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

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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?
 
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.
 
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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)
 
kevinkr said:
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.
 
Erik,

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

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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.
 
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
 
I've done that too (a delay with just a big cap on the ADJ pin). But you need a big cap, and you definitely need the reverse diodes to protect against output shorts. The transistor allows for long time constants without the need for a huge cap. For the big-cap-only design, for starters scale the two voltage-setting resistors up so that the resistor from OUT to ADJ is around 1.2K. Then you can make the cap from ADJ to ground about 470uF or 1000uF, or more. That's still only a 1/2 to 1 second time constant, not all that slow. With a 1.2 K resistor, the minimum output current of the LM317 is not satisfied unless the heater is connected, so testing without a load may not give you the results you expect.
 
Not to beat a dead horse, but here's a schematic I found from an old project of mine that needed slow turn-on for 6.3V heaters. This particular supply delivered 1.2 A. I had a lot of 22uF/16V caps that I put to use. The time constant here is 1.3 seconds, but could be stretched a bit by making either the 59K or the 22uF on the base bigger. Note that I used an LM117 (a military version of the LM317) in a TO-3 case, on a heat sink dissipating 4.5W. A TO-220 version on a sink would have worked too. I usually use pots on the ADJ pin for heaters since the voltage drop in the wires with all that current can be significant. I measure at the tube pins for the desired heater voltage. That might mean 6.4 or 6.5 volts at the LM317, especially if the supply is remote as I prefer to do.
 

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Wavebourn said:
Brian;did you try a current limiting voltage regulator?
The trick is, to limit the current on the level a bit higher than needed for hot filaments. It starts slowly without any capacitor, just waiting for filaments to heat up.

No, although that could work too. I suspect it might be more complicated than just an R, C and a junk-box PNP though. Any ideas?
 
Brian Beck said:


No, although that could work too. I suspect it might be more complicated than just an R, C and a junk-box PNP though. Any ideas?

For my ongoing project I planned to use CCS to feed "E" kind tubes. I like to see the voltage coming up slowly on the tubes filament, although for some tubes (the ones terminating with A, like 12BH7A) this doesn't make a difference. As I am using separated trafo's for the B+ in some projects I thought about connecting a relay parallel to the filament...the relay will only activate and turn on the B+ when the tube is pretty warm. This is another issue that may be of no importance (example by SY: datasheets state max allowable voltage for cold tubes).

Morgan Jones warns that the 338 regulator may fail to work when feeding high current tubes (as the 13E1). Has something to do with to high inrush currents, that deactivate (or something like that) the 338. The solution is a resistor from input to output of the regulator.

Alain Dupont writes the following, which he applied to feed the current hungry 6C33C
"The great problem is the current at startup... >15 AMps
so the idea of starting as a shunt reg with a 0.5 Ohms limits current at <10 Amps
and 3 second later a relay shorts the 0.5 Ohms resistor...
Then with a pot the voltage is adustable ..."

But now we are talking about power tubes, and most agree those don't need DC on their filaments.

Erik
 
ErikdeBest said:



Morgan Jones warns that the 338 regulator may fail to work when feeding high current tubes (as the 13E1). Has something to do with to high inrush currents, that deactivate (or something like that) the 338.


Yes, they protect themself by decreasing a maximum current, instead of just limiting it as shown on my diagram in the previous message.
 
Brian Beck said:


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.


I thought the heaters need to be hot before applying B+ so the slow turn-on must be on the B+ instead of the heaters?
 
jarthel said:



I thought the heaters need to be hot before applying B+ so the slow turn-on must be on the B+ instead of the heaters?


On both.

See "slow turn of B+" schematics in my topic about tube VS solid state voltage regulators.

Cold heaters have much less resistance then when they are hot, it causes high inrush currents when you switch them on. The thinner parts of them heat up faster then the rest, causing higher differene of temperature across the filament. It causes evaporizing, mechanical damages, even knots on filaments that eventually can short on cathodes... Limiting inrush current prolonges life of tubes and minimizes risk of casual accidents.
 
jarthel said:
I thought the heaters need to be hot before applying B+ so the slow turn-on must be on the B+ instead of the heaters?

Well, yes, the cathodes of some tubes can be stripped if the B+ is on for long without the cathode being hot. But we're only talking about less than 2 seconds of heater ramp-up. The heater would have taken several seconds to get hot with or without the slow-start. We're trying to nix the turn-on current surge through cold heaters, not delay the ultimate heating by very much. Furthermore, I usually delay my B+ for about 10 seconds, and even then I've sometimes used a slow ramp up there. My power supplies are a bit complicated...
 
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