power supply math

[danotron]

I not sure if I am doing this correctly or not. Please feel free to set me strait.

I am trying to calculate the fist capacitor in the power supply. I am using this formula:

C = (t * I) / V
Where:
I = average load current drawn
V = ripple voltage peak-to-peak
t = duration between charging cycles and is equal to: 1/twice mains frequency. 60Hz, so: t = 1/120 = 0.008 seconds.

I am going to use 4 12Ax7: 2 for preamp, 1 for PI and 1 for reverb. The current draw would be 40mA.

Power amp: class AB2, 6L6GC Push/Pull. From data sheet looks like 210mA.

I= 220mA + 40mA =260mA

Ripple should be 10% of transformer voltage. Transformer is 325-0-325.....325X1.4=455volts.......10% of that is 45.5volts

Back to the formula

C=(.008x.260)/45.5
C=(.00208)/45.5
C=45.7uF

All the schematics show a 100uF or 200uF cap to be the very first cap. Isn't this too big? Will this cause unwanted "ghost notes" to be produced?

Did I do this correctly?

[Structo]

That is beyond my knowledge base but I can tell you that the D'Lite has two 100 mf caps in series for the first filter so that would be 50mf total right?

[JimiB]

ghost notes are caused by not unough filtering.

[Bob-I]

I used the Duncan power supply calculator and came up with about the same. I used 2x80uF on one amp and it sounds great.

[danotron]

That is beyond my knowledge base but I can tell you that the D'Lite has two 100 mf caps in series for the first filter so that would be 50mf total right?

ooops! four years of school (electronics engineering) and I missed that......thanks fellas.....been a while since I actually did some calculations

[danotron]

I used the Duncan power supply calculator and came up with about the same. I used 2x80uF on one amp and it sounds great.

Just down loaded it..very cool.......thanks

[danotron]

How accurate is Duncan’s Power Supply Calculator? What I designed and what I measure are way different. What curent sink values should I be using for2-6L6GC Push Pull and 2-12AX7 preamp and 1-12AX7 reverb and 1-12AX7 PI?

[llemtt]

Just two small suggestions

- your 4 12AX7s should draw approximately 10ma not 40ma

- ripple should be as little as 4-5v don't know where your "10% rule" comes from

my .02
teo

[Bob-I]

- ripple should be as little as 4-5v don't know where your "10% rule" comes from

I was wondering the same thing. Soldano uses 220uF of primary filters on the SLO and there's no ghosting, and tons of bottom end.

[Tonegeek]

Keep in mind that some of the magic of the Dumble tone has to do with the supply filter. Many of the schematics show the main caps in series for more voltage handling but half the capacitance so what you see might be correct. General rule for D style amps is 100uf for a 100 watter and 50uF for a 50 watter. Thats the main caps only. YMMV.

[jdh]

Increasing power supply capacitance will decrease ripple current and increase energy storage/joules. This increases the amps ability to rapidly supply larger changes in output current (slew rate). This is similar to damping in that the amp has a firmer grip on the signal, and the amount of sag or sponginess will decrease.

Dennis

[talbany]

Types of caps are also pretty important to me.. Blue Spragues are my fave followed by IL's.. The smaller stand up Xicons are OK for preamp and loop as well as bias supply... I personally don't care for them too much on the plates and screens..F&T's are also a good choice for multi stage can type in 500v and the JJ's are a close 2nd..A good cap will still still give you some sag help with the low end note bloom as well as the amps ability to sustain... My .02 cents as well


Tony VVT

[Fischerman]

I was wondering the same thing. Soldano uses 220uF of primary filters on the SLO and there's no ghosting, and tons of bottom end.

And FWIW, the SLO uses a total of 4 each 220uF caps in series-parallel for the main filter (still 220uF total capacitance)...then uses another 2 each 220uF in series for the screens (110uF capacitance). I just built one...very tight! Also very quiet.

I've not had any problem with the blue Spragues but I've read several 'respected' builders have given up on them. They claim that they are experiencing more bad ones AND the price is just rude. They recommended Nichicons and that's what I've been using with good results (but again...I didn't have 'bad' results with the blue Sprague Atoms). I've used both the F&T and JJ cans in Marshalls with no problems.

[Structo]

I think those F&T caps just plain look sexy too!

[talbany]

Been using the Blue Spragues for years and have yet to have a new one fry...Not to say they may have had a bad run!!
we just tried the TAD caps and so far have been real pleased with them..
They are a bit smaller than the IL"s and about the same price... so far work and feel pretty good for a smaller type cap... We've only built several amps with these caps but so far I think I like them a bit better than the IL's.. Though the jury is still out as far as consistantcy...
I've built several 100w with the F&T' cans D-Style and really are nice... Ken Fisher recommended them to us...
To my ears 220uf on the plates Chokes the low end


Tony VVT