Can increasing filter cap value cause problems?

[frank57]

I'd like to increase the 2nd filter cap in my Korean Hiwatt amp from 68uf to 100uf and see what happens.
Can it cause problems? It's solid state rectified. Also would ac readings with a multimeter help figure out how much ripple there is?

[jjasilli]

The extra filtering won't hurt anything.  One option is to jumper a 30uF or 47uF cap across the existing 68uF cap, maybe rig a sw too  Then you can easily do an A/B sound test.  Maybe use a breadboard to hold the test cap, for safety.  You can use a meter to measure AC ripple. You can also use a large cap (non-electrolytic) like 1uF, but probably .22uF is OK, between the test point and the meter probe, to block DC.

Note that the resulting tone, and whether you like it, is the key; not the amount of ripple that a meter reads.  (Nevertheless I too plan to take readings of ripple as I work on amps, for the informational value.)

[Dave]

I have read that raising the cap values on down the preamp chain can cause ghosting, but where you are wanting to do it shouldn't cause that.

Dave

[frank57]

Which side of the cap do I measure for ripple?
I put the black lead on the chassis bolt and the red on the negative of the cap?
I want to increase the filtering a little on the 2nd cap and see if it does anything at all.

I 'm pretty sure part of the problem on this amp is rectifier buzz leaking in somewhere.
But how do I track it down?It's centered around the 2nd tube.At least that's where I can kill it.

It sounds like it's 120hz. Also a lot of hum on the mid pot.I can reduce that by changing r13r from 100k to 47k.The tracks are so close maybe there's some coupling going on there somewhere.A 47k (like hiwatt does on the ol lead)resistor at r8 really helps things but the bzz does not go away.
I've attached a board layout of the amp as it is now with the filtering of the first filter cap off the board.
It did help. The back end of the amp seems to be sort of ok but there is some hum and bzz  there.
Is the bzzz simply from too may gain stages? I don't think so, because taking out v1 didn't work .

[tubeswell]

Was it humming before you did the mods? (if it wasn't, then I would look at the solder-work and lead dress in the mods first).  Those pcb traces need to have the right amount of solder on them and they need to be handled delicately.  The little solder pads on the trace-ends can easily tear from the board if you don't unsolder them properly when removing components. Check that you haven't damaged one/some as they may need repairing (if you have).

[sluckey]

Which side of the cap do I measure for ripple?

In this case, positive side. If unsure, just put your meter leads directly across the cap. But, it'll only take about a minute to jumper a cap across the suspect cap and let your ears tell you if the hum decreases.

Was it humming before you did the mods?

I'm pretty sure this amp has always hummed, at least ever since he brought it here.

[frank57]

So I could stick  say a 22uf cap on a perf board and run two wires to the 68uf pos and neg and see what happens?
Do I need a cap for ripple or do I just touch the meter probes to the respective pos neg of the caps?


The hum was always there like sluckey said. The buzz was sort of filtered out at the start and traces were cut.
Didn't really work and I don't know what exactly was done. Taking out v1 for example doesn't get rid of it.
I've reduced the hum some by moving the hv wires off to the separate board.The gain control and the preamp are amplifying that bzz.
 One thing I tried was getting rid of the efx loop and starting the master at that point. It doesn't work unless you lower r8. Buzz is still there after high noon.
Taking out gain stages sort of helps.
Also the signal loss from running the efx loop is buffered by that extra gain stage. There isn't that much of a difference in the sound with efx and without.Bit more bass without.

The pads are good as far as all the techs who checked it said but whoever put the amp together though,installed the snap in caps pretty roughly.I don't see anything lifting anywhere but they did tear into them somewhat.The big filter caps were all replaced but no difference.
Right now the bias is -15.7 or so  I think at 377-380 volts. About 60%.
So the voltages are up there on this amp.
The way the traces were reconnected contributes some noise.
But there is also radio coming in on the pin 7 of v2 when I ground it.
I tried putting in a 470k resistor in series with the gain pot, but the buzz went up .
Other thing I might try is bypassing v2b and see if I can just get around it.

 If the voltages are higher than intended will the original filtering still be as effective?
I'll try to find a soundfile and post it somewhere.

[67polara]

I did a search on the internet about these amps.  Everyone is complaining of the same buzzing or humming noise.  It is a bad design do yourself a favor and gut this thing and make it into a usable amp or sell it.  I know you have been trying to fix it for a long time and I suspect you never will.

Tony 

[frank57]

I might try one or two more things and then pack it in and try to sell it if I can.
It's certainly more useable than it was and you can use it to a certain point.
The clean is good on it and better than the soldano clean on the Jet city amps.
Much warmer and fuller.Maybe the gigantic output transformer has something to do with that as well.
It certainly kept up with the jet city amp bass wise even though it has a 10 inch speaker.

It would be a good project for a builder. I don't think I'm quite up to the task yet,
but I'm getting there.

[frank57]

One thing I noticed was the bzzz went away when I moved the grid wire on v1.
The connector, by mistake,was about 3/4 of the way pushed into it's holder on the tube pcb.
The contact was  sufficient though and the signal was going through.
Pushing it  away from the output transformer caused to buzz to go away.
If I pushed it in all the way the bzzz returned. An area to explore?

[jjasilli]

An area to explore?  Yes, lead dress can certainly be an issue where hum or noise is concerned.

Pushing it  away from the output transformer caused to buzz to go away.  Trannies cause magentic fields, which in turn can induce hum or noise into a signal lead.

The connector, by mistake,was about 3/4 of the way pushed into it's holder on the tube pcb  A poor mechanical connection can cause noise.  This should preferably be a firm, soldered connection.

[frank57]

I'll try using non-shielded wires and see if that does anything.
Then I'll try soldering shielded alpha wire directly to the pads and pin 7 on the tube pcb(conductor only)
and see if that bzz goes away.I'll make it long so I can move it around.
I extended the red wire from the PT at one point and the buzz went through the roof as well.
But I'm not sure if it's because I extended it or because it ended up closer to the PT.

If I check the ac on c20(non electrolytic) would that be an indicator of the ripple at that point?

[jojokeo]

I did a search on the internet about these amps.  Everyone is complaining of the same buzzing or humming noise.  It is a bad design do yourself a favor and gut this thing and make it into a usable amp or sell it.  I know you have been trying to fix it for a long time and I suspect you never will.

Tony 

+1 This isn't any help but there's a lot of reasons for not using a full pcb in a tube amp. I can see a number of things not to do in that build design. With that pooly conceived pcb you're stuck w/ what they gave you.

[frank57]

Did I do this right for the ripple?
I soldered one leg of a 102pf regular cap to the + of the second filter cap.
The other leg to a wire with a connector so I could touch the red probe of the meter to it away from everything.
On ac I got 0.175 to 0.180.The black probe I had on the ground bolt.