• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

#26 pre amp

its available on ebay, very expansive. look further i understand its more like 71a, you need source of gridbias voltage to operate it properly, so there will be input capacitor
Or an input transformer. I use an LL1676 in my 01A preamp with the bias voltage fed through the secondary. It isolates the grid bias voltage from the previous stage and the secondary also acts as a grid choke.
 
I just tried out 3 line stages into my PSE 4P1L output stage with O'Netics OPTs.
...
2. 2P29L into a single 126C used as plate choke, 0.2uF FT-2 coupling caps
...The 2P29L was closer to the 10Y in sound than to the 26.
...
If you want a considerably cheaper version of a 10Y line stage, use a 2P29L which will give you quite a lot but not all of the 10Y sound. It's also cheaper to use a single 126C as plate choke, and in fact this makes for a very good and very cheap choice if you like plate chokes. ...

Hi Andy,

I have noted this with interest. I have a pair of 126C and also a pair of ISO RC-160-15W that look to be a good match at 15mA, and eight 2P29L and it seems that I should try one of these combinations. What I do not have is data on the 2P29L valve...Ra, mu, and other pertinent data (other than Ale's mu tracer plate characteristics)...can you help with that? I'm interested in building a line stage.

Best, Robert
 
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Hi Andy,

I have noted this with interest. I have a pair of 126C and also a pair of ISO RC-160-15W that look to be a good match at 15mA, and eight 2P29L and it seems that I should try one of these combinations. What I do not have is data on the 2P29L valve...Ra, mu, and other pertinent data (other than Ale's mu tracer plate characteristics)...can you help with that? I'm interested in building a line stage.

Best, Robert
Hi Robert,

http://www.vt52.com/node/113

2P29L Ra, mu.jpg
curve_2P29L_triode.jpg
 
Hi Andy,
What I do not have is data on the 2P29L valve...Ra, mu, and other pertinent data (other than Ale's mu tracer plate characteristics)...can you help with that? I'm interested in building a line stage.
Best, Robert

This is what I have. You should be pretty happy with it. Using a choke allows you to put some current through it, like up to 15mA. I like good chokes!

For stocktrader: How is this better than 12AX7 or 6992? You would have to listen to it with your own ears. A lot of us build with directly heated triodes or trioded pentodes because we hear a nicer sound. If you're following this thread you probably have some curiosity to try a DHT.
 

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2P29L Triode Anode (or Anode + Screen?) Dissipation Limit?

Ale's muTracer anode characteristics plot (Attached) shows a power limit locus to be at 2.7W. The two attachments in euro21's post #5145 show a Pa of 2.0W. Is the difference between the two the dissipation of the screen?

Thanks for any elucidation you might provide...it does make a difference to selecting a design point. Best, Robert
 

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I use these diodes but there are other SIC diodes available. Voltage drop is 0.8v at these low currents like under 10mA. No need for any heatsink. I can't guarantee that SIC diodes sound good with all tubes - it's very tube dependent, some like it some don't to my ears, no idea why. I think adding a smallish resistor helps because they can sound a little aggressive - harder leading edge to sounds. They do have other virtues, though.

Screen Shot 2022-03-24 at 20.49.19.png
 
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I use the same ones, if I remember well 5 diodes per tube to have near -6V at grid. I want to try the resistor trick to know how it sounds what don't know the resistance value nor the power rating needed or better how to calculate?
It's just a question of experimenting using Ohms law. I haven't listened closely to alternative values of resistor. As a general idea I'd try something between 50 and 100 ohms, then listen to it and see.

If you are using filament bias you'd get a very different value resistor from standard cathode bias. You probably don't need to use a diode in filament bias, but you could try it.
 
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I'm measuring with my new toy DVM Brymen 869S the B+ AC ripple, but the voltage jumps to 900mV & drops to 700mV always and don't be stabilized with time also measure some weird frequency not 0Hz, the PSU it's a simple bridge rectifier 1000V 25A with a capacitor 820uF 550VDC. Of course no AC ripple when load isn't connected. I expected an AC ripple like the PSUD2 attached simulation. TIA for help & support.
 

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