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Using Damper Diodes for B+ Supplies
I'm contemplating using a pair of 6AU4GT damper diodes for a full-wave bridge, with a 700 VAC CT tranny. Yet the spec sheet for the tube has these limits: Heater positive with respect to cathode: DC: 100V DC + Peak: 300V Heater negative with respect to cathode: DC: 900V DC + Peak: 4500V I assume that during an AC cycle the cathode will "see" -350 volts relative to the heater which would normally be operating near ground. Looks like I'd be in trouble without floating the filament at a minus potential. Is that a real concern in this application? And if so, is floating the filament a -300V (using SS rectifiers) a practical work-around? Else would I be better selecting another device or topology? TIA, Jon |
#2
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Jon Yaeger wrote: al work-around? Else would I be better selecting another device or topology? Unless you are designing for nuke war-in which simply using tubes is not enough, you have to make use of some very specific shielding and grounding practices or small signal tubes will start losing filaments-you would be far better off using solid state recitifiers. Get a good book on building DC supplies that is simple and direct, such as that bu the late Joe Carr. First build a good DC filtered A+/- supply with either controlled warm-up time, foldback current limiting for the steady state requirements of the tubes used, or both. Then sequence your HT supply from the primary (line AC) supply for inrush limiting. Easy and simple. Derive your C- supply (you know what that is, right?) from the heater LT side so you have it as soon as you have heater voltage. If you have a D+ supply (an arcane term, so I will supply it-screen voltage) that is separate it needs to come up simultaneously with or AFTER the B supply. Think of it like starting a jet engine-or Tony Montana's soliloquy about, "First, you get the power..." |
#4
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On Sat, 01 Oct 2005 22:56:52 -0400, Jon Yaeger
wrote: I'm contemplating using a pair of 6AU4GT damper diodes for a full-wave bridge, with a 700 VAC CT tranny. I assume that during an AC cycle the cathode will "see" -350 volts relative to the heater which would normally be operating near ground. And peak of (350) x (sq-rt 2) = about 500 volts (during startup, and lower after warmup). Looks like I'd be in trouble without floating the filament at a minus potential. Is that a real concern in this application? And if so, is floating the filament a -300V (using SS rectifiers) a practical work-around? Else would I be better selecting another device or topology? I'd say that it's a non-issue, but if it bothers you, using the damper diode as slow-start device following silicon rectifiers may be more attractive to you. Serves most of the same purposes, but not all. Good fortune, Chris Hornbeck |
#5
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Jon Yaeger wrote: in article , Bret Ludwig at wrote on 10/1/05 11:09 PM: Thanks, Bret, that's all good general advice, but perhaps in my ignorance it seems that your answer is somewhat peripheral to the question. My interest in using these tubes is that they are reputed to produce the smoothest rectification. Because the cathodes are indirectly heated, there is an 11+ second warm-up time, which makes life easier on the cathodes downstream. Solid state rectifiers have their advantages, but I want an all-tube circuit here, EMP notwithstanding. With plenty of current available but no active regulation. You state that specific shielding is needed. Would that be different, say, from a 5AR4, and if so, kindly elaborate . . . . . Well, if EMP is anticipated, you are looking at voltages generated along a gradient so that if there is a loop formed by the layout a large differential can be developed across, that's where it will. What you are doing is the same basic thing done to reduce M-field interference-only much more carefully. I don't think smoothness of rectification is any better with a damper diode than any other tube, and I don't think smoothness of rectification is that critical IF you filter your supply properly. I wouldn't even spend any extra money on HEXFREDs. I would put my power supply on a subchassis or a whole different chassis if I were really serious, and I would heavily bypass all my lytics and put a RF choke in the supply path. Your heater supply should be fully energized at least one whole minute before ANY B+ whatsoever is applied for best life. You could use time delay relays or separate manual switches, whichever floats your boat. A tube rectifier of course will work fine, but if you want the maximum theoretical life you will have to provide for its slow warm-up as well. It can get complicated. Of course there is the alternate view, which is to say "screw it" and build for simplicity. You will go through more tubes but if everyone does that way they will keep making them and the price will be lower. With the CRT industry on the verge of stand-down in general this could be an important point-or the reduction in materials supply may end receiving tube production altogether. hard telling. |
#6
