#26 pre amp

[Rod Coleman]

Originally Posted by mogliaa

Rod,
Looking for some help here. Attached is the simulation of the 26 DHT with mosfet source follower as discussed. Frequency response is better than a 26 with a 300H choke. However, distortion is slightly worse 0.07% against 0.05%. This is due to the gyrator which I can't set properly due to the HT level I guess. I think if I were able to increase the HT above 200V should be able to provide headroom to the PNP to enable bias at 170V and 6mA.

Cheers,
Ale

Hi Ale, Yes, it is short of voltage. C2 should have about 10V across it to set a stable current in the P-FET, and The FET needs about 20-30V of Vds, minimum. When the drain-source voltage is too low, the current source shows a lowered dynamic impedance, which makes a bad current source. The LED string needs to increase to at least 25V for good CCS behaviour.

The 6Ж51П circuit looks really good!

[andyjevans]

Ale -

Thanks for the starved filament data! I'm reviewing the filament voltages in my amp. I just finished a new one tonight and listening to it. It's pretty damn good! First stage is 46 in unbypassed cathode bias (2.1K, 24v) into a Hammond 126C. This is an old favourite combination. Running the B+ here from a couple of glow tubes VR75 and VR150, giving 218v on the anodes. The 300b has cathode bias 1.3K with 100uF ICW PW polypropylene bypass, 66v right now and 388v on the anode. Running about 50mA but I can push this up since OPT is LL1660/80mA.

I'm a little short of volts in the power supply - about 400v HT under load. I'm using two 6BY5A double damper diodes with 3.3H chokes at 300mA and a bunch of 40uF motor run polypropylenes. Input cap is a 10uF polypropylene - about the max for a 6BY5. Mains transformer is 366v AC on the secondary. 400v AC would be better. I can see a super new power supply coming! Also need to convert to Rod's regs - using LM1084s right now. But sound is extremely detailed and nice smooth treble. The usual seductive 300b SET midrange! Listening to a bunch of female singers - what else with a 300b SET!

Happy tonight.

Andy

[dhtrob]

Hi there,
Just modded my existing '26 linestage into the filament biased version.
Having built and auditioned cathode biased, battery biased and led-biased '26 preamps, my ears are convinced this is the way to go..
Great, thanks to you all for sharing!
I use Tribute interstage 2:1 in reverse as load and power supply is a capacitance multiplier. As filament supply a C-L-C-R-C and VCCS is used. '26 is biased at 4.7Volts and 110Volt on the plate.

Worth mentioning: I experienced some serious noise (hiss) at the output of the preamp which came from the used 12Watt resistor which became too hot.. Replaced it for a 100Watt type and now the noise is almost gone. Guess paralleling is the way to go here? Anyone tried graphite resistors?

If interested I can post diagram and/or photo's.. :-)

Thanks, Rob

[Bas Horneman]

If interested I can post diagram and/or photo's.. :-)

Your post is worthless without pictures and schematics

[pieter t]

Originally Posted by Bas Horneman

Your post is worthless without pictures and schematics

And make sure that the brand logos are clearly visible

[dhtrob]

Hi Bas -- LOL

Posted as attachments some interior shots, with the small bias resistors and the larger ones.. and the diagram..

please comment!

Rob

[pieter t]

Not good Rob...try again....

[Bas Horneman]

Hi Rob,

Would you like to try my Rod Coleman filament supplies?

Kind regards,
Bas

[dhtrob]

Moi Pieter,
See attached picture..
Please elaborate on "not good, try again"? Help this novice out...

Bas. Might be interesting! Can I fit them in? Are they *big*?

Rob

[pieter t]

Nice Rob, But my wife wonders why there is no gap between the transformers.... shouldn't the sound be able to creep through? and how about cross talk?? OK we're a bit "melig" here...