The Latest Push-Pull Build

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

Post by Cressy Snr »

Bought half a dozen of these cute little Brimar top-cap grid pentodes.

These are cheap and plentiful and are the grid top-cap version of the Brimar 6BR7
which was the equivalent of the Mullard/Phillips EF86.

Image

I'll be replacing the 12SJ7s with these, when I rebuild the audio cct sometime this week.

There will be four on the Barbarik front end and the other two will be the front end of the mono amp
to be housed in the radiogram chassis.
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IslandPink
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#887 Re: The Latest Push-Pull Build

Post by IslandPink »

Cool !
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Thermionic Idler
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#888 Re: The Latest Push-Pull Build

Post by Thermionic Idler »

Good lord I didn't know Noval base valves with top caps even existed.
HT to the output stage has also been increased to 475V
How did you achieve this with the existing transformer - did you use the 75V secondary to make 525VAC into the bridge rectifier?
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Cressy Snr
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#889 Re: The Latest Push-Pull Build

Post by Cressy Snr »

I cheated with a small 3.3uF cap off the 450V winding, before the choke, so I suppose the HT supply is now, strictly speaking, cap input.
The 6BS7 is an unusual little valve isn't it.
There are plenty about. It wasn't a great success as an audio valve, though that was its original purpose (it was directly up against the EF86) but it eventually found a niche as an industrial valve in electrometer applications, such as pH meters and valve voltmeters.

More info: http://www.r-type.org/exhib/aab0164.htm
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Mike H
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#890 Re: The Latest Push-Pull Build

Post by Mike H »

Thermionic Idler wrote:Good lord I didn't know Noval base valves with top caps even existed.
http://www.r-type.org/exhib/aab0164.htm

Interesting blurb about the reason for the top cap!
 
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Cressy Snr
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#891 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Mike H wrote:
Thermionic Idler wrote:Good lord I didn't know Noval base valves with top caps even existed.
http://www.r-type.org/exhib/aab0164.htm

Interesting blurb about the reason for the top cap!
Indeed; it was designed for incompetent DIY'ers.
That's why I decided to use it. Right up my street! :lol:
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Paul Barker
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#892 Re: The Latest Push-Pull Build

Post by Paul Barker »

Well spotted as a valve find Steve. HAve you got it playing? IS it a good sound?
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Cressy Snr
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#893 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Have not got it playing yet Paul. That'll be a job for Fri, Sat and Sun.
I'll have to make some adaptor plates out of perforated steel to take the noval sockets, as the holes at the front of the chassis are octal sized and therefore too big. After that the adaptor plates and sockets can be fitted and the font end reconstructed.

The valves themselves look to be of extremely high quality. The anodes are bright nickel and the insides are built like the proverbial brick outhouse. I'm quietly confident.
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Cressy Snr
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#894 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Had a bit of a job on with the adapter plates for converting octal holes to take Noval sockets. It certainly took some thinking about as I do not want to ruin the looks of the chassis.
After much head-scratching and rejecting about half a dozen less than satisfactory solutions. I found four grotty old octal sockets that had seen better days and carefully hacksawed out the middles, freeing up their fixing flanges. The result is below:

Image

A satisfactory solution I think. Omce I get inside the amplifier, it is simply out with the old sockets and in with the new, using the same fixing holes. Nice little gap around the circumferences for airflow too. :)
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Mike H
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#895 Re: The Latest Push-Pull Build

Post by Mike H »

That is bluddy brilliant. Image

Me likes.
 
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IslandPink
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#896 Re: The Latest Push-Pull Build

Post by IslandPink »

Top notch !
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Dave the bass
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#897 Re: The Latest Push-Pull Build

Post by Dave the bass »

Shoddy!
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simon
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#898 Re: The Latest Push-Pull Build

Post by simon »

Impressive as ever Steve.
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thomas
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#899 Re: The Latest Push-Pull Build

Post by thomas »

Mike H wrote:That is bluddy brilliant. Image

Me likes.
+1! Very impressived!
Cressy Snr
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#900 Re: The Latest Push-Pull Build

Post by Cressy Snr »

Aye....It was one of those light bulb moments. :D

At the moment I'm on with rebuilding the front-end. I'll try to report on the results this evening, unless I fook something up.
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