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Old 26th November 2004, 04:04 PM   #1
Sherman is offline Sherman  United States
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Question SS vs. Tube Rectification (and chokes)?

I'm getting my stuff together for a pair of new KT88 SE monoblocs and have come to the PS.

The most basic question is SS or tube rectification. Tubes have the advantage of slow turn on of B+. SS is smaller and less expensive and I already have some 35A 600V bridges.

The next question is regarding chokes. If I go with tube rectification how do I go about sizing the choke?

Some basics about the circuit-
* KT88 SE monoblocs
* 6N1P driver
* 400V B+
* 75mA Bias

I was thinking of using Hammond 272BX PT and Hammond 1628SE OT. My first thought regarding chokes was Hammond 193D (8H, 150mA, 75 ohms, 600VDC).

Can someone with more experience (I have zero with designing/building tube rectification) help?

1- Tube or SS
2- If tube, choke size?
3- Choke even if SS?
4- Smoothing cap type and size?

Thanks!
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Old 26th November 2004, 04:11 PM   #2
Sherman is offline Sherman  United States
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Default Schematic

Sorry, here is the schematic-
http://213.67.45.187/se_kt88.gif
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Old 26th November 2004, 04:50 PM   #3
markp is offline markp  United States
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A 35amp 600v bridge wont work in a tube amp. You need a much higher(I like at least 3 times) working max voltage rating than the AC it will see. A current rating of one amp is likely enough, use three amps to be safe. The bridge you have is great for a high powered solid state amp.
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Old 26th November 2004, 07:42 PM   #4
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What he said.

The 8H choke will be fine as long as it handles the current. Typically, anything from 1 to 20H will be used. IMHO, the higher values (>5H) are only useful for choke input; any filtering advantage with the larger values can be handled better (i.e., a stiffer rail as a result...[hey stop giggling!]) with a larger second cap.

If you don't want to wind one yourself, you might only find larger (10H range) values in stock. That's fine. If you do want to wind, likely you won't want to put on 5000T, so you'll do a 1-2H size. Fine for CLC but no good for choke input unless you have a high current demand.

SS vs. tube: slow turnon is a bunch of bull (cathode stripping, etc.), and only works on the first shot anyway. If you turn it off then back on within a few seconds, it comes back in the same time as with SS. Tubes burn a very large amount of heat, 5V at 3A is 15W just for heater, plus the 10-40W (SWAG) plate dissipation caused by the voltage drop. Regulation is also horrible in comparison.

Tim
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Old 26th November 2004, 08:13 PM   #5
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what Sch3mat1c forgot to sait is that tube rectification sound better.

the best power supplie you can do is a LC (big choke, small caps)
then, for SE, you need better regulation, so LCLC (big choke small caps)

I try the opposite (big caps, small choke) but it`s not as goot to my ear...

Martin
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Old 26th November 2004, 08:26 PM   #6
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You may not care at this stage but SS rectification generally sucks sonically. Once you've built it you may want to investigate how vacuum rectifiers, damper diodes and mercury compare. Vacuum devices generally drop a lot of volts so comparisons won't be fair but you'd get the idea.

Chokes usually help the sound (apart from doing filtering); in particular the bass. Using SS rects you may be able to get low ripple with large caps but often the sound is better with small caps and large chokes. As you may notice i generally avoid making strong statements and comparing chokes to sliced bread. Many PS chokes don't sound good at all, probably due to high parasitic capacitances and high dc resistance. Sonically i was about to give up on them until i heard some decent examples.

Finally, you may want to investigate a choke input supply. For some strange reason these are not very popular although usually sound better.
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Old 26th November 2004, 09:21 PM   #7
SHiFTY is offline SHiFTY  New Zealand
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With SS diodes, the B+ might rise to 500V before the output tubes warm up. So make sure your filter caps are rated to at least 600V or so.

Valve rectifiers are much more fun to work with. How often do you see SS diodes glow. If you are going to use a valve, use a GZ34 for slow warm up and low voltage drop. As it is class A you don't need to worry too much about regulation, the current draw is reasonably constant. If you use a tube, the maximum first cap will be 47uF or less.

I would use a choke whatever topology you go for, just make sure it is rated for the current.
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Old 27th November 2004, 01:28 AM   #8
Sherman is offline Sherman  United States
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Thumbs up OK, then; tubes all the way

It sounds like the consensus is that tubes would be the way to go. Even though I haven't done tube rectification before I find myself happy with this result and I'm happy to give it a go.

There are other chokes available with similar specs (current capacity, inductance etc.) as slightly lower cost (about 1/2) but I think I want the iron up top with the tubes (gotta show off that expensive hardware!) and those open frame chokes are ugly !

So it seems I need to-
* use a choke with plenty of current capacity for both the driver and output valve
* use as high an inductance as I can afford (10 or 12 H)
* and balance the above with the lowest DC resistance I can find
correct?

Now for the PS caps-
* for 400V B+ it seems the caps should have 600V minimum capability
* what about capacitance?

Thanks to everyone for their input! I'm looking forward to this project.
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Old 27th November 2004, 01:55 AM   #9
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Hi,

Quote:
correct?
Correct.

Choke input is also my prefered way to go about it but it just wastes an awful lot of volts.
Use chokes that are designed for this app. "Swinging chokes" is what we call them over here.

Quote:
* what about capacitance?
Enough for the task at hand, it's basically a constant current circuit. You won't be able to cancel ripple so that rail should be clean for the word go.
More importantly use good quality caps, think MKP, ASC oilers, BGs if you have deep enough pockets.

Cheers,

No one mentioned the RFI pollution those SS diodes spit out?
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Old 27th November 2004, 02:08 AM   #10
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Default A LA CARTE........

Hi,

Oh...as for the circuit proper:

I'd replace the 6N1P and use a ECC88 of decent pedigree, ommit the cathode bypass on that one too.

Use a Mundorf silver in oil coupling cap for the 0.220F (600VDC or thereabouts), use a BG 220F/160VDC not 63VDC.

If the 100K volctrl is a pot put a gridleak of about 470K to 1M behind it too and pray the wiper of that pot won't fry on you.
Try a carbon film, they tend to sound better than those plasticky thingies.

Put a 10K (carbon comp.) gridstopper on the 6N1P or ECC88's grid unless you like to listen to birdies...
A 1K carbon comp. as a gridstopper on the KT88's grid would help overload recovery on that KT too....

Voila, my two pence worth of free advice.....

Cheers,
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