Where can I get ??

[TimS]

just havin' fun David :-)

[David W]

There will be varations in the full load voltage, depending on the voltage and current ratings and the regulation of the transformer used.
Lets say we get 8V under load and we want around 6.3V.
That gives us a required voltage drop of 1.7V.
If the heaters draw 5.2A then we need an R of 0.3269 ohms
It will dissipate 8.84 watts.
You could use a 0.33 ohm 50W resistor from RS ($8.14) part # 160-887 and bolted to the chassis with a tad of heat sink grease.
In parallel with a 6.8 ohm is 0.3147, with 12 is 0.3211, 15 is 0.3228, 18 is 0.3240, 22 is 0.3251, 27 is 0.3260, 30 is 0.3264, 33 ohms is 0.3267 and 39 is 0.3272 ohms.
Using a W21 Series 2.5W axial from RS ($2.00, but you have to buy 5 pcs)

If the current drawn was 5.5A then we would need 0.3090 and a 5.1 ohm resistor would give 0.3099 ohms

If the input was 9V ( at this point I would be looking at a low drop out 5V regulator with C (ground) raised via two diodes and bypassed with a 10 -27 mfd cap, the valves will still be happy at 6.0V heaters) and current is 5.2A then we would need a 0.5192 ohm resistor.
In this instance the next value is (50W) is 5.6 ohms the the parallel R would be 62 ohms (5.1360 total).

There is a small problem and that is the mains voltage is rarely 240V.
So when you do this, measure the mains at different times of the day, because this will effect the voltage out of the transformer..
It is normally a small variation, but try and keep the heater volts between 6.0 and 6.5 VDC.

I hope this will help the DIYer in selecting the correct heater voltage as an alternative to using the 50 W rheostat

Ross

Sorry to re-hash an old thread but Ross Giles calculations with this were 100% spot on.
I now have a working Poor man 670 . I am eagerly awaiting a new sidechain board (with original 670 time constants) being offered via "The Prodogy Forum" as the time constants are very slow.