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DC voltage drop Q

12/21/2004 11:11 PM
DC voltage drop Q
Useing 5v3a winding to a ss fwb to a 10m cap I get an unloaded voltage of 6.6vdc. Running this to a single 12ax7 heater drops this to 5.4vdc, I know that dc req's 40% more current than ac, but even at 420ma Im still way under 3a, why such a large voltage drop? Is there any way to get the minimal 5.7vdc 12ax7 req out of a 5v winding?  
12/22/2004 6:58 AM
Don Symes

It's all about the transformer's source impedance ... at least that's the impression I get fiddling with it under Duncan's PSUD-II (Power Supply Unit Designer).  
With the load down to 10mA, you get 5.5-ish volts, adn darned noisy at that.  
...geeze - with the load at 500k ohms, you don't quite get to 5.7V.  
Pick up a copy at in the software section. Measure your 5V winding's DCr and enter that (I was using 3 ohms).  
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12/22/2004 7:39 AM
Wild Bill

10 Mfd? Way too low, IMHO.  
These are low voltage 5vac rectifier pulses and you are asking much more current than you'd draw from a plate B+ supply. So that 10 mfd can never charge up very much - essentially it's not enough to be a cap input filter.  
Hang a 470 mfd or even a 1000 mfd in there and see what happens. I find that when using these windings for DC filaments 500 or 1000 mfd is the minimum value, especially to reduce ripple. With only 10 mfd I'd expect to get a bad filter hum in the 12AX7.  
---Wild Bill
12/22/2004 8:34 AM

Thanks Guys  
Im going by KOC's TOT filter supply that should be able to feed 2 12ax7s  
same PT,  
same bridge  
same cap 2200u, i played w/whis value a little but w/little of no chnage  
I just dont get why Im not replicateing KOCs loaded voltage. I tried dong this in the past w/the 6.3 winding and not having any success. Can some one point me to a solid method of a good dc supply for 2-3 12ax7's? Getting a dedicated filament xfer a choke and a bunch of hi u caps is no problem, I just want to get a clean dc heater supply  
12/22/2004 10:38 AM
Don Symes

"Getting a dedicated filament xfer a choke and a bunch of hi u caps is no problem"
Choke? What's the choke for (except to drop some voltage)? Just my stupid opinion, but that seems like like overkill.
12/23/2004 12:22 AM

Ya a choke if itll smooth things out. Ive got 2 small spares from early tweed builds, whether their appropriate or not I dunno.  
Thanks for the PSUD Ive been screwing around w/it, but must confess Im haveing difficulty interpreting the results.  
I did however figure out the heater has a resistance of 5r, why this totally escaped me I dunno.  
Just a noob taking baby steps  
12/23/2004 7:30 PM
Don Symes

It's not the MOST intuitive tool, but if you screw around with it a bit using PSU circuits you know or understand, the light slowly comes on.  
In the tool, you select a (major) section and tell it what topology you have in mind. Then you can select individual components in each section and get fairly detailed about its properties (try double-clicking in the various menus).  
Once you've completed a run, you select the V or I node(s) you want graphed - like in some SPICEs - and you get a semi-usable picture. You may find it useful to futz with the time parameters to delay and scale the graph.
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