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PostPosted: Sat Dec 28, 2019 3:51 pm 
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Location: Winchester VA
Greetings, everyone!

It's been several years since I visited this forum. I have built several amps and still regularly gig with my TC15 - great amp! Presently I am tweaking my 5F2 build, assembled in an old Supro chassis and a handbuilt combo made of cherry with dovetail joints, wicker grille, and TT 10" ceramic.

I notice there are several variations on the filter cap layout for this Princeton (Champ) circuit: Robinette and Triode have 8uF, 8uF, and 16uF. The Trinity Triton has three 16uF. The Tramp has two 50uF. Hoffman suggests 8, 8, 16, with a fourth cap at 16uF - but strangely (to me) that fourth cap is in parallel with the other 16uF with no resistor. If my audio physics is correct, that would make the two 16uF become 8uF (1/C total = 1/C1 = 1/C2), so I don't understand how that fourth cap helps.

My own build has 10uF, 10uF, and 16uF (the 16uF at B+1 and with a beefy PT and JJ5Y3 yielding 370V at B+1. I like the headroom of my amp, preferring to get distorted tones out of my pedals. But I do have a little bit of hum that I think is coming from the PT and filter cap arrangement of my amp. I have triple checked everything else, and I can live with the minor amount of hum. But I wonder if increasing the capacity of my filter caps, or adding a fourth, might help? I also would appreciate if someone could explain to me why the Hoffman circuit with its fourth cap in parallel without an added resistor works.

Bottom line, would adding a fourth cap or increasing the capacity of my 10 or 16uF caps make any significant difference? i.e., more headroom? less hum? My PT is a Classic Tone 40-18085. What do you folks recommend?

Many thanks, and happy holidays!


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PostPosted: Sun Dec 29, 2019 2:14 pm 
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OK, I found an important correction to my post: capacitors in parallel are additive as to capacitance (but not voltage, but I have adequate voltage capacity already). So adding a fourth 16uF capacitor in parallel increases the total capacitance of the power filtering. So, depending on whether my present circuit is adequate already or not, adding the cap might help smooth out DC ripple and reduce the slight hum I am hearing.

I'd still be interested in hearing any comments before I try this out.

Thanks!


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PostPosted: Sun Dec 29, 2019 8:17 pm 
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16 at the start should be fine. Original was 8-8-8. No harm in trying . Take a 8-16 cap and just touch the leads to the first filter cap. See if the hum drops.
How is the heater voltage referenced to ground?

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PostPosted: Tue Dec 31, 2019 10:44 am 
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Thanks, I'll try that - good trick to avoid soldering unless necessary. I have a spare 10uF I will try. I'll also check to make sure the heater voltage is balanced in ref to ground, but I did check when I built the amp about a year ago and it was fine. Don't think it would change.

Also, my PT wires are a bit sloppy and could be neater. I think I'll re-twist and re-solder them to the rectifier. Heater wires are twisted tightly and away from other wiring in the seam of the chassis. The hum is pretty quiet, but noticeable enough to annoy me when there's no other sound in the room.


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PostPosted: Tue Dec 31, 2019 9:38 pm 
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I added a 10uF filter cap to my 16-10-10 and the hum is gone. Maybe my stout PT wanted more capacitance than is usual for the 5f2 circuit? Anyway, it worked. Curiously, that change also made my volume control work more smoothly. Though it is a log taper 1M pot, the volume always came on so quickly it was hard to adjust done to low a volume. Now I have more useable control in the very low settings - awesome! It makes no sense to me how adding a filter cap would alter the way my vol pot operates, but I'll take it. With the added VRM I now have excellent control over all the parameters of my sound.

Thanks for all the help, Stephen. Happy New Year everyone!


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PostPosted: Wed Jan 01, 2020 10:00 am 
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Good news. Enjoy!

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