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 71A's in push-pull - How much Audio Power?
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 Return to top of page · Post #: 1 · Written at 10:54:25 AM on 11 October 2017.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 382

Being only into post war radios, but having old 1928 chassis from swap-meet, was wondering how much audio power from its push-pull 71A's? As I want to listen to Amplifier only with hi-fi speaker for esoteric "all transformer triode" sound -- sort of like a poor man's Western Electric.


 
 Return to top of page · Post #: 2 · Written at 12:57:17 PM on 11 October 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 2747

I would have thought there were better tubes than that, even a super expensive #50 could do better.

By the looks of it it has similar Grid bias requirements for the space charge. You can always use a Pentode Triode wired?

You are not going to get much out of it.

SE .180V plate, Load 4.8K 0.79 Watt OP. Grid -40.5V . Nothing in the data here re push pull. Try "Franks Electron Tube Pages".

A 1929 Lyric 70 Series actually used a #24 wired as a rectifier on its own winding to supply the -84V grid . 450V Plate that its #50 can take.


 
 Return to top of page · Post #: 3 · Written at 2:09:37 PM on 11 October 2017.
Ian Robertson's avatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 624

The 71A was primarily intended for battery radios. Interpolating from the curves, you might get about 2 watts from a pair, transformer coupled class B. Push them beyond the ratings and you might do better....

If you want triodes and transformers how about the 2A3?

http://www.jj-electronic.com/en/2a3-40

But you are only doing this for sentimental reasons I assume, and to watch the bottles glow? A properly designed amplifier based on the venerable LM1875 will outperform a pair of 2A3s. And you can emulate the academy-curve sound of a WE amp with a DSP! Choose your preferred distortion!


 
 Return to top of page · Post #: 4 · Written at 7:30:13 PM on 11 October 2017.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 382

I'm guessing there may be 3 or 4 watts in push pull as radios went to 45s (black plate) by early 30s from white plates I see on 71s.
The chassis is Majestic-model 70 with nice steel cased audio transformers (like Western Electric). A 27 in the voltage gain stage drives transformer coupling to a 26 driving a phase splitter transformer that drives two 71As to the output transformer (probably to ~30 ohm voice coil as my Majestic speaker measures 20 ohm DC voice coil.) I got two other chassis (all at swap meet throw out auction) that take 45s in push pull. All good for audio projects as they have sockets etc. All chassis were stripped of tubes: Looking through my boxes of pre-octal's I find dozens of 26s & 27s, but only five 71As and just two 45s (National Union brand). Will be intrigued to hear the sound character of the transformers as, after all, it's the transformers that are the secret of the revered Western Electric sound from that era.


 
 Return to top of page · Post #: 5 · Written at 7:47:19 PM on 11 October 2017.
BringBackTheValve's avatar
 Location: Linton, VIC
 Member since 30 December 2016
 Member #: 2028
 Postcount: 67

"Choose your preffered distortion"

Hah, thank you Ian for many happy memories of arguing with older techs about valve amps.

These guys were my mentors, professionals and good friends, and they all argued against my passion for "valve sound' guitar amps.

"2nd. harmonic distortion--" they would argue. "Pleasing to the ear, but distorted--" "Less distortion with semiconductors---" etc. etc.

And they were right, of course. But by gum, you just cannot beat that valve sound, can you?


 
 Return to top of page · Post #: 6 · Written at 12:10:10 AM on 12 October 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 2747

It really depends on how berserk one wants to go & what transformers are available to achieve it. You really don't get decent power until you get into the thirties.

I worked on a 1932 AWA special that used a 6A6 (later 6N7) in its OP: Also an American Midwest that used I think PP 37's to drive four 6F6's into, Jensen Woofer & Tweeter both field coil types. A pair at 375V plate is 18.5 Watts RMS.


 
 Return to top of page · Post #: 7 · Written at 1:31:53 AM on 12 October 2017.
Frank F's avatar
 Location: Brunswick, VIC
 Member since 3 May 2017
 Member #: 2100
 Postcount: 8

You probably won't get much more than a watt or two out of an amplifier using push pull 71As but that would be plenty loud if you run it into a decent field coil speaker of the era mounted on an open baffle. The valve sound is as much to do with the interactions of the speaker with the relatively high output impedance of the amplifier and the resonances as anything else.

The best way I can explain it is that your valve amplifier will tend to output constant power independent of load. Your vintage speaker will tend to vary in impedance because of resonances all over the place, but because it is being driven by constant power that it was designed for, it tends to even out.

Your HiFi speaker will have those same dips and rises in impedance too, but it will be designed to work from constant voltage and the assumption you have plenty of power. Generally the results won't sound anything like the valve sound you are after.

