Planning my BA-6A

[abbey road d enfer]

So I think 300v @ 200mA works out to 450v @ 125mA.

Yes, but no! 300V @ 200mA works out to about 420V @ 140mA, so you're not too far off.
But
The xfmr is 300 0 300 @ 200mA, so it's equivalent to 300 @400mA, which works out to 420V @ 280mA DC.

CJ mentions below that the 5R4GY rectifier tube drops 67v @ 250mA.  So would that be a 34v drop @ 125mA?

Not exactly; the voltage drop is not linear; at low current it is higher than expected and at high current it is lower; you may assume 40V drop.

I need to get this down to 285Vdc, by dropping 118.5v.
How does that sound?

Sounds like an overrated xfmr.

 

[letterbeacon]

Ha! I'll get there eventually!

How about this one: http://www.edcorusa.com/products/664-xpwr068_240.aspx
200-0-200 @ 150mA which is equivalent to 200 @ 300mA, which works out to 400V @ 150mA.

400v - 40v = 360v after the rectifier tube.

360v - 15v = 345v after the choke.

This means I need to drop 60v to get to the 285Vdc B+ needed.

60V / .15A = 400r @ 9W.  So I'll use a 390r resistor which should get my in the ball park.

How does that sound?

 

[abbey road d enfer]

Ha! I'll get there eventually!

How about this one: http://www.edcorusa.com/products/664-xpwr068_240.aspx
200-0-200 @ 150mA which is equivalent to 200 @ 300mA, which works out to 400V @ 150mA.

No. 200v @ 300mA works out to 280V @ 210 mA. Not enough voltage.

 

[letterbeacon]

Could you explain to me the maths behind that please?

No. 200v @ 300mA works out to 280V @ 210 mA. Not enough voltage.

But isn't the current draw of the BA-6 around 120mA, so the voltage would be higher?

I appreciate your patience!

 

[abbey road d enfer]

Yes, it would be slightly higher, maybe 5-10% more, still not enough for the targeted 300-something before choke.

 

[letterbeacon]

No. 200v @ 300mA works out to 280V @ 210 mA. Not enough voltage.

Would you be able to explain to me how your arrived at 280V @ 210mA?  Or point me in the direction of a website with the info on it?

How about this one that lassoharp recommended earlier in the thread: http://www.edcorusa.com/category/37-xpwrseries.aspx?secondary(output)highvoltage=447
225-0-225 @ 500mA.

 

[CJ]

ok.

How much plate voltage would you like to run on the 6V6 tubes in this compressor?


i believe that a 300-0-300 rated at 100 ma will give you 100 ma.
yes, i screw this up all the time, so let me explain it to myself  :-\

if the transformer sec has a spec of 100 ma, that means that the wire in the sec will handle 100 ma without overheating.

you have a less than optimum use of the transformer windings in a center tapped circuit as opposed to a full wave bridge.


if you look at the diagram, you will see that current can only flow one way in either leg of the center tapped circuit.
the rect tube only conducts during the positive half cycle.
so each leg is only contributing current during the positive half cycle.

so leg A contributes 100 ma for half a cycle, then leg B contributes 100 ma for half a cycle.
this is because the rect tube will not conduct during the negative half cycle.

with the full wave bridge, you have two extra diodes which are configured to conduct so that they pass both neg and pos half cycles.

see that since the center tapped xfmr only conducts half the time in each leg, that you can use smaller wire since you have a 50% duty cycle.

this is why the full wave transformer is more efficient, current in the secondary winding is flowing 100% of the time.

 

[lassoharp]

How about this one that lassoharp recommended earlier in the thread: http://www.edcorusa.com/category/37-xpwrseries.aspx?secondary(output)highvoltage=447
225-0-225 @ 500mA.

That one won't get you enough for the 320V B+ line at that current, even with the overkill rating.  I had posted that based on the earlier post stating 225-0-225 was what you were looking for.  BA6A would work at lower voltage but wouldn't meet spec.  The 600-700V CT PTs are more common and it sounds like both Hammond & Edcor have several to choose from.


Using Dave's current draw figure it looks like a 300-0-300 will give you a starting voltage of ~ 355ish VDC with a 5AR4.

