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Old 24th August 2008, 08:12 PM   #1
jkeny is offline jkeny  Ireland
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Default Too low heater voltage?

I'm at the final stage of building John Swenson's BDT pre-amp using 6AR8 TV Beam Deflection Tubes datasheet here: http://www.r-type.org/pdfs/6ar8.pdf

I'm installing it in an existing amp chassis for an integrated amp & space is at a premium. The heater voltage is 6.3V (300mA) which is best fed DC for hum prevention. I have a small SMPS of 5.8V 1A which I wanted to use

Is this voltage too far off the mark & could it lessen the life of the tube or starve it of space charge? How low is too low?

There is an existing 6.3V from the PS trafo which runs A/C to 4 ECL86 tubes (4*660mA cureent = 2.6A) & also powered the original preamp 3 ECC807 (3*300mA) before I removed them. So it should be powerful enough to feed the 2*6AR8 tubes (2*300mA)

I have a problem using this:
- I can't mix the existing 6.3V A/C supply and create a 6.3V D/C supply off it because 1 leg of the DC is grounded?
- I can't go DC all the way because 3.6A is a bit too high for simple 317 regulators.

Any ideas?
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Old 24th August 2008, 08:23 PM   #2
mcs is offline mcs  Denmark
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Default Re: Too low heater voltage?

Quote:
Originally posted by jkeny
- I can't go DC all the way because 3.6A is a bit too high for simple 317 regulators.
So use a 5A regulator like LM338?

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Mikkel C. Simonsen
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Old 24th August 2008, 08:31 PM   #3
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Default Re: Re: Too low heater voltage?

Quote:
Originally posted by mcs
So use a 5A regulator like LM338?
By the time you have sufficient capacitance, high current diodes (probably with heatsinks), and the necessary heatsink on the regulator, this will be a large, expensive, and hot solution.

Have you tried just doing AC all around? Do the normal of good lead dress and bias the heater up a few volts, and you may be fine.

Otherwise, 6AR8's are on sale at http://tubesandmore.com for $2.50 each. I say run them at 5.8V, buy a bunch, and replace them slightly more often if need be.
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Old 24th August 2008, 08:57 PM   #4
jkeny is offline jkeny  Ireland
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338 soultion is going to be too big as dsavitsk says - not an option.

A/C is a no-no according to John Swenson- hum sensitive tube even with good lead dress.

I'll try at 5.8V and see how it goes while looking for a small 12V SMPS which I can regulate down to 6V (I'll probably only ever run it at this lower voltage for better sonics, maybe?)

I wanted to give a SMPS a go for cleaner heater supply?
Do I need lots of capacitance on the output of the SMPS to kill any noise though?

I know 6AR8 tubes are cheap so I could buy a bunch as you say!
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Old 24th August 2008, 09:04 PM   #5
mcs is offline mcs  Denmark
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Quote:
Originally posted by jkeny
Do I need lots of capacitance on the output of the SMPS to kill any noise though?
An LC filter would be a good idea (if there's room for the L).

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Old 25th August 2008, 04:23 AM   #6
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I just did a 3.2A 6.3V lm338 supply and I agree it will take up a lot of space to do well. I was planning of fixing up an old integrated amp and was going to make a separate box for the heater supply. I was also going to run the AC for the amp go through the separate box so that I could do standby switching though that separate box as well. That way I could keep it looking original but still have everything work as I want it to.

You want to look at how much plate current you have with these tubes you are going to run at 5.8V. If you are way below limits, you should be fine. If you are pushing the limits, you may damage the cathode.

Oxide coated cathodes are meant to be protected by a cloud of electrons. If you deplete that cloud, nothing stops ions in the tube from smashing into the cathode and damaging it. Running the heater at a lower voltage makes that cloud smaller and easier to deplete. Again, not a problem if you run the tube conservatively.
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Old 26th August 2008, 12:08 AM   #7
jkeny is offline jkeny  Ireland
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Actually it seems like 5.8V is fine - tubecad goes as far as to suggest 5V is even better! http://www.tubecad.com/july2000/page10.html

There is a graph showing transconducatnce over time with the 5V showing the flatest line line i.e least decline

Quote:
Why not try a 5 volt heater power supply and see if the extended life and greater consistancy in Gm, albeit lowr Gm, is not worth the effort.
I can live with the lower Gm so I'll give it a go!
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