Reduce hum in BF Twin Reverb(AB763)

[Alan0354]

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.

Good, that is perfect. You want to keep the ground return current path away from the preamp section.
You try the power tube yet? It is important to find the cause of the hum by process of elimination.

[Alan0354]

Maybe I was wrong about pairing the 46mA and 16mA on the first half and 36mA and 20mA for the second half. I was trying to balance it. But I think that was wrong.

I think the best should be putting 46mA and 20mA on the first half side( say the two left tube), then 36mA and 17mA on the second half( say the right two tube). With this, you have obviously the first half is pulling more current. Then you just adjust the balance pot the even out the current so hopefully the higher current two tubes will be running at about 40mA and the two lower current pull about 18 to 19 mA. Then you get a balance circuit. try different ways as this should not hurt anything.

As long as both sides are balance, the 60 or 120 Hz should be cancel out by common mode rejection. I think from you second post, you have V9 and V8 both carry high current of 44 and 36mA on the first half, then you have 20 and 17mA on the other half, your balance pot is out of range in adjusting it.

from looking at your layout and talking to you, I think you did a very good job in the layout. Where did you get those boards with the eyelids?

[Twin]

Alan0356,

Thanks for the tips on matching the power tubes. Tried swapping power tubes & didn't change hum... I pulled each of the preamp tubes 1 by1 to try & isolate the hum & it is evident that the source is coming from V2. Any ideas on what to look for next?

The tagboard was included in the Mojotone small parts kit:

Search Results

Thanks!

[Alan0354]

Alan0356,

Thanks for the tips on matching the power tubes. Tried swapping power tubes & didn't change hum... I pulled each of the preamp tubes 1 by1 to try & isolate the hum & it is evident that the source is coming from V2. Any ideas on what to look for next?

The tagboard was included in the Mojotone small parts kit:

Search Results

Thanks!

I don't know V2 or V?!!! Please upload the schematic. I looked at the Vibrosonic on Fender site and V2 is the preamp tube of Vibrato channel. I looked at the picture. You did something different, the 68K resistor from the input is away from the jack.

1) Unsolder the Red wire from the resistors and just ground at the chassis. If that don't help then
2) Take the RED wire out from under the board and hook back up to the resistors and see any improvement, if that don't help then
3) Ground it close to V2 and see whether the hum still there.
4) If the above change or lower the hum, use a coax that grounded one side at the input jack just like the Fenders. Try that first and upload the schematic. Take one or two close up pictures of the back of the input jack, I want to see how you connect the wires to the 68K.

Also what is the blue wire from the left side that comes out of the gromet? You twist that around the input area of the Vibrato channel, try move it away and see what happen.

Where do you get the chassis and the cabinet?

[Twin]

Alan,

Here is the schematic & layout:

twin_reverb_ab763.pdf

The red wire is the input signal going to the 12AX7, not sure I want to ground that, however I did unsolder it & moved it around to see if it would help... nothing. I can try coax, but none of the vintage Fenders that I've seen have shielded inputs(that I remember, anyway).

The blue wire, I believe you are referring to the left leg of node "D", it comes from the 22uf filter caps in the doghouse.

I got the chassis & cabinet including the speakers off ebay. Someone gutted it & left it for dead until I came along!

[Alan0354]

Alan,

Here is the schematic & layout:

[ATTACH=CONFIG]17327[/ATTACH]

The red wire is the input signal going to the 12AX7, not sure I want to ground that, however I did unsolder it & moved it around to see if it would help... nothing. I can try coax, but none of the vintage Fenders that I've seen have shielded inputs(that I remember, anyway).

The blue wire, I believe you are referring to the left leg of node "D", it comes from the 22uf filter caps in the doghouse.

I got the chassis & cabinet including the speakers off ebay. Someone gutted it & left it for dead until I came along!

I know that's the input, it is safe to ground the red wire as it is at ground level through the 68K. The input is ground level so the cathode is at +1.5V or so to self bias to about 1mA. In fact it is more dangerous to let it float as it is very high impedance and if it float to +V, you will increase the current and heat up the tube.

I am trying to open up stage by stage to see where it stop. The gird of the tube is grounded through the 68K if you don't plug anything into the jack. Read the schematic and you'll see. If you don't want to do that, then the next step is to disconnect the middle wire solder to the middle of the treble pot( the wiper). If the noise goes away, then you have to go back and do the red wire test. If the noise is still there, then this proof that the noise is from the second half of the tube and not even from the first half. The treble pot is AC coupled to the volume pot, so you don't loss bias on the second tube. Try this then to see whether you stop the noise by unsolder the middle of the treble pot.

