space charge 12fx8 amp or preamp schematic?

[Bob_345]

on sopht amps sight, they say the 12k5 can put out 1/4 watt.

[astouffer]

Originally Posted by Bob_345

on sopht amps sight, they say the 12k5 can put out 1/4 watt.

1/4 watt but not for very long judging by the datasheet. It claims 0.035 watts typical. Maybe a pair in push pull...

http://www.mif.pg.gda.pl/homepages/f...137/1/12K5.pdf

[Arnulf]

I had a close look at the 12FX8 datasheet ... Man is this one irritatingly tough nut to crack or what ? The easiest way to use it is with grid leak bias afterall, due to its internal heater-cathode connection.

Anyhow, if I had one of these tubes and wanted to give it a try, this is how I would go about it (see attached pic).

Value of R2 must be determined experimentally as there is way to much variation allowed by the datasheet. How to determine its value ? Hook it up to a power supply (capable of supplying at least 5V) as shown. You can omit RP6 and C3. Hook a miliamperemeter between JFET source and supply ground. Adjust value of R2 until current measures no more than 1.3 mA. Note the value and pick RP6 approximately 5 times the size of R2.

RP6 may be omitted if you don't feel like tweaking anode load current (I'd omit it for initial testing, then put it in in order to tweak the sound once everything is working). C4 accidentally says 0.1 uF, should be 1 uF but isn't critical because both reference and load are resistive and voltages across them stable.

You can use 2 PP3 (9V) batteries in series for tinkering if you don't have an 18V power supply handy. A standard battery will give you 1-2 hours of operation with vast majority of power wasted on heating.

Remember to hook unused heptode terminals to the ground through large (1M-10M) resistors so some negative bias can develop and grid current will not flow.

[Bob_345]

tahnks for the help arnulf. that is exactaly what i was looking for.