Voltage drop in a power supply

[ward]

When designing a powersupply for a guitaramp, how can you predict the voltagedrop after a bridge rectifier ??

My example, (please correct me where I'm wrong.)

I've got a Powertranny with 110, 120, 220, 240V in. / 240V out.
Wall voltage is about 240Vac here.
I want to get somewhere between 340 and 360Vdc out.

If I use the 220Vprimary (is this safe to try?)
Bridge rectifier 4X 1N4007 with 2 x 200uF in parallel gives me, without any load: 345 Vdc.

My questions,
will the 2x200uF do for lowering the ripple enough ?
how do I calculate voltage drop when under load ?
I guess I need to know load current to calculate that ?

This is the schematic I'd like to follow

 

[mikka]

I'm so dumb, I would just build it and see what comes out. :shock:

 

[Kit]

will the 2x200uF do for lowering the ripple enough ?

Oh yes, certainly. Thats a lot of filtering for a 15-18W tube guitar amp.
Note that more C also puts the power tranny under greater stress.

how do I calculate voltage drop when under load ?

Thats a little complicated, it has to do with the internal R in the transformer windings.

Rod Elliot has some great info on his site.

But as mikka says, just try it and tweak later.

 

[ward]

[quote author="Kit"]Thats a lot of filtering for a 15-18W guitar amp.
Note that more C also puts the power tranny under greater stress.[/quote]

The reason why I went for bigger caps instead of the 2x32uF that is in the original design is because I thought : the bigger the caps, the larger the rms voltage I'm gonna get out.
And since my voltage is rather low ...

That could be totally wrong ?
Better stick with lower caps ??
I have no idea how capsize will affect tone/feel of the amp.

[quote author="Mikka"]I'm so dumb, I would just build it and see what comes out.[/quote]
I will, I'm anxiously waiting for parts to come in. :roll: :roll: :roll:

 

[Kev]

[quote author="ward"]The reason why I went for bigger caps instead of the 2x32uF that is in the original design is because I thought : the bigger the caps, the larger the rms voltage I'm gonna get out.[/quote]
:shock:
what ??

put in bigger caps .... and get more volts ?

is it just me and Sunday morning
perhaps I need another coffee

 

[mikka]

You might find that big caps will take the "warmth" out of a valve amp. Check out the filtering on the Fender circuits..... sometimes less is more. I suggest .....best to build it as drawn for now.

 

[Gus]

with a push pull tube guitar amp I sometimes use the lowest uf, lowest ESR caps for the plate and screen output stage in a tube amp that I can

looks kind of like a marshall 18 watt circuit.

try it remember DIY. Don't trust anything you read on the web.

 

[Svart]

240vac * 1.38 = 331.2VDC - diode drops of ~ 1.5 * 4 =

roughly 325vdc

The amount of capacitence you need is directly related to how much current you will be drawing plus a safety factor of 2.

1 amp is roughly 1000uf *2 so for every ampere use 2000uf, 2200uf is fine.

 

[Guest]

There are 2 types of transformer calculations: by voltage drop, or by temperature increase. When designed by a voltage drop it is usually 10% on nominal power.

 

[ward]

[quote author="Kev"][quote author="ward"]The reason why I went for bigger caps instead of the 2x32uF that is in the original design is because I thought : the bigger the caps, the larger the rms voltage I'm gonna get out.[/quote]
:shock:
what ??

put in bigger caps .... and get more volts ?

is it just me and Sunday morning
perhaps I need another coffee[/quote]
Hmm, ...
that is indeed silly the more I think of it.
rms is average voltage, so why would changing the ripple change the rms voltage.


I did try the power supply, and if I use the wrong primary 220Vac, I get 345Vdc.
Thats enough according to the designer.
I was just wondering if, when under load it would still be ok.

The design uses a 290Vac secondary transformer. I'm using this 240Vac out, and that is just because I have it laying around.
And that's also why I'm going solidstate on the rectifier and not following the design as it is.

Maybe it would be safer for a beginner like me to just follow the design completly and buy the transformer from the designer. But that wouldn't be much fun, would it ?
I would still be thinking that raising caps in a ps raises the voltage

Any advice on using the wrong primary for slightly raising the voltage ?
I would think, try it out and see if it get's hot ...

 

[mikka]

I figured you'd use a solid state bridge rectifier.... that would help with more volts.... and is a good idea. I've tried using bigger caps to push voltage up also .... don't worry.. you're not alone. Using the 220v primary seems fine to me.....it would just be a tap on the primary I assume. That transformer looks like a monster. I'd bet it would be idling on what you're drawing.

What I meant by "build it as drawn" I meant the filtering and basic circuit. Remember to have fun! :thumb:

 

[PRR]

> more C also puts the power tranny under greater stress.

> I thought: the bigger the caps, the larger the rms voltage I'm gonna get out.

Neither of these is really true.

A too-small cap will give lower DC voltage and less tranny strain, true.

But once you get to the point that ripple is low enough for audio, more capacitance does not increase voltage or tranny-heat enough to care about.

> I guess I need to know load current to calculate that?

Well, yes. Zero current gives zero sag. Infinite current causes infinite sag. You gotta put a number on the current to get a number for ripple and sag.

1 uFd per mA is usually plenty. In tube guitar amps, it is usually way more than you need. The EL84 pair will pull around 20-24 Watts max, or around 80mA for supply voltages like this; the little tubes a few more mA. 80uFd-100uFd would be ample, 32uFd is probably not a problem.

