[CButterworth]

It seems to me that the Cornet2 PSU is very quiet.

This has led me to think about replacing my solid-state, choke filtered PSU in my Aikido preamp with a tube-rectified PSU as in the Cornet2.

The Cornet2 uses a B+ of 356VDC, whereas my Aikido would prefer around 320VDC for my 6SN7s.

I thought that a bit of modelling in PsudII would allow me to change the Cornet2 PSU to give 320VDC.  Obviously, to be sure, if I could get 365V modelling the Cornet2, then I can be a little bit more sure that my modifications would give close to 320V.

However, I cannot get PsudII to model Jim's Cornet2 PSU and give 365V.

Any idea why not?  Also any idea on how I might modify a Cornet2 PSU to give 320V?

Right now, my Aikido uses ss recitifcation, 2 10H chokes and 2 45uF motor-run caps.  This has resulted in much ironwork and weight.

Thanks,
Charlie

[JoshK]

I am just fully learning the ins and outs of PSUII, but did you model the power transformer's impedance correctly?  There is an article in Bas's diy rag that provides the instructions on how to do that.  It does make a difference.  I got quite a bit different voltage from my quick and dirty estimation, which is to say I didn't do it right at first. 

If you PM me the cornet's PSU schematic or better yet email me the .psu, I'll have a look at it.  Don't post it in public, as that is Jim's IP and I promise I won't go blabbing. 

[CButterworth]

Josh,

I can email you the schematic, although it is available from Jim's website in the Cornet2 manual.  It seems that Jim won't mind me co-opting his design for the Aikido, as I paid and built the Cornet2, and I most certainly do not plan on modifying his design for monetary profit.

If you read this, let me know if I am correct.  You can send a PM.  I will not be offended as I have much respect for your designs.

Charlie

[JoshK]

I didn't know he put the design in the public domain.  I planned on sending Jim $$, and still do, if and when I get to building the cornet.  No point in struggling to build a proven design when the board and plans are so cheap.  I think guys like Jim need all the support of the community since he is offering such a great and afforrdable service to the diy community.

Anyway, send me the .psu you have Charlie... pm me for my email address if you don't have it already.

[hagtech]

It's the resistance of the windings in the transformer secondary.  Add that to your puzzle, and then you'll see that the actual voltage output of the secondary is simulated much higher than specified.  Only at full output current do the windings produce the rated voltage.  I use this to my advantage in the CORNET2 to bring up the voltage a little. 

Also, I see nothing wrong with lots of iron (chokes) in a power supply.  They'll end up making it even quieter, if done right.  In fact, all of my new designs (those on the draing board) will be using chokes in the supply.  The reason the CORNET works is because of all the extra RC filters and relatively constant current load.

jh

[CButterworth]

Jim,

Thanks,  I'll measure the secondary resistance on the transformer (all my transformers) tonight.  It is likely that I am clutching at straws in trying to think that another PSU will make my Aikido better (quieter).  I have a very small amount of hum,  I cannot tell if it is 60Hz or 120Hz, it is too low in volume for me to be able to tell.  My Hammond 269AX (125-0-125) does sing very slightly, if I put my ear on it.  Although dampened with grommits (as VoltSecond's method), some of my more microphonic tubes may simply be picking up so of this resonance.  I had wondered if it may be a faulty transformer, or maybe the 300 series are better.

In all likelihood, my PSU problem in the Aikido may be improved with one from the Cornet2, but if a choke-input PSU is uspposed to be quieter, then I really need to work on the current PSU and fix it.

If only I had access to an oscilloscope!

Charlie

[hagtech]

Here's one trick you can try.  Put a power resistor in series with the transformer primary.  You need to drop the voltage on that 269ax to 115Vac.  Then it will be much happier.  A value of maybe 10 to 20 ohms will do, use 5W to 10W.  Depends on power draw.

Do this, estimate the total power drawn by the secondary.  That is B+ voltage times current plus heater voltage times heater current.  A guess at the CORNET would be 360V * 0.02A + 6V * 0.9A + 5V * 2A = 23W.  So figure with conversion losses it pulls about 30W from the ac line.  Line current is then about 30/120 = 0.25A.  We need to drop line from 120 to 115, so for a 5V loss we have R = V/I = 5/0.25 = 20 ohms.  Power dissipated is V*I = 1.25W, and you need a 4x derating, so resistor should be 4*1.25 = 5W.

Do same analysis for Aikido.  Try something like a 20 ohm 5W flameproof resistor in series with the tranny primary.  It will quiet it down, run much cooler, and greatly reduce the sinewave distortion on the secondary, likely improving the sonics.

jh

[JoshK]

Good trick Jim! Thanks for sharing and makes total sense. 

[CButterworth]

Jim,

You stated:   "Put a power resistor in series with the transformer primary.  You need to drop the voltage on that 269ax to 115Vac"

Do you mean a single resistor in series with one of the primaries, a resistor in series with each primary (ie. two resistors required)?  The transformers have two primaries, so a resistor on each?

I am going to tweak the Aikido PSU by removing the 20K bleeder resistors which will allow the B+ voltage to increase to around 330VDC.  Then, given your reply, I'll buy either one or two flameproof 5W resistors to tame the mains input voltage.

Charlie

[hagtech]

a single resistor in series with one of the primaries, a resistor in series with each primary

It doesn't matter.  One resistor in series with the winding is the same as two resistors (of half value) in series with the winding.

jh