CC vs Shunt filament supply?

[phrarod]

I need to create filament supplies for my indirectly heater triode signal tubes. One friend says Constant Current is the way to go while another insists that Shunt is better.

What do the filament experts here think? Much appreciated. I don't have a lot of experience with different filament methods. I usually just build the standard regulated supply.

[Sch3mat1c]

Shunt?

Constant voltage AC has worked fine for the last 70 years, I don't see any reason to change it.

Tim

[phrarod]

From what I've read people here and on other forums really like Constant Current which has been around almost as long.

You mean AC heaters? Yeah, I don't think my layout skills are up to solving the hum of AC heaters although many people I respect think that is the best.

[Wavebourn]

Originally Posted by phrarod

Yeah, I don't think my layout skills are up to solving the hum of AC heaters although many people I respect think that is the best.

DC heating requires even more attention & layout skills.



Once more:

enjoy!

[Holger Barske]

Shunt regulation on tube heaters is a desaster in terms of efficiency. Shunt regs are said to be really good if they draw at least twice the power needed by the load, so this is becoming a rather hot deal.
Contant current is fine, as it avoids the turn on current surge due to the cold filament wires. Your tubes will perhaps be thankful in terms of life span.

[phrarod]

Thanks Wavebourn! I didn't know that. I have a small question on your schematic but I want to study it before asking.

Thanks Holger. Is there any truth to the theory that CCS on heaters could lead to cathode stripping if the heaters come up slower than the cathode voltage?

[Holger Barske]

It is said to be. I know a number of commercail units where care is taken that the heater voltage is settled before B+ is turned on. If this is really an issue - no idea.

[SemperFi]

Hehe, this happens to be my favourite topic today

First: A 'filament' is the glowing part on directly heated tubes, indirectly heated tubes have a 'heater'. Yeah. I'm nit-picking

here is nothing difficult in achieving silence with AC heaters. Read the thread I just started about the same time as you. What a pleasant coincidence eh?

Simply attach one end of the AC heater wiring to a voltage divider which gives you 20-60volts. As always with such wiring, tightly twist them. Should give no noise at all.


For subduing inrush current on cold heaters, if you want, this simple semi-passive circuit does a good job: (Yeah I like this circuit)

The cap is there to provide a relatively smooth DC for the relay, but untill the voltage is > coil rating, the relay starts to click as the voltage rises. Gives a nice indication the circuit is getting ready...

[Wavebourn]

Originally Posted by phrarod

Is there any truth to the theory that CCS on heaters could lead to cathode stripping if the heaters come up slower than the cathode voltage?

Yes. In radar tubes and other high power transmitters.

[Sch3mat1c]

Originally Posted by SemperFi

For subduing inrush current on cold heaters, if you want, this simple semi-passive circuit does a good job: (Yeah I like this circuit)

Oddly enough, I had to do that for my tubescope supply. The CRT heater, HV and PWM controller supply are created by a self-excited oscillator, which starts with fairly low bias (fractional mA). If the load is light, it will easily start oscillating, supply voltages rise and the regulator comes online (increasing bias as required). But when it's shunted by a cold heater, it takes a very long time to start up. At cold temperatures and low line voltages, it may never start! So I added a small NTC in series with the heater winding, and now it starts up immediately.

An unusual circumstance, but it will work just as well for other situations. Note that you need a fairly low resistance NTC, because you want a lower quiescent voltage drop than an offline supply requires. A few volts out of 120 or 240V isn't a big deal, but out of 6.3VAC, it is!

It's not all bad, of course; most of the time, the heater winding is over spec anyway (~7.3VAC?), given modern line voltages. A little drop would be quite healthy for the tubes.

Tim