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#26 pre amp

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andyjevans

andyjevans

Member
Joined 2003
artsandmedia.com
merlin el mago said:
I ask because I'm not happy with my 26 sound, I'm using resistor filament bias & 1660 OPT with SSHV2 & Rod Coleman heaters.
Click to expand...

How "not happy" are you? That's a good and well tested combination - 26 into LL1660 in filament bias. It shouldn't underperform unless there's something wrong with the build. And it depends what you're driving.

I think you can go better. I've built several DHT line stages and my current favourite is Ale's 01A Gen 2 preamp. This is the best I've ever heard, and if there's anything wrong with that design it's going to be the build or what it's driving.

I would recommend you buy a Gyrator off Ale (Bartola Valves), and use it with something like a teflon FT-3 output cap. It will work with either a 26 or a 01A. It even works with a 4P1L at 25mA. It won't be hugely better than the 26 into LL1660 but it could be audibly better if you are a very critical listener.

One other thing I should say is that if you're not happy with filament bias, improve the filament supply. That is definitely audible. I only use choke input supplies because I can hear a difference in smoothness. You want something like a Hammond 159ZC. You might get away with a 159ZA if you starve the filaments down to 850mA or so. But go choke input for sure. Same with the 01A Gen 2 preamp. The 159ZA works for that as well.

A choke input filament supply will do you more good than playing about with battery bias, in my opinion. Do it on your 26 preamp and see how it sounds.
 
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merlin el mago

merlin el mago

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Joined 2009
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These?
 

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andyjevans

andyjevans

Member
Joined 2003
artsandmedia.com
merlin el mago said:
These?
Click to expand...

Yes, exactly those. Use a BF862. They're surface mount but if you're careful you can solder them under a magnifying glass with a fine tip iron.

You need 200nF on the output - that will give you more body than one. Use 2 caps in parallel. I use FT-2, saves space. I use a FT-2 100nF cap in the gyrator with the recommended 10Meg resistor. I also tried some Soviet 100nF silver mica caps - square brown ones. They were good but I preferred the teflons. Some prefer the silver micas.

I've also been playing around with filament resistors - currently using some Soviet ones. I posted on this in one of the threads - not sure which, maybe the 01A one.
 
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