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Transformer choice for DC heater supply?

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How do designers choose power transformers, if they want all their tube heaters supplied by DC?

I like the idea of choosing flexible and generic parts, so the design can be tweaked or repaired easily.

I want to choose a Hammond 272JX transformer for a design I am thinking of, but it seems like Hammond does not make its heater secondaries so that you can create a regulated 6.3VDC heater supply from the two available secondaries. The Vpp is 6.3*sqrt(2)=8.9 Vpp - not enough of a safe margin to handle the rectifiers plus the regulator voltage drop.

I have a couple of the O'Connor books. And, Merlin Blencowe's books are ordered and being shipped!

O'Connor's designs mostly use Hammond transformers with AC heater supplies created from the 6.3VAC secondary for the power tube stage; and, DC heater supplies created from the 5VAC secondary, which are rectified/smoothed, but not regulated for the input tube stage.

Merlin Blencowe, on his site, argues that unregulated/smoothed dc can be MORE noisy than AC. So, the 5VAC route is out. I need more amps anyway than it can supply.

Are there other transformers on the market specifically designed for DC heater supplies, or do I just not use the 272JX's heater secondaries and add a dedicated 9VAC transformer to the design?

I also wonder why Hammond does not make a transformer with a HT secondary and a 9VAC, 8A or 12VAC, 8A secondary for the heaters? I am very new to this, so I imagine I am missing something obvious.

Thanks,
Dave
 
I now see that this topic has generated some rather 'heated' discussion (pun intended). :>) http://www.diyaudio.com/forums/tubes-valves/116603-tube-heaters-ac-dc.html

I stand on the DC side of the fence.

Ignoring the heater secondaries on the 272JX and using a separate transformer could be an advantage for a noob like myself. If I fry the relatively cheap heater transformer, I am not taking out the more expensive HT 600VAC CT supply with it.

If I will be supplying one 12AU7, one 12AX7, and two 6550's. So, that makes 2*300mA + 2*1600mA = 3.8A. So, at 6.3VDC, I need 24W power.

So, I think a 12VAC CT, 3A transformer will suffice.

We will see how my thinking changes once I get Blencowe's book on power supplies.
 
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There is absolutely no point in running the 6550's from DC.
Use the main filament supply to run those.
The 12au7/ax7 gives you lots of flexibility, you could put the fils in series and run a 24 volt supply which only needs to supply 150mA, that would be a piece of cake to arrange.
Regards
Henry
 
There is absolutely no point in running the 6550's from DC.

This should be clear to all diyers, but I wonder why it is not.
If there is somebody who knows a good reason to use DC for output tubes - especially in PP stage - please tell us.

The situation is fully different with preamplifiers and similar.
Anyhow, a typical integrated tube amplifier with input sensitivity of some 0, 5 Vrms can be built with all AC filaments and with no audible hum. (< 80 dB), assuming the contsuction and design is OK.
 
I stand on the DC side of the fence.

Why ? Do you like creating unneccessary work and expense for yourself ? I can understand using DC in the case of filaments , which are in the signal path or with DC heaters in sensitive circuits such as phono or mic pre's but not within a power amp . Your best bet is to ditch DC altogether , form a centre-tap across the heater chain and float this at a pre set DC potential .

316a
 
Hello dfro

I have done personal experiments privious amplifiers (6550PP ones) I have built running the output tubes with DC vs. AC while all the earlier line stages and phase inverter stages were kept with DC for the heaters. There is absolutely no audible improvement by having DC on the 6550's as there is very little gain from in the output tubes themselves. A good twisted pair run on AC is all that is needed. If you insist on running DC on the output tubes, you will face much increased expense to doing the all DC topology due to the high current demand for the heaters of the 6550 finals. Good luck to you which ever path you choose.

Mickeystan
 
Thanks guys for the comments!

I told you all I was a noob. I will now definitely run the power tube filaments with the 6.3VAC secondary.

In all the books I am reading on tube amps, it looks like the power tube filaments are supplied with AC. I am interested to learn how the signal to noise ratio (ac heater hum: signal) is derived for this stage, and why it is not an issue with power tubes.

The 272JX's 5VAC secondary can supply 4A. If I do a voltage doubling circuit, smooth, and then regulate, would I have enough capacity to supply 600mA to the two signal tubes without a brownout on the regulator or too high a current draw on the transformer? I do not know the math for estimating this?

Dave
 
Probably easiest to do a simulation, or to do a temporary mockup with a resistive load and whatever parts you have available. Diode peak currents will be high. Schottky's will need to be rated for a few amps cont. Electrolytics will need to be rated for a few amps ripple. Any regulator will need to be low drop/ Parts may need heatsinking. Switching noise may couple in to other transformer windings.

I concur with others - its an ugly approach, but if you've got the parts for a temporary setup then at least your experience level will go up a notch.

Ciao, Tim
 
I am not opposed to adding another small-in-comparison 9VAC (?) transformer to the amp for the signal tube filaments.

I need to learn about ripple current ratings for elec. caps. I have not heard about that, yet.

I would like to perf-board a voltage doubled, cap smoothed, regulated circuit with the 272JX 5VAC winding, put a scope on it, and learn some things.

I will try a spice simulation next. I used to know how to do that using gschem and ngspice. I just got LTSpice working through Wine on my Ubuntu box. It seems to be the best free(dom) alternative at the moment.

Thanks,
Dave
 
It's worth building up experience when you've got the interest. I've used a small doubler off a heater winding for 12VDC nominal to power a small fan to keep 2x KT88 cool in a tight head unit. Alternatively could do a tripler off 5VAC for fans. Lot's of great small fans around from PC's - just find some small leaded schottky's - and run them slow for negligible noise.
 
I am interested in a toroidal mains transformer. I have built a 4 stepper motor control box using one, and I have read that they contain their magnetic field much better than a standard transformer.

Does anyone have suggestions of where to source toroidal power transformers?

I found Antek's website and I am interested in these two:

AN-2T300 - 300VAC/.300A, 6.3VAC/4A for HT+ and power tube heaters:
Antek - AN-2T300

AN-1209 - 9VAC/5.6A for input tube heaters:
Antek - AN-1209

The prices seem to be very reasonable!
Any thoughts?

Dave
 
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