RCA BA6A limiter

« **on:** January 05, 2005, 08:43:49 AM »

Hiya folks im new on this forum tho I recognise some names from a previous one, I would like to say a BIG!!! THANKYOU to anyone involved in obtaining info & even a BA6A interstage tranny so that Mr Sowter could do his stuff and recreate this beast, about a year ago i started to bug poor Mr Sowter about this piece of iron, I already have the in & out trans from him and my BA6A is 90% done tho on very tight budget & now thanks to you chaps I can order the final piece. My electronics knowledge is at best patchy I have much to learn & any thoughts or info would be very welcombe & if I can give you any info I would be more than happy, but bear with me as I said my knowledge is a bit limited [pardon the pun] thanks once again
PS i tryed posting similar message but think it failed so sorry if i posted twice some how.

« **Reply #1 on:** January 05, 2005, 11:30:35 AM »

Hi Gary

Have you managed to work oot how much current is being drawn on the HT supply. Or what rating doe s your mains transformer have ?

« **Reply #2 on:** January 05, 2005, 12:16:36 PM »

Hiya, got lots of pics don?t expect too much tho its not as pretty as lot of the stuff on here, im not tryin to copy the orignal looks & layout wise & funds are low so I have to make do with what iv got, enough excuses.As for transformer it looks like it says 250 ma on the tran im using does that sound right, it?s a PX 87 made by Danbury electronics near where I live, I bought it from Maplin electronics. Someone advised me that this would be enough to power the tube complement of the BA, iv powered up my BA with resistors & capacitors in place of the inta tran for now & I just put a meter inline with the B+ & it reads about 130Ma.Im not sure if how the BA would perform in its current state & how that would effect the amps drawn, tho it does amplify for sure.

« **Reply #3 on:** January 05, 2005, 02:37:49 PM »

Thanks for that info Gary. There will be some current drawn throught the interstage transformer since it runs like any other push pull output stage, although from what I can see the Lion`s share of current looks likely to be passing elsewhere.

I`m intending to build a Ba-6a & have all the valves for this, but as you know the expense is in the transformers.

Did you bother with the valve tester meter switch, & are you using a daven bridged T attenuator on the output.

I would like to see some pics if thats possible.

« **Reply #4 on:** January 05, 2005, 06:48:08 PM »

Yes the iron does cost a bit, I started my BA with none & soon learnt that I needed a input tran that would split the phase, so I borrowed one from a mic pre I made & used resistors & caps instead of the others to get me started.Ive got a 2 pole 3 way switch for the meter for gain reduction balance & fine balance.Is that what that type of attenuator is called then a daven bridged T.I tried to make one with a 2 pole rotary switch & some fixed resistors but got confused so have taken it off for now,I made a 12 position attenuator for the input didn?t have the 20 pos switch that I believe is on a real BA but seems to work ok for now. I made my own separate power supply B+ is 280V & have 12.6V DC for filaments except for 6H6 which has 6.3V AC that also provides the hum signals for the balancing.Its mounted on a much smaller chassis than a real BA but as I said power supply trannies are not on this, ive probably squashed it all a bit too close together, im learning as I go you see & the soldering is messy point to point due to constant changes & experiments, once I get things working I?ll tidy up, I?ll see if I can put some pics on here soon, but don?t laugh she?s a bit of a mess.

Thanks Gary O.

« **Reply #5 on:** January 05, 2005, 07:40:00 PM »

Thanks for the info Gary.

Where did you get the top caps for the 6J7`s, was it somewhere in the UK?

« **Reply #6 on:** January 05, 2005, 09:25:06 PM »

Got the top caps ,valves,valve sockets & a choke from a chap near you Robin in Sussex Billingshurst in fact perhaps you know of him his name is Phil not sure his company name, think I had the last 2 top caps at the time one is 2nd hand, his prices are good too, ive bent his ear quite a lot over this BA project, he has been very helpfull, I can give you his details if you need them, Be nice to put bit of trade his way,having bit of an issue with the top caps at the mo as it goes, moving them while the BA is on while playing a walkman thru it upsets the voltages on the 6J7s pin 3s anodes is it? I would of thought both anodes volts should be the same, kind of balanced & sometimes they are 272 volts in fact but then no audio passes its like they cancel each other out & when top caps are wiggled audio blasts thru but voltages read roughly 104V on one anode & 57V on other any ideas or is this right or is it coz I have no inta tran in place?

Cheers?? Gary O.

« **Reply #7 on:** January 06, 2005, 02:40:23 AM »

When audio is passing I wouldn`t expect the anodes to be at exactly the same voltage from an a.c point of view, but I would think dc wise they should be the same at this point in the circuit.

You probably want to make sure the connections are very clean. Since valves are high impedance devices dirt can be an issue. The fact that the audio blasts through is not a good sign

« **Reply #8 on:** January 06, 2005, 11:38:02 AM »

I serviced a BA-6a about a year ago and had a lot of trouble with snap crackle and pop from the unit. I took out all of the tubes and polished the pins with #0000 steel wool, then washed them with alcohol. (Boy did they sparkle!!!) I cleaned the socket contacts using a teeny-tiny brush and some more alcohol. Blew-dry the sockets afterwards with some compressed air, then let it sit for a half-hour before a final blast of compressed air. Plugged the tubes back in and voilà!... perfectly silent ever since.

I think I'm possibly going back for another service call next month, we'll see.