If you do want to use modern "speakers" try a set of headphones with moving coil speakers. Power won't be a problem then and the impedance will be flatter because you are not running through crossover networks etc..

As for the sound of a push pull amplifier, most people would have said that push pull was a great way to save batteries but didn't sound as good as the power hungry single ended approach back when it was invented.

I have probably opened a can of worms here. All I can say is I personally prefer my HiFi speakers driven by my solid state amplifier, but I have heard some high efficiency speakers driven by valves that sound damn good. The advantage valves have though is that even a simple badly designed all triode transformer amplifier will never suffer from the slew induced distortion that almost all solid state amplifiers suffer from.

So BringBackTheValve you may be right in your choice of distortion. At least you will only have had things added to your music rather than bits missing.


 
 Return to top of page · Post #: 8 · Written at 10:35:28 AM on 12 October 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 2747

A lot of it comes down to what part of the market the set is aimed at and that is across the board SS, or Valve.

The AWA is an example of a custom build: It had all the same valves, save one, as the production model: But! At the wiring stage it to a severe left turn. The IF frequency changed, the "band pass" filter for IF1 was retained, Bigger transformer for the 6A6, this time on board PP; Backbias abandoned & self bias introduced, choke for add on PP amp added to B-; Field coil retained in B+. 3 position Local / DX switch added, despite having delayed AGC. Station 100Km away in daylight, needed the second position to hold it back.

Base model OP tube retained this time as the PP driver & self bias used to hold it back: Transformer coupled PP. As I have said before; I have rarely seen sets before & after it that could outperform it. Full volume no signal: Silent.

Just shows that even in the thirties, what could be achieved where cost was not part of the equation (Golden handshake perhaps) and it was designed & built for perfection.


 
 Return to top of page · Post #: 9 · Written at 3:56:22 PM on 12 October 2017.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 382

Marcc said: Also an American Midwest that used I think PP 37's to drive four 6F6's into, Jensen Woofer & Tweeter both field coil types

That must be the same 'Midwest' brand that sold radios and televisions by mail order? Am surprised they had a no-compromise 'flagship' model like that. I know someone with a (rare) Midwest 16" TV set that incorporates radio tuner on same chassis! Fancy cabinet, fully restored and running. He has pic of it plus their catalogue online: search 'Midwest TV set'.


 
 Return to top of page · Post #: 10 · Written at 5:06:08 PM on 12 October 2017.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 382

Frank F said: As for the sound of a push pull amplifier, most people would have said that push pull was a great way to save batteries but didn't sound as good as the power hungry single ended approach back when it was invented.

I think I saw on the Majestic-70 schematic (may have been a battery radio - outboard power supply) with the 71a's PP having about the same bias (-40V) as the class-A spec for 71A but will have to look into that for class AB operation.


 
 Return to top of page · Post #: 11 · Written at 10:26:05 PM on 12 October 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 2747

The one with actually 2 x #37 feeding 4 x #45 was the 16-34. As far as I am concerned these were not legendary performers. 18-36 had a single 6F6 transformer coupled to 4 in PP. 16 and 18 were the number of tubes: Other number year.

The use of some of these tubes was dubious. The Faraday cage little more than decoration and one was extremely unstable. Things like running RF wires into the audio and the tone wire past a hole in the Faraday cage, not helpful. Oscilloscopes detected a lot of RF in the audio.
When you have to shield valves inside a Faraday cage, its a dust cover.

Odd things with some of these was the use of Collpits oscillators (four gangs): Autodyne mixer, one amplified AGC and virtually needing a welding transformer Two #80 / 5Y3's with the plates wired parallel in each leg of the HV secondary.

Scanting on the chassis was also something that I was not impressed with, these squirmed (flexed), that could easily result in joint breakages.


 
 Return to top of page · Post #: 12 · Written at 1:45:25 AM on 13 October 2017.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 382

I'm looking at the 16-34 schematic now, certainly interesting. Wonder how the audio sounded through all those trannies without negative feedback? Of course similar Western Electric [WE] amplifiers sounded superb, but they made their own transformers. Mid-west did not have the engineering resources of the major brands/WE.

There was guy with transformer rewinding business here a while back and I queried him on WE and its "mystique": As one who had opened up their potted packaging and got inside them, he surprisingly dismissed and even disparaged them!! But WE did have unlimited funding and engineering, maybe they do justify today's prices in the esoteric audio market.

So it was low risk to pick up three chassis with transformers for $2 each and only one chassis had an open circuit phase splitting tran - as interstage transformers have that very fine secondary windings for high-Z - cannot take any accidental DC current. I doubt if the transformer rewinding guy would have done these or if it was ever economically viable?


 
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