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Converting AC Filamenta to DC Filaments

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kannibul, Jun 26, 2006.

  1. kannibul

    kannibul Member

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    After rectfication, what else is needed?

    This would be for a 2x 12AX7, 1X EL84 amp, using a Hammond 269EX PT (2.5A Heater Supply)

    Thanks!
     
  2. hasserl

    hasserl Member

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  3. kannibul

    kannibul Member

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  4. donnyjaguar

    donnyjaguar Member

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    I used a R47 power resistor on my DC filaments. I had 2 filament windings to what I did was run the output tubes on AC and all the 9 pin tubes on DC. I believe I settled on 4700uF and a 3A bridge. This netted me 6.8Vdc (probably still okay) and I used the one resistor to bring it down to exactly 6.3Vdc. Put the resistor between the bridge and the capacitor as this will more effectively filter out the hum. You won't kill the tube by putting 6.8Vdc to it for a short time while you experiment with resistor valutes to bring it to 6.3Vdc.

    I thought also about using a regulator but the inrush current just drove the regulator into shutdown.

    DJ
     
  5. danieldroukas

    danieldroukas Member

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    You need to regulate the voltage, and the type of circuit you use will depend on the amount of current your tubes will be pulling. Your current draw will be somewhere in the 1.5 Amp area for AC filaments, but running the filaments in DC actually makes the current draw about 140% of that. So you'll need to design a circuit with the current capacity of at least 2.1 Amps.

    O'Connor details a fairly nice 5A DC filament circuit in TUT1 in figure 2-16 if you have it.
    Here he uses a circuit based around the LM338 adjustable voltage regulator.
     
  6. hasserl

    hasserl Member

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    Well, here is the part of the schematic for the DC heaters on the Carvin VT50. No dropping resistors. Note that there is actually about .7 voltage drop across a diode, which combines for about a 1.4 voltage drop for the two tubes. This ends up with a dc voltage in the low 5's. It is enough to light the tubes. Note that this does cause some problems with certain tubes in the MTS series amps when running EL34's in the power section, as the added draw tends to tax the supply.
    [​IMG]
     
  7. hasserl

    hasserl Member

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    Well, here is the part of the schematic for the DC heaters on the Carvin VT50. No dropping resistors. Note that there is actually about .7 voltage drop across a diode, which combines for about a 1.4 voltage drop for the two tubes. This ends up with a dc voltage in the low 5's. It is enough to light the tubes. Note that this does cause some problems with certain tubes in the MTS series amps when running EL34's in the power section, as the added draw tends to tax the supply. I've measured only 4.7vdc at V1 on an MTS, it was real picky about which tubes would work. I've never had a problem with the Vintage series amps.
    [​IMG]
     
  8. donnyjaguar

    donnyjaguar Member

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    If that's the case, its not a good design and I wouldn't model my own circuit after this one. :) I don't like the location of R44 and R45. It would make more sense to put these on the AC side of the circuit.

    DJ
     
  9. hasserl

    hasserl Member

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    The point is that voltage reduction is not needed, just lots of filtering. BTW, I have one of these amps on my bench right now, so last night I stuck a meter on the pins to measure the voltage, 5.1vdc at V1, the supply going into the diode bridge was just about 6.0vac. There was no problems with the filaments lighting in the tubes, none at all.

    The 100 ohm resistors to ground at the end of the string form a virtual center tap. If your PT has a center tap these are not needed.
     
  10. donnyjaguar

    donnyjaguar Member

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    Basically what happens if the filament isn't at a high enough operating temperature the cathodes are more likely to "strip". The hot-spots on it conduct the bulk of the current and chew them up prematurely. This is the same as what happens when you apply high-voltage to tubes before they are warmed up properly. I don't believe this is conjecture as I've read some well researched articles on it.

    Many of the common tubes types with 6.3V filaments have 5 Volt filament equivalents. If it didn't matter if 5V were applied to a 6.3V tube they wouldn't exist.

    PS: anyone want some 5BQ5s? ;)
     
  11. hasserl

    hasserl Member

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    Well, all I can say is there are no tube life problems with these amps, none at all. This series of amps have only been around for about 10 years now, which is not a long time compared to others, but I've seen plenty with the original tubes still working fine. If 10 years isn't long enough, oh well. (most people around here don't hold on to an amp for 10 months, let alone 10 years. And if they did they'd have changed tubes several times just because they wanted to!)

