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Old 1st April 2013, 04:39 PM   #1
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Default Power supply options for tube amplifiers.

I wanted to start a thread to discuss power supply options. The power transformer is a fairly large and heavy component in a tube amplifier. Are there other options?

Why aren't switched mode power supplies common in tube amplifiers?

It dawned on me that active power factor correcting computer power supplies first step up the line voltage to ~400v before stepping it down again to 12v, 5v, and 3.3v. There are other voltages but those 3 are the high current outputs. One other output to make note of is the -12v output. Maybe it could be used for bias. The 400v DC source inside the supply would seem ideal for the output stage of a tube amp. The 12v may be ok for some preamp tubes. Would the 5v be enough for lighting 6v heaters?

I'm not sure how isolated a tube's filament is from the rest of the tube. Could a pair of PP output filaments be wired in series for 12v without effecting the function of the tube?

Another cheap switched mode supply is electronic ballasts used in fluorescent lighting. Maybe it could be used as is or modified for tube use. My concern with it is that some may not be 100% isolated from the ac line.

Thoughts?
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Old 1st April 2013, 04:47 PM   #2
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The 400V DC rail of PFC type switching supplies is usually on the mains side and should not be used for anything except driving the PFC circuit in this case.

Supplying DIY circuits directly from the mains without an insulation transformer is dangerous and should be avoided under any circumstances.

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Old 1st April 2013, 04:49 PM   #3
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Besides, 12V anode voltage does not make a preamp, but a quite annoying effects box instead. Also, 5V is way to low for 6.3V heaters, seriously reducing valve life.

Switch-mode supplies can be used for valve circuitry, but they have to be designed for that purpose. Computer stuff will not work.

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Old 1st April 2013, 05:04 PM   #4
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I was thinking the +12v could be used as filament power for tubes like the 12AX7. Obviously, if there is 400vdc available, I would not suggest a 12v anode.

I'll have to look at the supply again but I believe the step-up in the pfc stage was a push-pull invertor and isolated. I'm almost sure the negative leg of a 450v cap was on the "pc side" of the circuit.

Power is power and as long as the voltage and current requirement is met (and isolated). I'm sure there's an alternative out there.

Now that you've explained under-volting filaments is bad... "Could a pair of PP output filaments be wired in series for 12v without effecting the function of the tube?" Does the filament have to "float"? If one side or the other of a filament had a positive or negative dc voltage present, would it effect the tube's function? In other words, if two output tubes filaments were wired in series, you would see 0v(gnd) and 6v on one tube and 6 and 12v on the other. How would this effect the amplifier?

Last edited by lazzer408; 1st April 2013 at 05:20 PM.
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Old 1st April 2013, 05:14 PM   #5
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You're pretty much stuck with a big iron transformer of some sort,short of a custom SMPS.

One trick I've used in quick-n-dirty prototypes and such is to use a power transformer from a big old SS amp. I had one that was 50-0-50 (0-100),and fed it into a voltage tripler for a B+ of around 320V,IIRC. if the transformer has other windings available,you might be able to use them for filaments/bias/etc.

Really though,whatever you do,the key is ISOLATION.
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Old 1st April 2013, 05:32 PM   #6
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Quote:
Originally Posted by DigitalJunkie View Post
You're pretty much stuck with a big iron transformer of some sort,short of a custom SMPS.

One trick I've used in quick-n-dirty prototypes and such is to use a power transformer from a big old SS amp. I had one that was 50-0-50 (0-100),and fed it into a voltage tripler for a B+ of around 320V,IIRC. if the transformer has other windings available,you might be able to use them for filaments/bias/etc.

Really though,whatever you do,the key is ISOLATION.
There's a ton of switching supplies out there for anything from laser supplies to lighting. Just have to find it.

I've done the same as you using SS power transformers and voltage multipliers. Mine was from an old Pioneer amp that had a lighting winding perfect for filaments. Then there's the old step-down > step-up +multiplier trick to get filament power.

I understand the importance of isolation and wouldn't suggest otherwise.
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Old 1st April 2013, 06:31 PM   #7
hpeter is offline hpeter  Europe
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hammond trafos are not expensive, cheaper than isolation 230/230 trafo
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Old 1st April 2013, 07:34 PM   #8
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For the brave ones i can recomend a plasma tv PSU.

Plenty of low volt for heaters and 1000+ V for HT.
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Old 1st April 2013, 07:59 PM   #9
pilli is offline pilli  France
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Is it considered ok to use back-to-back transformers, to get 220V (minus some losses)?

Like for example 220:12 into 12:220.


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Old 1st April 2013, 07:59 PM   #10
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Quote:
Originally Posted by pilli View Post
Is it considered ok to use back-to-back transformers, to get 220V (minus some losses)?

Like for example 220:12 into 12:220.


-
That was mentioned. I'm interested in making a small high-power tube amp. Just curiosity.

Quote:
Originally Posted by flatheadmurre View Post
For the brave ones i can recomend a plasma tv PSU.

Plenty of low volt for heaters and 1000+ V for HT.
Don't start giving me ideas for a quad 300b PP amp.

Last edited by lazzer408; 1st April 2013 at 08:01 PM.
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