steve
Rank: Trio
Posts: 206 
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Post by steve on Jan 12, 2016 15:12:07 GMT
JBL Paragon Jeez You're not asking a lot then Martin  |
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Post by MartinT on Jan 12, 2016 15:19:00 GMT
 Wouldn't it sound good driven by one of your valve designs, though? |
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steve
Rank: Trio
Posts: 206
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Post by steve on Feb 7, 2016 16:37:02 GMT
 Here is the latest version of my push pull amplifier. The input stage has been changed for some cute little Brimar small signal pentodes with top cap grids. A pair of double-triode, cathode followers, now drive the output stage. It sounds even better now. |
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Post by MartinT on Feb 7, 2016 18:04:22 GMT
I've never seen such small valves with anode caps!
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steve
Rank: Trio
Posts: 206
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Post by steve on Feb 7, 2016 18:30:58 GMT
Yep, they are certainly a rare configuration. The top caps are actually the grid connections, so no high voltage on them. Here's a bit more info on them: www.r-type.org/exhib/aab0164.htmThey were high gain pentodes, and designed for incompetent customers apparently They were not widely used as being seen to be using them would have labelled you as incompetent. Consequently there are plenty about and they are cheap |
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Post by MartinT on Feb 7, 2016 18:41:06 GMT
Grid caps, then? Bizarre! Why for incompetents?
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steve
Rank: Trio
Posts: 206
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Post by steve on Feb 7, 2016 18:50:02 GMT
Who knows Martin.
I think it was something to do with the pinout of the 6BR7 sister valve putting the grid pin in a hum field, which would have required skilful heater wire twisting and dressing, so they brought the grid out on a top cap to make it easier for builders to get a hum free first stage.
Backfired on Brimar because nobody would have wanted to appear incompetent by putting them in a hi-fi design.
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steve
Rank: Trio
Posts: 206
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Post by steve on Apr 22, 2016 11:10:50 GMT
So, here is my next project:  Basically it is a monoblock version of my push-pull pentode amp. Small case in the middle is for a silver-wired passive control unit incorporating a Khozmo stepped attenuator, feeding Sowter phase-splitting transformers, to give a balanced output. This will then feed the balanced signal to the L/R monoblocks. All the cases will have polished walnut end cheeks. |
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Post by MikeMusic on Apr 22, 2016 12:03:22 GMT
This is where I sit back and wait, having neither confidence or competence to go further Waiting.....  |
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Post by Sovereign on Apr 22, 2016 16:40:26 GMT
As Mike said, looking forward to your developments
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steve
Rank: Trio
Posts: 206
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Post by steve on May 2, 2016 17:42:46 GMT
Power amp cases have been given oiled walnut front and rear panels:   Next job, when the transformers arrive, will be to punch out the top plates and start fitting all the ironwork and valve sockets. |
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Post by ChrisB on May 2, 2016 18:18:26 GMT
Very nice Steve. I like the mirror image layout, even down to the red and black speaker cable terminals.
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steve
Rank: Trio
Posts: 206
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Post by steve on May 9, 2016 17:40:38 GMT
Chassis' have been marked out:  |
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steve
Rank: Trio
Posts: 206
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Post by steve on May 9, 2016 17:42:22 GMT
Below is a rough idea of the mirror-image layout of the monoblocks:  |
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Post by MartinT on May 10, 2016 5:38:06 GMT
I love seeing projects come alive.
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Post by speedysteve on May 10, 2016 12:42:25 GMT
Looks great! Real sense of achievement.
Takes me back to my amp build.
Led by one of the guys at work about 20 were built.
Chassis were cut and pressed in a small Swedish metal work factory for the exchange of a built amp.
I brought them over special baggage back when I had an SAS gold card:)
6SN7 driver/splitter tubes, KT88 power tubes. Lundahl output xformers and a massive 15kg power transformer ' the Heffa clump ' which gave the amp series their name.
Dual mono apart from sharing the same power transformer.
Ultra linear soft start silicon power circuitry.
Pure class A. Got very warm.
We soon realised that dual power supplies were needed!
Built in 2002, still in daily use today, but not in my system. I sold it on a couple of years ago.
I look forward to following your story.
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Post by MikeMusic on May 10, 2016 14:01:44 GMT
Like a time lapse. Love seeing it come together
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steve
Rank: Trio
Posts: 206
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Post by steve on May 12, 2016 14:12:47 GMT
Chassis' are drilled, punched, and grommeted up. Transformers can now be mounted.  |
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steve
Rank: Trio
Posts: 206
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Post by steve on May 15, 2016 13:14:06 GMT
OK... All the external hardware is now mounted. I've stuck some valves in to give an idea of how it'll look when completed. Valve between the 2 x KT88 is the output stage screen grid voltage regulator. EL34 will be used as the output stage in the final amp. Valve at dead centre is an E182CC cathode follower, for driving the output stage. Two front valves are Brimar 6BS7 input pentodes. The pot is an adjuster to set up the two input pentodes so that their screen grid currents can be balanced. This brings their static operating characteristics into line with each other.  |
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Post by MartinT on May 15, 2016 15:38:54 GMT
I like EL34s but I've never heard a particularly detailed output stage using them. Better than the harder 6550s though, I think.
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