An illustrated guide to building an F5

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Ok I’ve spent some more time reviewing the original posts, the build guide and another website and think I understand the transformer wiring (hopefully).

I drew out this diagram for myself. Is this correct for 120v wiring?
 

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Thank you! Here is the trans diagram just in case.


I just got the terminal block in the mail and should be able to finish soon. Here is what it’s looking like right now.

I noticed the aluminum faceplate has tapped holes perfect for the PSU board. However I don’t see other build pictures mounting the PSU like this so hopefully it’s ok. It’s pretty tight if I try putting the transformer, the PSU, terminal block and diodes all on the bottom so mounting it frees up some room.
 

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Ok I’ve spent some more time reviewing the original posts, the build guide and another website and think I understand the transformer wiring (hopefully).

I drew out this diagram for myself. Is this correct for 120v wiring?

I think you are bypassing the CL60's in your drawing.

Should look like this if you use a conventional barrier strip.
 

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Question -- What's the relationship between the bias of an F5 and its wattage? If .6v/1.3a equals 25 watts, what is .7v/1.5a, and so on?

There is no relationship. The wattage does not go up with more bias as the amplifier is limited by the rail voltage.

What more bias does give you is more Class A envelope...F5 biased at 1.3A will give 25W of Class A into 8 ohm but only 13W Class A into 4R. Biased at 2.5A we can has 25W Class A into 4 ohm. But the amplifier will still only deliver 25W before clipping.

If we raise the rail voltage to 32-ish (like F5 turbo) volts we can get 50W into 8ohm. Class A envelope still is related to bias and since we have higher rails the amplifier will produce more heat compared to the 24V rails even at same bias. So, assuming the same chassis and dissipation an F5 turbo will have less bias than a standard f5 even though it has more wattage.

So, in real life higher wattage amplifiers usually have LESS bias than low wattage amps because they have higher voltage rails.
 

6L6

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711194d1540397599-illustrated-guide-building-f5-img_2184-jpg


Nope.

Cap needs to be across the Live and Neutral, not downstream of the thermistors as you have shown here.

Also, as drawn, you have both thermistors in series with both primaries, not one thermistor in series per primary as it's supposed to be.

I can't begin to say how much I wish Antek used 8 different colors on their transformers, there would be no confusion...
 
There is no relationship. The wattage does not go up with more bias as the amplifier is limited by the rail voltage.

What more bias does give you is more Class A envelope...F5 biased at 1.3A will give 25W of Class A into 8 ohm but only 13W Class A into 4R. Biased at 2.5A we can has 25W Class A into 4 ohm. But the amplifier will still only deliver 25W before clipping.

If we raise the rail voltage to 32-ish (like F5 turbo) volts we can get 50W into 8ohm. Class A envelope still is related to bias and since we have higher rails the amplifier will produce more heat compared to the 24V rails even at same bias. So, assuming the same chassis and dissipation an F5 turbo will have less bias than a standard f5 even though it has more wattage.

So, in real life higher wattage amplifiers usually have LESS bias than low wattage amps because they have higher voltage rails.

Thank you for the detailed explanation. I'm running my F5 at 1.5a into 4 ohm speakers.
 
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Went to fire it up this morning to test just the PSU, nothing..... Doh!!!

After poking around with the DMM, I forgot to put the fuses in the mains socket.

Off to Home Depot to get some fuses and some more DMMs.

I probably should build a bulb limiter while I’m at it so it doesn’t blow up.
 
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Ok so I keep blowing the fuses that are in my mains outlet. I either got the wrong fuses or something else is wrong. When I put the fuses in, I didn’t hook up the live and neutral wires to the terminal block. I flippped the switch ON and verified with the DMM that the pins out were hot at 120v. Flipped the switch off, connected Live and Nuetral from the terminal block to the outlet. Flip the switch and the fuses blow.

I bought 125v 5A fuses. 120v operation with a 400VA transformer. Are those the right fuses?

If they are then it means I have a short somewhere?
 
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Yes, it would work. And quite likely safely. But it's not what's drawn the schematic, and when dealing with AC, I'm going to default to doing exactly as it's designed. :D


On that note, four questions:

  1. What's the purpose of C9 as shown on the PSU schematic?
  2. What's the spec for C9?
  3. I can make out that the capacitance is 3300 µF but why is this the right value for a 120 VAC primary (possibly answered by resolution of Q1)?
  4. What should the capacitance of C9 be for a 240 VAC primary?

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If I refer to relevant documentation from First Watt, such as:
http://www.firstwatt.com/pdf/prod_f5_man.pdf
http://www.firstwatt.com/pdf/prod_f1_srv.pdf
... the purpose of C9 is not explained.


Rod Elliott, here offers an explanation for an equivalent (as far as I can make out) C9:

http://sound.whsites.net/power-supplies.htm#s14
... and provides this advice:
EMI is not usually a problem with a linear power supply, and most will pass the regulations used in all countries without any filtering...

... The first method is to use a mains rated (Class X2) capacitor in parallel with the transformer's primary ... - it must be an X2 mains cap.
For a 240 VAC primary, Rod Elliott suggest the following additional points:

  • X2 caps are specifically designed for use across the mains
  • the material of concstruction is usually (but not invariably) polypropylene
  • a common voltage rating (for 240 VAC service) is 275V AC
  • a capacitance of around 470nF is generally suitable.

Notably, the capacitance suggested for C9 in the First Watt documentation is about 7000 times greater than Rod Elliott's recommendation. I doubt that the difference in primary voltage will account for this.


What would be useful for me is to understand the purpose of C9, the guidance for specification beyond capacitance selection and the method used to determine its capacitance.


Thoughts and useful advice appreciated... :cool: