Hi All, my first post so please pardon me if I didn't find the answer and it is already somewhere. I just restored a Bogen CHA-20 PA amp. It is about 60 years old and after replacing all the electrolytics and waxies and a few tweaks to bias the tubes it is making sweet music again. I also replaced the old mike input with a 1/4 inch jack for guitar use.

My question is regarding a hum (I think 60 cycle) that is getting introduced somewhere between the input jack to the grid of the first tube. Bogen chose to use a 330Mf cap in between the input and the grid and 2 Mb to ground. It really improves the sound as I removed it and replaced with a 68Kohm resister (what fender does), but then the sound of the amp lost a lot of high end definition and my guitar would make loud popping noises when I switched pickup positions. Any piece of static in the volume control would make loud scratching noise as well.

So I added the 330MF cap back into the circuit and it sounds great except the buzz as you turn up the volume. Interestingly, the buzz is louder when there is nothing plugged in to the amp. When I plug the guitar in it goes down a lot, but is still present. So I carefully re-wired the input shielded wire etc. and had the cap band side facing the input direction (to maximize shielding). Still buzz. When I poke around with a chopstick the (new) Zoso 330mfd cap makes noise and the hum in that area gets louder as I move the chopstick closer and goes down when I move it away.

Any ideas on things to try are greatly appreciated. This is my first amp restore so the answer may be obvious. Thanks.

Hi. Try increasing the 300pf cap to something like .01uf or bigger .1uf maybe. Leave the grid resistor what it was you said 2meg.

I assume the new 1/4 inch jack is grounded directly to chassis at the mount. Was the original mic connector? If not your ground may have introduced a hum loop. You would need to isolate the jack from chassis and ground it back at the same point the original mic jack picked up ground.

The cap between the mic input and the grid of the first 12AX7 section is supposed to be 0.033-uF. Hopefully you did not use a 330-uF cap to replace it!

Thanks guys for the feedback. It is a .033mfd cap sorry for the typo. Schematic for reference. The input grounding is a good question. I will try to isolate the jack from the housing and ground it exactly as the Mike input was. Thanks for the help!

Tip: You may want to use the self-shorting phone jacks ( like Fender, Gibson, etc used on amps) to cut 'noise' coming from unused jacks on inputs. That may help some.

@caverat, thanks for the grounding tip. I reworked it to be as clean and close to the original way as I could. Sounds much better!

Still some audible hum remains, but much less and none of the microphonic stuff at the first stage input. manageable until I turn the volume above 4 (which is really loud).

Any ideas on how to track down the remainder of the hum? I can keep replacing caps like the small ceramic ones for the tone etc. Not sure how to use a scope but imagine that would be a good tool to find it. Suggestions appreciated. Thanks.

Try grounding the grids of each pre-amp tube one at a time, then the power stage grids, if needed, and see it the hum goes away at any time.

Keep leads as short as possible from the jack to the grid. Sounds like that is your hum source. Does the hum disappear with the 1st stage muted?
Keep all filament wiring as close to the chassis as possible, with the signal path components away from those wires. Sometimes pushing things around with a chopstick will make a difference.

It might just be the nature of the beast, in combination with a guitar rather than a mike.
Sometimes 'star grounding', where grounds are combined at just a few points, helps to eliminate ground loops.

Hi, thanks again for the ideas. It seems to me like the first stage is ok now lead is shielded and short. Poking with a chopstick in that area doesn't increase hum seems pretty robust now. Don't no how to mute the first stage short of disconnecting the plate out into the grid of the 2nd half of the 12ax7 is that what you are suggesting by mute the first stage? Thanks.

Like this, with alligator clips.

Are the tube heater leads grounded at either side? If so, try floating them above ground or connect a 100 ohm 1W resistor from each side to ground (Effectively balancing the heater above ground equally.)

Another possibility, have you swapped out the 12AX7s? Possibility of heater to cathode leakage in one of them. Try grounding the control grid of the first stage to the same location the cathode resistor gets ground. Turn up the volume, and the amp should have no hum. If it does the hum is getting in upstream from the input stage. Ground the next stage control grid to the point hwere its cathode resistor is grounded When yo come to a stage where the hum disappeard you found the stage where it is coming from. Troubleshoot accordingly.

I had a very slight residual hum in a jukebox amp I had restored. It wasn't loud but could be heard during very quiet music passages. Annoying. I tried all the usual shielding, lead placement, tube changes, etc. to no avail. This exercise was repeated several times over several years.

Then one day I decided that I was going to keep looking until I found the problem. That time I did. While measuring everything I could think of, I noticed that there was a very small ac voltage (barely moved the needle on 1.5 VTVM range) on one of the tube socket mounting rings. Bingo. Corrosion (invisible) had added a tiny resistance between the socket ring and the chassis. The ring was used as the filament return for a 6L6 and it's grid resistor. Soldering the ring to ground completely eliminated the hum. Then I soldered all the socket rings to the chassis.

RRM

Wow great feedback guys. Thanks again! I will keep working this as I have a feeling I can make this amp dead silent. What a fun project!

instead of grounding grids you could always just yank the pre-amp tubes out one at a time and notice any changes. it may be easier than poking around in the underside of the chassis

Thanks, very good forensics... Never met it myself, but forewarned is forearmed.
On the wider issue, I have changed most of my old tube power amps to 100K or 250K input resistance to discourage hum pick-up... never understood why they chose 1 Meg back in the day. All my pre-amp outputs are cathode follower (~500 ohms) or modern solid state (very low), so no source problems.
Cheers,
Roger

Hi all, even with your good feedback I haven't been able to sort out the hum issue when nothing is plugged in. Goes away as soon as I plug in my guitar. One thing I realized that is different - I added a 3 prong cable to this system (was 2 prong before) and connected the ground from the power cable to one of the bolts holding down the input transformer. Could that have anything to do with this? Thanks again for your help.

Do these two tests separately.

1. Disconnect that ground wire. If hum goes away, that means it may have a ground loop with something else plugged into the wall in the house.

2. On the input jack, put a jumper across the two conductors. If hum goes away, you need a different type of Jack as others suggested.

It's not mentioned before: the piece of gear is old -- did you replace the old paper and electrolytic capacitors? If not, one or more of these old capacitors could be leaky or defective (very common). Can you also list all the mods currently installed?

Thanks all! It turns out that the remaining hum (when the jack was not plugged in) is fixed by going to a closed circuit jack. I may be jumping the gun but the short test proved positive.

I didn't understand it until AJJ and Kevin suggested a test to short the input jack and see if the hum went away (it did!). Thanks to Tbirdkid for the original suggestion in this thread (which I didn't understand at the time).

John

if the jack has grounding contacts check to see if they are making contact when idle no plug. burnish if needed. a small flat screwdriver blade is sometimes a good burnishing tool