| Electronics > Projects, Designs, and Technical Stuff |
| Calling all vacuum tube pros: A power supply question |
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| cvanc:
Hi all- Please take a look at the attached schematic. I give you (drum roll please) the power supply of the legendary McIntosh C22 preamplifier. This is a highly regarded classic piece from The Golden Era. One of these in good condition can fetch thousands of dollars. Except... I don't have one of those. I have one of the other ones :-DD . It's in really sad shape right now. I don't work on tube stuff every day and might be missing something basic here, but can someone explain the nuance of this power supply? I have a couple of specific questions but would love to hear any comments. The 6 filaments are all 12AX7s. The DC filament supply ties to the midpoint of 2 of the 12AX7s. These 2 tubes serve as the cathode followers at the output of the line stage. In other words they are the output tubes of the entire preamp. So... What's up with the plate supply connecting down to the filament supply? Is there some kind of plate/filament/ground offset relationship that is considered best practice going on here? I'd love to get clarity on that. I might be forced to build an entirely new (and perhaps external?) power supply to make this unit right. I want to understand what tricks are being played and make sure to emulate anything that needs to still exist for best performance. And by 'performance' I mean low noise and low hum. OK gang, let me have it :-+ Thanks. |
| 001:
it is oldtimer trick to eliminate hum from heaters It simply bias parasitic diode behind heater and catode |
| Benta:
Sneaky circuit :-+ What you have is a capacitive voltage doubler with D1, D2, C29 and C30, the last is split into several sections. D2 sets the DC reference level. |
| T3sl4co1l:
No, it's not for heater bias, it's all common ground. That is a common addition though. Appears they just wanted to save ~18V by stacking the HV on top of it. Also, it's 18V filament supply, oddly enough, but that's fine. I'm not convinced that doubler is drawn correctly. It seems like it's missing a ground return. Tim |
| cvanc:
--- Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pm ---No, it's not for heater bias, it's all common ground. That is a common addition though. --- End quote --- I've heard of this and I wondered but do not know the rules of thumb that apply. Is the general idea to make the filament(s) more positive than the cathode(s)? Is this done for noise reasons, or something else? --- Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pm ---Appears they just wanted to save ~18V by stacking the HV on top of it. --- End quote --- I kept coming back to that! It's like the transformer order was already out the door when they decided they wanted a bit more B+. I also thought of more far out stuff, like maybe the ripple signals from the 2 supplies nulled each other out if blended together "just so". --- Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pm ---Also, it's 18V filament supply, oddly enough, but that's fine. --- End quote --- 12AX7 tubes have a center-tapped 12.6V filament. The drawing is a bit obscure if you don't know that, but it shows the 18V supply connecting to the CT of two of the tubes, and the overall arrangement of all the filaments is as 4 18V branches each of which goes to common. --- Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pm ---I'm not convinced that doubler is drawn correctly. It seems like it's missing a ground return. --- End quote --- I kept coming back to THAT as well. The voltage doubler part (drawn unlike any doubler I've ever seen before, not that I've seen that many) clearly AC couples to common via the filament filter caps, but it doesn't DC couple to common anywhere. That has to be a drawing error, right? Thanks everbody. |
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