BTW, your schematic show the layout of the grounding, the components and the wires. You should follow the grounding point of the drawing. You are bundle the grounds from the input stage together with the later stages at the strip where Fender ground onto the chassis at different points close by to the circuit. You know the Fender layout work. It is very hard to trouble shoot this kind of problem by writing. I really need to have my hands on the amp to do it.

[Twin]

Alan,

Thanks for the wealth of info in this & former posts! I will do like you recommend & ground the input to see if it fixes the hum(Normal channel right?). If that doesn't work I'll pull the wire off the center lug of the Treble pot. If that doesn't work I will purchase some adhesive-backed copper tape & create a ground bus similar to Fender's for the pots & the negative side of the tagboard in order to directly solder them where the arrows point. Thanks again for all your input & patience as this is not an easy troubleshoot, nor first time build.

[Alan0354]

Don't pull the wire out just yet, do the treble pot first, maybe it's with the second half of the tube, then it'll save you the trouble to re thread it back. I thought your problem is on the vibrato channel as I posted to verify with you yesterday!!! If it is the normal channel( on the left side, let me know, I have to look at the picture again!!! Yes, if the hum goes away when you remove the preamp tube, we start from there. What I am trying is to determine which half of the tube have the problem. post back and we'll keep going.

[Twin]

Hey Alan,

You are correct, the problem is on the Vibrato channel & the hum goes away when I remove V2. I will check the treble pot as you said & report back...

BTW, I bought some of the copper tape today to simulate the brass grounding bus like Fender used... Do you think it needs it even if we fix the hum?

Thanks again!

[Alan0354]

Hey Alan,

You are correct, the problem is on the Vibrato channel & the hum goes away when I remove V2. I will check the treble pot as you said & report back...

BTW, I bought some of the copper tape today to simulate the brass grounding bus like Fender used... Do you think it needs it even if we fix the hum?

Thanks again!

I would follow the Fender's grounding. The reason I said that is right now, you have one point ground for the preamp to the phase splitter, all the signal return concentrate at one point. I always like to look at the signal return path. as the signal current return tends to follow the signal path on top. It would be much more important at higher frequency, at low frequency, the current tend to spread out a little more. But still ground is not just ground.

But I don't think you should do anything about grounding for now, make it easy, trouble shoot one problem at a time. Disconnect the treble center wire and see what happen. Removing the V2 kill the noise, the problem got to be at the around that area.

[Twin]

OK Alan, will do. Gonna check the treble pot wire in the next hour... Thanks again!

[Alan0354]

If removing the center wire of the treble pot stop the noise, then the noise is from the tone circuit and the first half of the tube. Then lift the 100K plate resistor of the first 1/2 of the tube. Just lift one side with do, that will just turn off the first 1/2. See whether that stop the noise. If noise still there but stop when remove the center of the treble pot, the noise is from the tone adjustment circuit.

If the noise is still there after open up the center tap of the treble pot, then the problem is in the second half of the tube. The next step is to pull the 12AT7 reverb tube as see whether that would stop the hum. I just want to double check the reverb circuit is not causing the problem.

[Twin]

Removing the center wire of the Treble pot does NOT stop the hum... Then I tried removing the 12AT7(V3) & the hum is still there...?

-I should also mention that I didn't reattach the middle wire to the Treble pot of the Vibrato channel when I pulled V3.

[Alan0354]

Can you double check removing the V2 to make sure the hum is off. Then replace the V2. Leave the V3 off for the moment. Remove the 7025..call it V4, this is the one that follow the reverb tank. That should stop the hum also.

If removing either V2 or V3 stop the hum, then the next step is to lift one side of the 0.02 cap that connect the plate of V2 to the grid of V3 and 10pF//3.3M. Doing this will cut the connection between V2 and V3. If that stop the hum, the problem is in the second half of the V2. If that don't stop the hum, then there are contradiction as I cut the connection of V2 to V3 and it should stop the hum. This is a little redundant because I want to double verify as I don't have my hands on it.

Does the volume of Vibrato channel affect the hum?

[Twin]

Yes. Pulled V2 and no hum(same as always). Replaced V2 & removed V3 & V4 & NO hum. Will pull one side of .02 cap now... What am I looking for once pulled? Should I power up with all tubes & listen?

I should mention that if I ONLY remove V3 the hum is still there. Also the volume of the Vibrato channel does NOT effect the hum.