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/5/5AR4.pdf 

EDIT:

I got 404 error too.  No idea.  Try main page and search 5AR4.

http://frank.pocnet.net/

 

[CJ]

error 404 on the above link...

remember, there is no spec for the B+ voltage given on the voltage chart, they are only giving you the plate voltage on the 6V6 tubes,

but the output transformer is going to eat up some voltage due to the high resistance of the secondary.

figure another 10 to 30 volts, depending on the xfmr.

there are transformer catalogs that have all this garbage written out already,

let me see if i can dig up a chart,

one company  has choke input vs cap input specs ion their catalog, believe it was chicago transformer,

 

[lassoharp]

I got 404 error too.  No idea.  Try main page and search 5AR4.

http://frank.pocnet.net/

 

[CJ]

here is a clip from the Chicago Transformer catalog,

sorry, prices have gone up.  ;D

notice the DC voltage does not always go up in a linear fashion, different regulation with different current draw i guess,

you can find a transformer that will require no dropping resistor, which is the correct way to run a power supply.

from the williamson lab web page>

" Full Wave Center Tapped
A full-wave rectifier uses only one-half of the transformer winding at a time. The transformer secondary rated current should be 1.2 times the DC current of the power supply. The transformer secondary voltage should be approximately 0.8 times the DC voltage of the unregulated power supply per side of the center tap or the transformer should be 1.6 times VDC center
tapped.
120 Hz Ripple
Depth of ripple slope is dependent on Capacitance and Load.
Full Wave Bridge
The full-wave bridge rectification circuit is the  most cost effective because it requires a lower VA rated  transformer than a full-wave tapped rectifier. In a full-wave bridge, the entire  transformer secondary is used on each half cycle, unlike the full-wave center  tapped which only uses one- half the secondary on each half cycle. The  transformer secondary rated con-rent should be 1.8 times the DC current  of the power supply. The transformer secondary voltage should be  approximately .8 times the DC voltage of the unregulated power supply.

 

[abbey road d enfer]

remember, there is no spec for the B+ voltage given on the voltage chart, they are only giving you the plate voltage on the 6V6 tubes,

Voltage chart gives the screen grid voltage 285V.

 

[letterbeacon]

Thank you very much for your replies, I think I'm slowly grasping it... I think!

i believe that a 300-0-300 rated at 100 ma will give you 100 ma.

I'm not trying to start a fight, but is this in contradiction to abbey road?

"Full Wave Center Tapped
A full-wave rectifier uses only one-half of the transformer winding at a time. The transformer secondary rated current should be 1.2 times the DC current of the power supply.

Based on DaveP's DIY BA-6A, the unit has a current draw of .12A.  .12A * 1.2 = .14A.  So I'm looking for a secondary rated current of 140mA.

[quote author=CJ]but the output transformer is going to eat up some voltage due to the high resistance of the secondary.  figure another 10 to 30 volts, depending on the xfmr.[/quote]

According to the voltage charts, the B+ is 285Vdc.  Shall we say 310Vdc to account for the output transformer and the voltage drop of the choke?

The transformer secondary voltage should be approximately 0.8 times the DC voltage of the unregulated power supply per side of the center tap or the transformer should be 1.6 times VDC center tapped.

Most of the Edcor's seem to be center tapped.    Meaning I want a secondary rated voltage of 310V * 1.6 = 496V CT.

How about something like this:  http://www.edcorusa.com/products/887-xpwr195_230.aspx
250-0-250 @ 175mA.

 

[CJ]

ok, so we finally have this seemingly simple problem sorted out,

we need 285 volts B+ after the filter at about 120 ma.

120 was measured off orig. RCA by nice forum member.

B+ voltage is seen on the chart in the original manual.
and we also have a cool link for the compressor thanks Brian.

www.brianroth.com/library/rca-ba6a-docs.pdf

the screens are indeed across the highest B+ voltage so 285 is correct.
there is no screen resistor so no voltage drop on the screen itself.

there will be a voltage drop across the 5R4GY rectifier and the choke.

here is a data sheet for the 5R4GY so we can figure out voltage drop.


http://www.mif.pg.gda.pl/homepages/frank/sheets/049/5/5R4GY.pdf

looks like we need to add 40 volts dc to our 285 B+ spec,

285 + 40 = 325 VDC.

the choke DCR is on the schemo, listed at 160 ohms DC

so we can use ohms law to figure out the voltage drop across the choke coil,

0.120 amps * 160 ohms = 19.2 volts,

325 + 19 = 344 VDC.

divide by 1.2 and we have the AC RMS voltage spec that we search for online.

344/1.2 = 287 volts AC (rms)

now mains voltage will be different depending on what power grid you are on, figure a 10 percent difference

but that is alright, tube circuit voltages always had a wide 10 to 20 percent tolerance,

maybe compressor circuits are more fussy as they need to be well balanced and all that, but do not sweat it if your B+ is off a bit.

