How do I bias up a constant current series filament circuit?

[sgerus]

I’m building a stereo amp (5691/300B), with the 5691’s in SRPP for the input stage.

Because of the SRPP, I need to bias up the filaments to around 90V.

Can I use a constant current filament with my SRPP (biased filaments)?

If Yes, how do apply the +90V to the filament circuit?

[dhaen]

You need a "floating" filament supply, exclusively for this SRPP stage. The whole supply needs to be isolated from anything else except the biasing components, 2 resistors and a cap perhaps?.
That means: Its own tx winding, rectifier, smoothing and regulation.

I'm just wondering.....why...? Since it's indirectly heated, the 5691 will be joyfully happy with AC heater supply. And I notice that for a small signal valve it is rather greedy with heater current - 0.6A.

[kevinkr]

Constant current operation is not required with indirectly heated tubes, however in some cases the reduction of cold inrush filament current may result in much longer life. (I have lost several Mullard ECC83 to this problem on low Z regulated dc supplies.)

I don't think the 5692 benefits particularly from constant current heating, but it doesn't hurt. Note that warm up time will be substantially greater due to the limited heating current.

The benefits of constant current heating in dht's has been addressed in other threads, suffice it to say I usually find it beneficial to sound quality.

Definitely be sure to use the cap Dhaen mentioned in his post or you may find that you have noise problems. If you are using a single tube as an SRPP (and I do) make sure that you put the filament bias voltage somewhere around the mid point of the difference in cathode potentials on the two sections, while bearing in mind that when the difference between cathode and filament gets to around 90V the insulation may start to breakdown. Also note that I have gotten away with 100V filament bias and 200V on the upper cathode for years without problems in 6SN7. (A little over the rating actually)

[Brian Beck]

Originally posted by kevinkr
Constant current operation is not required with indirectly heated tubes, however in some cases the reduction of cold inrush filament current may result in much longer life. (I have lost several Mullard ECC83 to this problem on low Z regulated dc supplies.)

I agree, but if you use regulated DC VOLTAGE heater supplies as I almost always do, you can add a resistor and capacitor to the adjust pin of an LM317 or similar to create a slow-start circuit. Make it any time constant you want. No flashes and you can get the exact heater voltage you want.

[ErikdeBest]

Hi Brian

I don't exactly understand how to implement the resistor + capacitor on the 317. I imagine both in series, one end of the resistor connected do adjust and one end of the capacitor connected to ground?

Erik

[Brian Beck]

Erik,

This is straight out of the app notes. I forgot to mention that you need a common PNP too. Just pennies though...

[ErikdeBest]

Thanks Brian! Interesting idea!

Erik

[sgerus]

Dhaen & Kevin,

Thanks for the reply... you talked me out of ccs heater.

Once again it seems the KISS method ie best.

-Scott

[kevinkr]

V regulator with delay works well, I've used it in later designs. You can do it even more simply with a large electrolytic on the reference terminal as well, if the voltage ramps up to full supply voltage over a couple of seconds this pretty effectively suppresses most of the inrush current. Just make sure to use the app note current steering diodes with any cap over 10uF.

[ErikdeBest]

Hi Kevinkr

I am being dumb today, hence one more question. When you tal about the reference pin, do you mean the 'adjust' pin on the lm317? The solution with the big cap is better suited for my existing supplies...just having to add a large cap to the existing circuit is easier.

Erik