Universal filament regulator

[mogliaa]

Thanks Tom.
I forgot to add that I'm running the valve with filament bias, therefore I will probably need 7V.

I will do the initial tests with the filament boards I have and will send you a PM to get yours and do a comparison, ok?

Thanks!

[Rod Coleman]

Originally Posted by tomchr

The filament obeys Ohm's law regardless of what voltage/current source is connected. So if you want lower current in the filament, just lower the output voltage of the regulator. If you want the current to be 10 % lower, make the output voltage 10 % lower.
~Tom

It is not so simple.

Ohm's law is always obeyed, but that does not mean that the filament resistance has to be constant.... and indeed it is not. This is because of the temperature-dependence of resistance found in any common metallic conductor.

Typical DHT filaments show an increase in resistance from cold to working temperature of 400% to 1000%, and even when at working-temperature it shows a temperature-driven rise.

Over a 10% change in voltage, some filaments show only a small change in resistance, others change substantially.

.

[tomchr]

Originally Posted by mogliaa

I forgot to add that I'm running the valve with filament bias, therefore I will probably need 7V.

Nifty. With the low filament current of the 4P1L and relatively low Vgk, that's even realistic. Unlike the #26 amps that require 50 W power resistors for filament bias.

For 2.1 V, I suggest using the "low voltage" version of the regulator. That uses the LM22673-ADJ IC and is optimized for that. If you are committed to 7 V (-ish), then I suggest building the "high voltage" version based on the LM22673-5.0 IC. You'll get the best performance that way. If you want to be able to try both, then I suggest going with the -ADJ version of the IC and see if you can find a good set of components in WebBench that'll give you decent performance at both output voltages. The circuit board is the same for the two regulators, so you could get more boards and build both versions to perform an A/B comparison.

I like your Mecano "chassis" by the way.

Originally Posted by mogliaa

I will do the initial tests with the filament boards I have and will send you a PM to get yours and do a comparison, ok?

I'm not going anywhere... When you're ready, just jump on my website and place an order. Easy as pie... Just keep in mind that I close down for the holidays starting Dec 13th. Any order placed after that date will ship in early January.

~Tom

[tomchr]

Originally Posted by Rod Coleman

Ohm's law is always obeyed, but that does not mean that the filament resistance has to be constant.... and indeed it is not. This is because of the temperature-dependence of resistance found in any common metallic conductor.

That's true. Some experimentation (or knowledge of the resistance curve for the filament) is needed to obtain a given current by changing the voltage. Easily done with a lab power supply and transferred to the filament board.

~Tom

[SpreadSpectrum]

I finished two of the boards and tested them with load resistors and a lab supply. Using the values in your spreadsheet for the 801A I got 7.4V out which I think is perfect. I'd rather be a bit low than a bit high.

Is there any danger to the parts on the board if you power this thing up without a load, say if the tube is unplugged?

[tomchr]

Originally Posted by SpreadSpectrum

I finished two of the boards and tested them with load resistors and a lab supply. Using the values in your spreadsheet for the 801A I got 7.4V out which I think is perfect. I'd rather be a bit low than a bit high.

Did you use 68 uH + 100 uF (designed for use with one tube) or 22 uH + 47 uF (two tubes)?

7.4 V is that measured at the tube or on the output connector of the regulator. I think WebBench rounds down, so you could be up to 2 % low (7.35 V). But the feedback is taken at the output connector - not at the tube socket, so you will have some resistive drop across the wires going to the tube socket. Not enough to matter in practical terms, but enough to make you go, "but... I thought the output voltage was supposed to be 7.5 V"). The voltages I'm seeing are all slightly below the target value, but within +0/-2 % measured at the output connector pins. Component tolerances...

Originally Posted by SpreadSpectrum

Is there any danger to the parts on the board if you power this thing up without a load, say if the tube is unplugged?

Nope. It's fine without a load. In that case, the resistive divider R1, R2 provides enough load for the regulator to remain happy.

Attached are Transient Response (1140 mA DC + 100 mA step); Line Regulation (ripple rejection) in dB; Output Impedance (1.24 A DC + 50 mA AC) in Ohm for the 7.5 V, 22 uH, 47 uF, LM22673-5.0 option.

~Tom

[SpreadSpectrum]

I was going to build one for two tubes and test it and see if there was an improvement in building a separate regulator for each, but I just got lazy and built four boards with the 68uH+100uF.

I'm using these tubes well below their max DC cathode current so I would be comfortable running them down to 7V or so.

Soldering the DAP went well. At first, I was worried because we had no large tips but those Metcal stations handle it fine even with a smaller tip. I used like a 3mm wide chisel tip. Actually, I think I have used one of the fine point hooked tips for surface mount soldering to solder an N-connecter onto a piece of semi-rigid coax before. Those are good solder stations.

Thanks for making this board. It is just so cool to be able to provide 5A of filament current and only draw half that from the raw supply.

[tomchr]

Originally Posted by SpreadSpectrum

Thanks for making this board. It is just so cool to be able to provide 5A of filament current and only draw half that from the raw supply.

You're quite welcome. Thanks for your feedback. I'm glad to hear the build went well.

Yeah.... The whole Power IN = Power OUT (almost anyway) of the switchers is what attracted me to the project in the first place. I've run over 2 A out of these boards and they may get lukewarm. In the 5 V, 1.4 A boards I use for the 300B's, the regulators reach 31 deg C with 20 deg C ambient temp... No need for huge heat sinks. No thermals to manage. Yay. I can focus on the tube part of the amplifier design.

~Tom

[wicked1]

Any problem using these for tubes w/ 6.3v 3a filaments? It looks like I use the "6.3v 1.2-2.5A" parts, or have I overlooked something?

[tomchr]

I don't see a problem per se, my personal preference is just to leave a little bit of margin in my designs. The regulator is spec'ed to handle 3 A continuously. Just make sure the ripple current in the inductor doesn't hit the saturation current for the inductor or the worst case current limit for the IC.

Whether the inductor current hits those limits depends on the input voltage to the regulator. It's pretty easy to check with WebBench.

If you're running a lot of indirectly heated tubes with the heaters in parallel, don't forget to check that you don't violate the heater-cathode voltage spec on any of the tubes. In many amps, the cathodes are at vastly different voltages, hence, if you put all tube heaters in parallel you'll violate that Vhk spec somewhere in the amp. I'm not saying it's a problem in your amp (as I have no knowledge of that) just a common pitfall to be aware of.

~Tom