Power filter bigger caps causing hum?

[Stevie B]

In an attempt to get more "umph" out of my amp, I tried bigger caps for the main B+ filter.
I am using a solid state rectifier (diodes).

Instead of the stacked 220uf 100v (with 100k .5w bleeder) and 47uf 500 v (with 470k 1w bleeder) I put in stacked two 220uf 600v (with 470k bleeders) caps.
This ends up with 110uf with a 1meg bleeder.

But I ended up with hum.

What did I do wrong?
Do I need smaller value bleeders to keep current flowing through the diodes?

[Jazz P Bass]

I do not get why you changed the bleeders but the higher voltage on the caps will certainly not give more umph. Higher capacitance, maybe.
Put the old bleeders back & see what happens.
What amp is it?

[R.G.]

+1

Is the 220uF/100v the bias supply? Is the 47/500 the main filter? And you replaced the main filter with 220uF stacked?

There are several things that could have gone wrong, including attaching wires to the wrong places on the grounding scheme.

Schemo is needed.

[Stevie B]

I do not get why you changed the bleeders but the higher voltage on the caps will certainly not give more umph. Higher capacitance, maybe.
Put the old bleeders back & see what happens.
What amp is it?

The reason I increased the bleeders is that I was looking to reduce the load on the power transformer - I realize that it's probably insignificant but I thought it wouldn't hurt. But it takes like 10 minutes for the caps to discharge. So now I think I'll try again with half the value (total of 470k instead o 940k).

I have heard that bigger filter caps give a different response for bass attack. A friend had an amp with two filter choices for that specific reason. I think it was Mesa Boogie M50 or something like that.

The reason they are 600v is that was the only higher voltage and higer value caps they had at the store.

But the stacked values they used (it's a Fender Deluxe Reverb re-issue) is a very efficient choice for size and cost. They used the bleeders as voltage dividers to use the 47uf at the higher voltage and the 220 at a lower voltage.
But the whole thing ends up at only about 39uf total filtering.
Works fine for no humming but I wanted to see if I can get a bass response change.

I think I'm gonna put in a switch to bring in the extra filters IN ADDITION to the existing 39uf.
That'll make it a total of 150uf!
Is that too much???

I think the power transformer can handle the inrush and I'm not worried about the diode rectifier plug in for the 5AR4 .

[Stevie B]

+1

Is the 220uF/100v the bias supply? Is the 47/500 the main filter? And you replaced the main filter with 220uF stacked?

There are several things that could have gone wrong, including attaching wires to the wrong places on the grounding scheme.

Schemo is needed.

Yes, that is correct. It's the main filter caps. The first ones after the power tranformer for the B+ supply. Before the dropping resistors.
I basically went from 39uf filter to 110uf filter.
But the bleeder load went from 570k to 940k.

But it had a hum - not very bad but very noticable when not playing.

I simply disconnected the exisitng caps and added the new filters and bleeders right off of the rectifier tube socket (with the solid state in it). Caps in series (parallel bleeders) - neg side to ground.

If the caps were backwards then I think the hum would be huge, but it's not, but it's not acceptable.

I just don't know enough about this stuff to know what works right for filters.

[Stevie B]

Can't figure it out

After looking around and thinking about it;
I see no reason that the bigger filter caps and bleeders should cause any hum at all.

In fact, I see many filters without bleeders at all (that's where all the warnings come from).

SO I'm gonna hook it up again and see if its still humming.

Before I swithced back to the original filter caps I noticed I had pushed the heater wires pretty close to the power tube sockets.
Maybe that was the source of the hum?


I did not bother to "form" the caps. They were pretty new and they are a higher temp version (all they had).

[diagrammatiks]

I just looked at the schematic for the DRRI.

If you are using the vacuum tube rectifier you can't go higher then the 40 that's in there. If you are using ss diodes you can use whatever.

You don't need to use a stacked cap if they are rated for 600vdc. Just use the one.

