The Latest Push-Pull Build

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Paul Barker
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#931 Re: The Latest Push-Pull Build

Post by Paul Barker »

Should do shouldn't it? There was a 6550 parallel push pull article in Glass Audio. I think it came out with about 90 watts. It would have been a Williamson Clone probably, that's what was going around at that time. When I get some time, I'll see if I can find the article. My glass audio's are still in tact around the place, but as dog eared as the shared single page of a health and efficiency of my all male boarding school in the 1970's.
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Cressy Snr
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#932 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Aye.. the CF buffer should be fine driving a parallel push-pull set of output valves.
Double the transconductance as well, which is a good thing as the plate-to-grid feedback scheme thrives on the output valves' transconductance. Within reason, the more gm in the output stage, the better. :)

You've got me thinking now about a pair of monoblocks consisting of:
a quad of EL34s in class AB, parallel push-pull, driven by a pair of triode strapped, 6AQ5 power cathode followers, with a differential pair of EF86 or the Brimar equivalents on input stage duty. I reckon that would be an awesome setup. Dynamics would be positively frightening :shock:
Biasing the output stage safely, would be the bit that would need some thinking about.
I think I'd much rather mess about with something like this than 300Bs.
It's a sound I like and the thought of a Barbarik on steroids is quite an attractive one.
Electro-Harmonix EL34 are cheap as chips and sound fantastic in plate-to-grid feedback, pentode mode. NOS 6AQ5 are equally good value at around £6 each. So we are looking at, £108 (Hotrox) for an octet of EL34, about £30 for 4 x 6AQ5 and another £30 or so for 4 x 6BS7 ultra low noise small signal pentodes. So a complete set of 16 valves for less than £200.
I feel another design materialising in my head. :wink:
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Paul Barker
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#933 Re: The Latest Push-Pull Build

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Paul Barker wrote:Should do shouldn't it? There was a 6550 parallel push pull article in Glass Audio. I think it came out with about 90 watts. It would have been a Williamson Clone probably, that's what was going around at that time. When I get some time, I'll see if I can find the article. My glass audio's are still in tact around the place, but as dog eared as the shared single page of a health and efficiency of my all male boarding school in the 1970's.
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Paul Barker
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#934 Re: The Latest Push-Pull Build

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The amp is 6 6550's per channel, so the picture shows a monoblock. There are 50 ohm resistors on each plate as well is control and screen grid stoppers "to prevent oscillation"

He uses an unusual parallel 5687 input valve and also unusually parallel 6sn7 long tailed phase inverter.

Claims 100 watts from 44v RMS, all of which is generated by the parallel 5687.

He has an interesting take on adjusting to balance the phase inverter and further on the output valves. Rather than test for null, he advises run 35hz through it and adjust for minimum distortion. You could say it would make the same difference if I just adjusted to null. But at the end of the day least distortion is the objective, so why not adjust by that parameter?

In respect to the copywriter holder I won't publicise the circuit but I can answer questions in full detail.
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IslandPink
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#935 Re: The Latest Push-Pull Build

Post by IslandPink »

What does it sound like ? :D
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Paul Barker
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#936 Re: The Latest Push-Pull Build

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'Production of vocals and high frequencies at a very natural and realistic state" is the only mention of sound. The rest is pure instruments.
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Paul Barker
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#937 Re: The Latest Push-Pull Build

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But I have to admit one should sit up,and notice claims of 20/20k 80 watts 30/16k at 100 watts. At 100w output distortion is claimed to be at 30hz 0.8% and at 15khz 1.6%
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andrew Ivimey
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#938 Re: The Latest Push-Pull Build

Post by andrew Ivimey »

The best amp I've ever heard was fully balanced push pull with 3 x 6550 in parallel. Through monster Tannoys and an ideal listening space the music could be as loud as one could wish. The quality was superb. Dropping back to 2 x parallel 6550 was still fabulous.

Ah,but I have just reminded myself - the valves are in triode mode. Totally fab,though. Best bar none.
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Thermionic Idler
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#939 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

I really must get off my backside and get my balanced preamp built. Right now I'm 3D modelling the layout.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
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Paul Barker
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#940 Re: The Latest Push-Pull Build

Post by Paul Barker »

Cressy Snr wrote:Full circle, right back to the start.

Image

I haven't used EL34s in any recent versions of this amp; abandoning them early on for big power tetrodes.
Pleasantly surprised is the expression I think.
Running an HT of 465V and 232V screens at 35mA per valve, they really do the biz on vocals, brass and guitars, both acoustic amd electric.

The conclusion we can draw from this project is that with EF86 or equivalent input stage and 300-330K feedback resistors, it's a pretty versatile amplifier now the output stage is being driven properly by those 5687 type cathode followers. Simply choose the output valve you prefer, bias it up and balance the input and output and it'll play anything you want it to.
I think this is the thread where "The Barbarik by SJS" belongs.

Bought this from Steve a few eeeks ago but had to wait for Anth to meet me with the sowter phase splitter balanced preamp yesterday before I could use it.

She got home safely. Found balance on the input valves, which sets the balance right through, was perfect. One chanel output valves were perfectly balanced. Other channel 0.2 mA out of balace, so that was sorted out straight away!

Only thing I'm nervous about is the balance at the start on grid 2 of the EF86 (we'll call the equivalent that so people reading from fresh won't get lost not knowing what the valves are.) is adjusted by a twiddle knob right at the front on top of the amp. Obviously one per channel. Other people can play with that. Bad idea in my house. For now I've phorographed it. One day I'll have to drop it inside. Actually I could fit a piece of glass acroos the front to keep pot twiddlers out.

Wife who hasn't noticed it before as it was stashed until it could be played, came down and said "oh well at least it sounds good". I think that means I got away with buying an amp.

It sounds bloody good.

By the way I let it burn in a bit before I corrected the shocking 0.2mA imballance.
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Cressy Snr
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#941 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Good to read tht it's found a new home and is working nicely. :)
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