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300b all-DHT SET
300b all-DHT SET
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Old 22nd August 2012, 07:40 AM   #11
gaborbela is offline gaborbela  Canada
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Nice and very clever. I'll save your schematic.
No coupling caps....
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Old 23rd August 2012, 11:22 PM   #12
Ejam is offline Ejam  Australia
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Default Power Supply Details


Can you please give some details on your power supply or supplies for the amp?
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Old 24th August 2012, 11:38 AM   #13
palmas is offline palmas  Portugal
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Originally Posted by radiostar View Post
Nice project!!!

I need to add one stage to my 300B 10Y DHT SE Amp, but here is hard to find 4P1L ...

Thanks for idea Andy!
Not that difficult, I even beleave I have a pair or two...
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Old 24th August 2012, 01:06 PM   #14
andyjevans is offline andyjevans  United Kingdom
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Power supplies:
300b filaments: 2x9v transformer, Schottky diodes, 20,000uF caps, Rod Coleman filament supply board, available from him
4P1L filaments: 2x24v transformer, Schottky diodes, 280mH choke (2.4A), 20,000uF caps, Rod Coleman filament supply board

Amp HT: 390v secondary on mains transformer, 2x 6BY5G damper diodes in parallel in a hybrid Bridge, 10uF cap, 5uF choke, 80uF, dropper resistor, 80uF. All caps polypropylene.
4P1L driver stage: 180v glow tubes (voltage regs) off same HT

Preamp HT: 230v secondary on transformer, 80 rectifier, 2uF input cap, 10H choke, 80uF cap, 10H choke, 40uF cap. All caps polypropylene.

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Old 24th August 2012, 01:42 PM   #15
Rod Coleman is offline Rod Coleman  United Kingdom
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300b all-DHT SET
Thanks Andy.

Filament Bias can work very well - all amps sound better without the electrolytic capacitor bypassing the cathode resistor. The payback is that the filament supply must be VERY quiet - the current noise in the heating supply is directly impressed on the cathode resistor AND mixed into the cathode current within the filament itself - as dictated by Kirchoff's Law.

I can prepare special high voltage versions of the Filament Regulators that can handle the ~25V rectified dc supplies noted on Andy's circuit. They are self-assembly kits. These are all ultra-low noise and have caused a lot of surprise by how much they improve the sound in any DHT - (see 'New DHT Heater' thread, last few pages)
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Old 24th August 2012, 02:49 PM   #16
andyjevans is offline andyjevans  United Kingdom
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Location: London
Hi Rod,

I credited Rod and Thomas Mayer because they've really put filament bias on the map. Thomas revived it from some dusty archive which everyone else had forgotten and made it work using chokes in the filament supply - excellent idea, and I use choke input in my filament supplies to the first stages. Rod happened to have developed a really good DC filament supply at about the same time and then quite a few people including myself started using filament bias with Rod's nice clean DC supply with their 26 preamp projects. It's pretty ideal for a 26, but it's really good for a 4P1L as well - this is where Ale, Anatoliy and some others came in and convinced us to use this rather forgotten Russian tube that was originally an old Grerman Wermacht design. Though it must be said that Klaus (klausmobil) had circuits for the 4P1L driving 6C4C dating back years and years if anyone had the sense to try them out. Turns out it's just brilliant, and it's even kicked out my beloved 26s. The combination of all this has taken the sound of my amplifiers up a whole level.

It's funny that every now and then you think you've reached the end stop with building an amp, and then a whole lot of new ideas come out and you're jumping with joy again! Never ceases to amaze me that there's always something better round the corner. And most of all this tube stuff dates back to the 1920s. Astonishing.


Last edited by andyjevans; 24th August 2012 at 02:54 PM.
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Old 24th August 2012, 08:06 PM   #17
mogliaa is online now mogliaa  United Kingdom
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300b all-DHT SET
Well done Andy!
This reminds me when I did my first side by side comparison between my 4p1l preamp against the 26. I preferred the 4p1L but soon tried to convince myself that my 4p1l design and build was improved from my original 26.
Definitely will incorporate some of your ideas in next designs...
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
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