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Here's my 26 circuit.

2.126.106.47

In Reply to: RE: Help with a 26 preamp. posted by amnesiac on October 22, 2023 at 07:40:58

Lately I've been using a 26 stage to drive my EL12n finals in a 2 stage SE. So I've been doing a lot of work on it, and pretty happy now. I attach the schematic. What I've learned in many revisions is this:
1. Rod Coleman filament regs are the way to go. I have V4 but V9 is the new version, needing only 3.5V headroom. V4 was 6.5V
2. For me resistor anode load gives the purest sound. This needs to be at least 39K or 47K better, otherwise the bass is rather loose. You want around 5.6mA through the tube so you're looking at a B+ of around 380V ideally.
3. The anode resistor really matters. The best NOS wirewound of at least 3W is what you need, like vitreous enamel.
4. In filament bias the cathode resistor will get hot, so mount it above the top plate otherwise the insides will cook. Again, a good wirewound is needed.
5. You can use a 12+12v 60VA filament transformer, or better still a 15+15V at 100VA
6. You need a good heatsink again mounted above the top plate or on the rear panel.
7. Output caps are preferably Russian teflon like FT-2
8. PSU caps should be DC Link for first and last caps. All rated 500V +
9. 1200V Schottky diodes work nicely for rectification. Cree etc.
Different concept in my build, no interstages. Took me a long time to modify this circuit to my satisfaction. Result is a lot better than a 6J5. I also prefer a CV1135 to a 6J5 and I like the 6AH4 too. But a 26 will be a step up. I like the ST shape ones.
Andy

Edits: 10/22/23 10/22/23

Follow Ups:

RE: Here's my 26 circuit. - amnesiac 08:41:45 10/24/23 (1)
Hi Andy So no preamp at all just two stages? And a pentode! your kit has changed in ten years since I have been away. I still have got my ten y amp but it's got a few millivolts of hum enough to hear with aircon off. When I put my chinese mx50seamp I think its called Its just a simple class ab amp a copy of a british diy magazine amp. I bought the boards and built it up with lots of good parts like russian paper bypass caps. Its ok but its silent I dont want to go back to any hum so I might end up with a idhp final too.
I have also built a 600hm /5k magnequest input trans to 10y electraprint choke driving a cap and a grid choke with a 2a3se electraprint amp finished. Its a beautiful build but I'm not confident it will sound good with the input trans. And with the grid choke anything could happen. I need an oscilloscope to test that.
I need to drive an f7amp too. I'm tired of low power amps. I want a loud system even if I have to go buy something even.
For this 26 preamp I got some 600/1200 ohm input trans Iam wiring in. That will give me the gain. I will go through with it if no one else has got any opinions on whether the lundahl trans have sufficient inductance.
I have been using a 6ak5 driving a resistor loaded 12b4 with a stax 8ohm trans with a tube reg but it just went bad and I'm doubtful if the stax trans has the bandwidth. I think when I went to the solid state amp the preamp couldn't swing enough volts somewhere. I want to build a preamp that will be dead quiet and drive any amp. Sound quality is secondary. I have a ten y preamp 90% finished too but just these regs. I will try a 2c22/6j5 if the 26 sounds bad. maybe I try a 2c51 I don't know. That's what I originally bought the trans for.
What made me reevaluate everything is getting into in ear monitors. I have some good ones and they are exceptional (penon serial)they have shown me my hifi just isn't good enough. If I can get sound that good from a phone a dongle and a headphone amp I don't need tubes to get good sound I Am prepared to make any compromises needed to up my sound quality.

Edits: 10/24/23
- IDHT choices - andy evans 10:57:26 10/24/23 (0)
  I think I mentioned that my 2 favourite IDHTs are 6AH4 and CV1135. The CV1135/CV6 is better sounding than 2C22. Beware also that it has the anode and grid top caps the other way around.
  
  Mu EL12n outputs are in triode. Gain in triode is x18 (mu=18) which gives me an easy life driving it with a 26. So only 2 stages.
  
  The 10Y sounds wonderful in filament bis with Rod's regs, but it's a more complex build than a 26. These days I use DC Link caps in the power supply and a large value choke, like Hammond 157G, 30H.

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