• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12B4 Linestage Woes

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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
 

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iko

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Joined 2008
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.
 
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
 
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Joined 2004
Paid Member
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!
 

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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
 
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Let's go back to the original drawing. The grid is NOT floating. The volume control pot. provides the grid to ground resistance. If a cheapy control is employed, adding a 1 MOhm, safety, resistor between grid and ground is wise. Buy PEC hot molded carbon controls and rest easy. ;)

Yes, "12" V. heater power is indicated. Pin 3 is grounded by a cap. and biased off B+. Look at the drawing, carefully.

The value of the "fudge factor" cap. in the pseudo choke I/P filter B+ PSU needs to be tweaked, on the bench. The 47 nF. value shown is merely a starting point. Use the smallest capacitance consistent with providing sufficient voltage compliance for the CCS load chosen. The larger that cap. is, the poorer PSU regulation becomes.

A 9K Ohm bleeder is correct for a 9 H. choke and a 10 KOhm bleeder is correct for a 10 H. choke. Don't use wimpy power "iron" in this project. While low cost, a Triad N-68X has the "cujones" needed and it has dual primaries, making it suitable for use in both "120" and "220" VAC mains zones. :D
 

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