AB763 deluxe reverb reissue conversion B+ and buzz problem

[Mrsedor]

Could it be as simple as bad tubes? I don't have 6V6 spares yet to try it...
What do you think?

Thanks

[sluckey]

Yes. Tubes always need to be eliminated as suspects. Try a new set of 6V6s.

[Mrsedor]

Yes. Tubes always need to be eliminated as suspects. Try a new set of 6V6s.

Thank you,

I have ordered a new pair of 6V6gt TAD.
I will be able to try them at the end of the week I guess.

In the meantime, in your opinion is there any test I can conduct?

Thanks!

[sluckey]

Ear test.

[Mrsedor]

Ear test.

Just so I don't do any harmful mistake :
You mean, playing and hearing with the tubes set at a bias corresponding to -37V at the 220k junction, even if they are redplating?

[sluckey]

No!

[Mrsedor]

No!

I didn't.

I Just didn't understand what you wanted me to test by hearing.
Sorry if I don't get everything, please forgive me, English is not my mother language.

[pdf64]

The DRRI uses a different PT than used by an original AB763 DR. So, replace your 470Ω with a 220Ω like the DRRI uses. You'll get a little more -VDC.
...

Just to note that R69 is 22Ω, rather than 220Ω.
https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_manual.pdf

[Mrsedor]

The DRRI uses a different PT than used by an original AB763 DR. So, replace your 470Ω with a 220Ω like the DRRI uses. You'll get a little more -VDC.
...

Just to note that R69 is 22Ω, rather than 220Ω.
https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_manual.pdf

Thanks!
Yes I will change for a 22Ohms.
I notice 2 other changes in this circuit:
A 100uf100V instead of the 25uf50V
A 22uf63V is added at 220K Junction.

Wouldn't it benefit the bias circuit if I change them as well?

[sluckey]

But, you already said, "So now at the grids I can read from -45V to -22". That's a perfectly good bias voltage range for 6V6s that will likely be set to about -36V when biased properly. There is no issue here.

[Mrsedor]

But, you already said, "So now at the grids I can read from -45V to -22". That's a perfectly good bias voltage range for 6V6s that will likely be set to about -36V when biased properly. There is no issue here.

Ok I understand, there's no issue here. So the issue should be either the tubes (I shall have a new pair at the end of the week) or somewhere around theses tubes. I will double check the sockets, resistors and connections.
Thanks

[sluckey]

When the bias is set for -45v on pins 5 are the tubes redplating? How does the amp sound?

[Mrsedor]

When the bias is set for -45v on pins 5 are the tubes redplating? How does the amp sound?

When it is set at -45v, it is not redplating.
The amplifier sounds about ok, nothing obvious but the hum I described previously.

[Mrsedor]

Yes. Tubes always need to be eliminated as suspects. Try a new set of 6V6s.

So I finally received a new pair of 6V6 and part of the problem is solved:

With bias to the minimum I have 16,9mV across the 1 Ohm resistors. I can set the bias now around 23/24 mA for both tubes, safely and without redplating.

Thanks a lot for helping me finding this, I learned a lot!

Now I still have the hum.

I can hear it with V1, V6 and power tubes installed. it is not related with volume. It is low hum, not a lot of harmonics, so I think it is more 50hz than 100hz.

Is there any test I could do to troubleshot it?

Thanks a lot for your advices 

[Esquirefreak]

I suspect a grounding issue. Most likely a ground loop or possibly a shunt/breaker tip on the input jacks not making proper contact when unused.

/Max

[Mrsedor]

I suspect a grounding issue. Most likely a ground loop or possibly a shunt/breaker tip on the input jacks not making proper contact when unused.

/Max

Thank you.

I still have the hum with V1 pulled out. Only pi, 6v6 and rectifier installed.
Does that rule out a failed input jack tip grounding?

Is there a good method to test ground loops?

Thanks a lot

[Mrsedor]

Today I moved the center tap for the 100ohm heater resistors from bias ground to 6v6 cathode.
I felt it helped a bit with the hum, but it might be just an impression, as there is still to much 50hz.
Would an humdinger be of any help here in your opinion?

One thing I never noticed before is, with nothing plugged, and input gain to maximum I can hear a slight buzz added.  It is higher pitch than the other hum and much much lower in level. This buzz disappear whenever I touch the chassis or jack inputs. I must say it is almost not noticeable, and I could definitely live with it, but I mention it because it could maybe be a symptom of something wrong.
Would it imply bad grounding? The amp is plugged in another outlet in the basement, earth may not be perfect...

Thanks for your help ☺️

[Mrsedor]

Well, I'm making progress here !

I changed the bias capacitor, 25uf /50v Atom for another Vishay 100uf/100V.
No more hum !!!

I guess somehow either the original capacitor was shot, or it was not happy with the DRRI transformer, or both. But now this annoying hum is totally gone. Very happy !

Ok it is not the end of the story, now I've got to figure something else:
With the amp open, and only V1, V6 preamp tubes I get a bit of buzz (100hz and harmonics), that goes away when I touch the chassis and put my hand 10cm above the circuit (not touching it). So I guess it's picking up RFI.
With all tubes inside it is way more, but same behaviour. With me touching the chassis and my hand above the circuit, there is no more hum.
It is the most with reverb, as could be expected.
As I said previously, earth in this basement is suspect. I will try it tomorrow in a good known electrical environment.

Any idea if this could indicate a ground fault? Or is it just RFI because the chassis is still open and not properly shielded / earth grounded?

