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Old 7th January 2011, 01:22 AM   #1
dfro is offline dfro  United States
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Default Transformer choice for DC heater supply?

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
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Old 7th January 2011, 06:45 AM   #2
dfro is offline dfro  United States
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I now see that this topic has generated some rather 'heated' discussion (pun intended). :>) Tube heaters AC or DC?

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.

Last edited by dfro; 7th January 2011 at 06:49 AM.
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Old 7th January 2011, 06:56 AM   #3
dfro is offline dfro  United States
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What about using a voltage doubler and rectifying it down to 6.3VAC?

Simple DC Heater Supply

I wonder if the 6.3VAC secondary can supply enough current to keep it from sagging?
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Old 7th January 2011, 08:14 AM   #4
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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
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Old 7th January 2011, 09:51 AM   #5
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Quote:
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.
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Old 7th January 2011, 10:35 AM   #6
316a is offline 316a  England
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Quote:
Originally Posted by dfro View Post
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
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Old 7th January 2011, 11:17 AM   #7
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hey-Hey!!!,
So far I have run every amp I've built on AC heaters, including the one using 813's. Even with 10VAC filaments it is quiet. I'll add my thought that it is not at all required in thses cases.
cheers,
Douglas
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Old 7th January 2011, 03:28 PM   #8
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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
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Old 7th January 2011, 05:10 PM   #9
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You will also be challenged to keep hash and noise to a minimum after rectifying all that current. Ask me how I know this
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Old 7th January 2011, 07:17 PM   #10
dfro is offline dfro  United States
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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
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