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Old 1st October 2012, 02:51 PM   #231
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I'm definitely NOT going to buy a box of 7,500, and I'm also NOT going to tell you where I'm not getting them from.......

And in any case, they can only be used in filament bias, and that requires large chokes in each of the DC filament power supplies for each tube to sound good, and you really don't want to go there. The chokes put out huge hum fields and can't be used with any speaker systems over 82db.

Personally I think the 12AX7 is very underestimated...... they're cheap and nobody knows about them and they'll be a ripping good investment when it dawns on people that they can also be used in stage amps.

Last edited by andyjevans; 1st October 2012 at 02:58 PM.
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Old 12th October 2012, 04:52 PM   #232
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Now listening to my previously 300b SET with the output stage changed to PSE 2 x 4P1L.
I have 245v on the plates, a 340 ohm cathode resistor and 19.5v bias which works out at around 30mA each tube, 60ma for the pair. About 225v a-k. That makes for around 13W dissipation for the two tubes. This is a rough-and-ready experiment, but the voltages aren't too far wrong.

So listening impressions. For a start it's considerably louder! More gain in the output. Sound is very neutral. It's all there - treble, bass, clean mids, very transparent. It's slightly lost that lushness of the 300b and a bit of the bloom in the bass, and I have to say that the 300b still has the edge in terms of tone. Sound is not quite so euphonic, now more towards a 2a3, and is pretty much up there with that kind of quality as far as I can tell.

The secret weapon here is filament bias for the output stage. I haven't started on that yet. It's quite easy to do and will eliminate the cathode bypass cap. Yum yum!! I make the cathode resistor for filament bias 16.6 ohm at 24 watt. This will get hot, so I over-rate x 3, so that looks like a 60 watt part at least. Not a problem. The filament supply will need to be 27v into Rod Coleman's regs. I run the 4P1Ls at 1.9v in parallel, so two at 600mA makes 1.2A. I'll eventually make the supply choke input. I want to see how far this can go.

Sitting here listening happily as I type. This works fine, and it will be interesting to optimise it. It's very strange looking at a row of 4 squat little tubes in place of the majestic 300bs and hearing what they're doing.

Last edited by andyjevans; 12th October 2012 at 05:05 PM.
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Old 12th October 2012, 05:13 PM   #233
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nice done ...
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Old 12th October 2012, 08:53 PM   #234
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Quote:
Originally Posted by DimZ View Post
I think that 4P1L prices will rise ....... soon ....
Still $5 shipped. But only 2 sellers left.

4 x 4P1L / 4LS1 / RL2 / 4P6 PENTODE. LOT OF 4 NEW TUBES | eBay
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Old 5th November 2012, 11:08 PM   #235
tcqanh is offline tcqanh  Viet Nam
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Hi Anatoliy,

You use Vak 220V, 50mA/ tube for 4P1L. So dissipation losses 220 x 0.05 = 11W. Is it safe for long life ? Data shows Pa max 7.5W, Pg2 max 1.5W.

Did you compare the sound between 2 connections: G3 strapped to anode and G3 strapped to kathode ?

Regards,

QA.
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Old 5th November 2012, 11:15 PM   #236
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I don't know about long life, but half an year of occasional usage they survived. However, it is far from enough for conclusions.
About 3'rd grid; I did not mention any difference, except when it is positively biased curves in triode mode as if shift left, so less of positive drive is needed on the 1'st grid for the same current.
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Old 6th November 2012, 12:50 AM   #237
tcqanh is offline tcqanh  Viet Nam
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Does G3 draw any current in triode mode with G3 connect to anode ?
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Old 6th November 2012, 01:51 AM   #238
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Quote:
Originally Posted by tcqanh View Post
Does G3 draw any current in triode mode with G3 connect to anode ?
I did not measure, because I doubt the current it draws is of any significance. G2, though, is quite dense, looking at pentode curves.
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Old 6th November 2012, 09:56 PM   #239
Ejam is offline Ejam  Australia
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Default Andy, Have you a Schematic to Post

Andy

Can you please post a schematic of your 4P1L amplifier.
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Old 28th November 2012, 03:53 PM   #240
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Sorry I've been late in responding. Here's my current setup. I actually have a different arrangement for the input tubes with glow tubes totalling 180v - 0A3 plus 0C3 - but a single 150v glow tube should be simpler. So my operating point is a little different. Current is always 15mA through the 126C. This is a remarkably cheap amp to build, though the filament supplies are complex. I have choke input DC supplies on both stages. Clean filament supplies are crucial with filament bias.

The output stage 4P1Ls should be matched for gain and filament draw. I tend to run all 4P1Ls at 1.9v wired in parallel. Cathode resistors should be the best you can find. I use combinations of NOS Dale wirewounds plus Welwyn W24 for trimming. Don't be tempted by aluminium clad ones or thick film or ordinary white ceramic. Good sound depends on the best possible parts here, together with the DC filament supply. I use all polypropylene caps in the PSU.

Hammond 126C should be wired IN: Brown>HT, Blue>Anode, OUT: Black>Grid, Green>Ground. If you reverse the outputs you'll get awful sound and wonder what you did wrong!

I prefer this amp to all the 300b SETs I built with the same input stage and with cathode bias and bypassed cathode resistors on the output. Filament bias on the output stage gives a very clean and detailed sound. Grand piano sound is incredible - this is truly an amp for pianists and lovers of the piano. I'm not afraid to leave it on continuously - the tubes are cheap enough to replace!
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Last edited by andyjevans; 28th November 2012 at 04:20 PM.
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