Help me finish up this SET project (multiple questions / ramblings)

[zobsky]

I've been working on this on and off over the past year or so and it finally works. It has a couple of issues and there are some improvements I'd like to make to it, hence this thread. Some of my questions are naive so bear with me.

It's a single ended 6bq5 pentode driving a single ended 46 tube for a little over a watt output. I may consider switching to a 2a3 if i feel the need for power when I'm done optimizing this but for now, it's plenty for a rather small 8 x 10 room driving 6.5" fostex full rangers.

A picture:



Questions (mostly on the driver stage based off Thorsten's legacy amp http://www.fortunecity.com/rivendell...cy/Legacy.html , .. the power stage is quite conventional, as is the C-L-C-L-C power supply ):


PART 1 !!

1. With zero signal, I can get the hum down to practically zero by adjusting the hum pots on the power tube heaters. But as I open up the volume control with / without any source connected, I seem to pick some hum up. Would this be stray PS / heater hum, or should I look at my circuit some more?

2. The original SV83 driver maintains the screen considerably below the plate voltage . I'm using a 6bq5, .. which I have recently come to understand is typically run with the screen much closer to the plate voltage, as opposed to the SV83 which is run with the screen at a much lower potential.

The voltages I've measured on the driver stage are
B+ 295 V
plate 221V
screen 93V =========> does this need to be increased to something near the plate voltage?
cathode 3.5V


I've managed to find some rules of thumb about designing pentode drivers here.
http://db.audioasylum.com/cgi/m.mpl?...ubediy&n=71983

The three guidelines mentioned here are
i. Use much less than nominal anode current, try 1/2 to 1/3rd of the current given under typhical application as "Class A Amplifier" in the datasheet. =========>i'm a bit confused how to determine this on a pentode curve. On the datasheet, .. the current in pentode mode is 48mA. Going by this rule, we're need between 16 and 24 mA. Looking at the curves and locating my operating point bias of 3.5V , it would appear that my operating points and bias are quite hopeless ) . Any suggestions ? The curves look a bit better for lower screen voltages (though still not great) but I couldn't find any curves for the 100V point I'm using.



However, looking at the valve Thorsten used (the SV83), the bias value of 3.5V suddenly looks much better, especially with the B+ value of 400V used to get past the kink in the curves.


Also, the curves look much moer linear. I may just have to try using these valves instead of the 6bq5 (esp. since they're so cheap). Any comments?

ii. Have around the Bias of the output valve between screengrid and anode. ==========>i think this is satisfied by my design (output valve cathode bias is around 34V)

iii. Have around twice the output valve bias across the anode load ===========> what does this mean? Does it mean I should drop around 68V on the 6bq5 load resistor?

[zobsky]

PART 2 aka simplification / optimization:



1. If I end up using a bias of between 3 and 6 V (depending on which tube I use), would a battery below the cathode work in this application? Or perhaps a string of LEDS? How about zeners ?

2. I cut out 2 empty sockets when I built the chassis. I was thinking of fitting in VR tubes to regulate the screen voltage. I initially thought of something like stringing 2 VR150/0D3 http://www.nj7p.org/Tube4.php?tube=0D3 in series and tapping off 150V for the screen and 300V for the driver B+ (after bumping up the B+ a bit by increasing the first filter cap). The issue I see with this (if my calculations of 18- 20 mA per channel + the screen current are correct) is that I get dangerously close to exceeding the 40mA current capability of the tubes.
a. Is it as important to regulate the plate current in a pentode as the screen current?
b. depending on (a), are there any simple hollow state alternatives that might work to regulate both screen and plate voltages?
c. and if possible / reccomended, regulate the B+ of the power tubes.




Thanks very much in advance.