Planning my BA-6A

[Brian Roth]

Recalling one thing which I had written last weekend before "the forum ate my homework"....

I would take the various voltages shown in the chart which LetterBeacon posted awhile back (and which I also included in my "compilation" PDF that I recently posted) with a GRAIN/boulder of salt.

I found another BA-6A file in my collection, and the info above the chart says "approximate voltages" when using a 20,000 Ohm/Volt VOM.

Alarm bells went off <grin>, since we are all SO used to using modern DVMs with readouts going out to multiple decimal points, and accuracies well better than 1%.  My current Fluke DVM supposedly is accurate to +/- 0.1% on the DC ranges!

I also have (among many other instruments) a Simpson 260 analog multimeter, which would have been somewhat State of the Art in the era when RCA made the BA-6A.  Looking at the Simpson specs, I see it is accurate to 2 or 3% "of full scale" on the DCV ranges.  Thus, as I recall from those types of specs.....on the 500V scale, the accuracy will be only +/-  10 or 15 VDC.  That assumes the meter itself is in PERFECT factory calibration, and also if the readings are made while the meter is sitting in a horizontal position.

As another example:  I also seriously question the "ultimate accuracy" of the 13.1 VDC number for the cathode voltages shown for the 6V6 valves, as shown in the BA-6A info (which we have all been so closely scrutinizing).  On my Simpson 260, I would have to be on the 25 VDC range/scale to be able to measure that, and I'll be damned if I could make out any reading THAT accurate...even if I used a magnifying glass for my ancient eyeballs.  <G!>  That also ignores the possible +/- 0.5 or 0.75 VDC inaccuracy, which would be with "in spec" for the meter operating perfectly.

Also, according to my RCA tube manual (RC-28), the "anode voltage drop" across an OD3 can be anywhere from 142 VDC to 163 VDC, with a "center" at 153 VDC.  That is approx. a +/- 10% variation...which fits into my ancient recollections that "10 to 20 percent variations" were expected for the voltages shown in "Boat Anchor" valve device schematics.

I think the "vultures are circling" around 300-0-300 at maybe 150 mA for the power transformer, especially if the filter choke with 140 Ohms value is selected.  With a lower resistance choke, then the PT voltage should maybe drop to 290-0-290...IF you can find anything like that!

I still dunno where the 315-0-315 or 320-0-320 RCA xfmr. spec came from, unless it had a relatively high resistance for the B+ windings.....

Best,

Bri

 

[letterbeacon]

Thanks for that insight Brian - very interesting!

I suppose what you're saying is, when dealing with these old designs a certain amount of tolerance either way is expected.

In the end I went for the Hammond 370FX which is 275-0-275 @ 173mA.  The older version of the 370FX that CJ linked to earlier in the thread, that rated at 150mA isn't available anymore, but 173mA should be fine I think.

The guy I bought the Hammond from said it's a good idea to put some low value resistors on the anodes of the 5R4GY to help preserve the life of the tube (so an even current flows through the tube).  He said if the B+ voltage is too much then I could use those resistors to drop the voltage while also protecting the 5R4GY.  What are the board's opinions on that?

Thanks a lot!

 

[emrr]

I found another BA-6A file in my collection, and the info above the chart says "approximate voltages" when using a 20,000 Ohm/Volt VOM.

Yes, the standard mentioned on pretty much every pre-1970 schematic.  I don't know that peak AC power voltages were ever mentioned on small signal amp schematics in the era of analog meters.  Important to consider the tools used in the era the info was compiled in; what else could it refer to?

Cathode voltages will refer to the reading on the RCA rack panel meter sold for taking such measurements, when looking at the various amps having outboard measurement taps. 

I'm surprised DaveP's build measured so high for current, when thinking about the SA-39 at 90ish mA, also a PP 6V6 output.  Gates built with a 90mA PT for about 25 years.  Field replacements almost always the r-14 instead of the stock r-11, at 120 mA. 

 

[DaveP]

Check out this ref for the minimum value of anode resistors required.
http://www.diyaudio.com/forums/tubes-valves/150665-rectifier-limiting-resistors-capacitor-input-2.html
You can always increase them to adjust your B+

best
DaveP

 

[Jean Clochet]

Post removed.

 

[emrr]

The earlier RCA voltage charts show socket drawings, with all DC voltages referenced to ground. 

 

[AZ999]

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.  ???

Hi CJ,

as i understand it, current in class a is pretty constant, so voltage drop with tube rectifiers should be constant too. The same thing happens with SS rects, with lower drop. How would this make compression? BA-6A looks like PP class a to me...

 

[CJ]

tube rectifiers cause something called "sag", which is just the voltage dropping for an instant due to the higher fwd voltage drop of the tube rectifier.

solid state diodes will drop about .6 to 1.2 volts depending on full wave bridge or CT circuit.

so if you compare solid state to tube, you get 1 volt vs 50 volts.
some musicians like the natural sag caused by the tube rectifier,

what if you hit the compressor with a positive half cycle square wave for an instant,

does the class A amp still have constant current draw?

i am trying to figure this out right now,

jump in if you know the answer.

while we are at it, let us spec out a transformer for solid state diodes in the RCA compressor,

285 volts plus 10 volts choke plus 1 volt diode drop = 296 volts.

divide by 1.2 to get RMS volts from DC plus losses and you get

296/1.2  =  247 VAC secondary volts.

so order a 250-0-250  VAC trans with only the 6.3 volt heater winding as you no longer need the 5 volt rectifier winding as there is no longer a tube rectifier in the circuit.

 

[abbey road d enfer]

what if you hit the compressor with a positive half cycle square wave for an instant,

does the class A amp still have constant current draw?

For class A push-pull, yes. Not strictly constant, but close enough. While one side pushes, the other pulls. In fact, the square-law response of vacuum tubes introduces some unbalance between the increase of current on one side and the decrease on the other side, but in practice this difference is marginal.
Different thing in guitar amps, since the "sag" thing happens when the amp is outside its linear operation; then, the amp drifts significantly from class A to the realm of class B.

 

[CJ]

ok so run the compressor in class A and no worries about rectifier sag.

and you probably wanna build the original circuit just in case that tube rectifier adds some type of sonic signature which it is known for.

build the original first, then see if the modifications add or subtract from the sound.

Fender runs closer to class AB i believe, Vox would be class A with the cathode resistors.

 

[AZ999]

Some time ago i meassured psu output voltages with DWM on PP and SE class a preamps, sources were sine waves and pretty complex music tracks. In all cases (reg. and unreg. psu's) i found very constant voltages, with unreg. psu it changed a few volts during dynamic input signal to preamp. HT was built with back to back transformers where output transformer didn't have enough juice, it got pretty warm over time. I think this added to changes in output voltage.
But even if voltage was constant i could hear difference in sound between reg. an unreg. psu. Preamp with unreg. sounded a little compressed. When i ordered proper transformer (no problematic back to back wiring) and wired it to reg. psu, i thought sound got better, but unfortunately i didn't repeat sound tests between two psu's. I wish i did.
Did anyone ever noticed sound difference between reg. and unreg. psu's? I mean with proper transformers and circuits (tube or SS) on preamps or compressors?
Btw, i noticed power transformers something like BA-6A are very expensive in the US or EU. Similar transformer, custom ordered in Croatia costs around 30 euros, toroid even less. I can't believe it when i see Sowter's prices for their replacement power transformers. Ok, buying from them saves you from having to write thread like this (for those who don't want to learn) and they might look nicer, but it's still a lot of money.

 

[abbey road d enfer]

Some time ago i meassured psu output voltages with DWM on PP and SE class a preamps, sources were sine waves and pretty complex music tracks. In all cases (reg. and unreg. psu's) i found very constant voltages, with unreg. psu it changed a few volts during dynamic input signal to preamp. HT was built with back to back transformers where output transformer didn't have enough juice, it got pretty warm over time. I think this added to changes in output voltage.

There are two different phenomenons happening here.
PSU voltage sag reduces the anode voltage and thus reduces gain, whatever the configuration, PP or SE, class A, AB or B, however very marginally, unless the sag is exaggerated by high DCr transformers, high dynamic resistance vacuum rectifiers and low value caps.
For cathode-biased tubes, the increase of average current increases the negative bias, which in turn tends to reduce the quiescent current and thus gain is also reduced, in a more significant way than the anode voltage reduction.
The effect depends on the topology, if there is a single resistor shared by both tubes in a PP or two separate resistors, and also the variation of mu vs. bias.

In SS circuitry, the effect is generally very different, because they rely on higher NFB, so the voltage drop has almost no effect until the circuit runs out of juice and starts to breakdown badly.