And.... don't think. Steal. Look at other similar designs.

But don't obsess about getting the "same" voltage as another amp. Small change of voltage, even 20%, is just a small change of maximum power, not so you'd notice. You need a big change of voltage, 40%, to make a real change of power. And going 40% high on an existing happy design will usually get you in trouble.

> Wall voltage is about 240Vac ... If I use the 220V primary (is this safe to try?)

It may not smoke.

But the difference is too small to risk any smoking. You will get less than 10% increase of DC, less than 10% increase of Volume, which is just not audible. You can make a heck of a racket with "only" 320V-330V DC.

That "Trinity" plan works out to about 354VDC and about 16Vpp ripple. Tranny RMS current is 0.13A due to high tube resistance.

A 240VAC winding on a fairly large PT, with a bridge and the same 32uFd, gives 325V and about 17Vpp ripple. Transformer RMS current is about 0.20A.

Going to 200uFd gives 327VDC, 4Vpp ripple, still about 0.21A RMS in the tranny. 400uFd gets you below 2Vpp ripple, 0.22A winding RMS current. 2,000uFd(!) brings you to not quite 328V, 0.3Vpp, 0.23A.

Note that going from a reasonable 32uFd to an insane 2,000uFd got you not quite 1% more voltage and only 15% more RMS winding heat. The ripple scales directly with C, so you pick C for reasonable ripple. Nearly all P-P pentode amps can get by with 20uFd-40uFd per channel.

 

[ward]

Thanks PRR,
that is a great answer thanks for your time

The PT came from a big radio that was using:
2x ef89
ech81
ecc85
eabc80
ecc83
2x el84
So I guess it should do ok for current draw

and I'll use the 240V primary, and stop worrying about a 30V difference.

 

[Kit]

I forgot to mension, with that LTP phase inverter, AC ripple is really not an issue.

Any ripple voltage that creaps through the plate resistors will cancel in the primary of the output transformer.

Save your stored energy for the preamp single ended triode stages.

 

[Gus]

You might want to raise the fil voltage above ground for the CF stage heater.

A simple way is connecting the fil CT to the output stage cathodes. That will bring you up to the voltage drop across R20

Might want to try a 12at7 for V2

 

[ward]

thanks all for your comments.

Gus,
I'm trying to understand this.
the 12at7 suggestion: what would that do ?
less gain to the eq stage ?

CF = cathode follower
CT = center tap?
fil voltage = ???

I think you're talking about raising the heater ground on the power tubes to reduce hum.
Like this:

Gerald Weber advocates using a 270K/27K resistor divider from B+ to raise the filament windings in a DC sense above ground. This keeps electrons from the filament from hitting the plate, another source of hum.

A simple way is connecting the fil CT to the output stage cathodes.

I'm confused, does this dc voltage needs to be connected to the heater or to the cathode of V4&5 ?

 

[Gus]

The 12at7 Yes you MIGHT want a little lower gain or you might not It is a try it and test it. Things like high output humbuckers or lower output single coils etc. A personal taste issue.

Look at http://blueguitar.org/ for more fun schematics

CF cathode follower

CT center tap

The output tubes being cathode biased have a cathode resistor. The cathode resistor node at the output tubes is above ground. So an old timer trick(from what I have been told)was to connect the center tap to the output cathode node raising the filiments above ground by just moving a wire.

I changed an old 10 watt PA amp with cathode biased 6v6s to a guitar amp and I wanted to use the smallest uF caps I could for the plate and screen without hum this trick worked.

Raising the filiment voltage above ground helps with noise and hum
AND in this amp the cathode to heater breakdown voltage at the cathode follower.

 

[ward]

Ok thanks,
I'm gonna built it stock first and then try this easy mod to see if it makes any difference.

 

[PRR]

> with that LTP phase inverter, AC ripple is really not an issue. Any ripple voltage that creaps through the plate resistors will cancel in the primary of the output transformer.

Well, sorta.

Driver hum should not be a problem. Signal level is high; the driver only draws a few mA. You can tap plate or screen supply through a 5K resistor bypassed with a 22uFd cap, and get driver ripple low enough.

Yes, at low levels the output plate supply ripple tends to cancel in push-pull. This is one reason why only fools (like me) build single-ended amps over about 5 Watts. SE needs better iron and better filtering, so the economic path is push-pull for anything bigger than "small".

But the cancellation is never perfect.

And at HIGH levels, when you clip, the ripple is impressed directly onto the output. You get fuzzy clipping and cross-modulation with the B-flat-flat power frequency harmonics. This is not a big problem in hi-power Hi-Fi and PA amplifiers: you don't clip them, or not long enuff to hear the ripple. (It can look very bad on bench-test numbers.) But small guitar amps are pushed to clipping for a large fraction of many notes.

That power ripple is probably part of the Tweed Tone. Also why the Silverfaces sound less tweedy, and a Peavey 5150 has no tweed at all. The Tweeds have modest power and very modest filtering, Blackface has more, and a 5150 has really quite ample main plate filtering (as well as so many excess gain-stages that you can't hear power buzz through all the heavily-layered distortion).

 

[mikka]

This is one reason why only fools (like me) build single-ended amps over about 5 Watts.

I'm a sucker for single ended in parallel.....it's a tone thing... :green:

That power ripple is probably part of the Tweed Tone.

Years ago I built a few power supplies for guitar amps using choke input filters..... less ripple on the power tubes(edit: and the OT)....just to try. They were quieter, but noticeably "colder". The sound might be more in fashion now.