PSU

This project began simply because of circumstances. I have 2 chassis which are eventually intended to be push-pull 6l6 amps but I also have a number of other bits. Unfortunately I can’t afford to continue with the monoblocks at the moment and I couldn’t find a suitable mains transformer to power a stereo SE amp, even though I have some output transformers & valves. Then I found a computer PSU that I bought years ago. The spec included 5A 5V via a 78H05 regulator. The transformer was more interesting though. It had obviously been intended for some other purpose originally. The secondary windings are 9-0-9v at 7A and 110v at 0.7A, the latter appears to have been intended to power a monitor. The unit also included a pcb containing the rectifier and a few more bits to provide +12v and -5v supplies. I thought this could be useful, producing a 6.3v DC circuit by tweaking the regulator and voltage-doubling the 110v to get nearly 300vDC for the HT supply. So I carved it to bits… 🙂

This shows the 78H05 regulator re-housed on a heatsink rescued from some scrap years ago. Nice shape though! The PCB shown here has been very heavily modified to perform its new duties. The rectifier is the only original component.

Then a bit of chassis-bashing ensued… Perhaps I should explain a bit more about this PSU. It is intended to be a general-purpose unit with full remote control from the equipment that it is powering.

Two relays are fitted. One switches the heater and a bias supply derived from another voltage doubler. The other switches the 110v feed into the HT voltage doubler and also switches the HT output between the voltage doubler and a large 1k discharge resistor. The two relays are interlocked so that HT cannot be applied without the heaters being on. It is intended to add some automatic sequencing later. As it is, unplugging the amp end automatically cuts the HT off the plug.
The (almost) final unit hasn’t been fully tested yet. It is producing suitable voltages and is switching them correctly, so it stands a good chance!

 

 

 

 

 

 

 

Specification:

HT: 290v 300mA
LT: 6.3v 5A DC regulated
Bias: -23vDC
Also: 12vDC continuous

Connection cable is 8-core and carries:
Chassis earth
Ov (not solidly earthed)
Heater supply
+12vDC (unswitched, continuous)
Bias supply
HT supply
Off/On control line (active low)
Standby/On control line (active low)

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