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Old 31st March 2010, 12:58 PM   #361
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Quote:
Originally Posted by coolzero View Post
Hi,

Attached schematic shows what exactly I have now.

For CCS between choke and B+ supply, I used K&K audio, compact CCS (K & K Audio - Lundahl Transformers, audio DIY kits and more). Filaments are in series, fed by a SMPS used in DVD players (SMPS fo DVD SMPS - 226 SMPS for DVD CN;GUA products). SMPS works well to reduce the overall hum drastically, which I couldn't control using any other simple filament supply. Plate chokes are from EP (Electra-Print.com High-Fidelity Audio Tranformers). Most of the components I used which I had already. It is not necessary to have the exact same part mentioned. You could easily find alternatives for some of those for a lesser price.

However, to make my pre a pure DIY I am planning to replace B+ PSU with Salas simplistic regulator and Filament supply with Rod's Gyrator/CCS. Hope it will give me a better result. You could easily find the details of those in this thread if you wish to use those as well. This thread includes pretty much everything you need to build a hum free wonderful sounding 26 pre. Good luck.
Thank you so very much for taking the time compose the list for me. I will print this out and give it to my builder. Hopefully this will be a success.
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Old 31st March 2010, 03:18 PM   #362
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by regal View Post
I have a Broskie PS-2 power supply which is similiar but it has the Lm reg on the output after the 10m45S, so it is a series regulator. I had to go this route because I'm using a SS buffer after the tube which has a DC servo that goes nuts if the B+ voltage drifts even 100 mA which is common for shunt regulation, but this combo looks like it might hold a constant voltage?
Depends on what you mean by that. Usually a servo is used where a small DC offset will be amplified by a following stage. So it has to be very tightly regulated to a specific voltage. The servo typically references the stage output voltage, not B+. That's not an issue for a B+ supply as used here. A modest amount of thermal drift will not cause any noise on the output. As the VR tubes age, voltage may change a little. What we want is very low ripple. The type of shunt supply I posted (and the Salas supply) will hold the voltage within a volt or so, with very low ripple.

Sheldon
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Old 1st April 2010, 09:42 AM   #363
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Default Cathode Caps and Resistors

In discussing filament heating, I mentioned connecting the cathode capacitor to the negative side of the filament for best sound.

When I went to my 300BSE to mod something today, I found that I had not wired it like that in 2004. And when I tried moving the cap to the negative filament, the sound degraded badly - especially, the image was very unstable.

If you use a cathode capacitor, be sure to try it on both sides, to ensure you get best sound.

The drawing shows exactly what I use on 300Bs. The cathode resistors are split, to try to ensure that the anode current drawn from each side is the same.

C2 is the Western Electric cap, designed to improve supply rejection.
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File Type: jpeg 300bwecathodes.jpeg (65.6 KB, 771 views)
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Old 1st April 2010, 11:32 AM   #364
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Hi Rod,

Some advice needed here. Right now I just have a LM1084 as a current source for the 300b filaments. No gyrator. I've connected the plus to pin one and minus (off the capacitor) to pin 4. I've put the cathode resistor on pin 4. Should I put it on pin 1 which is the filament positive? I haven't used a cap yet - where do I connect that?

Best,

Andy
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Old 1st April 2010, 12:07 PM   #365
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I should just add that I also have a PP 300b amp building, which has a shared cathode resistor, so what's best to do there?

Andy
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Old 1st April 2010, 12:43 PM   #366
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hello Andy,

The SE connexion sounds correct. As far as I'm aware the 300B is symmetrical (not the case for every DHT) and so either filament pin can be +ve.

If you only have one cathode resistor, connect it to the FIL -ve. Then connect the capacitor to the FIL+.

If you don't use a gyrator, try adding an HF choke in its place, to be sure rectifier current pulses and mains noise (from the filament supply) are minimized. This is especially important if your trafo is toroid.

As you've found out, the current source makes more difference than the gyrator in most cases.

With PP, depends on whether you're using a common cathode resistor, or two separates. With 2 resistors you can use 2 circuits just like SE, which I suspect is the highest quality route.

With a common cathode resistor, you CCS must be a source (not a sink), and the circuit becomes like the one I posted for PP at post 348:
#26 pre amp


have fun building!
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Old 1st April 2010, 02:53 PM   #367
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Hi Rod,

As you know I'm not an EE but a humble hobbyist, and I sometimes get thrown by the theory if I'm unfamiliar with it. so I looked up current sink and source on Wiki and by the time I was knee deep in biological membranes and trolls lurking in rivers I gave up. It's a wacky read!

So am I right in thinking that a "source" is in the positive line? That's what I have - LM1084 with the input positive from the reservoir cap and output going to pin 1 which is nominally the positive pin. The negative of the cap then goes to pin 4. I'm attaching the common cathode resistor to pins 4 and 4 joined of the two 300b outputs.

I don't have room for chokes, but I'd be interested in adding a simple gyrator. I have a few LM1084 so tend to use those. Can you suggest a circuit that would add a gyrator to the LM1084 current source (or would it be sink then, if the LM1084 were in the negative side - bit hazy here?).

Andy
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Old 1st April 2010, 03:59 PM   #368
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Andy, the quickest gyrator is just to take the ST logic level FET named in the FET version of the circuit (in this very thread).

Connect the CCS to the negative side.

Connect the V+ to the drain of the FET. connect a 1M or 1,5M resistor to V+, other end to a 1 to 5uF MKP capacitor (cap's other end to ground). Connect the RC join to FET gate via 1K resistor (carbon composition or 1206 thick-film chip). Minimise the gate wiring absolutely, or there may be subtle, or not-so-subtle oscillation trouble.

The FET source is your gyrator output.

Make sure there's 2V across the FET, and that heatsinking is adequate, before using on a filament (I use a pair of 2 Ohm 25W resistors to test them)
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Old 2nd April 2010, 01:42 AM   #369
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Quote:
Originally Posted by Rod Coleman View Post
In discussing filament heating, I mentioned connecting the cathode capacitor to the negative side of the filament for best sound.

When I went to my 300BSE to mod something today, I found that I had not wired it like that in 2004. And when I tried moving the cap to the negative filament, the sound degraded badly - especially, the image was very unstable.

If you use a cathode capacitor, be sure to try it on both sides, to ensure you get best sound.

The drawing shows exactly what I use on 300Bs. The cathode resistors are split, to try to ensure that the anode current drawn from each side is the same.

C2 is the Western Electric cap, designed to improve supply rejection.
Hi Rod Coleman!

Do you want say about the circuit???
It is good or no??? i want try use it for my 300B Se. What is wrong in the circuit???
Attached Images
File Type: jpg CRICUIT-DTH-FILAMENT-2010.jpg (137.0 KB, 703 views)
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Old 2nd April 2010, 10:23 AM   #370
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Quanghao a resistor before and coils between caps can be bad
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