Too much power filtering....

[Mike T]

What is the downside of too much filtering of the B+? I need to get rid of some more hum and I can't figure out where else to look.

[Blue Strat]

What amp? What's the status? What's been done to it?

[Mike T]

What amp? What's the status? What's been done to it?

It started out as a Champ. It's now in an old Musicmaster cab with a V30. I put another 6V6, a Deluxe OT(mercury) and used diodes for the rectifier. The power tubes are cathode biased. I punched the chassis for another 12ax and put in a long tailed driver and a plate fed tone stack with treble, middle, and bass. It really sounds good expept for a little hum. Not a big hum, but it is annoying, especially in clubs with neon beer lights. I got rid of some of the hum a few months ago with a 22uF cap from the B+ into the OT... I think that is the point, the amp isn't in front of me now. The lead dress isn't all that bad, into the grids on the first two triodes is shielded. The 6VAC is grounded at the pilot light through (2)100 ohm resisters.

I have been using the amp regularly on the weekends. But the hum is starting to get to me..

[Mike T]

Always had hum or just recently?

It seems to have slowly increased over time... It could be me becoming more aware of it though, but I do not believe that I wouldn't have let this hum go at the beginning....

[PRNDL]

What is the downside of too much filtering of the B+?
More filtering usually makes the amp "stiff", which means it doesn't respond to playing technique. Most like guitar amps to be "loose" and "lively", which is opposite of HiFi amps.

I need to get rid of some more hum and I can't figure out where else to look
There are a ton of places to look, and its not easy to tell in advance. Grounding schemes and tube quality issues are typical. Excess gain is another issue (some use lower gain PI tubes because they also have lower noise). The preamp cathode resistor and capacitor can also be suspect.

It seems to have slowly increased over time
That suggests tubes. Perhaps the phase inverter or output tubes have drifted into mismatch.

Schematics and a chassis photo will help.

[mark norwine]

Is it 60Hz or 120Hz? That usually tells a lot...

[Mike T]

I think the hum is 60hz. It increases with the volume control. Like I said it is the barroom lights that brings it out. Otherwise it is manageable. A shielding problem maybe? I have to admit I haven't really moved into the 21st century in a few ways....I have no digital camera and the only way I can imagine to get a schematic to you would be to draw it by hand and then take a picture.....then I have to work up the guts to display it. :eek:
I'll see if I can borrow one. I'll try to get it done within a week. thanks, guys

[phsyconoodler]

Put a metal mesh or plate in the cabinet on the inside top the full size of the chassis.Make sure it makes a solid contact with the chassis.It will reduce the noise induced hum from things like flourecent lights and dimmer switches.

[tremolux]

Conductive self-adhesive metal tapes are a quick fix in addition to psychonoodlers suggestion for using other heavier shielding products in the top of the amp (depending on how the chassis is mounted of course).

+1 to Johns suggestions re: tube shield on V1 and 120 hz vs. 60 hz. etc.

...and its not a bad idea to solder a well insulated wire from the chassis (usually from the spot where the green ground wire on the AC plug is grounded) to the metal side of the tape......same basic principle as connecting the adhesive metal tape shielding on the back of a pickguard to a ground point on the back of a pot thats grounded to the - side of the input jack/tremolo block etc. Try running the wire thru a hole in the tape so it can be soldered and makes contact with the adhesive also.

You may have to burnish or rough up the smooth tape surface to take the solder.

[Mike T]

The new tube sockets I used don't accomodate tube shields, but I believe I have some that do. Also I have some adhesive copper tape I use for guitars and I may have enough to cover the inside top of the cab. The hum isn't that bad and I think it is more of a shielding problem, but we'll see. I'll still try and get photos of the chassis and a schematic.

Time right now through the end onf the year is at a premium because besides for my day job the bass and keyboard players quit last week and we are in the process of breaking in a new drummer (the old one is going out front) a bass player and a rhythm player for at least 9 gigs already booked in December. January will be light because then we will be tightening up, making a demo and photos and all that but I will have more time to work with my equipment. Till then I'll just use my Reverand.

