Help me identify the hum and buzz

[deuce42]

Hi guys

I've made a single channel AB763, no trememlo just reverb.

I cannot seem to work out where the hum is and whats wrong with the amp.

I've attached a sound file below hopefully you guys could hear the noise and offer me some advice as to the problem. It is there with the volume turned completely to zero. If I turn up the volume other whooshing noise comes in but really the humming is the same irrespective of volume or tone control changes.

I cant work out whether its:

1) Heater wire grounding issue
2) Heater wire layout/dress
3) Filter caps in power supply - seems unlikely as they are only relatively new
4) some grounding problem with the signal path.

Any help would be so grateful.

Thanks

[pompeiisneaks]

Hi guys

I've made a single channel AB763, no trememlo just reverb.

I cannot seem to work out where the hum is and whats wrong with the amp.

I've attached a sound file below hopefully you guys could hear the noise and offer me some advice as to the problem. It is there with the volume turned completely to zero. If I turn up the volume other whooshing noise comes in but really the humming is the same irrespective of volume or tone control changes.

I cant work out whether its:

1) Heater wire grounding issue
2) Heater wire layout/dress
3) Filter caps in power supply - seems unlikely as they are only relatively new
4) some grounding problem with the signal path.

Any help would be so grateful.

Thanks

here's a direct playable link for others of the sound.

Sounds kinda like 60 cycle hum to me, could be a ground loop, and/or poorly shielded parts of the amp, could be a bad tube, lots of things can accept hum from the outside. To better assist it really helps if you have high quality pics of the amp, so we can look around for potential sources of the problem.

Have you tried using a wooden chopstick and taping around to see if any component causes the problem? Have you tried moving some of the wires around with the chopstick to see if lead dress impacts it?

How are your heaters grounded? Did you follow the original fender ground one side all the way, or did you run them separately and have either a real center tap to the heaters that's grounded, or are using a pair of 100 ohms to ground and/or hum balance pot?

~Phil

[deuce42]

Ok thanks Phil - I will take out he chassis and take some photos.

So if you were to think it it was 60hz hum, I am I right in saying this would be a signal ground issue rather than heaters?

As for the centre tap of the heaters, the power transformer has a a centre tap wire and this is grounded at the same point as the power supply circuits grounded signal ground as well.

[pompeiisneaks]

it could really be either, if the heaters have a center tap, but their lead dress is a bit too close then they could send hum.

More likely a ground loop, we'd like to see the way you grounded the different sections of the amp in the pictures that will help see if there's something that could be contributing noise too.

~Phil

[deuce42]

I'm not sure how revealing these photos really are. Its hard to get the phone into the grounding points. I wish I could take photos that would actually show you guys what I'm trying to do.

Basically I took an old fiberboard from an original silver face AB763and the pcb from the power section of a new 65 reissue super reverb and tried to put them into an old disused combo chassis. The wirings a little messy but the best I could do so far. I'm still a bit of a novice.

When I turn the volume down completely the hum is at the same level. It doesn't change with the volume knob. As I turn up the volume, the hum gets drowned out by the guitar signal a bit but is always there.

I tried adding an extra node to the power supply for the first gain stage but that didn't change the hum.

I have isolated the output sockets from the chassis and also made sure the output xformer secondary ground is attached to the speaker grounds. No change in hum.

I've connected the grounds of the preamp and its filter caps to the input socket which is grounded at the chassis. All other grounds go to a lug attached to one of the metal circuit board stand offs that's attached to the chassis. No change in hum there either.

The only other thing I can think of is the problem may be around:

a) the toggle switch near the volume knob - this is not a bright switch but has a cathode bypass cap and resister attached to each lug on the switch. It was so that I can switch cathode bypass caps/resistor combinations - ie a 2.7k.0.68 on one side and a 22uf/1.5k on the other side or:

b) I inserted a larmar post phase invester volume control. Maybe this is causing a difficulty although I've inserted them in two other amps without any problems.

I just cannot ascertain what the problem is. Perhaps the heater wires for the first 4 tubes (the 9 pin tubes) which are lying flat rather than twisted? The chassis that I used as a donor already had the heater wire laid in first 4 tubes without any twisting. The amp I pulled the chassis from never had any humming.

Any help would be appreciated before I blow my own fuse...

[pompeiisneaks]

The pictures are quite dark, and hard to see. Also any way you can get them larger/higher resolution? Do you have a sound clip?

