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 Post subject: Single-ended push-pull Better design found?
PostPosted: Apr Tue 30, 2019 4:18 am 
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A circuit I pulled from my saved files had this schematic, designed for the 7694 or 7754 output tubes.
The input signal goes to only the lower tube's grid, and the upper tube functions as a cathode follower/constant current source. This is said to be an improvement but was not used much. Does anybody have some insight?

The 7754 pdf linked below, near the bottom of page 1 describes the 'bogey' tubes, and would that be the upper tube, lower, or both?
This circuit does indeed date from the Bogey years, 1942.

I am wondering if I could build with reasonable results, this circuit using two 50C5 tubes and a typical single-ended output transformer?
Should I expect noticeable improvements to the audio quality?
I suppose I should be comparing the 50C5 characteristics with those of the 7694 and 7754 tubes.
Anyway, it is interesting and I have a chassis waiting to be rescued from the junk box. 8)

Bogey tubes here:
Attachment:
7754.pdf [121.09 KiB]
Downloaded 17 times


Attachments:
7695or7754amplifier.png
7695or7754amplifier.png [ 115.44 KiB | Viewed 618 times ]

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Last edited by westcoastjohn on May Sun 05, 2019 8:03 pm, edited 1 time in total.
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 Post subject: Re: Single-ended push-pull questions
PostPosted: Apr Tue 30, 2019 5:12 am 
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Such circuits do work, and once in a while saw production.

The load resistance can be so low, that with a high impedance speaker, no output transformer is required.

For regular low impedance speakers, a line to voice coil transformer may be a good choice.

Watch out for the heater to cathode voltage rating of the upper tube.

A 50C5 may require a higher cathode resistor value to keep the dissipated power reasonable.

The upper tube is not constant current at all. As the lower tube plate current falls, the negative grid bias on the upper tube falls as well. So the upper tube plate current increases.

Likewise, an increase in the plate current of the lower tube results in less plate current in the upper tube.

The circuit only works in class A, since the lower tube must conduct at least a little if the input signal is going to do anything.

A "bogey" tube is usually one selected to closely match the published characteristics for that type. Usually provided by the tube plant to larger customers to aid in product design.

A bunch of negative feedback might help the fidelity quite a bit. This circuit will not be super linear open loop.

Ted


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 Post subject: Re: Single-ended push-pull questions
PostPosted: Apr Tue 30, 2019 6:52 am 
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SEPP was an attempt to make 'transformerless' amplifiers, for the obvious reason that eliminating the output transformer would cut costs. They can't drive a load as low as 8 Ohm, though, so they needed a high impedance speaker, which is problematic and introduced new problems. I.E. winding that much wire (mass) on a voice coil (wire must be REALLY thin, so they tend to burn up easily). And then it didn't save as much cost as one might think because it required two output tubes for essentially the same power as conventional circuits got with just one.

It never caught on to any significant degree although Philips made some sets using that technique.


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 Post subject: Re: Single-ended push-pull questions
PostPosted: Apr Tue 30, 2019 3:51 pm 
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Thanks, that explains it pretty well. The schematic shows the added feedback loop, but without resistance values, so that would take some experimentation, along with determining the ideal output load.

And thank you, Ted, for the definition of a 'bogey tube'.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: Apr Tue 30, 2019 6:31 pm 
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westcoastjohn wrote:
… The schematic shows the added feedback loop, but without resistance values, so that would take some experimentation, along with determining the ideal output load.

If you're referring to the two resistors with a "?" for value, those aren't part of a feedback loop. They form a voltage divider for the B+. Note that the top end of the plate resistor for the triode is bypassed by the 22uF cap.

This divider would be used for two reasons. First, to isolate any variation in the triode plate supply from variations in the 280v B+. In this role it is an anti-feedback circuit. Second, it is used to optimize the plate voltage for the triode for the required triode stage gain and signal handling characteristics.

John

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Wed 01, 2019 4:03 pm 
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Thanks for the clarification, John. Cheers.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Thu 02, 2019 3:36 pm 
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I mulled around the idea of something that was SEPP a few years back but never got around to building anything.


