The Latest Push-Pull Build

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Cressy Snr
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#901 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Have decided to call it a day after having been at the thing since 11am.
I just have three top caps to make up with screened cable, but my eyes are on stalks.
The front-end rebuild has been comprehensive, with improved earthing arrangements via renewed and repositioned ground and HT busbars, so extra-careful checking and testing will be needed, once things are ready for fire-up.
State of play below:

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Mike H
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#902 Re: The Latest Push-Pull Build

Post by Mike H »

Only to comment if you're going to put grid stoppers on those 6BS7's preferably close to the top cap.
 
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Cressy Snr
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#903 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Mike H wrote:Only to comment if you're going to put grid stoppers on those 6BS7's preferably close to the top cap.
Grid stoppers are indeed fitted right next to the top cap, with shrinkdown sleeving as strain relief. The strain relief goes right over the solder bucket within the top cap and goes back out an inch along the lead itself, so protecting the resistor from any mishap.

The front-end conversion is finished, and very nice it looks too, though I say so myself :)


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Mike H
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#904 Re: The Latest Push-Pull Build

Post by Mike H »

OK then!
 
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Cressy Snr
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#905 Re: The Latest Push-Pull Build

Post by Cressy Snr »

:D

First impressions are positive. These little valves are far less noisy than any of the 12SJ7s I have been using.
There is no hum whatsoever and tapping the valves with a pencil produces only a very faint thud at the speaker, and I mean faint. Hiss is entirely absent and the sound seems completely neutral, with human voice reproduced better than ever.

I am using a small OA3 VR tube to drop the HT to 300V for the plate supply to the 6BS7s and the cathode followers, which means that the front row is once again fully occupied. :) The 0A2 is however getting caned with 50mA of current (10mA total for the 6BS7s and 40mA total for the cathode followers) and so won't survive for long under those conditions.
The plan for tomorrow is to swap out the 0A2 for a pair of 75C1 neon VR tubes, which are specced to be able to deliver 60mA.

The feedback needs a slight tweak to get the gain back up to where it was before, but this is to be expected when the front-end has been changed. All in all I'm pleased.
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Paul Barker
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#906 Re: The Latest Push-Pull Build

Post by Paul Barker »

Fantastic.
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IslandPink
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#907 Re: The Latest Push-Pull Build

Post by IslandPink »

Good effort - should sound great & looks very nice.
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Thermionic Idler
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#908 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

Great news - interesting result from those noval pentodes, I had had my eye on the old RCA 5693's but if these are a) cheaper and b) take up less space and c) sound better... winning.

With regard to the reappearance of the VR tubes, I take it this means the B+ to the input stage is now effectively a VR regulated supply. How have you got the two OA3's and OA2 configured - are they strung together to total 300V? (so 75+75+150)
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'
Speakers: FF225WK Big Mets
Cressy Snr
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#909 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Thermionic Idler wrote:Great news - interesting result from those noval pentodes, I had had my eye on the old RCA 5693's but if these are a) cheaper and b) take up less space and c) sound better... winning.

With regard to the reappearance of the VR tubes, I take it this means the B+ to the input stage is now effectively a VR regulated supply. How have you got the two OA3's and OA2 configured - are they strung together to total 300V? (so 75+75+150)
Yes, the 6BS7s are nice valves. I wouldn't say they are better than the 12SJ7 type in terms of sound quality, apart from the voice reproduction. But then that could be simply to do with the rebuild. There have been too many changes at the front as well as the actual valves, so it would be tenuous to put all of the improvement in voice reproduction down to the valves alone.
However they are far superior in terms of noise, hum and microphonics; outstanding noise performance overall. The 6BS7 is a very good pentode by any standard. :)

The VR tubes are just series voltage droppers.
The two larger ones drop the HT by 300 or so Volts to a level suitable for feeding the input stage screen regulator (NickDS MJ statistical reg board)
The two neons drop the voltage to a level suitable for the input stage HT.
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Cressy Snr
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#910 Re: The Latest Push-Pull Build

Post by Cressy Snr »

A copse of glow tubes. :D
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...And no, the neons are not that bright!
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Thermionic Idler
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#911 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

Oh very nice!

I think for the moment I'll just sit back and watch progress on this whilst I get my balanced preamp built (the dieselpunk VU meters arrived today).
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'
Speakers: FF225WK Big Mets
Cressy Snr
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#912 Re: The Latest Push-Pull Build

Post by Cressy Snr »

The feedback has now been tweaked up to suit the 6BS7s. They are set up with 300V HT, 100K anode resistor and 1K shared cathode resistor, and run at 1.5mA per phase.
RFb is 470K and could do with a very small adjustment downwards in value, but we are virtually there. It's a fine line between keeping the bass right, and the gain correct, on the one hand and losing gain and buggering up the leading edge speed on the other. But that's the way it goes with feedback.

The thing I like about this new arrangement with the cathode follower driver is that the input pentode can live in its own little world, whilst the CF does the heavy lifting. You get a lot more flexibility in the choice of pentodes for input valves than you would without a CF buffer.

Personally, I would no longer build one of these kinds of amp without a CF...well not unless it was a source follower instead. But that's only my opinion.

It has taken me a week to replace the front end and set it up properly. A long process, but worth it. I acquired another four of these little pentodes (from a different supplier) this morning. I needed 10 altogether to find two matching pairs, but that goes with the territory with high- gain pentode input stages in a differential pair arrangement. Gary Pimm's screen grid balance adjustment worked like a charm when it came to setting up the front end. All that was needed was to tweak the pot until the voltages on the outputs of the cathode followers were identical and Bob was your Uncle.

I'm starting a nice late night listening session. :wink:
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Cressy Snr
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#913 Re: The Latest Push-Pull Build

Post by Cressy Snr »

I don't know whether or not to do schematic, version 393 quite yet.
There's one or two things that mght raise a few eyebrows, such as the off centre op point on the input stage
that I haven't quite sorted yet. I'm 50V off from where the curves say I should be, due in part to incorrect screen voltage.
Last edited by Cressy Snr on Sat Feb 06, 2016 5:02 pm, edited 5 times in total.
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Paul Barker
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#914 Re: The Latest Push-Pull Build

Post by Paul Barker »

Wait 'till you ve fettled the feedback resistor.
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cressy
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#915 Re: The Latest Push-Pull Build

Post by cressy »

get a couple of 500k linear pots instead of the feedback resistors and tweak that way, or try that 500k stepper i gave you, it might have the right value somewhere around the 3rd step
these would do
http://www.ebay.co.uk/itm/Potentiometer ... SwNSxVKqxq
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