#26 pre amp

[Vinylsavor]

Hi Andy,

nice job!

I agree with others that the 0,1uF cap is a bit on the low side unless your power amp has very high input impedance.

Why the 1k grid stopper resistor? 26s have low transconductance and low mu. You have to really try hard to get them to oscillate. No grid stopper necessary with those.

Thomas

[massimo]

Originally Posted by Vinylsavor

No grid stopper necessary with those.

Good to know!

What do you think about that operating point, Thomas?

[andyjevans]

I think the input grid resistor is 560K on my power amp - that or 270K, can't remember.

Yes, the 0.1uf is probably low. I have a larger FT-3 one which would probably fit in the space - just used what came to hand.

Keep the ideas coming! The 156C plate chokes are each 8mA and 150H, 3.7K resistance.

Yes - those cathode resistors. I went through about 8 different types before I settled on those - they came from USA and were on ebay. Redundant stock Dale I believe. Best I tried.

Andy

[dvb-projekt]

Now the main parts for my pre arrived today



Two Arcturus 126 and one Telefunken AZ1 mash. Rod´s filament supply also arrived.

Let´s wait to the results from AE-Europe to make a replica UTC HA-133...

[merlin el mago]

Looks very good.

How much will cost the AE-Europe replica?

[dvb-projekt]

I gave them all needed parameters and waiting to the results.
Perhaps they could make a bit better with actual materials

I am also planning a top version with silver secondary windings.

[coolzero]

Originally Posted by andyjevans

Who's using plate chokes in their 26 preamps? I've been playing about with four 156C Hammond chokes - 150H at 8mA - using them in series as pairs. I put them bottom-to-bottom and wired them out of phase since I read that somewhere. Resistance in series is 7K.

Sound is damn good! Quite surprising. I'm using Teflon FT-3 output caps and filament bias with 10 ohm resistors. 140v on the plate, 10v on cathode. Rod Coleman filament supply - his original one which was posted early on in these pages. This is a really cheap and effective way to build a 26 preamp, except that in my case I'm using choke input supply to Rod's boards with a 280mH choke, 2.7A, and 20,000uf caps.

Now I know that this works I can look at some more exotic plate chokes!! I was thinking output transformer, but Thomas is always telling me to have more inductance and it really does sound very clean.

Andy

Andy,

Plate chokes gives far better sound than any other plate loads I tried on my 26 minus output transformers which I haven't tried yet. I use EP 200H chokes with slightly hotter parameters than yours. Indeed my 26 pre gives great sound with all these including Rods/SALAS PSU's.

[dvb-projekt]

Originally Posted by merlin el mago

Looks very good.

How much will cost the AE-Europe replica?

Got the refusal from AE-Europe today. So no UTC HA-133 replica

Any concerns to use the Lundahl LL1660/5mA for the #26 with 150V B+ and -9V Bias?

[Bas Horneman]

Got the refusal from AE-Europe today. So no UTC HA-133 replica

That must be a bummer for you?

How about the Lundahl 1680. a Replacement built for the UTC ls-27

http://www.lundahl.se/pdfs/datash/1680.pdf

[andyjevans]

I have the LL1660/5mA. I used it in 1:1 to keep the gain. I think I slightly preferred a plate choke.

Andy