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Old 22nd October 2007, 07:33 PM   #11
Stixx is offline Stixx  Germany
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Charlie,

sorry for bringing more confusion, but as for the current draw YOU are right...
I checked my own values (I am in the process of building an aikido headamp with 6CG7 and 6H30), and current draw as listed is per TUBE, so your calculations are correct. You flipped the numbers, but total current draw should be around 23.4mA.

As far as I know (being quite a novice myself) you can calculate the current draw using Ohm's law ( I= V/R). Measure the voltage across the cathode resistor and put it into the equation. Example: cathode resistor 470R, measured voltage 3,5V at the bottom Rk, I=7,4mA.
(Hope nobody's beating on me now... )


Quote:
My new PSU consists of: 550VAC; 5V4GA; 0.68uF; 10H; 60uf; 10H; 60uF. When I model this in PSUDII for 23mA, I get a voltage of 317VDC. If I model it for 46mA, I get more like 260VDC. In reality, I see something like 311VDC on the output.
You're almost dead on...311 to 317V is a deviation of 2%!!
I'd call that perfect...

Good luck with your modding,
Oliver
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Old 22nd October 2007, 08:30 PM   #12
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I am glad that was cleared-up. I tried Ohms Law, but plugged in 300V, then 150V. Where did you get 3.5V. I suppose I must go back to the schematic and see what lies between B+ and the plate to figure this out.

Anyway, the upshot of the current-draw is that 24mA with 312V means that my chokes don't have enough henries to properly filter out ripple. Prior to going tube-rectified, I had an extra current draw of around 20mA using a resistor across the final cap. I may simply reinstall this. The B+ will then drop, which I may be able to bring back up using a 1.5uF cap instead of the 0.68uF. I sense a degree of synergism here!

Thanks for your input. You'll not get any beating whatsoever from me - look at the ridiculous number of posts I make to this group and you'll see that my blundering gives me no reason to complain at all!

Regards,
Charlie
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Old 23rd October 2007, 07:19 AM   #13
Stixx is offline Stixx  Germany
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Quote:
Where did you get 3.5V.
The voltage across the cathode resistors is actually that low, it sets the bias. The mentioned 3,5V were just an example, but it should be in the neighbourhood...
Put the probes of your DMV across the bottom cathode resistor (see image) and note the voltage you read. It should be somewhere between 2 and 4V ....put this into the equation...et voila!

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Anyway, the upshot of the current-draw is that 24mA with 312V means that my chokes don't have enough henries to properly filter out ripple.
I wouldn't worry too much about the chokes. I am using chokes in both of my current headphone amplifiers (C3G and a Morgan-Jones variation), both are drawing between 23 and 32mA only, but hum is "almost" indiscernable....over headphones!

Try the bleeder resistor across the final cap, do not worry about voltage too much at this point and listen whether there is an improvement. When there is still hum, I'd put in more capacitance as someone already suggested. It's the cheapest and easiest way instead of pulling your chokes....just try a "lowly" electrolytic cap in the last position with your motor cap.

I know how bothersome hum can be from my own experience...it took me months to get my C3G headamp hum-free, but it was a good learning experience being my first tube project. In my case it was improper wiring and grounding, but my endless fiddling and trying to improve helped in the end...now it sounds truly glorious and is deadquiet!

regards,
Oliver
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Old 23rd October 2007, 04:26 PM   #14
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Oliver,

OK, I'll make the measurement. I think it may be 4.5V for 300V B+ which is what Broskie uses in his original schematic. I expect that with 312V, I'll not notice any appreciable deviation, but you never know.

I have modelled my PSU with differing degrees of capacitance. If I use 107uF on the final (the 60uF, plus 47uF electrolytic), and add another 22uF to the first 60uF cap, I get really low levels of ripple 0.6mV or thereabouts.

What are the negative aspects (if any) of adding extra capacitance? Is there a maximum amount?

When I get chance, I'll drive out to my local surplus place and see what motor-run caps they have. I am sure I can find a 100uF there and that it'll cost less than buying a new electrolytic.

Regards,
Charlie
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