#26 pre amp

[Salas]

I am just briefed it works right from the start, no noise no hum when on the #26 heater. Disco has to listen in a couple of days in his proper system out of the lab for qualities, let it show if it will present any kind of issues or if its reliable too. He also has a current source special reg so its fortunate that he will be able to compare voltage and current methods at a point, beyond chip solutions.

In the pictures you can see the simple shunt, the trafo and the simple rectification and filtering, 1.5V trimmer set when about 1A was on the load and Vbe was 0.66V on the bootstrapped one Vbe Mosfet CCS controller BJT. 1.18A was running in total. The cables may look more than needed, but remote node sensing is included since heater cabling is always rather long.

''There's millivolt wandering (0,002V max) within 5 minutes, I guess because of parts heating'', ''The old 120MHz scope did not show anything more than its own residual noise'' Disco said to me for now.

[Richard]

change out that 7805 for a 7806

Hi Kevin,

Thanks for the tip, I had 7806s in there, but for some reason indicated 7805 on the schematic. I moved some of the wires and resistors around and got the hum down in 1 channel, I'm getting there.

cheers,

Rich

[coolzero]

[QUOTE=Richard;1995736]I now have a 3 chassis 26 preamp.

Voltages were all good, no smoke but I do have this bloody low frequency hum and some hissing noise. I officially feel like I’m part of the 26 problems club. A view of the wiring is also attached (Fig 4).

Hum could be because of the way those chokes were mounted close to the 26 tubes. I had a similar issue and it went away when I fix the chokes at normal standing position little away from the 26 tubes. I tried various positions of the chokes (even the way you fixed) but normal vertical standing got rid of this low frequency hum and hiss.

[coolzero]

Originally Posted by andyjevans

Here's some numbers from my latest attempt to get all the voltages right.

PSU HT = 365vDC
Dropper resistor to glow tubes - 3.2K, 9W
Glow tubes - 150v and 105v total 255v
IXYS cascode active load - sense resistor 470R
Anode voltage 178v
Cathode voltage 11.2v
Current 6mA
Cathode resistor 1.85K (2.4K // with 8.2K) wirewound

Seems to sound pretty good - full and detailed.

andy

Andy,

I adjusted my 26 pre with almost similar parameters. Sound fantastic as you said. Big difference from standard 135V-5.5ma config.

[martyh]

Back again with what is probably a silly question... Can I bias a 26 like this if I am using a floating current regulated filament supply?

Thanks Again,
Marty

[Richard]

Hi Coolzero,

Thank you for the suggestion, I’ll make the mods this weekend. I think you may have hit the nail on the head, it may explain the low hum during the testing stage. Also a lot of the other 26 preamps I’ve seen have a lot of space around that tube.

Martyh,

I’m not sure of that method of battery biasing, but as I understand it, one needs to have the potential difference (grid voltage) between the grid and the cathode. Have a look at Kevin’s write up on his 26 preamp there is a schematic of battery biasing there. Also, I have attached a schematic of a battery biased 01A-01A-71A amp by one of the forum members Joel. He uses a neat way of biasing. I hope it helps.

Cheers,

Rich

[Sheldon]

Originally Posted by martyh

Back again with what is probably a silly question... Can I bias a 26 like this if I am using a floating current regulated filament supply?

Yes. Make sure the battery can handle the current.

[kevinkr]

Originally Posted by Sheldon

Yes. Make sure the battery can handle the current.

NiCads are the best sounding in this cathode connected fixed bias application, but that said I actually don't like the way they sound. Forget NiMHs altogether.

I have also tried series battery bias in the grid circuit (battery - to grid, battery + to volume pot) but this does not sound all that great with the typical alkaline battery (-9V or so required) and needs to be bypassed with a good quality cap.

I recommend using conventional fixed grid bias on the grid instead and this can be provided by a 9V alkaline battery (+ to ground) and a ~100K grid resistor - of course a coupling cap is required and this should be of high quality.

Sometimes the simplest circuit approach is not the best - I have found many instances where placing a battery in the audio path sounded no better than any alternative and often worse. It is however very dependent on the quality of the battery, and some are much better than others. (I tried a few) I found the Sanyo Cadnica series batteries sonically superior to anything else I tried. (Their internal resistance is lower than most competitors) Note that for cathode circuit battery bias the battery should be rated such that in normal operation the trickle or floating charge current rating is not exceeded. (Typically no more than 5% or so of the MaH rating of the battery)

Note that in the 26 application you would have to stack the equivalent of 6 - 7 cells in series to get the right voltage, and IMO this will sound worse than a comparable number of LEDs in series or my preferred grid bias arrangement.

[Sheldon]

Kevin,

Any cap-o-phobes try something like a mosfet shunt supply under the cathode?
Very low impedance. Wouldn't even need an extra winding. Could steal some B+ from a center tap return.

Sheldon

[Salas]

There was a small amp once doing that by wrenchone.