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DC Heaters for Phono Preamp
DC Heaters for Phono Preamp
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Old 11th October 2018, 01:46 PM   #41
JJMontague is offline JJMontague  United States
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Are each of the 12V windings center-tapped or will I need to use a resistor bridge to ground?
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Old 11th October 2018, 02:16 PM   #42
Steve Morley is offline Steve Morley  Canada
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No C.T. Not needed if you are using DC on the filaments. Go back and look at the circuit attached in #34 in this thread. The filament supply is referenced to ground after it's rectified through the two resistors across the B+ supply.
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Old 11th October 2018, 02:38 PM   #43
JJMontague is offline JJMontague  United States
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Are DC heater supplies usually elevated? I'm using elevated AC for the line stage heaters referenced to 70V using a resistor divider off the B+ rail. Any reason not to reference the DC heaters from the same resistor divider?
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Old 11th October 2018, 03:01 PM   #44
kylej1050 is offline kylej1050  United States
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DC supplies are normally only elevated when a heater-cathode spec is being exceeded.
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Old 11th October 2018, 05:52 PM   #45
JJMontague is offline JJMontague  United States
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In regards to using the separate 12V secondary windings to give each channel it's own heater supply, how concerned should I be about this warning on the Triad datasheet:

Primary and secondary windings are designed to be connected in series or parallel. Windings are not intended to be used independently.

Is this warning meant for the primaries only?
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Old 12th October 2018, 03:12 PM   #46
kodabmx is offline kodabmx  Canada
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It says "Primary and secondary" in what you quoted... My guess is that if the current drawn is the same on both windings, they can be used independently. I can't see a reason not to parallel them though. Or, if you want, you could connect them series and use "full wave" rectification instead of a bridge.
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Old 12th October 2018, 04:33 PM   #47
Lingwendil is offline Lingwendil  United States
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DC Heaters for Phono Preamp
I'm pretty sure the warning is just to ensure that the secondaries aren't left floating with regards to eachother, as they would in some cases develop a pretty large voltage between them. If both are referenced to ground, or some other common voltage you should be fine. When I use Antek toroids I routinely use one heater winding per channel, and then tie both windings to a common ground or heater lift reference. it ends up symmetrical and easy to route, as well as troubleshoot later on.
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Old 12th October 2018, 06:43 PM   #48
kylej1050 is offline kylej1050  United States
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The most important part of that warning is the primary. If you only connect 1 of the 2 primaries you have to derate 50% secondary load.
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Old 12th October 2018, 08:05 PM   #49
JJMontague is offline JJMontague  United States
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So the Triad VPT24-1040 takes care of my 12VDC heaters for the phono stage. Nice.

Could I do something similar for my AC-heated line stage (separate heater supplies per channel)? I'm currently running AC heaters off a 6.3V CT secondary winding of the main PT, and I want to stay with AC. Can I use something like the Triad VPT12-2080 and use the dual 6V secondaries for the two channels, creating an artificial center tap via resistors for ground reference for each secondary winding? This would also serve the purpose of ensuring all heater supplies are separated from the main PT which provides B+.
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Old 12th October 2018, 09:18 PM   #50
Lingwendil is offline Lingwendil  United States
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DC Heaters for Phono Preamp
Sure, that would work fine. I do the same thing off of my single transformers, and I pretty much use AC heaters exclusively in my builds. I like to put the heater wiring to the highest-current demand sockets first, and then daisy chain additional twisted pairs to any additional sockets from there, usually ending at the input tube socket. Here I run a pair of 100R resistors as an artificial center tap, and then ground the junction, or run a wire to a heater lift from there. Dead quiet if tightly twisted and routed correctly away from other sensitive bits. Do this with one wireset per channel from the transformer. Easy peasy

I've only had to resort to DC a couple times these last few years, and I suspect even then that it was due to noisy power coming from the mains. I only run DC if it shows absolutely necessary, or if it's just plain easier (single tube builds with on-hand or junkbox transformers, etc) when I can help it.
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