287 * 2 =574 volts line to line.

so get a 580 volt center tapped transformer,

reads 290 VAC @ 150 ma in the catalogs,

one thing still puzzles me, the RCA print says 320 VAC on the pwr trans,
but 320 * 1.2 = 384 volts.

subtract 60 volts for the tube and choke and you have 384-50=324 VDC.

285 to 324 is a 39 volt discrepancy,

either the choke coil has massive dcr, or the engineers were on crack, i do not know.
welcome to the world of boat anchor schematics,  ;D

 

[CJ]

how does this look? (see jpg at page bottom)

buy this from

http://www.tubesandmore.com/

or get Ed to wind you the same thing.

or, for a bit more plate voltage,

P-T272FX
Transformer - Hammond, Power, 300-0-300 V, 150 mA

    Primary 115 VAC, 60 Hz.
    Secondary DC current measured with cap. input filter, full wave (two diode) C.T., rectifier circuit.
    Enclosed, 4 hole chassis mount.
    Minimum 6" long leads.
    Class A insulation (105 degrees C).
    Hi-Pot test of 2,000 V RMS.
    Conservative designs - CSA certified (# LR3902).

Secondary: 300-0-300 V, 150 mA
Filament winding 1: 5 V, 3 A CT
Filament winding 2: 6.3 V, 5 A
Mounting centers: 2.5" x 2.19" vertical mount
Weight: 4.6 lbs.

 

[CJ]

attached jpg showing utc pwr trans ratings, VAC vs VDC.
L is choke input, C is cap input.

note industrial model (bold type) is same xfmr run at slightly higher current which drops B+ due to more voltage drop across windings due to more current.

RDH4 has graph showing output voltage vs capacitor size.
your filter caps will raise or lower the B+ depending on how many uF they are.

 

[letterbeacon]

Thank you CJ, I think I've finally got it straight in my head now.  I think I was still confusing RMS with peak to peak.

That 275-0-275 Hammond looks good.  I've found this Edcor equivalent: http://www.edcorusa.com/products/765-xpwr163_120.aspx but it's only available with 120V on the primary.  I've asked them if they'll add a 240V primary.

The choke I'm planning to use is rated at 400Vdc.  Do you think that's enough overhead for 344Vdc?  It's also only got a 105DCr so would drop 12.6V @ 120mA, but that's still within specs I think.

one thing still puzzles me, the RCA print says 320 VAC on the pwr trans,
but 320 * 1.2 = 384 volts.

They also give a figure of 315v in the voltage chart, so who knows what they were smoking!

RDH4 has graph showing output voltage vs capacitor size.
your filter caps will raise or lower the B+ depending on how many uF they are.

So if the B+ comes out higher or lower than what the schematic requires, it's better to adjust the uF on the filter caps than use a dropping resistor?

I'm not sure what "RHD4" is, where can I find the graph of voltage vs capacitor size?

You also recommend a 300-0-300 if I wanted a higher plate voltage.  Is that just 'in case' with the idea that I could reduce the B+ with the size of the filter caps?


Thank you very much!

 

[abbey road d enfer]

I'm not sure what "RHD4" is, where can I find the graph of voltage vs capacitor size?

Radiotron Design Handbook can be downloaded; just google it - large file! 38 chapters.
Chapter 30 shows that voltage under load will vary depending on the smoothing cap; lower cap => lower voltage, but it's at the expense of quite large ripple.
This was a concern at the time this book was written. Capacitors were expensive, the larger the more expensive. Today a 47uF 450V cap is just the price of a capuccino at Starbuck's.
Using anemic smoothing caps in order to decrease the voltage is not my idea of good practice.
RDH graph shows that the voltage difference between 4uF and 40uF is less than 10%.

 

[letterbeacon]

Thanks abbey, I'll have a look for it!

 

[CJ]

you can find RDH4 in the books section of the Meta, ready to download.

that choke sounds fine.

400 is probably the working voltage,

they hi-pot those things to at least 2 kv i would hope.

yeah, caps are better and cheaper now days, except the teflon stuff,

make sure you do not go over the max cap value for the rectifier you use,

you could use solid state rectifiers, but then you would have to re-calculate your trans voltage, which will have to be reduced a bit to get the same B+ you wanted.

tube rectifiers cause a natural compression due to their large forward voltage crop.

a compressor with compression.  ???