, maybe I will do that...
 
13 H is pretty low,  should be more like 130, a  test with 20 hz and a current meter should prove this out,

what is the DCR on that 500 ohm wind?
with the generator still hooked up, take measurements at 500 Hz (midband freq)with and without sec shorted and see what you get,

to get Henries, divide voltage across winding by current thru the wind, (ohms law) then take that reactance number and divide by 2 pi and then divide that by the freq and you have Henries



 
vinyvamos said:
Well this may be a decent Kenyon as I stripped two of them from a sorry looking 1940s mic pre which was originally built for National Broadcast use.

Pics?  National Broadcast?  NBC?
 
take pri inductance measurements at the mid band (500 Hz) and then short the 8 ohm wind and take hem again, then we can compute he bandwidth.
 
OK so I am now trying to calculate the inductance. I have a 100R 1% resistor in series with the primary to measure AC current using my Fluke multimeter. I have a sig gen giving 7.45VAC (about 20Vpp?) into the setup. At 20Hz I am getting 88mVac across the resistor so this translates to an AC current of 880uA, yes? Then going through the formula I get 66.6H at 20Hz.

For 500Hz I am getting 24H with sec open, and 326mH with 8 ohm sec. shorted. Can you explain CJ how I will calculate the bandwidth from all this data?

But what baffles me is when comparing my manual readings to the ones I get from the Protek LCR meter they are larger, by a factor of 2 at 100Hz and about 30% larger at 1KHz. Can you think of any reason for this? Other than the LCR meter being out of cal, which it might be????
 
sounds like you got it down,

the reason your meter reads different is the amount of current it puts out compared to the generator.  since the core is non linear (see BH curve) then you get different readings at different current (flux) levels.

now measure the pri dcr and the 500 ohm wind dcr and we can compute -3 db hi and low points and center frequency.

(spread sheet required!)

get rid of the 100 ohm resistor as it will mess up the calc, if only one DMM use it to measure voltage across pri wind then insert it as ammeter into circuit.
 

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you can see an example of the calcs on pg 482 of this book:

https://babel.hathitrust.org/cgi/pt?id=uc1.b4154646;view=1up;seq=510

we made a spread sheet that uses this method, those poor guys back then, all they had was a slide rule,

it shows a LF of about 6  Hz which is good,  but only 5 K Hz for the HF, but this HF spec is based mostly on the leakage inductance, if we put in .08 Henries (80 uH) for the leakage, then the HF goes up to 20 K. from 5K to 20 K your center freq goes from 180 to 360 Hz which  makes more sense, 500 Hz is average but this Stancor seems to have good LF response.

so maybe we should take the leakage reading at a higher frequency like 5 K since this is where it becomes important, current will probably stay the same as you run the freq up from 500 Hz to 5 K Hz as the core is out of the equation, with the sec shorted, we are measuring air inductance,

PM me with an email of you want a copy of the spreadsheet file,

cj





 

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PM sent :).

OK well the LF outcome is a relief. I did some freq. sweeps and spot measurements of the whole unit  last night and it's looking very promising regarding the trafos I have used. LF is quite flat all the way to 20Hz. As I went up above about 1K though it started to fall off, ending at  6dB down at 20K. I then thought that maybe my input atten pot was loading down the input trafo too much, as I am using a dual 10K, as Drip used as I am still waiting on more dual gangers from the postman. The input sensitivity was also way too high...

I then put resistors in series with the pot to drop the level, ending up with 100K which gave a total load of 220K to the Kenyon. Freq. response was now much better, albeit with a slight increase of a dB or so at the top end. But now I think I have not enough sensitivity at the input. With a full +4 studio line level drive I can only get about 10dB of GR, and I can't overdrive the input stage, and that's no fun!

What do you folks think about these dropping resistors? The Kenyon calculates out to be 1:17.6 Vratio (600Rct:186Kct) which is WAY too high for this project. The one Sowter makes is 1:4.08 (600Rct:10Kct). It would be nice if I can make this Kenyon work as it is all fitted to the chassis, plus I have no better suited trafo in the junk box right now...

Also, I wanna lift that grounded CT to make the input fully floating, is that a bad idea? I can't see why... :)

I ran audio through the unit last night and maaaan it sounds soooo good!!! The only thing I would say is that I would like the option of HPF on the sidechain so it doesn't grab the bottom end so much, but I can try that on a switch :)
 
Here's the guts, more or less finished. Still considering to add a bypass relay at the back...
 

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He He! Well this is the last place that I thought I would get interest in a sale CJ :p. Not gonna sell this one as it's going straight into our Tape Studio here in the East Fjords of Iceland. And I don't think it will be feasable to build these to sell as the amount of work hours in it is huge. I am gathering parts now though for a stereo BA-6A build. Main thing I am missing and might have to fork out cash on is a pair of good interstage trannies...

The control on the bottom left is a 2K wirewound pot made by Sanken(?). I am using it as part of the STA Level style H attenuator, but I have changed a lot of the values (unmathematically of course!) to reflect correct impedance to the 6V6s and to also give bigger attenuation range compared to the STA. Right now I'm getting about 8.5dB and getting a steady 6-8K on the 6V6s.
 

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