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    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

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#862
Vinylsavor said:
Hi!



Not optimal. That would load the tube too much. The point of transformer coupling (among others) is to give the tube a high impedance load to achieve a load line close to horizontal. I like to load a tube with 5 times it's rp or more.

In your case that would be less than 2 times the rp

better would be a 9:1 output trans like Lundahl LL1689

Thomas
Click to expand...



Hi Thomas, thanks for your comments.

Lundahl describes the winding of the LL1689 as 9 : 9 : 1 : 1 : 1 : 1 . Is it possible to hook up the output taps to get 4.5:1 for the pre-outs and 9:1 for the 600-ohm headphone outs?

Is there a multi-tapped OT that could feed the newer headphones, which are often low-impedance and high sensitivity. Say, 30 to 100 ohms. I was originally thionking of a 6as7 for the output, since 30 ohms might be too much for #26???

Hi Thomas, thanks for your comments.
 
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#865
Erwin, Could start with Kanishka's post at:

http://www.diyaudio.com/forums/tubes-valves/151421-26-pre-amp-12.html#post2264317

His design uses a 200 Henry anode-load choke. This is an important part of the design, and must be a high quality part, with most of the inductance available at 12 to 15mA, say.

The grid is biased with a 9V consumer alkaline battery - Duracell or Panasonic powerline.

The filament can be heated with constant current [dc], and will sound better with one of my filament heat kits for #26. Don't use ac heat on a 26.

Whether this is the right schematic for you depends in part on what you are driving:

- long cables? with high capacitance?
- power amplifier input impedance?
- how much signal does the power amp expect? Can you alter the gain [DIY valve amps].

With long cables, transformers provide a good solution, but the cost is high for high quality items made by Lundahl or Sowter.
 
#869
No but planning on trying the salas shunt,
Click to expand...
Hi Regal,

The Tentlabs bias control has a different function to the Salas shunt. It is to bias a tube. Very useful if you have a push pull amplifier. Or a something like a dht preamplifier that is "universal" so you can use many types of tubes without having to change the cathode resistor. Etc. Etc.

In that sense this would be a competing product: DIYHS Active Bias Supply Module (Single pcb to Control 2 Power Tubes) | Diy HiFi Supply
Diyhifisupply's bias control.

Would be great if someone designs a diy version so us frugalphiles can try them 🙂 Hint hint nudge nudge wink wink.

I had an article by David Baron in my diymag on just such a thing. But it looks suitably complex that a diy version may not be interesting. And I suppose it might not even be cheaper.
<:cop: redacted basaudio links>

The salas shunt reg is soon te be implemented in my power supply
 
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#870
Bas Horneman said:
Hi Regal,

The Tentlabs bias control has a different function to the Salas shunt. It is to bias a tube. Very useful if you have a push pull amplifier. Or a something like a dht preamplifier that is "universal" so you can use many types of tubes without having to change the cathode resistor. Etc. Etc.

In that sense this would be a competing product: DIYHS Active Bias Supply Module (Single pcb to Control 2 Power Tubes) | Diy HiFi Supply
Diyhifisupply's bias control.

Would be great if someone designs a diy version so us frugalphiles can try them 🙂 Hint hint nudge nudge wink wink.

I had an article by David Baron in my diymag on just such a thing. But it looks suitably complex that a diy version may not be interesting. And I suppose it might not even be cheaper.
<:cop: redacted basaudio links>

The salas shunt reg is soon te be implemented in my power supply
Click to expand...

Hi Bas,

I've been working at a solution to the PP biasing problem, too. Maybe I can aim for the same ballpark of cost as the filament regs - do you think that will be frugal enough??

I don't like the microcontroller idea much. Won't it get a virus?
 
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#878
Oliver,

It is true - the Filament supply should be floating, and then ONLY connected to Ground at the tube socket.

Here's my view of the general schematic for Transformer Line Preamp - use the transformer type (Lundahl, Sowter) to suit the DHT you are using.

There's more details on connecting the Filament Supply in the PDF Manuals - sent today!
 

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#880
Well, I finally tried out battery bias on the grid instead of filament bias! My impression is that I prefer it to filament bias.

That's something I need to know - it'll certainly make my life easier - no need for high current resistors and large filament transformers. It's not night and day - filament bias is good when it's really clean with choke input etc. So I'll need to do a bit more comparisons to be sure.

Right now I have battery bias on my 10y preamp into a Hammond 126C transformer. My 26 preamp with LL1660/5mA still has filament bias, and that one sounds better than the 10y preamp when it had filament bias. So next need to try the 26 preamp with battery bias.

Any preferences for an input cap? I'm using a 0.1uF Russian silver mica. I also have the Russian teflon ones of various kinds I could try - thinking FT-2.

Andy
 
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