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PostPosted: 24 Oct 2010, 17:03 
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I actually just finished the supply design based on the Edcor XPWR077 and the 47µf input capacitor. I have been working on the write up for the forum. Everything works out pretty well assuming a 5U4GB rectifier. The efficiencies are within tolerances for the tube and the voltages are close to the target.

My preference would be to stick with the Edcor power transformer if at all possible. If not then my next preference would be the Hammond 282X or 382X. As a last resort the Hammond 714 will work but I'll have to add a bunch of series resistance to the primary to keep from blowing the 5U4. It would also mean two additional filament transformers. Please see if you can convince your client to let us use the Ecdor. (Maybe you could spray paint it black. ;) )

The 300B filament transformer can be anything that supplies a set of dual 6.3v windings which can supply at least 1.2A each. The Hammond 266K12 was just a convenient choice. For that matter a pair of Hammond 166L6s would also fit the bill. These are only $16.65 (USD) each here in the states. A mere "drop in the bucket" compared to some of the other components going into this amp. I don't see anything from Edcor for the 300B filament transformer except a pair of PWRC6.3V3A-1s. But these have an asking price of $23.03 (USD) and are bigger then the Hammond 166L6s.

As a side note. A lot of the calculations for the power supply are based on some "educated guesses" concerning the characteristics of the various transformers. I am going to finalize the design now, however after the transformers arrive, I would like you to measure the primary and secondary DC resistances of the power transformer and the primary DC resistance of the output transformer (tube side). Running the 300Bs as close to the plate dissipation limit as this amp does, I would like to make one last check with the measured component values. This would essentially be to set the value of a single dropping resistor in the PS.

I like the idea of a remote power supply. I agree about the 300Bs being picking up AC and I'm doing everything I can in the design to ensure very clean power. A remote supply will help keep things very clean.

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PostPosted: 24 Oct 2010, 19:48 
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The client is happy with Edcor power tranni. I now need to look for the filamnet tranni. Does the Edcor power tranni you specify have any filament windings?

So 47uf Mundorf is OK?

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PostPosted: 24 Oct 2010, 21:19 
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Mark;

The Edcor XPWR077 has filament windings for both the 5U4 and the 6SN7. The Hammond transformers I am suggesting are only for the stand alone DC 300B filaments. (The 300B filaments must be isolated to provide adequate channel separation.)

The Mundorf 47µf and 100µf look good. We'll need two 47µf and one 100µf caps.

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PostPosted: 25 Oct 2010, 02:51 
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Suncalc wrote:
The Edcor XPWR077 has filament windings for both the 5U4 and the 6SN7. The Hammond transformers I am suggesting are only for the stand alone DC 300B filaments. (The 300B filaments must be isolated to provide adequate channel separation.)

The Mundorf 47µf and 100µf look good. We'll need two 47µf and one 100µf caps.

Is there an Edcor Filament tranni I can use? My local Hammond store doesn't stock the listed one.

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PostPosted: 25 Oct 2010, 06:05 
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When it comes to filament trannies all I need is 5V at 1.2A. Is that it?? Did you say I needed twin 5V at 1.2A??? Total drain 2.4A?????

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PostPosted: 25 Oct 2010, 18:53 
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Well actually not quite. What I am suggesting is two independent windings (or transformers) with 6.3V 1.2A (or greater) windings.

Let me explain. In a DHT the noise induced on the signal by the heater voltage is equal in magnitude to that due to B+ supply ripple. This is why DHTs are specifically designed for DC heaters. It is possible to use a balanced AC heater but the noise immunity for such will always be suspect. As such, given the requirements for high fidelity, a set of DC heaters are a must.

The requirements for the heaters are being driven by ripple (noise) and channel separation. The target ripple requirement for the B+ supply is .003% or -90dBv. Since the heater ripple is induced directly on top of the bias voltage this means that the requirement for heater ripple is...

