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300B 5V DC filament supply from 5V AC windings
300B 5V DC filament supply from 5V AC windings
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Old 27th November 2016, 11:36 AM   #1
cowneko is offline cowneko  France
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Default 300B 5V DC filament supply from 5V AC windings

Hello,

Here is my problem; I have a 300B SE amp and, until now, they where driving a pair of 91db sensible 3 ways speakers (Davis Matisse HD), the Hum was then manageable with the help of the Humbucket pot.

I recently switch to a pair of Klipsch HeresyIII speakers witch have a 99db/1w efficiency....... the hum is quite there now, witch was predictable.

I think it's time to think about switching to DC filament supply for the 300B, and I need recommendations in this regards.

My 300W PT as the following secondaries:
- 390-320-0-100-320-390 0.3A (B+)
- 5V 4A for the rectifier
- 6.3V 4A for the two 6sj7
- two 5V-2.5V-0V 5A secondaries for each 300B/2A3 power tubes filaments

Can I regulate a 5VDC from those 5V-2.5V-0V 5A secondaries or must I add another transformer (for witch i don't have much space, since I'll have to find available clearance for the regulation boards too)
Witch should be the most tiny/efficient circuit for the regulators?
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Old 27th November 2016, 12:39 PM   #2
Eli Duttman is offline Eli Duttman  United States
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Each 300B filament draws 1.2 A. So, you will be OK, if you phase up the 2X 5 A./5 VAC windings, in series, and FWCT rectify the composite with 2X Schottky diodes. The low forward drop (approx. 0.5 V.) of a Schottky is important in this situation, as is dealing with only a single forward drop. Use appropriate, low drop out, adjustable, IC regulation, after substantial cap. I/P filtration.
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Old 27th November 2016, 12:50 PM   #3
Ketje is offline Ketje  Belgium
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Probable he needs two independent 5volt sources.You could try rectifying the 5volts from the transformer with a bridge followed by a capacitor of at least 10mF (10V).Gives allmost 5voltsDC with a little ripple, but much better then full AC.
Mona
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Old 27th November 2016, 01:53 PM   #4
kmaier is offline kmaier  United States
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I'm guessing this is a stereo amplifier, yes? As the power transformer has a 100-volt tap, are you using fixed bias or cathode bias?

Two options:

1- If Cathode bias is used, you're using one of the 5V winding per tube. With a 5 amp capacity, you could make a full-wave voltage doubler and have sufficient current and voltage to provide a clean regulated supply per tube.

2- If using fixed bias, you could do as above, or take both 5V windings in series, and use a pair of diodes for a full-wave center-tapped supply with heavier filtering (CLC) and likely get ripple low enough without regulators.

Regards, KM
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Old 27th November 2016, 02:05 PM   #5
cowneko is offline cowneko  France
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Thanks a lot for your answers,

The amp is a stereo amp, cathode biased.
So I may use a FW voltage doubler on each secondaries?
Can you show me a schematic?
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Old 27th November 2016, 02:09 PM   #6
MelB is offline MelB  Canada
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You can use the 6.3v supply for one of the 300B's and both 6SJ7's. Rectify the 6.3V and you will get ~6.6v => large cap => small dropping resistor => 6SJ7s and also to a low drop 5v regulator then one of the 300B's.
If you need separate windings for the 300B's
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Old 27th November 2016, 04:21 PM   #7
Eli Duttman is offline Eli Duttman  United States
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The doubler idea is good. Approx. 1/4 of a winding's RMS rated current is available as DC. 5 A. rated windings yield 1.25 A. of DC, which is adequate.

The "hen scratch" schematic I've uploaded shows the topology needed. Higher current diodes, larger stack caps., and a low drop out regulator are in order.
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File Type: jpeg Phono DC Heater Supply.jpeg (554.5 KB, 339 views)
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Old 27th November 2016, 04:35 PM   #8
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by cowneko View Post
Can you show me a schematic?
Look how another Frenchman did it
Mona
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File Type: jpg Mahé300B.jpg (108.4 KB, 336 views)
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Old 27th November 2016, 04:48 PM   #9
Yvesm is offline Yvesm  France
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+1

Click the image to open in full size.

or (with less sand !)

http://dissident-audio.com/SE_300B/KISS/SE300B-Eric.pdf

Yves.

Last edited by Yvesm; 27th November 2016 at 04:51 PM.
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Old 27th November 2016, 04:53 PM   #10
nautibuoy is offline nautibuoy  United Kingdom
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The thing to remember is that with DHTs like the 300B the filaments are actually part of the audio path so its not as simple as just getting 5VDC if you want to optimise the SQ. I would do it properly with something like the Coleman or TentLab DHT filament supplies, though you'll need an additional transformer (with a separate winding for each 300B).

For my 300Bs I use DHT filament supplies that were developed on a UK audio forum;

DHT Heater using VCCS - PCB or Kit (Possible Group buy) - audio-talk
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