• 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.

LT1085 Current Regulation for DHT filaments

I attach below a circuit (from internt) of using the first LT1085 as a voltage regulator and the second LT1085 as a current regulator for feeding the filaments of 2A3.

I know how to calculate the value of resistors for the LT1085 as a voltage regulator, but the datasheet of LT1085 does not contain any information of using it as a current regulator.

It appears that the R6 (520 ohm) shown in the circuit does not tally with the formula I = 1.2 / R for LM-338K. How to calculate its value when using LT1085 as a current regulator?

How about the R7 (1K ohm) shown in the circuit?

Is the formula of LT1085 (3A) the same as LT1083 (7.5A)?

Can anybody lead me the way? Thanks.


  • lt1085_current_regulated.jpg
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Basically they are very similar parts, I recommend looking them up on the Linear website and downloading their datasheets to compare.

IMHO you don't need the voltage regulator section at all in power tube applications, I have designed and built them both ways and the sound was indistinguishable - the important thing is not to run them too close to their drop out voltage, typically under low line conditions you want a minimum of 3V across the regulator ic.

The current in current regulator mode is vref/sampling resistor value.

For 1.2A something slightly over 1.0 ohm is close enough.

I actually use a precision 1 ohm power resistor (3W) with a 50 - 100 ohm pot across it , wiper to the ref pin on the regulator - if a good pot is used this allows me to set the current very precisely. Tie the wiper through a 1K or greater resistor to the end of the pot connected to the load, that way if the pot fails it will default to the minimum current. I use a large cap on the output and I don't usually bypass the ref pin in current mode. (If you bypass this point I doubt the ground is the right place, probably should be referred to the high side of the load - your filament. Note I haven't tried this.)