12B4 Linestage Woes

[Doug R]

Hi All,
I am making Eli Duttman's 12B4 Linestage.
I have 120 to 140 volts to the CCS, depending on the first cap size.

Problem is, I am dropping to 45-50 volts in the CCS (with tube in place.)
Same result for either cascaded DN2540 or Bottlhead C4S.
If I replace the CCS with 6K plate resistor, I get about the same result.

I can't figure out what is wrong. Why would the CCS drop so much voltage?
I'm going crazy trying to figure this out. It should be such a simple circuit - and I did have it working before I redid my preamp.

Any suggestions?
TIA
Doug Rice

[Zen Mod]

tube is dictating it's voltage from K to A , when biased per schematic

CCS will adjust it's working to tube's dictate

it's already written at anode - 90V/20mA

[euro21]

Cathode voltage?
500R * 20mA = 10V

[iko]

If you biased the tube to pass 20mA at idle and use a 6k plate resistor, you get a voltage drop across the resistor equal to 6000*0.02 = 120V. To get 90V on the plate, you need to have a B+ of 210V. The ccs will have a similar voltage drop. No free lunch.

[Doug R]

Thanks all. I guess I misunderstood Eli's previous posts about the power. I thought he was feeding about 125 volts into the CCS. Ikoflexer - I learned something new today - CCS drops the same voltage as a resistor. Thanks!
Time to go bump up the b+!
Doug

[DF96]

More accurate to say that with a given grid bias and given anode current a valve will have the same anode voltage.

[SY]

Exactly- if you set the B+ to 150V (or 140V, for that matter), you'll still have 90V at the plate with a CCS.

[euro21]

Originally Posted by Doug R

CCS drops the same voltage as a resistor

It's a mistake.
CCS pumping definite current to the anode. Cathode current on cathode resistor create bias voltage. Bias v. and anode current define operating point, thus define anode voltage (over cathode).

B+ minus anode voltage is CCS dropping voltage. Cascode (DN2540) CCS happy with 10-20 V drop!

[Doug R]

OOOH my head hurts.
Re-configured the trafo for 220 V AC output instead of 115 V. Getting a B+ of about 165 V with FWB - is it low due to 9K bleeder?? Anyway, still have about 165 V (under load) going into CCS and about 53 V on plate. Dropping about 3V across R-bias (180 ohm - so, about 17 mA current.) Yet another dumb question: I don't have grids connected to anything yet. Trying to get the right voltage first. Is that OK? Measured cathode V at +10 V above ground. I am using DC for heaters, with 500 ohm cathode resistor with no bypass cap.
Jeez. I can usually follow a schematic, but this is confusing.
Thanks to all.
Doug

[DF96]

The grid must be connected to something, otherwise nothing will work. No point in measuring anything or testing anything with the grid floating. No point in showing us a circuit which you have not built.