 

[CJ]

anybody agree that choke input guitar amps suck the big one?

old fender circuits, tried them once and boy are they gross,

 

[abbey road d enfer]

Yes; choke input is for constant-current load.

 

[lassoharp]

anybody agree that choke input guitar amps suck the big one?

old fender circuits, tried them once and boy are they gross,

Which one?  If you mean a true choke input filter I haven't seen an official one yet.  The closest thing is that mystery schematic in The Tube Amp Book (pg 380) labeled "Old Bassman".  None of the known Bassman schematics show a choke input.  I've wondered myself if choke inputs were ever used and haven't seen them on audio gear either.  Probably deemed "too expensive".  Guitar amp would lose a lot of HT voltage too.

 

[CJ]

choke input does have advantages,

like less peak current from the power trans,
better regulation,
smaller filter caps can be used

probably popular with the ham radio guys,

morse code at 1 kw is pretty hard on the power supply,

ok this>input choke pwr supply


"The ICPS is particularly well suited for realising the HV power supply of an RF linear amplifier, mainly because of its inherently stable output voltage. While looking through recent editions of the ARRL Handbook for Radio Amateurs, we noticed that it no longer dwells on the ICPS, probably because of the extra cost and bulk of the filter choke, and of the lower Vout that is obtained for a given Vrms, compared to the CPS. They just mention that ICPSs have become less popular than they once were, because of the high surge-current capability of modern silicon rectifiers allowing to use high-value filter capacitors, thus making it possible to obtain an acceptable regulation performance even with the simpler CPS circuit.

 

[letterbeacon]

My Hammond transformer arrived yesterday and I've done the following calculations to determine what the resistors on the 5R4GY plates should be.  Would someone be able to check my maths?

Check out this ref for the minimum value of anode resistors required.
http://www.diyaudio.com/forums/tubes-valves/150665-rectifier-limiting-resistors-capacitor-input-2.html
You can always increase them to adjust your B+

From the above link:

When you know it ,there is a very simple ( but with good results ) formula to calculate the impedance :

Rt = Rs + Rp . N2 where :

Rt = Total impedance ( transformer natural impedance )
Rs = Secondary DC resistance , measured with a multimeter
and :
Rp . N2 = The influence of primary impedance over the secondary
( called reflected impedance )
where :

Rp = Primary DC resistance , measured with a multimeter
N = Transformer ratio ( Secondary Volts / Primary Volts )

I’ll give you an example :

Rs = 35 ohms
Rp = 6 ohms
Transformer ratio ( N ) = 300 VAC output / 117 VAC input = 2.56

Then Rt = 35 + 6 . ( 2.56 )2 = 74 ohms , that way , you’ll need TWO 27 ohms resistors ( preferably , wirewound 5 watts resistors ) between each secondary tap and each plate of the 6BY5GA ( the tube that I had used as an example ), because 74 + 27 = 101 ohms > 100 ohms ( minimum impedance per plate )

If you use this formula with a split secondary ( center tap ) , you must to consider that :
( Rs ) will be the secondary DC resistance between one tap
and the center tap .
( N ) Transformer ratio will be the
HALF total secondary voltage / primary voltage

The minimum plate impedance on for a 5R4GY is 125 ohms (see attached file)

In my case, with a Hammond 370FX (which is centre tapped):
Rp = 9 ohms
Rs (CT) = 51 ohms on one tap and 53 on another
N = 550Vac output  / 240Vac input = 2.29 / 2 = 1.15

53 + 9 . (1.15)2 = 83

125 - 83 = 43.

So I need 47 ohm resistors on the 5R4GY plates minimum to meet the datasheet requirements. This will drop about 5v.

Once I've built the unit and find I need to drop the B+ further I can increase these resistors as required.

 

[DaveP]

Letterbeacon,

This is how I work it using your figures:-

Rt=Rs+n2xRp+Ra

Call rs 52

n=275/240= 1.14583

n2=1.31293

x9=11.8

+52=63.8

125-63.8=61  0hms

The diagrams on the Philips tube page seem to call n one side of the secondary which makes this difference.

So there we are 47 ohms or 61 both are in the right ballpark.

DaveP

 

[letterbeacon]

Great, thanks for that Dave!

 

[Brian Roth]

So, have there been any further developments?

Best,

Bri