HT center tap versus balancing resistors - help...

[HTH]

hi,

I have a question regarding balancing the voltage on series connected caps...

The first method is where the HT winding's centre tap is connected to the junction of the two series-connected smoothing caps right after the diode bridge.

The second method doesn't use the CT and instead uses 220k resistors accross the smoothing caps to balance out the voltage (so each cap gets the same voltage and one isn't working harder).

My question is this... which way is 'better' - are there pros/cons to either method? The PT I'm installing has a CT on the HT winding but I could use either method.

Cheers... Ian

[Tom Phillips]

hi,
The first method is where the HT winding's centre tap is connected to the junction of the two series-connected smoothing caps right after the diode bridge.

In my experience, that topology is used when you are making a dual supply with equal positive and negative voltage outputs. The CT is at ground potential. The caps rally aren't stacked in series. It just looks that way because of the schematic layout.

I'd use the other method for your guitar amp. You only need to use two caps in series if your voltage is too high for one to handle.

Regards,
Tom

[HTH]

this is how the amp in question is wired... http://www.schematicheaven.com/marsh..._100w_1959.pdf

see how the CT goes to the junction of the four series/parallel caps? - what I'm asking is whether it's better to not use the CT and just put 220k resistors accross each of the caps to balance out the voltage on each of them.

the raw HT voltage right after the diode bridge is in the 500v range so it's right on the edge of specs for most can caps - I'd rather connect them in series for safety sake.

[Ray Ivers]

Connecting the caps as per the schematic posted will guarantee 1/2 B+ voltage across each cap - as would the 220K resistors - and will also discharge the caps much quicker on power-down (no biggie IMO). If you're not using the CT for a half-B+ supply node, either way should work equally well.

Ray

[Tom Phillips]

OK. Got it. Thanks for posting the schematic link. In addition to what Ray said I'd vote for staying with the stock arrangement if you can find a suitable replacement PT. Not because one or the other method is "better" but because it should be less work and because it could be less confusing later.
Regards,
Tom

[stokes]

Whatever you decide,if you dont connect the CT as per the schem dont ground that CT if you choose to use the 220k resistors.The CT,with that rectifier,should not be at ground potential.

[Enzo]

I think we are overlooking that we are talking of two different circuits.

If we want to use lower voltage caps, we can wire two in series to effectively double the voltage at the cost of halving the capacitance. In that case we put a couple high value resistors across each cap to act as a voltage divider to even thiings out by brute force. This is used to filter the common plain old rectifier.

When you see one end of the power transformer winding going to the junction of the two caps, it is not a simple filter, it is a voltage doubler circuit. In that case, the rectifier diodes seem to be wired a little odd as well. And if you think it through - or look up and explanation - you will see that the current flows one way and charges one of the cap, then whjile flowing the other way, it charges the second cap. The two caps being in series, we now have twice the voltage.

So these two methods are NOT equivalent, they are utterly different jobs being done. In the first case we have simple rectification, and we are wiring two lower voltage caps to take the place of a single higher voltage cap. In the second case, we have a lower voltage transformer charging each cap separately, and the caps add up to some desired total.

[Ray Ivers]

Enzo,

The Marshall 1959 PS circuit in the posted link is a little different... it uses a conventional bridge rectifier across both ends of the HV secondary, but also uses that winding's CT to 'bootstrap' the junction of stacked filter caps connected to the BR output - possibly to save the cost of two large-wattage resistors and still provide a bleeder function, although the CT must have added some small cost to the PT as well.

I like this variation because the CT provides a half-B+ node for the screens, preamp, or whatever.

Ray

[Enzo]

Oh... never mind. You are right, I didn't open the link. it sounded like he was describing a voltage doubler, and I thought it through no further than my assumption. Sorry.

besides, I m out with the flu this week. There, that took care of things...

[Amp Kat]

You guys do realize there are two different schematics on the link Ian posted. And they are basically the same except for some value and parallel differences. Both use the center tap to stabalize 1/2 the voltage as Ray pointed out at the CT connection. The second or lower schematic that Ian posted on the link however goes into different screen filtering coming off the rectifier thru a choke and additional balancing network with the 56ks and 100uf network. It's ground is connected to the output tranny common. Either way Ian as Stokes pointed out is that center tap is NOT a grounding point but a stabilizer or half voltage point and shouldn't be grounded as it has half of the voltage potential at that point.

[Ray Ivers]

Enzo,

Illness, fever, lack of sleep - these things are not excuses...

Just kidding - hope you feel better soon (remember the vitamin C !).

Ray

[HTH]

Either way Ian as Stokes pointed out is that center tap is NOT a grounding point but a stabilizer or half voltage point and shouldn't be grounded as it has half of the voltage potential at that point.

thanks for pointing that out (really) but I'm aware that the CT of the HT winding has 1/2 of the voltage accross that whole winding (hence the name centre tap). btw, I know that sounds a bit pissy, but it's not supposed to - thanks for the concern.

I'm just gonna use the CT as Marshall originally intended. btw - Dagnall still supply Marshall with the same PT for their plexis except that Marshall no longer uses stacked caps at this point in the circuit so the CT is not used.

[Enzo]

...no, I don't wanna go to school today, mommy....

[Ray Ivers]

I'm aware that the CT of the HT winding has 1/2 of the voltage across that whole winding

Half the AC voltage at the center tap is obvious, yes - but as a circuit-analysis exercise, you might want to consider how half the DC voltage appears at the CT, as the secondary winding is floating and has a low DC resistance.

I had some standby-switching current-surge issues when using the CT as a half-B+ node, so I worked it up in Circuitmaker and did a pretty exhaustive analysis - I found it quite interesting, FWIW. When using the CT as a voltage source, this arrangement actually works like a FWCT and FWB at the same time.

Ray

[Amp Kat]

Agree Ray, and I did the circuit at the shop today of the second drawing with the two caps/center tap in the middle and as Enzo stated it is indeed a voltage doubler.