Keith

« **Reply #9 on:** January 06, 2005, 05:20:53 PM »

Snap crackle & pop mmm I fancy some rice crispies.Ive since checked my wiring & found an extra resistor connected to one of the 6J7s pin 3 & to B+ silly me, whats more I cant see how I managed to do it, it doesn't seem to go anywhere else, on my previous balls up I could at least see what I meant to do..strange.The 6J7 area is very sensitive tho,connecting a meter seems to cock it all up but when left alone the voltage on the 6Js pins 3 are about 68V each, dont no if this is right but as a mic pre its sounds great! even without the interstage, but when in daul or single limit modes it makes pops as the gain reduces, is this the thumps ive read about I wonder, maybe im asking too much to work without its interstage tran.Anyway was gonna add a picture so does anyone no how to add pics cheers.

Gary O.

« **Reply #10 on:** January 06, 2005, 05:59:36 PM »

If your hearing thumps that is probably the control signal sitting itself on top of the audio signal. The Ba-6a has quite a precise presets to balance this thump out, so it may need setting up & tweaking a bit. Although you only have this adjustment on the 6sk7`s if you adjust it correctly it won`t get through to the 6j7. This is reduced further with the interstage transformer, because it electrically isolates the 6j7 from the previous stage & its secondary operates fully balanced.
In a nutshell you use R3 to match the cathode voltages & R8 to match the screen grid voltages on the 6sk7`s. This needs to be repeated abit because the interact.

pin 3 on the 6j7`s at 68v is fine and well within tolerance.

« **Reply #11 on:** January 06, 2005, 06:55:18 PM »

Yeah I built that signal injector part & the meter part to read it into my BA & it works a treat, with the audio out connected you can hear the hum signal & hear it dissapear when you get the sweet spot as the meter falls, you can almost balance it by ear,Got no output attenuator on at the mo so put 200 ohm res across the output seems to balance better this way cant get it perfect, bin singing thru a solid tube sounds lovely but gets caught out now & then & makes a thump but im hopeing all will come good when the interstage arrives on the door matt....well after iv soldered it on of coarse.

Cheers Gary O.

« **Reply #12 on:** January 06, 2005, 08:37:55 PM »

Check to see if your rev has the current limiting resistors feeding the pri of the innerstage.

« **Reply #13 on:** January 06, 2005, 09:04:21 PM »

Hello cj, sorry not sure what you mean by rev, at the mo I dont have a inta transformer just a couple of resistors & capacitors the rest is as the schematic from the manual, or it should be at least, made a couple of cock ups, do you mean R14, R15 the 4k7s yes I have those.Thanks for your efforts regarding the said inter tran I ordered one today.

Cheers Gary O.

« **Reply #14 on:** January 06, 2005, 10:19:26 PM »

sorry, rev means revision. I have seen schematics of BA6a's that have no resistors feeding the innerstage. I think they later added the resistors to cut down on thumps. On most schematics, there are some 22k resistors feeding the pri, R5 and R6 I believe.

« **Reply #15 on:** January 07, 2005, 07:51:39 AM »

Ah revision,cj your diagram is from an Audio Engineering article from 1950, someone told me that it was just to illerstrate how the limiter worked like a block diagram I spose, theres quite a lot of bits missing when compared to the manual schematic, pins 4 on 6J7s going nowhere, some caps missing, the meter parts not there.Ive tried to make my BA as the manual schematic, tho I couldn?t see some of the resistor values for sure & couldn?t get some of the capacitor values.Anyway I have 22ks from pins 8 of 6sks to my pretend interstage, & 4k7s from the 22ks & inter to the centre tap as on the manual schematic, if anyone wants a copy of the manual schematic I have 2, ones inks a bit splodgy the others ink a bit thin.

Cheers Gary O.

« **Reply #16 on:** January 07, 2005, 07:59:41 AM »

I have a PDF manual that´s not very readable... Some parts are a total mess... If you can scan the ciopy you have and post it, it would be good.

Thanks!

« **Reply #17 on:** January 07, 2005, 08:27:59 AM »

Quote

tho I couldn?t see some of the resistor values for sure & couldn?t get some of the capacitor values

What values can`t you read for sure ?

« **Reply #18 on:** January 07, 2005, 08:43:01 AM »

Hiya sorry might have mislead you there my copies are not originals, got them off the web one is PDF maybe the same as yours rafafred, but I can read mine fairly well & when cross referenced with the other one I think theres only two resistor values I cant quite see Robin, R38 R39 390R & 870R looks like to me anyway, I couldnt get hold of .25uf caps at the time so have got .33ufs on the 6H6 & all the 40ufs are 47uf on mine dont no if this will make much difference. let me know if I can help anyone,I can post my copies back on here anyway if you like, once I work out how to do that,that is.

Cheers Gary O.

« **Reply #19 on:** January 07, 2005, 10:11:05 AM »

R38 is 390R R39 is 820R.

The 47uF will be fine for the PSU. The only thing you have to be a bit careful about there is the first capacitor after the regulator valve. The Regs normally have a maximum capacitor value that they can see in front of them. This is because at switch on the current to charge up the first cap can be over the current rating of the reg if the cap is too big a value. You should be well within the limits with 47uF.

The .33uF caps will allow lower frequency signals through to the side chain. But the cut off freq is very low anyway, so I don`t think you will notice any difference.

My name is Rob as in Robert not Robin

Subject / Started by	Replies	Last post
RCA Ba6a Started by kpeek The Lab	0 Replies 2180 Views	September 08, 2004, 06:37:54 AM by kpeek
RCA BA6a Pics Started by CJ The Lab	10 Replies 5618 Views	September 28, 2004, 08:24:01 PM by Rob Flinn
Finished BA6A Limiter Started by schlamn « 1 2 » The Lab	32 Replies 6973 Views	April 07, 2014, 10:04:11 PM by EmRR
Enclosure for my DIY BA6A Started by letterbeacon Machine Shop	12 Replies 1987 Views	April 30, 2018, 11:24:47 AM by boji

Related Topics