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Old 18th July 2012, 08:57 AM   #211
mogliaa is offline mogliaa  United Kingdom
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Sorry Lars, missed that: Gyrator!
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Old 18th July 2012, 04:59 PM   #212
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Quote:
Originally Posted by 316a View Post
I did a 4P1L---4P1L SE DC coupled amp which suffered quite bad microphony , if the chassis was tapped there would be a noticeable 'poing' that lasted a few seconds , sometimes when I hit pause there would be a bit of a poingy sound too . I find as output stages microphony is fine but as linestages microphony poor although I do use 98dB speakers so just about any hum/buzz/filament clanging comes through . Running the fils under 4v seemed to help a little to tame the microphony

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Your setup might be experiencing oscillation.
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Old 19th July 2012, 01:50 PM   #213
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Right - some results! This for a projected driver stage using the Hammond 126C interstage. I don't have a 126B so higher currents than 15ma not available right now. This is in filament bias

Vp = 160v
Ip = 13mA
Rk = 20 ohms, 20W
Vk = 13v
Vfil = 1.9v
Ifil = 580mA
Supply voltage to filaments = 15vDC

Pins 2+3+4 connected to plate, filaments from 8 and 7+1

Sounds very nice - full bass, plenty of detail, very lifelike. Not too microphonic in my system - not an issue.

This is using it as a preamp. Next step to install it in a 300b SET and see what it does.

Andy
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Old 19th July 2012, 04:32 PM   #214
mogliaa is offline mogliaa  United Kingdom
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Nice one Andy. I did run my preamp's filaments starved at 500mA which minimised significantly microphonics and distortion. Can you do a quick check (if possible) to get your impressions on sound compared to your latest design? Should have a slight impact on the bass in theory...

Keen to get your impressions.
Cheers,
Ale
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Old 19th July 2012, 05:37 PM   #215
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Bass is deeper at 13ma than at 7mA. Just installing the stage in my 300b amp today - hoping to listen soon and will report results. It will be 26 filament bias with 2x 156C and teflon coupling cap into 4P1L into 126c into 300b. Actually, I'll probably just connect up the 4P1L to the 300b in the amp and then try different preamps.

Andy

Last edited by andyjevans; 19th July 2012 at 05:42 PM.
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Old 19th July 2012, 08:38 PM   #216
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Well - 300b SET is operational. Basically it sounds very good, though far from optimised. I now have a "generic" sound of 26 into 4P1L into the output stage. So I can compare the SET with differential pairs of 26, 4P1L into 6C4C/2a3. I'm happy with this combination of tubes, though it's possible that another 4P1L stage may replace the 26 input. There's arguments for each - 26 more silky, 4P1L cleaner but not by much. I'm a bit suspicious that I slightly prefer the 4P1L.

The differences between SET and PP above are exactly what you'd expect - 300b is a touch nicer on strings (vln, cello) and female vocals. PP is better on bass and cleaner in orchestral climaxes. My speakers could do with more than the 18W plate dissipation of the 300bs at present - they're running at 330v p-k. So I don't know where the overload is coming from - intermodulation or just plain not enough watts. The 4P1Ls are running at about 12.5mA and that could be higher but not into the 126Cs.

But the basic sound is there to be improved on - very promising and now replaces the previous 46 driver stage, which wasn't in filament bias. This is and that makes a difference. So yes, it can be done and I think can be done well.

Incidentally, 7N7 has done all the building and a lot of the operating point calculations here so he's very much part of the whole project.

Andy
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Old 19th July 2012, 09:12 PM   #217
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Andy, how can you prefer 4P1L, if it is much cheaper!? Can billions of flies do the same mistake in food preference?
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Old 19th July 2012, 09:32 PM   #218
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Flies like dead meat - the 4P1L is so full of life it would scare them shitless...

Incidentally - has anyone ever seen a RL2 4P6?

Andy
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Old 19th July 2012, 09:55 PM   #219
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by andyjevans View Post
There's arguments for each - 26 more silky, 4P1L cleaner but not by much. I'm a bit suspicious that I slightly prefer the 4P1L
Interesting feedback, this reminds me of a conversation we had about a year ago at your place . Good to have Paul around, projects are coming out really fast!
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Old 19th July 2012, 11:35 PM   #220
7N7 is offline 7N7  United Kingdom
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Thanks for the credit, Andy and for your nice comment Ale.

I'm enjoying all this R&D, and delighted to have become acquainted with the excellent 4P1L, the first small directly-heated valve I have come across that has a creditable specification (ducks...) with reasonable mu and good gm.

I should like to know how far one can push the upper voltage limit; with its beef and good gm it would make a superb driver for a big power stage if it could be persuaded reliably to swing sufficient voltage. 845 would be out of the question, but triode-strapped 813 at 900V and 100mA requires only about -75 - -80V bias.

The only other small DH valve I know of that has an interesting specification is YL1130, again relatively new to me, but it does not have 4P1L's linearity; I suppose it might though make an interesting differential pair, but then 4P1L makes an excellent diff. pair!

7N7
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