Reduce hum in BF Twin Reverb(AB763)

[Twin]

Hi All,

Just thought I'd repost a new thread concerning my Blackface Twin Reverb that I just built from scratch to '65 vintage specs.

I thank the folks that had input on the last thread & warmly welcome any input you might have for me.

Quick rundown of current issue: I finished the build last week & had wired the ground switch incorrectly(got an incorrect wiring diagram off guitarfiles.com). After that was fixed I had a pretty loud buzz after the amp was switched off standby. I then made a few adjustments to the ground wiring by reading several threads here on MEF & have reduced the noise at idle to about 30% of what it was to begin with. In particular, I took the CT & Filament wires from the PT & grounded them to one of the PT mounts & adjusted the heater wires to come straight up out of the tube sockets.

Thanks in advance for any input!

[soundguruman]

You have some wiring mistakes but we could not possibly tell what they are unless you post several pictures.

[Alan0354]

I think you should post pictures and point to all the ground points. There is a chance you have ground loop problem also and it cannot be trouble shoot with schematic. Also it is important to look at your layout, make sure the preamp tubes are away from the PT.
I supposed you try adjusting the power tube balancing pot to get minimum hum also. Do you have a scope? That is one important piece of equipment to have to check for oscillations.

[Twin]

Hey All,

Thanks for the replies!

I am posting pics & will relay ground points. I tried to bias the quad matched 6L6 GC's(new JJ's) & came up with some VERY different #'s... V9 was 46ma, V8 was 36ma, V7 was around 20ma & V6 came in around 17ma. As I rotated the wiper(bias pot) the numbers all either increased or decreased incrementally and never "balanced". All of the plate voltages were within 1 volt of each other or less...? Something HAS to be wrong here!

The grounds from the cap can are grounded near the PT(blue) & in between V1 & V2(green). The grounds for the pots are located near the PT(green) & Vibrato input(green). The grounds for the power tubes are grounded near the PT(blue). The bias cap is grounded to top left PT bolt. CT & Filament are grounded to bottom right PT bolt. The power cord ground is soldered right where it comes into the chassis.

[Twin]

A few more gut pics...

[txstrat]

I would have a look if the mA readings stay with the tubes or the sockets, by pulling the tubes and interchange them. Even if it's a MATCHED quad the tubes might not be.
If the readings stay with the sockets there might be a wiring mistake, if they stay with the tubes I'd try a different quad.

Proper grounding is essential for a non humming tube amp. I've learned that the hard way. You might have a look at this thread: http://music-electronics-forum.com/t26045/ - especially post #41 where "defaced" posted an image of a grounding scheme, which is really great.
After changing the grounding in my amp the hum decreased to hardly noticeable.

I assume the heaters on all tubes go to the same pins, right?

[Twin]

txstrat,

Thanks, I'll swap tubes & check readings again.

Yes, all heaters go to the same pins.

Thanks for the link!

[soundguruman]

Welp, you may have bad tubes or a bias issue.
If the tubes are running all out of balance like that, then there will be hum as a result.

[52 Bill]

Is the filament winding center tapped?

I think that you should rethink your grounding scheme. Check out the one referred to earlier, I think that it will help.

[Alan0354]

Hey All,

Thanks for the replies!

I am posting pics & will relay ground points. I tried to bias the quad matched 6L6 GC's(new JJ's) & came up with some VERY different #'s... V9 was 46ma, V8 was 36ma, V7 was around 20ma & V6 came in around 17ma. As I rotated the wiper(bias pot) the numbers all either increased or decreased incrementally and never "balanced". All of the plate voltages were within 1 volt of each other or less...? Something HAS to be wrong here!

The grounds from the cap can are grounded near the PT(blue) & in between V1 & V2(green). The grounds for the pots are located near the PT(green) & Vibrato input(green). The grounds for the power tubes are grounded near the PT(blue). The bias cap is grounded to top left PT bolt. CT & Filament are grounded to bottom right PT bolt. The power cord ground is soldered right where it comes into the chassis.

I looked at your layout, you did a good job doing the fiber board. How do you make that, it is very close to the Fender boards. I want to comment on the layout first:

1) I notice you use red wire for input from the guitar, and you tuck under the fiber board. I remember Fender use coax that grounded on one side of the shield to shield the wire. You might be picking up hum because of that particularly you tuck under the components after the amplified signals.

