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Old 24th September 2009, 08:45 AM   #1
alexg is offline alexg  Philippines
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Default Help on outboard PSU for tube amp

Am trying to design an outboard psu for three amps (one with B+ of 400VDC for an SEKT88 , the other two amps with 315VDC for 2a3 and a PP EL84).

Using the same power transformer, I can get the different voltage by varying the first capacitor on the PSU.

My power transformer have all the heater windings that I need and I usually use AC for heater.

My question is, can I use a same cord (with multiple wires) for the B+, ground, and AC heaters? Or I need to have a separate connectors for the heaters since I will be using AC heaters?

Thanks.
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Old 24th September 2009, 04:22 PM   #2
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Hi!
I'll advise You to use 2 separated cords /for AC and DC wires/.
AC wires must be twisted. The wire for "ground" must be in DC cord.
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Old 25th September 2009, 02:44 PM   #3
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H.J. Leak main amps and preamps were inter-connected via a single multiple-conductor "umbilical" cable. The Leak preamps did not have their own power supplies, so they had to be supplied from the main amp. Despite this, the on/off switch was built into the volume control in the preamp.

Thus, the "umbilical" for interconnecting the Stereo 20 (or Stereo 50/60) and Varislope stereo preamp had to carry:

* mains live and return from the switch (2 AC conductors)
* 6.3 volt heater supply (2 AC conductors)
* common (1 conductor)
* L and R signal (2 conductors)
* B+ (1 conductor).

The total number of conductors in this single cable was 8, and Leak used an octal plug and socket at each end. Of course, the cable was constructed with the necessary shielding.

It worked well with no objecionable hum, despite the high sensitivity of the main amp, in the order of 150mv for full OP.
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Old 25th September 2009, 03:11 PM   #4
Gordy is offline Gordy  United Kingdom
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You can do it either way.

Tightly twist the current send / return wires in pairs...
Examples;
B+ high voltage twisted with audio supply ground return
heater + twisted with heater -
etc.

Remember that you may need two grounds: one is chassis-to-chassis for safety purposes only, and does not attach to audio ground. The second is audio power supply ground (and is twisted with B+).

If you place everything in one cord then you could shield pairs of wires if you get a problem. Also, an overall outside shield to the cord could be installed, and attached to chassis ground at the Power Supply box.

Remember also, the end of your cord will be live even when not connected to the amplifier, so it is a good idea to fit fuses in each voltage output in the Power Supply chassis just before the output.

: )
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Old 25th September 2009, 03:33 PM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by alexg View Post
Am trying to design an outboard psu for three amps (one with B+ of 400VDC for an SEKT88 , the other two amps with 315VDC for 2a3 and a PP EL84).

Using the same power transformer, I can get the different voltage by varying the first capacitor on the PSU.

My power transformer have all the heater windings that I need and I usually use AC for heater.

My question is, can I use a same cord (with multiple wires) for the B+, ground, and AC heaters? Or I need to have a separate connectors for the heaters since I will be using AC heaters?

Thanks.
Don't bundle the AC heaters with the DC power supply. Route the two as far apart as you can. This would be true even if the PSU was in the same chassis, you'd always keep the AC heater leads twisted together and far from the other leads. This rule should apply even more if the wires are longer

Of course you want to twist the AC lines into twisted pairs but you may want to do the same with the DC supply. Twist each with it's return line to keep the long run of wire from picking up noise present in the room. Also consider using coax cable. It works much better than twisting and provides good mechanical protection two.

The safety conciderations above really need to be followed. Every pair of wire leaving the PSU needs to have a fuse AND inportantly the fuse has to be rated for less amps than the wire it protects. You want the fuse to blow before the wire. The wire that provides chassis ground needs to be at leat one size larger then any of the power wires in the cable. Once the wires leave the PSU chassis they will need some kind of mechanical protection. You can just have plain wire with lethal voltage running along the inside of a stereo rack unprotected

Last edited by ChrisA; 25th September 2009 at 03:40 PM.
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