6V6 line preamp

[merlin el mago]

Salas, just curious how do you calculate the resistor value of 2R7 between the LM317 & the filaments?

 

[Salas]

I calculate Iccs=Vref/Rset (LM317 has 1.25V Vref at adj pin with some tolerance)

 

[merlin el mago]

You don't use any resistor at adj pin of LM317?

 

[Salas]

That one resistor (or any value from parallelism) between Vout & Adj is all you need because now we treat LM317 as a CCS device.

 

[merlin el mago]

Understand, you don't use as a voltage regulator.

Rset = Icss/Vref

462mA = 1.25/2.7

 

[Salas]

6V6 has 450mA +/- 10% Ih original spec

With LM317 Vref tolerance and resistor tolerance all combine and what you look for is 6.1-6.5V Vh on the tube

 

[merlin el mago]

Using 10VAC power tx ok, but what happens if I use 9VAC or 12VAC or other values?

 

[Salas]

Prefer 9V transfo to save on LM317 dissipation. You basically want no less than 9.5V raw DC after rectification and filter. To accommodate LM317 dropout minimum margin.

 

[merlin el mago]

I have the power tx, time to look for 2R7 10W resistor

 

[Salas]

Experimenting with 5W parallel resistors is also practical. Connect heater when with a likely combination and see what Vh voltage the LM317 & setting resistors achieve on the actual tube brand and vintage. After a minute it will be fully lit.

 

[Salas]

It will take a while because CCS does not apply voltage to the heater. It only feeds the right current until the heater develops the right voltage. It's soft start heating.

 

[merlin el mago]

That's good for cold filaments.

 

[Salas]

Exactly. And subjectively rather better in my experience. The LM317 just floats there as two pin in-out CCS device. No extra Adj & output caps.

 

[Salas]

That's good for cold filaments.

Goes well with slow starting tube rectifier. If with solid state bridge, better voltage mode heating to come up fast. Or preheat with a separate switch before engaging B+. Which is generally a good idea regardless of rectification style.

 

[merlin el mago]

Look what I have ready, are the DC heaters for 307A that likes better voltage regulation.

 

[merlin el mago]

But looking the simplicity I will make your design.

 

[merlin el mago]

Exactly. And subjectively rather better in my experience. The LM317 just floats there as two pin in-out CCS device. No extra Adj & output caps.

Sometimes less is more.

 

[merlin el mago]

Goes well with slow starting tube rectifier. If with solid state bridge, better voltage mode heating to come up fast. Or preheat with a separate switch before engaging B+. Which is generally a good idea regardless of rectification style.

Jadis does like you said "old school".

 

[Lingwendil]

Hey Salas, do you still like the Maida regulator PSU for these? I'm currently working on PCBs for power supplies and was thinking of putting one together as a spiritual companion to the other boards.

I'm also putting together a simple mosfet ripple filter PSU on a 50x50mm PCB as well since it's what I use in 75% of my own builds, same basic circuit as the one Pete Millet used in his Engineer's Amplifier, but just with a diode bridge-

It would be easy to start with this and expand it to 50x100mm to work in an LM317T to make a Maida regulator board.

 

[Salas]

But looking the simplicity I will make your design.

Voltage source mode heater PSU as you already got has the characteristics of coming up instantly and maintaining fixed voltage to all tubes if you roll a lot.

CCS mode is minimalist, occupies less space, comes up soft. But various tubes can develop different Vh. Depends on exactly made filament for nominal spec current. You are happy if within 6.1V-6.5V range. Else you tweak Rset.