2) You are truely using one point ground using the green wire. I don't believe in one point ground and neither is Fender. I remember Fender ground to the chassis at multiple points. Download schematic of the Fender with the assembly drawing and copy their grounding points. Grounding is a very important thing, unless you know what you are doing, doing one point ground can be dangerous. I do so many pcb layout for high speed and RF analog circuit that has so much more gain and I always use ground plane concept ( which in your case is the chassis.). But I don't want you to just ground anywhere unless you know grounding, copy Fender.
I can tell you I designed a cascade front end into a Bassman by ripping out the board and everything, I tape the metal chassis with copper tape to get better conductivity as steel is not the best. I take every single ground point right down to the closest possible solder point onto the tape. All pots are ground right onto the chassis at that point. It was dead quiet. People don't know grounding and start making all different assumption. I worked on signal integrity and system grounding throughout the years, one point ground only work if you have separate rack of electronics and you have no choice but to ground at one point using star configuration. Within each rack where you get solid ground, you don't play with the grounding. I use this principle and never once I ran into grounding issue.

3) The twisted blue and yellow wire that goes to the 220K resistors for the differential stage( phase splitter) are tuck under the preamp wires along the way, I don't think it's a good idea as the signal might cross talk. Again look at the assembly drawing of the Fender.

4) I can't see how you ground the volume pots and others, copy Fender.

Now about the tubes, they are unbalance. I think you should put the 44mA and 36mA on the opposite sides, then pair up so the 44mA next to 17mA, and let 36mA pair with 20mA so the total current on each side are closer.

[Twin]

Alan0354,

Thanks for the reply!

1) Yes, I did use unshielded wire for the inputs & on an actual vintage Super Reverb(AB763) they did use unshielded wire for the inputs as well. I am attempting to keep it as close to original Fender design to avoid any issues that may arise from "mods". However I do not have the vintage brass ground buss like the originals have, nor do I currently have access to an actual vintage Fender ground layout(arrows don't aid in placement), so I did the best I could. Trouble is that my best isn't quite enough as I still have a fair amount of hum...

2) There are several points that I have grounded to the chassis. I tried to explain this in good detail in the #4 post, but if you need me to be more descriptive let me know.

3) The twisted wires(A,B,X,Y etc.) layout were not clearly defined on the Fender layout, so I just copied my buddy's vintage Super Reverb(AB763). Could you please elaborate on which wires need to be moved?

4)The volume pots(and others) are grounded to the chassis via the green wires.

Thanks again & I hope that my reply has been usefully informative.

[Alan0354]

Alan0354,

Thanks for the reply!

1) Yes, I did use unshielded wire for the inputs & on an actual vintage Super Reverb(AB763) they did use unshielded wire for the inputs as well. I am attempting to keep it as close to original Fender design to avoid any issues that may arise from "mods". However I do not have the vintage brass ground buss like the originals have, nor do I currently have access to an actual vintage Fender ground layout(arrows don't aid in placement), so I did the best I could. Trouble is that my best isn't quite enough as I still have a fair amount of hum...

2) There are several points that I have grounded to the chassis. I tried to explain this in good detail in the #4 post, but if you need me to be more descriptive let me know.

3) The twisted wires(A,B,X,Y etc.) layout were not clearly defined on the Fender layout, so I just copied my buddy's vintage Super Reverb(AB763). Could you please elaborate on which wires need to be moved?

4)The volume pots(and others) are grounded to the chassis via the green wires.

Thanks again & I hope that my reply has been usefully informative.

I actually just came back because I forgot to look at one thing. How do you ground the 4 power tube? I remember Fender soldered it right onto the chassis next to the cathode pin. I can't see in your picture.

Try the power tube matching first. You can verify if you pull the two lower current ones out, put the 44mA and 36mA two on the extreme opposite side so they become the push pull pair. It is safe to do so as Mesa cut the grounds to one pair to lower the wattage. Then adjust the balance and verify that you can get rid of the hum. If you can, you don't need to do anything to the wiring, just pair it up like what I described. If the hum side cannot be adjusted out, then you problem is not the tube mismatch. The amp is not in front of me, so you just need to do one step at a time.

[Twin]

Alan0354,

I made an error when I stated that the grounds for the power tubes were blue in color... They are actually BROWN & terminate on a ground bus near the bottom right corner of the PT.

Will try swapping the power tubes around to see if that has any positive/adverse effects on the hum situation. Thanks again for your input.

[Alan0354]

I see the ground bus, I also see the blue wire from the ground bus going towards the preamp section and disappeared under the fiber board. This always worry me. Other than going to the main filter cap on the other side, don't hook up to anywhere.

[Twin]

Alan0354,

Yes. The blue wire runs along the tagboard & goes through a rubber grommet to the cap can board & terminates on the 1st 100uf cap. You can see it in the picture of the doghouse in post #4.