Need Power supply advice for Andreas 845

[Rob11966]

Hi Folks,

I am just finalizing the specs for my power transformer and I think that I have found small a problem with Andrea's design - however he knows a lot more than me so it is unlikely!

The problem is this - the 845 10V filament windings are specified at 4A. Sounds OK as the filament draw for the 845 is only 3.25A for the RCA tube and less (2.5) for the Shuguang which I will be using. However, the filament voltage is full wave rectified and my understanding is that when rectified the AC current supply has to be divided by 1.8. Therefor the transformer as specified will only supply 4A/1.8 of rectified current = 2.22A.

Am I correct with this and if so should I specify a higher current rating (say 4.5A) for these filament windings?

Thanks for the advice, the circuit is to be found here if you are not familiar with it -

http://www.audiodesignguide.com/New845/New845v2.html

Rob

[Richard Ellis]

Don't know of a current drop that you point out, but I did notice the secondary winding of the 845 heater is rated @ 10VAC, 4A
Fullwave rectifyed gets you 14.1 VDC...assuming the rating is noted as RMS....perhaps that air-choke is used as a dropping resistor. I would look up the pure resistance of that choke & do the calculations for a voltage divider....also the ripple calculations as 20,000 mics seems a bit low. As the author has an up & running amp & this is the correct schematic undoubtedly this is his approach. Technically the schematic is incorrect as the mains has a bazzilion secondaries all on one core...so the 'Iron core" designation in the schematic has it all on one core.
Personally I would up the current rating to at least, Five amps, six would be great.........................but thats' just me, I like overkill.....lotsa headroom, safety margin.
_________________________________________Rick..... .......

[Rob11966]

Thanks Richard,

Here is an explanation of the current drop across bridge rectifiers -

http://www.tubecad.com/2008/09/blog0149.htm

I have upped the current rating to 5A and also busted that transformer up into two separate units.

Thanks for your advice, I presume that the voltage is OK an two counts -
1. The designer is a clever guy
2. The amp is up and running.

The explanation for the lower current on the 10V winding may be that the transformer appeared to be designed for an earlier version with no DC rectification on the 10v winding which was then re-used for this amp (but I could be wrong about this).

Cheers,

Rob

[Rob11966]

Sorry Richard,

One more thing, I have modeled it on PSU and the output voltage is less than 14 V but I always have trouble with PSU as I never know what to enter for the transformer resistance and the off-load voltage (and also the load resistance to be frank although I can make a good guess at it) so my results are always a bit questionable and look quite different to my real life bench tests.

Rob

[gingertube]

Rob,
Bookmark this link. It gives you the DC/AC Voltage and DC/AC Current "guidelines" for the most popular power supply configurations. I have it in hardcopy pinned on the wall.

http://www.hammondmfg.com/pdf/5c007.pdf

Cheers,
Ian

[Rob11966]

Hey Ian,

Thanks heaps - that is gold.

Cheers,

Rob

[Richard Ellis]

Thanks Rob for the new info!! I can't believe I missed learning this!
Back in '85' when I got my BSEE we were flooded with formulas.. SS circuit topology, microwaves, antennas, early microprocessors, analog TV, et.al......some I still have stuck in the brain, some written down & filed..pages & pages.
As I'm moving into the wonderful (Old) world of tubes, the old knowledge is being supplimented by the 'old school' new stuff....so more pages are being put into the 'brain bank'......I wonder if I should dive into transformer design...a bunch more formulas to file.
_______________________________________Rick....... .

[Rob11966]

Hi Richard,

It is interesting and seems to be plausible but it is not widely publicized and appears to be quite different from the Hammond advice posted by Ian - After reading the Hammond stuff (and they should know) I am really confused about the truth of the matter. In practical terms it is simple to go with the higher current rating but this does increase the size and cost of the transformer.

Cheers,

Rob