Your bleeders are fine but they don't need to be 470k. They can be 220k. Did you match them?
When you stack caps like that the bleeders also act as load sharing stabilizers for the 2 caps so you need them matched as well as possible.

It would be much better to not stack the cap and just use 1 bleed resistor so it's not doing double duty.

Stacking caps is unnecessary in this day and age.

[Stevie B]

I am not using the 5AR4 rectifier - I put in a diode plug in (full wave 4 diode jobber).

I used two 220uf caps in series to make it a total of 110uf filter. I wasn't sure at all about how much the inrush current (of 220uf) would strain the power transformer.

The bleeders are 5% (gold) so that is good enough to equalize the voltage drop in each cap - but since the caps can handle 600v each I probably don't need them at all? (except to drain the caps as bleeders) There are no other bleeders in this amp. With these big bleeders it takes over ten minutes to get down to under 100v! (I actually didn't time it, might be longer. I just kept checking voltage with the meter)

I was wondering if one of the caps might actually be bad.
But wouldn't both need to be bad to cause hum with them stacked? (or can one be bad acting as an open circuit?)

I also heard that increasing the filter for the screens (right after the choke) from the stock 22uf to 40uf will also change the bass response of the amp.
I think I'd like to try that as well but with this failure I don't have a whole lot of incentive.

So, can anyone tell me if the Deluxe re-issue power transformer is good with 220uf main filter and 40uf screen filter?
It's just pulling lots of current for the first couple of seconds right?

[tubeswell]

How have you grounded the new caps?

[diagrammatiks]

there's no reason that having 2 470k bleeders it should take 10 minutes for the caps to drain.

you could replace the bleeders with 220k and see if that changes anything.

but you really don't need to stack the caps at all since the deluxe is barely seeing 400 on the b+

[Stevie B]

How have you grounded the new caps?

I used the grounding screw next to the power transformer. It grounds a green wire with an eye terminal and I simply wrapped my bare wire around the screw and tightened them both down. It seems very secure.

[Stevie B]

there's no reason that having 2 470k bleeders it should take 10 minutes for the caps to drain.

you could replace the bleeders with 220k and see if that changes anything.

but you really don't need to stack the caps at all since the deluxe is barely seeing 400 on the b+

Yes, I know the caps can handle anything this amp can dish out. I just wasn't sure about the other way round.
Can the transformer put out enough inrush current without a problem?

I know it takes a while to bleed down the voltage cuz I was waiting for it with the amp open.
With my new wiring, all the filters (including these two 220s) had to bleed through 970K.
Which leads me to believe that these caps, holding a charge, are not bad.

One plan is to reduce the bleeder (equalizer?) values and see what happens - but I don't know why it would have any effect.

[pops]

Did you rebias the amp after adding the solid state rectifier? Voltage change from the rectifier may have changed the bias.
You need to solder that ground not just wrap it or tighten it down. I may work for now, but be sure to solder it before you are done.

[JoeM]

I simply disconnected the exisitng caps and added the new filters and bleeders right off of the rectifier tube socket (with the solid state in it). Caps in series (parallel bleeders) - neg side to ground.

?? Why didnt you just install the new caps on the PCB, replacing the old ones?

I used the grounding screw next to the power transformer. It grounds a green wire with an eye terminal and I simply wrapped my bare wire around the screw and tightened them both down. It seems very secure.

Probably not a good idea, I think you're using the AC ground point. You need a better ground IMO.

[Stevie B]

?? Why didnt you just install the new caps on the PCB, replacing the old ones?

Because they are too big to fit under the pan.

Probably not a good idea, I think you're using the AC ground point. You need a better ground IMO.

Oh, it attaches to the chassis, does it matter where it attaches? It's a very short piece of wire attaching to the cap.
The caps are located in front of the power transformer between the light and the pots - basically right behind the faceplate logo that says "Deluxe Reverb".