[kludge]

Does the amp hum much when there isn't a guitar plugged in? You need to distinguish between guitar hum and amp hum. Also, since you've modified it, is it single-ended or push-pull? You'll have a hard time getting rid of the hum completely with a single-ended amp.

At a certain point, you'll get diminishing returns trying to throw more filtering at the problem (and you're probably already there). Meanwhile, the amp will get less lively and responsive.

[Buffaloamps]

You wrote that you added another preamp for a gain stage. Are the heater wires wired the same as on the others?

i.e. pin 4,5 and 9 V1 to 4,5 and 9 V2, etc...

Is there a star ground for the filter caps? A good rule of thumb is to have all of your filter cap grounds going to one point on the chassis.

Are there any grid wires near any plate wires? They need to be perpendicular to each other.

Did you do the 100 ohm resistors trick with the heater wires or attach the CT of the 6.3V to the cathodes of the power tubes?

Are the input jack + side wires (if any) away from the volume/tone/trem pots?

Are the coupling caps soldered in with neg side (outer lead right of logo script on cap) to the grid of the power tubes and the positive side to the PI plates? Sometimes this can cause some minor hum.

Are any of your tubes micro phonic? Sometimes a tube will physically oscillate. Weber makes a preamp tube kit that has mounting springs to help eliminate this.

[Mike T]

You wrote that you added another preamp for a gain stage. Are the heater wires wired the same as on the others?

i.e. pin 4,5 and 9 V1 to 4,5 and 9 V2, etc...

Is there a star ground for the filter caps? A good rule of thumb is to have all of your filter cap grounds going to one point on the chassis.

Are there any grid wires near any plate wires? They need to be perpendicular to each other.

Did you do the 100 ohm resistors trick with the heater wires or attach the CT of the 6.3V to the cathodes of the power tubes?

Are the input jack + side wires (if any) away from the volume/tone/trem pots?

Are the coupling caps soldered in with neg side (outer lead right of logo script on cap) to the grid of the power tubes and the positive side to the PI plates? Sometimes this can cause some minor hum.

Are any of your tubes micro phonic? Sometimes a tube will physically oscillate. Weber makes a preamp tube kit that has mounting springs to help eliminate this.


The tube I added was for the long tailed driver. I am 99% sure I did 4-5 to 4-5 and 9-9. The filter caps are a 20-20-20-40 can. I did add 2 more 22uF caps but they are grounded to a lug of the PT right next to the cap can. I'll check for grid near plate wires... I do use the 100 ohm trick and the CT of the 6.3v winding is not grounded. I was not aware that the CT should go to the cathodes of the power tubes. Even if it is cathode biased? The coupling cap thing is new to me too. I'll check that. The tubes are not microphonic. Thanks, man......

[Mike T]

Does the amp hum much when there isn't a guitar plugged in? You need to distinguish between guitar hum and amp hum. Also, since you've modified it, is it single-ended or push-pull? You'll have a hard time getting rid of the hum completely with a single-ended amp.

At a certain point, you'll get diminishing returns trying to throw more filtering at the problem (and you're probably already there). Meanwhile, the amp will get less lively and responsive.

It hums without the guitar and it is push-pull.

[Tone_Terrific]

Easy enough to clip an additional cap into the PS and check for hum change.

[John Phillips]

If the hum is dependent on the volume setting it's nothing to do with the power stage or probably any of the filter caps except the last one.

As Tone_Terrific said, try temporarily adding another cap in parallel with the last 20 - try a really big one (eg 100uF), since it will make the result conclusive one way or the other... (it won't overload anything, that far down the chain).

My guess would be grounding or lead dress near the first tube stage.

[Mike T]

Hey thanks guys.... I put a tube shield on the first pre amp tube and that seemed to get rid of the hum last night at a gig with plenty of beer lights.....

[MikeMcK]

Does the amp hum much when there isn't a guitar plugged in? You need to distinguish between guitar hum and amp hum.

Great point... not that it helps much, but buddy of mine once drove his amp tech crazy chasing a hum that "suddenly appeared" one day.

Finally the tech asked a couple of questions and found that the hum started the day he traded a Les Paul Studio (w/ humbuckers) for an LP reissue Custom (w/ P-90's).