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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Thu 02, 2019 8:33 pm 
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Plans for the winter of 2019/20. 8)

My thinking was to retain the output transformer and just see if the output is improved. Maybe will continue reading and perhaps that will save some fruitless effort. In fact, I'm leaning towards dropping the whole idea. Aren't these discussions great?

Here's the radio that is begging for TLC


Attachment:
RCA C120.jpg
RCA C120.jpg [ 94.34 KiB | Viewed 445 times ]


And here is a link to the schematic:

http://pacifictv.ca/schematics/rcac120data.pdf

Just in case somebody is itching to draw up the schematic for modifying AA5 to single-end push-pull? 8)

Edit:
Plan B - a second output tube can drive another output transformer and another speaker. That would be simple but the 1st audio stage, 12AV6, needs to be up to the increased load.


Plan B2 - the input jack could then be doubled for stereo inputs. I think that would be a significant improvement.
But more parts are needed - a double volume control and another tube stage to drive the second power tube.

Could go with a 12AX7 or similar 9-pin duo-triode in place of the 12AV6 for the 1st audio stages for two channel output.

Here's a list of possible subs but have to stick to 0.15 amp heater and it needs to be High mu.
http://www.angelfire.com/electronic/fun ... iodes.html

Looks like a 12AX7 or similar will still be the best choice.

Will need an iso trans and power switch, as the clock will be gone. A 3 position on-on-on could be wired to turn on heaters only in the mid position for a controlled warmup.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Fri 03, 2019 2:44 pm 
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I believe optimum load for a single end 7695 is around 1500 ohms or approx 1000 less than a 50C5. Also the 7695 is more robust, were commonly used in better versions of single ended, transformerless stereos. Versions I've seen used silicon top hat rectifiers to net around 150v B+.

There is also a rather uncommon 50FK5 that's a octal version of 7695(or at least very similar). In place of 50L6, current draw is approx 30% higher.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Fri 03, 2019 3:14 pm 
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If you want to do push pull do real push pull provided there's enough room on the chassis.

Do it using two output tubes ditching the tube rectifier and then using a 12SL7 or 12SN7 (depending on how much gain is needed) as the driver and phase splitter. For the detector diode a 1N5711 will work.

Output transformer is a P-T291.

Use a Triad N-68X isolation transformer which will boost the B+ to around 150Vdc for the output tubes so they can produce more output. Then use a resistor to keep the B+ to the rest of the circuit stock.

I have an example of doing that to a Hallicrafters S-41W radio

Image

Might not be able to be seen clearly enough, but I can email a full size schematic.


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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Fri 03, 2019 6:11 pm 
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I know about this circuit and others, but with the cost of shipping, 2 transformers become 3 times more expensive for me here. Coming my way? Bring me some iron.

I need designs that use my surplus single-ended 3 Watt output transformers.
I was picturing two of them in series, but haven't ventured far enough down that path, yet. :lol:

Just having cheap fun with tubes. I have boxes of unwanted AA5's with damage and parts gone.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Fri 03, 2019 8:47 pm 
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westcoastjohn wrote:
I need designs that use my surplus single-ended 3 Watt output transformers.
I was picturing two of them in series, but haven't ventured far enough down that path, yet. :lol:

Just having cheap fun with tubes. I have boxes of unwanted AA5's with damage and parts gone.

I have seen two SE transformers used in expermenter PP circuits.

Eons ago there was a circuit in one of the electronics rags, I believe it was discussed here on ARF.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sat 04, 2019 7:02 pm 
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The output transformer primary impedance would ideally be about half that for a single output tube.

Or, you could increase the B+ voltage. Might need a separate heater supply for the upper tube, in that case.

With an existing output transformer intended for a single tube, the totem pole circuit should give lower distortion and better bass, since there will be no DC current in the windings. In this case you might run the tubes at lower plate current.

