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Old 8th February 2014, 12:32 PM   #1
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Default Check my PsudII

This is wat I get with a simulation is PsudII. I haven't measured resistance of C1 (c4 on pcb) and C2 (c5 on pcb) but made more or less an educated guess. The transformer is all measured and I have 233v power outlet.
I am making the SE for 300b's and 5k opt's.
I've attached the psu file for your information.

I allready burned a set of 300b's so i would like to know if I am on the right track before my next try.

greetings Mark
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Old 9th February 2014, 03:27 PM   #2
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Hi Mark: What is your target B+ for your 300B's?

Looks pretty good.....here are a few (possibly obvious) comments and questions.

Where is the 14ma coming from for stepping the current?

Increasing the Value of C2 will get you a little less ripple.

You are limited in high big you can go for C1 by the 5AR4, you probably don't want to go any bigger, although YMMV. You can reduce the B+ if required by reducing the value of C1 below about 8uf....as it gets smaller the B+ will drop (starting around 8uf or so).

The ringing/overshoot on startup shows that you are slightly underdamped; do you have the "soft start" bo checked under options?

I just use the default values for the cap resistance, never thought to change it....

You can zoom in (zoom way in) on the flat part of the waveform to check out your voltage ripple, or alternatively, use a time delay of say 8 seconds or so and read the max and min voltages from the left data table in PSUD to get steady state ripple. Less ripple=less hum.
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Old 10th February 2014, 07:28 AM   #3
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Hi Boywonder,
Thanks for the reply. I would like to go near 400v for B+, as advised by George.
Quote:
Some also use a 5K ohm OPT and a B+ nearing 400 volts, this is my preference
The 14ma stepped load, i got from another example wich was faulty at closer inspection. I should be using 20ma? (2x75ma for output tubes and 50ma for rectifier, Fet's and pre tubes).
With C1 47uf and 2 Ohm, C2 200 uf and 2 Ohm, I then get a B+ of 385v and ripple of 0.02v.
Soft start is off, I do get the rectifier IFRM warning of 0,76A at 0,17 sec.
With soft start on, the warning is "exceeded with value 1.09A at 0,3 sec." Is this a problem?
Actual measurement on board gives a B+ of 410v with rectifier and pre tubes in place.
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Old 10th February 2014, 01:52 PM   #4
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Reducing the value of C1 below about 8uf or so will drop your B+ voltage, so you can dial in whatever B+ you want by fitting smaller value caps for the first cap.

I don't think that you need to worry much about stepping the load, as this is a class A amp and will be drawing the full load even at idle. Stepping the load in PSUDII is an easy way to see if you are underdamped. If the output rings/oscillates, it's underdamped and needs more series R.

This should also eliminate the current exceeded warnings in PSUDII and you can probably fit a 630V rated film cap if you want. 47uf for C1 is probably OK, although smaller C1 caps make the rectifier's life easier. There is a limit to the value of the first cap+output impedance of the transformer secondary for a given rectifier; if this is exceeded the rectifier will arc over at startup....looks like fireworks in the bottle. Sometimes after this happens it leaves carbon tracks which will allow arcing to occur again sometimes destroying the tube.

The 5842's draw 12ma each with a 332R current set resistor, so your total ma draw will be roughly (75ma+12ma) X 2 = 174ma. Stepping the load by 20ma would be going from 174ma to 154ma or 194 ma......

If you already have the circuit running and it's close to what you want all the better.

Last edited by boywonder; 10th February 2014 at 01:59 PM.
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