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#26 pre amp

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#923
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
 
#927
andyjevans said:
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
Click to expand...

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.
 
#931
-9v bias and 150v B+ is around 8mA on a 26 tube so I would prefer 135v, in order not to stess the 1660 at 5mA. If you use a SSHV and have the proper sinks, you can adjust easilly. I drop around 90v on mine. I used a plate choke and prefered the 1660 transformer. Mind you that I use 4:1 for the 1660.
 
#938
Hi!

I have not compared the different cores for all transformers and all currents.

If you intend to run the 26 at 5mA or less and you want to use the transformer only for the 26 linestage, go for the 5mA. If you want to experiment with higher currents or maybe want to try other tubes as well. The 10mA will still work well with the 26. I use the 10mA version in my linestage which can be switched between 26 and 801A. Since the 801A runs at around 10mA, I used the 10mA transformer. I mainly listen with the 801A anyways since I prefer it.

Thomas
 
#940
To Thomas and all others:
Have you tried the LL1692A as line output ?
The PP-Version with 5mA gap has 210Henry. This should be fine for the tube 26. The LL1660-PP has 290Henry.
I was comparing the data sheets of both transformers:
Both with a gap for 18mA in Connection Alt. Q in SE Line Output:
LL1660 Freq. Response: 11Hz - 35KHz
LL1692A Freq. Response: 10Hz - 55KHz
The LL1660 has 4.5 : 1 ratio and the LL1692A has 3.5 : 1 ratio. The 5mA gap version should have better Freq. Response than 18mA.
In this connection Alt. Q the LL1660 has a primary DCR = 1250 Ohm and the LL1692A has primary DCR = 690 Ohm.
On the site of K&K i have found, that the LL1692A has a resonance point that is moved up in higher frequency and has a lower amplitude compared to LL1660. No comment about sound.
What transformer should i use ?
Thank you. Johann
 
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