    FWIW I believe the cathode stripping claims we see on the internet are way overblown. I have yet to hear of anyone who has actually witnesed this, only those who have heard/read about it. Many old tube amps, radios, tv's, etc from the old days never had standby switches, and never had cathode stripping problems. The same goes for cathode poisoning. It sounds neat, and grabs attention, but it is not a real life problem. Again, many old amps were turned on and never turned off for years. They did not have cathode poisoning problems. I think these are theoretical ideas with no real world application to our use of tube amplification.

    Also, I really don't care if you or anyone doesn't like the design. I didn't design it and I have no personal connection to it. I really do not see the advantage of dc operation anyway. Most amps do not use it and have no noise issues from the filaments. If they do, floating the ground seems to be a simple and effective way to deal with it. Besides, what are you going to do different from that design? All it is is a rectifier bridge, it's nothing special. What are you going to do that is so vastly different?
     
  12. Ray Jones

    Ray Jones Member

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    hi guy im new here but i am a ham radio opperator and i can tell that filament poisioning is a very real thing.....look up poisioned 811a 3500z 572b 3cx1200a7..z 8877 its a common and exspensive problem when you draw the highest power possible from a tube it strips the electron cloud away from the filament and exposes the bare tungsten in the filament .....in the case of my henry 5k classic amp that would be a pair of 3cx 1200 a7,s at 1600 bucks each....that range for this amp would be in the 4500 watt out put range....3.5 to 30 mhz and im only licensed to 1500 watts so i have tons of head room and a super clean signal.....
     
  13. Kyle B

    Kyle B Supporting Member

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    Hi new guy!

    You know that's a 12 year old thread, right?
     
  14. Ray Jones

    Ray Jones Member

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    im new to this kind of thing.....it had the info i was searching for kind of....i havent done an audio amp in over a decade .....also i can hardly play a lick been trying music on and off since grade school im 59 now so you get the ideal but i want to try one last insturment before i give up its called a theriman how ever you spell it but the important thing is you dont touch it when you play it...im gonna buy a cheap one to try out for a few months i have a 1950,s magnavox record player for an amp ...in the next few days im going down to corpus christi to pick up an old hammond organ its supposed to be in working condition.....its only got 2 6v6,s in its power amp but its got like 18 12au7.s in it so ill have ample parts to make an amp and i have the schematic for a 12au7 theriman so since its a single keyboard organ with an input ill be able to use with the theriman and try the keyboard to see if i can do that....plus it will have a nice jensen speaker with it ive read the speaker is worth more than the organ.....ill stop boreing you now...................i think the organ cabinet is walnut or mahoginy....forgive my spelling but real hardwood will make a great cabinet for the amp and the theriman............
     
  15. pdf64

    pdf64 Member

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    I think you may be conflating / confusing various issues, none of which have been credibly claimed / proven to be an issue with the 'low' voltage operation of receiving type tubes / circuits used in regular guitar / audio amps.
    Have a read of http://www.valvewizard.co.uk/standby.html
     
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  16. Ray Jones

    Ray Jones Member

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    ok i dont think i listed any low voltage tube....the 811a is the lowest voltage tube i listed and that one is 1800-1900 volt tube my 3cx1200,s run at 4250 volts but max out at 5000 volts these are what i deal with mainly any more....
     
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  17. Kyle B

    Kyle B Supporting Member

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    It's no problem my man - Just letting you know since you're 'the new guy' ;)

    Therimans are tons of fun. Enjoy!!!

    You'll find most audio tubes, they're operating around 200-400V at the anode. Cathode poisoning doesn't happen in our world.

    Getting your hands on an old organ isn't all that hard BTW. Don't make a long special trip for it. People are giving those away these days just to get 'em out of their basements. Watch your local Craigslist. The one caveat - Don't tell the seller what your plans are... they'll freak. The organ they're trying to get rid of used to be grammas, and it was her pride and joy - She loved it more than her kids - etc... They always think of it as some kind of heirloom. When I pick up old gear like that, I tell 'em I've always wanted XXX organ, or XXX stereo console or XXX reel-to-reel recorder or whatever the item is ... Don't tell them the minute you get home, you and your screwdriver are gonna strip it down and turn the cab into firewood. Let them think it's going to somebody who will really appreciate it.

    You may realize this already but worth saying --- If you use an old organ power transformer, keep in mind the heater windings are meant for however many tubes are in the original design. The one you're describing will probably over-voltage your heaters by 25% or more. You might need to load it down or add series resistance to keep the heater voltages under control.
     

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