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Old 13th April 2006, 12:33 AM   #1
phn is offline phn  Sweden
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Default Newbie PSU question

I plan to rebuild an old preamp. I don't know if rebuild is the right word since I plan to replace just about everything but the cabinet and transformer. It's my first tube project.

I would like to integrate the phono headamp with the preamp. It's not a necessity, but is my goal for now. But I'll keep it to the preamp for now. It will be based on the 6267/EF86 (or similar tube) in triode connection. The problem is that the transformer has 290 V and I need 250 at most, as I understand it.

What's the best way to get the voltage down to, say, 225? A resistor should do it. But is there a better way? I guess what I ask is, is there a way to not just burn energy, or to burn as little as possible?

A search here might give me the answer. But I doubt it. I'm completely in the dark here. So don't hold back on the details.

Well, I have a few phono headamp question as well. I'm considering one of these: EF86 preamp (in Japanese; image is 1/3 down the page) and EF86 pre.

Any opinions/input? Also, they have 12.6 V filaments. My preamp has 6.3. Would it work with these? Why different voltages for the same tube?
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Old 13th April 2006, 12:56 AM   #2
Sherman is offline Sherman  United States
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Questions-

Are you comfortable with working with lethal voltages? Tube amp voltages can kill.

Is the 290V directly from the transformer or is that what the PS is putting out currently after rectification and filtering?

What type of rectification are you planning on using? If the PTX puts out 290VAC and you use tube rectification you'll have no problem getting down to the voltage you are after.

Adding an additional RC (resistor/capacitor) section to the filter will also drop the voltage and filter the power supply further.

Answers-
Most tubes that have 12.6V filaments can be operated on 6.3V, the inverse is not true. Of course current draw will double. The method is to tie together two of the pins, often 4 and 5 and connect the 6.3V to the combined pins and pin 9. (Those numbers are tube dependant.)
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Old 13th April 2006, 01:30 AM   #3
phn is offline phn  Sweden
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Thanks for the reply and concern.

Comfortable? No. Secure? I would say, yes, but wary. (I just fried an SMPS, though I wasn't at risk, so I know what the consequenses can be.) Rest assured I will take any and all precaution. The preamp section is very minimalist with few things to get lost in. I wouldn't touch a big power amp, at least not yet.

I haven't looked into the details. For now I only assume the voltage is too high. I haven't gotten as far as to rectifier. My problem isn't follow some schematics, but to make practical use of it, like making small changes to make it fit my needs. I ask here in order to get some basic grasp on the fundamentals. It's very difficult to seek meaningful information when you can't even crawl.

Let's put it this way, how would I go about to get the 310 V of the PSU in the first link down to 290 V?

I don't expect to start this until after the summer. By then I hope to have it all, design, material and some very basic, but useful, knowledge.
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Old 13th April 2006, 02:55 AM   #4
jayme is offline jayme  United States
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As a first step, I highly recommend ordering Morgan Jones' book "Valve Amplifiers".

It is basically *the* textbook for DIY on tube-based amps and preamps. And he devotes a section to power supply design, as well.

Very well written, and very clear for a beginner. I learned quite a bit from it...and am now embarking down the road of building my first preamp.

Also, check out the Power Supply Designer software: PSUD2. Available at www.duncanamps.com This software will allow you to play with power supply components (transformers, capacitors, inductors/chokes, and resistors) to achieve the voltage you need. You'll learn a lot about how to configure a power supply just from playing around with that software.
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Old 13th April 2006, 10:01 AM   #5
phn is offline phn  Sweden
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I have added the Morgan Jones book to my Amazon cart. Yeah, that's probably the best way to go. I really need to up my general understanding of electronics, and not just for this project.
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Old 13th April 2006, 05:39 PM   #6
phn is offline phn  Sweden
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I will scrap the original transformer and get one suitable for my needs. I held on to it because it's there and I wouldn't have to try fit another.

I will use this PSU, or something very close to this. I might use diodes instead.

Anyone that would like to offer his thoughts on it? Ideas for bypass caps? Tube or diodes?

I'm planning to use the Elna Cerafine 47uF/500VDC and
220uF+220uF/350VDC. Any ideas for the 225uF cap? Chokes? Transformer?
Attached Files
File Type: pdf psu.pdf (23.8 KB, 94 views)
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Old 13th April 2006, 07:42 PM   #7
jayme is offline jayme  United States
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Hold on...your "290V transformer"....is that 290-0-290? or 290-0?

Because the transformer specified in the PDF is a 180-0-180 transformer, which can also be described as a 360V transformer....
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Old 13th April 2006, 08:51 PM   #8
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Quote:
Originally posted by phn
I will scrap the original transformer and get one suitable for my needs.
Don't be too quick to do that. First of all, it's not really clear what you have. As others have asked: are the voltages you've quoted unrectified AC (straight from the transformer wires) or DC voltages you've measured from the existing ps?

If they're AC measurements of the unloaded transformer, then those numbers will go down when the transformer is loaded with a functioning circuit.

In any case, a higher voltage isn't necessarily going to be unusable. It might work fine as-is, or you might just need to increase the plate load resistor of the EF86 a little.

Very often a particular power transformer is used simply because it happened to be available. The voltages written on the schematic are those that the builder happened to end up with. They might have been just as happy (or even happier) with something else.
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Old 13th April 2006, 08:57 PM   #9
phn is offline phn  Sweden
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Dave, we wrote at the same time.

I will start this over and post the details and make my questions clearer, what I want and such.

As said, this is my first tube project and I now realize I should have looked into things a little better.
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Old 13th April 2006, 09:51 PM   #10
phn is offline phn  Sweden
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Actually, there are now fewer details.

I have this Leak preamp I don't use. But instead of selling it I decided to have fun with it. (This may make sense to nobody but me. That's enough for me.) Also to turn it into a very good preamp. This is pretty much the preamp I want. I have no questions concerning the preamp, other than possibly the voltage.

I haven't checked the trafo in the preamp, but measured the output of the Leak power amp, 290 V. (It's moded, so don't think about the 125mV input.) Thinking about it, Lundahl should be able to supply me chokes and, if needed, trafo if I email the schematic.

I have also come to the conclusion that I will not be able to fit a phono headamp in the small Leak cabinet with those chokes. Which is fine.

The GZ32/34 may also be a problem. The Leak cabinet is quite small and I think it will be very hot in there. So maybe diodes will work better.
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