Ted


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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sat 04, 2019 7:19 pm 
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OK, I'm on board with it so far. 8)

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sun 05, 2019 1:26 am 
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Usually Lurking wrote:
The output transformer primary impedance would ideally be about half that for a single output tube.

Or, you could increase the B+ voltage. Might need a separate heater supply for the upper tube, in that case.

With an existing output transformer intended for a single tube, the totem pole circuit should give lower distortion and better bass, since there will be no DC current in the windings. In this case you might run the tubes at lower plate current.

Ted
The 7754 datasheet doesn't support your contentions. Power output for the SEPP is 5 Watt vs 4.5 Watt single ended. Distortion for the SEPP is roughly 10% vs 11% single ended. Good luck on being able to discern any audible difference between the two. Btw, plate current is roughly the same, as it should be, because there's nothing 'driving' the upper tube (unlike a 'real' push pull) other than the plate current through the lower. If it goes into cutoff the drive goes away so it has to operate Class A. And SEPP B+, being twice the single ended B+, means SEPP consumes roughly twice the power for essentially the same performance.

Removing DC from a conventional OPT won't improve the bass (without negative feedback) because the OPT is already gapped (for DC) and it's the inductance (which remains, for the most part, unchanged) that rolls the bass off on the low end. The 'improved bass' response of SEPP is with the transformerless version where there's no OPT and the speaker has to be wound to 500 Ohm (good luck finding one).


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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sun 05, 2019 1:59 am 
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Right, the plan was to build it and see if improvements are measurable with an O-scope, for example.
I can't trust my ears to hear any difference, unless the mod is switchable back to original, maybe by pulling jumpers.

I'm already thinking single-ended PP is a wasted effort if using old radio components. It will still be Lo-Fi.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sun 05, 2019 5:43 am 
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SEPP does work, I have repaired Philips sets long ago.

They used EL86 tubes and a special 800 ohm speaker.

Forget finding the replacement speaker. And that was in 1963.

I subbed in a 70 volt transformer and used the 5 Watt tap.

If you want to experiment using SEPP try the 12K5 (or 12DL8)
space charge tetrodes from 1959 era hybrid car radios.

Don't burn you fingers!

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sun 05, 2019 6:01 am 
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radiotechnician wrote:
SEPP does work, I have repaired Philips sets long ago.

They used EL86 tubes and a special 800 ohm speaker.

Forget finding the replacement speaker. And that was in 1963.

I subbed in a 70 volt transformer and used the 5 Watt tap.

If you want to experiment using SEPP try the 12K5 (or 12DL8)
space charge tetrodes from 1959 era hybrid car radios.

Don't burn you fingers!
Of course it works. It just doesn't give the benefits people think it does without the special speaker.


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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sun 05, 2019 7:32 am 
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I think Philips' approach was economy, and perhaps to promote the EL86
tube. There is far less cost in making an 800 ohm speaker than an output
transformer.

The speaker had a DC resistance of about 550 ohms so take 40 gauge wire,
the voice coil would be 500 feet long. 1 lb of that wire is 33,000 feet long,
making the voice coil winding mass 30 milligrams. (if math is correct 8) )

As to distortion, no B H from a cheap output transformer, and if I remember the
circuit, the drive is through a polarized electrolytic.

Energy saving, no i^2r loss in opt primary.

Also I think the plan didn't work because the voice coil just corroded away.

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 Post subject: Re: Single-ended push-pull questions
PostPosted: May Sun 05, 2019 7:11 pm 
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Right, so the design was successful in a limited way, if you had the specified hardware to go with it.

radiotechnician wrote:

If you want to experiment using SEPP try the 12K5 (or 12DL8)
space charge tetrodes from 1959 era hybrid car radios.

Don't burn you fingers!
Matter of Fact, I happen to have a '58 Chev hybrid radio in my back corner pile, will take a look for those tubes, thanks. 8)

According to the schematic, I do have one of unknown quality 12K5 space charge tube. Halfway there.

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Last edited by westcoastjohn on May Sun 05, 2019 8:42 pm, edited 2 times in total.

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