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Jon Yaeger wrote: I'm contemplating using a pair of 6AU4GT damper diodes for a full-wave bridge, with a 700 VAC CT tranny. Yet the spec sheet for the tube has these limits: Heater positive with respect to cathode: DC: 100V DC + Peak: 300V Heater negative with respect to cathode: DC: 900V DC + Peak: 4500V I assume that during an AC cycle the cathode will "see" -350 volts relative to the heater which would normally be operating near ground. Looks like I'd be in trouble without floating the filament at a minus potential. You are over analyzing this... In a simple full wave B+ supply (2 damper tubes with ct of transformer being ground) the cathodes are ALWAYS positive with respect to the heater (so that means that the heater is always negative with respect to the cathode). Heater is at ground potential, cathode at B+ potential. You are correct about all of the advantages that tube rectifiers have over solid state. The only drawback to them is that most consume A LOT of heater power; but if you have extra heater winding current to spare (or extra space to mount another heater transformer) then by all means, use them. They sound nice! For more info, get a copy of "Vacuum Tube Valley" issue # 12. There is an article about using damper tubes for rectifiers. Hope this helps... Daniel |
#7
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DanF wrote: You are correct about all of the advantages that tube rectifiers have over solid state. The only drawback to them is that most consume A LOT of heater power; but if you have extra heater winding current to spare (or extra space to mount another heater transformer) then by all means, use them. They sound nice! My years of serious listening experience have proven that tube rectifiers' only "sonic advantage" is if you want a saggy supply for a guitar amp. In hi-fi service there are no sonic advantages except in the minds of the nostalgia-ridden. In fact if the supply is properly, fully engineered there is no difference at all. A fully engineered tube rectifier power supply is more work and rigor than most people here know or will stand for. You would be far better to buy a Kepco or Lambda rack or bench supply and use that. They are still available despite the morons, some of them regulars here, who butcher them to make ****ty phat toob homebrew projects. They are well done pieces in almost all cases. |
#8
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Jon Yaeger wrote:
I'm contemplating using a pair of 6AU4GT damper diodes for a full-wave bridge, with a 700 VAC CT tranny. Yet the spec sheet for the tube has these limits: Heater positive with respect to cathode: DC: 100V DC + Peak: 300V Heater negative with respect to cathode: DC: 900V DC + Peak: 4500V I assume that during an AC cycle the cathode will "see" -350 volts relative to the heater which would normally be operating near ground. Looks like I'd be in trouble without floating the filament at a minus potential. Is that a real concern in this application? And if so, is floating the filament a -300V (using SS rectifiers) a practical work-around? Else would I be better selecting another device or topology? TIA, Jon I once used a pair of 6AX4's in a PS. They seemed to do just fine. The transformer HT was 400-0-400 as I recall. Cheers, JLS |
#9
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Clever Bret wrote
My years of serious listening experience have proven... To who? ...that tube rectifiers' only "sonic advantage"... No-one has suggested a sonic advantage. You have made this up. [snip irrelevant part] A fully engineered tube rectifier power supply is more work and rigor than most people here know or will stand for. Then perhaps we can look forward to some positive contributions from you. ... despite the morons, some of them regulars here, who butcher them to make ****ty phat toob homebrew projects... Morons? Here? Hope your not talking about my mate Patrick. Who else has made a ****ty phat toob homebrew project? cheers, Ian |
#10
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Jon Yaeger wrote:
Looks like I'd be in trouble without floating the filament at a minus potential. Is that a real concern in this application? And if so, is floating the filament a -300V (using SS rectifiers) a practical work-around? Why not float the filament completely? Adam |
#11
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Question - I use 6BY5GA in a hybrid bridge. I float the filament, which
also feeds the input valve. Is this, then, compromising the HK performance of the inpit valve, and should I reference this whole 6.3v supply to earth? And if so how? Thanks |
#12
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"DanF" wrote in message oups.com... Jon Yaeger wrote: I'm contemplating using a pair of 6AU4GT damper diodes for a full-wave bridge, with a 700 VAC CT tranny. Yet the spec sheet for the tube has these limits: Heater positive with respect to cathode: DC: 100V DC + Peak: 300V Heater negative with respect to cathode: DC: 900V DC + Peak: 4500V I assume that during an AC cycle the cathode will "see" -350 volts relative to the heater which would normally be operating near ground. Looks like I'd be in trouble without floating the filament at a minus potential. You are over analyzing this... In a simple full wave B+ supply (2 damper tubes with ct of transformer being ground) the cathodes are ALWAYS positive with respect to the heater (so that means that the heater is always negative with respect to the cathode). Heater is at ground potential, cathode at B+ potential. You are correct about all of the advantages that tube rectifiers have over solid state. The only drawback to them is that most consume A LOT of heater power; but if you have extra heater winding current to spare (or extra space to mount another heater transformer) then by all means, use them. They sound nice! For more info, get a copy of "Vacuum Tube Valley" issue # 12. There is an article about using damper tubes for rectifiers. Hope this helps... Daniel Jon, Daniel is right. I'm planning on using the same on my next project (someday!). I've characterized many different types for voltage drop (the dredded "sag" refered to elsewhere in this thread) and the 6CJ3 is an absolute beast, damn stiff for a tube. The 6AU4 is no weenie either but I think the newer 9 or 12 pin compactrons like the CJ are newer lower impedence designs. Good luck! Mark |
#13
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