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Eli
Been trying to bread board the 845 psu using the hybrid bridge you told me about. It seems the 6au4's are drawing a lot of current when I start getting up above 800vdc. Popped a 3 amp fuse on me. Does that sound right to you?
Thanks
Vince
Follow Ups:
Sounds like something might be wrong with the wiring. Are you using a variac to bring it up? Try powering it up with just the bridge components (no L, no C, no amplifier) and see if it still draws excess current.
Won't the voltage go over the top if I do that? The heavy current draw only shows up when it get into the upper reaches of the voltage, so I don't think it would be the L or C.
The peak voltage at the output of the rectifier will be virtually the same value as if the bridge were driving a L or C. Don't connect it to the amp like this, of course. It's only a test to be sure the transformer and rectifier are correctly wired. Current draw should be minimal.
That worked fine. Voltage went up to 590 vdc with no visable amps on the variac meter. This was with the rectifiers if full wave configuration.
Ok, I will give that try.
You are using a choke I/P filter, aren't you?Turn on surges are something seen when cap. I/P filters are used.
Also, to properly simulate real world operating conditions, a breadboarded choke I/P filter must be loaded down. A 10 H. 1st inductor needs a 10 KOhm HIGH wattage resistor in parallel with the 1st filter capacitor. By hook or by crook, the critical current must be drawn in order to obtain good regulation. If the critical current is not drawn, cap. I/P filter behavior is observed.
Eli D.
Yep, choke I/P filter. It seems that about the time the voltage gets up to 700 or so it really starts pulling the current (as seen on the meter on my variac, it goes from almost no draw to over 3 amps in about 150 volts). Why would it wait until the last little bit to really start drawing current?
Vinnie, exactly what is connected to the end of the supply? I understand there's an L and a C, but what's after that? An amplifier? A big power resistor? What?Also, are the heaters of the 6au4's also powered by the variac, or are they powered by a separate 6V transformer that's not plugged into the variac?
Dave
I have the front end module of the amplifier hooked up to the old psu module with the hybrid bridge and one of the two chokes and the capacitors standing along side.
The 6au4 filaments are powered by a separate tranny that is also brought up by the variac.
I actually got music out the other end before the fuse blew at about 850 vdc. It's running the actual load.
It's possible that the current comes on all of a sudden because the 6au4 heaters need to get up to a certain voltage before they start to work. I suggest plugging the heater supply in to a separate output (actually, all of the heaters/filaments) and let them warm up for a minute before you start turning up the variac. Odds are, you'll see different behavior.The 3 amps is on the primary side of the big power transformer. Is this the 1450V unit that you mentioned before? That's a turns ratio of something like 1:12, which means that the secondary is delivering something like 3/12 = 250mA. If it's a stereo amp, that's not a lot for 845's. Could be your variac is blowing fuses because it's just not up to the task (assuming the fuse is in the variac.) If it's a monoblock then it might be a lot, especially since you are not up to full operating voltage yet. If that's the case then I would look for a problem somewhere. Is this an amp that you had running good before with a different power supply? What kind of B+ voltage and idle current were you seeing then?
-- Dave
Yes to both. This is the amp that was running good before and it had about 750 vdc for B+ then, not sure about the about the idle current, but the bias was set at about -115vdc. And yes, this is the 1450 volt tranny.
I think it's more likely to be the 6au4's as they were not on the old amp and if I am reading the tube manual correctly they draw a lot of juice. I hope your guess about them conducting all at once is not the problem, because I am using them to eliminate power spikes with their slow warm up.
Vince,Mr. Cigna told you what needs to be done. The 6AU4s' filament trafo should NOT be on the variac. If you let the heaters warm up before you turn the applied voltage up, the surge should be SMALLER.
Until the heater voltage becomes a reasonable fraction of nominal, cathode emission is ABNORMALLY small. It's like a switch across a resistor was thrown. More current flows.
Eli D.
Eli
Please see above posts about calcs and see if my last post to triode is on the right track.
Thanks!
Vince
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