If there is noise without any signal present and volumes at 0, it often comes from the PI, output tubes, transformer placement of the transformers themselves or the wires that come from/to them. Another option would be the heaters adding in hum.

If you pull the PI tube does the hum persist, or go away. If it does go away, the problem is in the output section or transformer based (heater hum can inject at the power tube level but it's usually a lot less likely as they're pushing so much current it swamps most noise).

If it doesn't, put it back in, and try pulling and then replacing one preamp tube at a time, to see if you can find the noisy section of the amp. (where the noise is coming from).

~Phil

[Stevem]

If you yank the PI tube is the hum the same?
If you place a short across the return jack from the reverb does that lessen the hum?

Does jumping out the bias supply cap with another change anything?

[deuce42]

Hi guys, so the hum completely disappears with the PI tube being pulled out. You almost cannot even hear the amp is on. Put the PI tube back and it sounds like the room is being drilled!

Unfortunately don't have a return jack for the reverb as it isn't switchable. I just roll up the reverb volume when I want more. I like to play with reverb always on so didn't bother putting in a footswitch jack.

So if its transformer based problem, is it the leads dressing of the xformer thats the problem? Could it be PI section itself? - ie the leads to the PPIV pot?

[Stevem]

Well now that you know it's in the driver/ preamp section start unpowering / lifing one end of each plate load resistor starting with V1 and see which kills the hum.

I also hear a lot of buzz besides the hum in your sound sample so would get a peice of card board big enough to cover the chassis and tape some Aluminum foil to it and sit it on top of the amp and ground it with a clip lead, or ohm check it when on top of the amp to confirm it's grounded.
Does doing such make any difference?

Also does you voltmeter have a frequency test setting , if so hook it across the output of the amp and see what the strongest frequency is that the amp is outputting.

Also check all your parts in the preamp that go to ground for zero resistance.

[deuce42]

Ok thanks Stevem

For the life of me I'm just stumped - I've lifted the wire to the plate resistors of each tube up to the PI tube. The hum still persists! I just cannot work out what else it could be if the hum goes away when pulling out the PI tube.

Maybe heater hum??

Thanks for all your assistance

[Stevem]

In regards to the PI tube take its grounds , extend there wires and ground them at the input jack(s) and see what changes result.

In basic terms the ground for the first / main filter, screen filter , power tube ground and bias filter ground should all be landed at the same point near the PT along with the filiment winding center tap and the B+ winding center tap.
All of the other grounds should be separate and get grounded up at the input jacks.
You can parallel all the tube cathode resistor and bypass caps and just run one wire off of them to ground.
To do a duel start ground set up like this it's best if you take all of these preamp grounds and use solder or crimp on eye rings and stack them up on a long machine screw that's bolted down close to the input jack.

If you use crimp on rings I still solder them up after the crimp has been made.
They all need to check out at zero resistance otherwise you've made a antenna that will pick up noise.

[sluckey]

The speaker jack and OT secondary common must be grounded to chassis, preferably at the same point the PI circuit is grounded. Since the AB763 uses NFB, leaving the speaker jacks ungrounded will cause hum that will disappear when you pull the PI. Are your jacks well grounded?

[deuce42]

Ah ok I did the complete opposite with the speaker jacks. I isolated them so that there were only what I thought were supposed to be two grounds. The preamp grounded by the input socket and then the power amp, PI, bias, heater centre tap and power supply caps going to the other ground.

I've tried with both the speaker output isolated and touching the chassis. All paths lead to horrible hum.

[sluckey]

Disconnect the NFB wire from the speaker jack. Does the hum go away?

[Stevem]

You might try what I had to do to a reissue Deluxe reverb last night to find out why it had a 120 HZ hum out of the speaker.

This amp had 2 bad filters but replacing them still did not fix the issue and after checking and trying a host of things for 35 minutes I yanked out the two output tubes and the PI tube and it still had the issue, finally I yanked off the red wire feeding the OT center tap and the amp was fine, no 120 HZ hum!

It seems that my amp here has a bad OT as I can only say that it must be bleeding V+ voltage to the secondary winding even though I can not read and resistance between the two and the D.C. I am reading on the speaker is very small.
Tonight I will yank this OT out and wire it into a Bandmaster chassis I have out to confirm it's the OT .

I will report back tomorrow.