Need Power supply advice for Andreas 845 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th November 2008, 10:08 AM   #1
diyAudio Member
 
Rob11966's Avatar
 
Join Date: Jun 2007
Location: Australia
Default Need Power supply advice for Andreas 845

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
  Reply With Quote
Old 9th November 2008, 03:51 PM   #2
diyAudio Member
 
Richard Ellis's Avatar
 
Join Date: Sep 2006
Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
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..... .......
  Reply With Quote
Old 9th November 2008, 08:38 PM   #3
diyAudio Member
 
Rob11966's Avatar
 
Join Date: Jun 2007
Location: Australia
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
  Reply With Quote
Old 9th November 2008, 08:42 PM   #4
diyAudio Member
 
Rob11966's Avatar
 
Join Date: Jun 2007
Location: Australia
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
  Reply With Quote
Old 9th November 2008, 11:13 PM   #5
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
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
  Reply With Quote
Old 10th November 2008, 04:33 AM   #6
diyAudio Member
 
Rob11966's Avatar
 
Join Date: Jun 2007
Location: Australia
Hey Ian,

Thanks heaps - that is gold.

Cheers,

Rob
  Reply With Quote
Old 12th November 2008, 02:48 AM   #7
diyAudio Member
 
Richard Ellis's Avatar
 
Join Date: Sep 2006
Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
Default New formulas!

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....... .
  Reply With Quote
Old 12th November 2008, 04:22 AM   #8
diyAudio Member
 
Rob11966's Avatar
 
Join Date: Jun 2007
Location: Australia
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
DC Power supply questions, advice appreciated! gypsielectro Power Supplies 2 2nd April 2009 12:39 AM
need power supply advice for future power amp project. boricuaso Chip Amps 13 6th February 2008 03:24 PM
Some advice on this power supply Richard Tubes / Valves 11 23rd June 2006 06:21 AM
need advice on aleph 3 power supply jeapel Pass Labs 7 3rd September 2005 01:02 PM


New To Site? Need Help?

All times are GMT. The time now is 02:56 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2