Vr = 71v * (10 ^ (-90/20) ) = 2.25mV

Due to the incredibly low effective load presented by the heaters, 4.17Ω, acquiring this level of filtering on a high current circuit with just passive filtering will be virtually impossible. As such I am strongly recommending the use of LT1085 low dropout regulators to supply the heater circuits. These regulators give almost 70dB of ripple rejection by themselves and I believe that by using them the 2.25mV total ripple will be achievable.

This is the circuit I am proposing for each 300B heater. Please note that the heater circuit is not grounded in any way. Each heater supply must float approximately 71v above ground and will be ac coupled to ground via the driver stage cathode coupling capacitor.
Attachment:
Heater_circuit.jpg

I know that this is a violation of your normal approach to building amps (Retro-Thermonic) but I believe that in this case the use of these regulators is warranted. The LT1085 is a low dropout regulator with excellent startup characteristics that will handle the difficult job of initial heating of the cold filament. In addition, the snubbing of the heaters with a 0.1µf tantalum might also be a good idea.

The other requirement driving the heater supply design is the channel separation requirement. In order to preserve the soundstage of the amp, a channel separation of at least 70dB is required. One of the reasons that 300B amps do so well as mono-blocks is that the channel separation is guaranteed because the only connection between the amps is via the very low impedance mains. If both heater circuits are in the same box then either two magnetically isolated transformers are required or the DC circuits needs significant buffering to prevent crosstalk. The reason I recommended the dual secondary Hammond is because with each regulator offering 70dB of ripple rejection, the channel separation via conduction is guaranteed to be 140dB plus another 10dB or so for the coupling loss in the transformer. Thus preserving a deep and wide soundstage.

I hope this sheds some more light on the heater supply design. It's not final as there are a couple of prototyping experiments I would like to try, but this should give you some insight into my thinking on the matter.


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PostPosted: 25 Oct 2010, 19:46 
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Everything has been ordered now except the Edcor power tranni. I think we are going to bust the $4K limit!!!

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PostPosted: 26 Oct 2010, 06:00 
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Suncalc wrote:
I'll go with the Edcor XPWR077 for the power transformer and use the Hammond 193J 10H filter choke. Go ahead and order the 1603SEA output transformers and we should be set. I am also planning on using the Hammond 266K12 dual-primary/dual-secondary transformer for the 300B DC filament supplies. So just to be clear on the iron, here's the list:

Power Transformer: Edcor XPWR077 900V(450-0-450)@200mA, 6.3V@5A & 5V(2.5-0-2.5)@5A
Filter Choke: Hammond 193J 10H (200mA, 82Ω)
Filament transformer: Hammond 266K12 "Dual Primary/Dual Secondary" 18.9VA 2x6.3v@1.5A
Output Transformers: Hammond 1630SEA 3500Ω 30W 135mA Single Ended (x2)

This should cover the whole amp for iron.

A comment about the filter caps. I understand the desire to use motor run caps for filtering. The low cost and ready availability make them seem an attractive option. However in my experience these motor caps tend to be noisy in audio circuits. This is definitely a component construction issue with the capacitor design being optimized for other parameters. I'll post my power supply design in a little while. But I'll tell you up front the capacitors called for are 40µf, 100µf, and 47µf all at 500v minimum.

Matt: This power tranni is 120V and Australia is 240V. And Edcor only do a few 240V trannies. Also I have to order tonight or it will be 6 weeks before the tranni arrives. I'll try and find a Hammond equiv.

But I have found a local manufacturer which has a good range with 6.3V and 5V taps. I'm going to guess we can use the 460-0-460V tranni -> PTFWF460 or the lower secondary voltage -> PTFWF426
http://www.avat.com.au/Mambo/index.php?option=com_content&task=view&id=48

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PostPosted: 26 Oct 2010, 09:12 
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Mark;

The PTFWF460 should work with no problems. I'll rerun the power supply output and ripple calculations using this transformer.

Can you confirm for me which Hammond choke you ordered, the 193J or the 193M?

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PostPosted: 26 Oct 2010, 09:15 
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One more thing. Do you guys run at 50Hz or 60Hz down there? It affects the final ripple calculations because the omega-RC in the primary filter stage will be different. I think we're far enough from the knee of the regulation curve that it shouldn't matter, but I would like to check.

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