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Old 8th January 2012, 10:02 PM   #1521
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Quote:
Originally Posted by Rod Coleman View Post
For my part, I enjoy working with over-rated trafos, but maybe the chassis will wind up being very large.

If you would like 250V dc raw supply before the VR tubes, then my best recommendation would be to start with about 330V rms secondary with 150VA rating, and then use choke-input configured supply. With a valve-rectifier and choke input filter, you will get 260 to 280V dc raw output - and you can fine-tune this by using Duncan PSUD2 software.

I use industrial isolation transformers for this task, because the cost is low.

In the UK this means JMS:

150 VA
Interesting link Rod

Why so large though? Wouldn't even a 75VA trafo be well overspecced?
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Old 9th January 2012, 06:48 AM   #1522
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Quote:
Originally Posted by Original Burnedfingers View Post
It sounds like I should stay with the big transformers? No problem I have enough of them and two separate power supply chassis will make it lighter than one mega chassis with all the transformers on it.
I think that if you can raise the raw dc voltage required, then build with the transformers you have. If you really enjoy the preamp, you can upgrade the trafo, or if you want to try another power supply design, eg shunt regulated (SSHV), you can design-in the right trafo for the job.

Plenty of room on the chassis means you can try out different things later.
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Old 9th January 2012, 06:54 AM   #1523
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Originally Posted by simon2a3 View Post
Interesting link Rod

Why so large though? Wouldn't even a 75VA trafo be well overspecced?
I think the 150VA is the smallest size of JMS that is clearly visible as a split-bobbin design. The smaller clamp-mounted trafos may have the secondary wound directly on top of the mains primary - meaning much higher inter-winding capacitance. For a preamp, the mains trafo should have the lowest possible primary-secondary capacitance, and a split bobbin design keeps it to about 70pF, even at 150VA and 330V (measured on JMS builds).

An electrostatic screen makes an even better solution - but at a higher cost.
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Old 9th January 2012, 11:46 AM   #1524
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Originally Posted by Rod Coleman View Post
I think the 150VA is the smallest size of JMS that is clearly visible as a split-bobbin design. The smaller clamp-mounted trafos may have the secondary wound directly on top of the mains primary - meaning much higher inter-winding capacitance. For a preamp, the mains trafo should have the lowest possible primary-secondary capacitance, and a split bobbin design keeps it to about 70pF, even at 150VA and 330V (measured on JMS builds).

An electrostatic screen makes an even better solution - but at a higher cost.
Ah, I see, thanks Rod. Presumably low primary-secondary capacitance is good for output stages too? Eventually I'll box-up my 26-10Y-300B amp which currently has two mono power supplies because of weight and the iron I had to hand. But one option is to use a single large mains trafo, 300VA perhaps, or larger even. That would give plenty of scope for playing around with a shunt reg for the 26s too.
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Old 13th January 2012, 02:43 PM   #1525
Magz is offline Magz  United States
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Default Starved Filaments on #26 Tubes

Has anyone seen this work by Steve Bench re: starved filaments on DHTs? It appears that drastically reducing the voltage on DHT filaments (#26 included) can result in much lower distortion in small signal applications such as preamps.

Anyone ever give this a try?

DHT with starved filaments.
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Old 13th January 2012, 04:52 PM   #1526
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Yes and with excellent results in reducing THD and microphonics. See my thread around 4P1L DHT line stage
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Old 18th January 2012, 10:43 AM   #1527
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I have a question concerning R6's value on the Rod Coleman heater kit. The paper with the parts count and values list R6's value of 150 ohms. The paperwork shows 330 ohms. Which do I use for the #26?

It is a very nice compact neatly designed board in my opinion. It only took about 15min to put them both together. I can hardly wait for the weekend to start on the power supply.
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Old 18th January 2012, 11:05 AM   #1528
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Quote:
Originally Posted by Original Burnedfingers View Post
I have a question concerning R6's value on the Rod Coleman heater kit. The paper with the parts count and values list R6's value of 150 ohms. The paperwork shows 330 ohms. Which do I use for the #26?

It is a very nice compact neatly designed board in my opinion. It only took about 15min to put them both together. I can hardly wait for the weekend to start on the power supply.
Hi Joe,

Looks like a slip with the delivery note.

With your kits, please use R6 = 330. This should be present in the kit.

I hope you enjoy the sound with them!
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Old 19th January 2012, 10:32 AM   #1529
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And I used the 330's when I put it together
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Old 19th January 2012, 10:35 AM   #1530
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Pleased to hear that the assembly job was easy.... look forward to getting your impressions on the sound of them!
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