Thank you, I'm so happy I solved this annoying hum!!! 

[Mrsedor]

Ok so, now the amp is back in its casing, no more EMI affected, that's a good thing.

But everything is not solved.

Normal channel is good :
With V1, V6 and power tubes, no problem, just a tiny bit of 50hz hum, really low, that I probably can tame with a humdinger, I will try it soon.

Vibrato channel is more problematic:
With all the tubes installed, I get buzz even with everything at zero volume.
I get another buzz that is superposing to the first one with the reverb pot:
Reverb pot at 0, buzz added, at 1 or 2 less buzz, and it is coming back after 3. This is when reverb is out from the footswitch. When it is on, same behaviour, with a bit more buzz.
Looks like a faulty ground around here, or maybe a ground loop? should i try to isolate the plugs? not really easy...
Of course when I remove V4, everything is back to Normal channel only, with no noise.

[Mrsedor]

Reverb pot at 0, buzz added,

This is what's troubles me the most

[Mrsedor]

Looks like I was a little bit overenthusiastic yesterday... 

Back with only V1, V6 and power tubes and ... hum . I think less than before but still problematic.

Installed a humdinger. Could reduce the hum a tiny bit, but nothing signifiant.

I went back to all the grounds, nothing apparently wrong there.
I isolated the footswitch pedal and connected its ground to V4cathode ground on the preamp ground bus. No change. Next step is to isolate rca grounds and connect them to the same spot.

I'm starting to loose hope to save this amp...

Any idea about how tracking this hum?

[Mrsedor]

I isolated RCA plugs and footswitch pedal. Connecting their ground to V4 cathode ground (on the buss bar). No change.

[pdf64]

Soundclip of the buzz?

[Mrsedor]

Soundclip of the buzz?

Here it is, with all the tubes installed in. I switch on and of the reverb that has a lot of noise in this position. But there is idle buzz anyway.

Hope the level is high enough.

With only PI and Power tubes there is the same buzz, but less pronounced. I can also make a video of that if you want.

Thanks a lot for your advices

[pdf64]

Soundclip of the buzz?

Here it is, with all the tubes installed in. I switch on and of the reverb that has a lot of noise in this position. But there is idle buzz anyway.

Hope the level is high enough.

So the reverb buzz is much lounder than anything else?
How about if you unplug the footswitch?
If that doesn't help, check that the reverb tank input end isn't closer to the PT than the output end.

I'm not sure that the layout noted in the OP is ideal, particularly that the negative terminal of the HT C node cap seems to be directly connected to the A node negative.
Martin Manning has done a nice redesign of the 0V common layout of black panel Fenders, which might be of interest https://ampgarage.com/forum/viewtopic.php?p=428877#p428877

[Willabe]

If that doesn't help, check that the reverb tank input end isn't closer to the PT than the output end.

I'm not sure that the layout noted in the OP is ideal, particularly that the negative terminal of the HT C node cap seems to be directly connected to the A node negative.

Yes on both, check how you have the verb tank oriented and try moving the C node ground over to the D node ground. Won't hurt anything to try moving that C node ground, you can always put it back where it was if it doesn't help. It's only 1 wire.

[Mrsedor]

So the reverb buzz is much lounder than anything else?
How about if you unplug the footswitch?

With the footswitch unplugged it is the same :

Please note that I push the reverb to 10, which is a lot.

[Mrsedor]

If that doesn't help, check that the reverb tank input end isn't closer to the PT than the output end.

The reverb tank is not in its right position, it is now outside of the cab, as you can see on the right of the video.

[Mrsedor]

Martin Manning has done a nice redesign of the 0V common layout of black panel Fenders, which might be of interest https://ampgarage.com/forum/viewtopic.php?p=428877#p428877

I just registered to ampgarage to be able to see this document, but my account needs to be reviewed by the administrators, so I cannot see it at the moment.

For now, the reverb recovery noise is high, but the idle noise is also not quiet, that what is bothering me the much.
With only PI and Power tubes I have low 50hz hum, not that buzz, but still, it should be quiet:

[Mrsedor]

try moving the C node ground over to the D node ground. Won't hurt anything to try moving that C node ground, you can always put it back where it was if it doesn't help. It's only 1 wire.

You mean the preamp ground? (At the left of the picture)

[Mrsedor]

I'm hearing also a little bit of oscillation from the vibrato (not active at the footswitch) since I isolated the footswitch jack (I think, didnt hear it before).

[Willabe]

try moving the C node ground over to the D node ground. Won't hurt anything to try moving that C node ground, you can always put it back where it was if it doesn't help. It's only 1 wire.

You mean the preamp ground? (At the left of the picture)

Now that I see the dog house wiring, just disconnect the B node (B+2) screens cap ground for the C node (B+3) cap ground and now move that B node (B+2) screens cap ground to the A node OT CT (B+1) cap ground.

See attached pic below.

[Mrsedor]

Now that I see the dog house wiring, just disconnect the B node (B+2) screens cap ground for the C node (B+3) cap ground and now move that B node (B+2) screens cap ground to the A node OT CT (B+1) cap ground.
See attached pic below.

Thanks a lot,
will do in a minute and report.

[Willabe]

If that doesn't help try this;

The K R and bypass cap on the reverb recovery tubes pin 3 and 8, move those 2 parts off the board and onto pin 3 off the verb recovery tube socket and ground them with the recovery tube 220K grid leak R.

Parts shown and where to ground them in pic below. But I didn't show them moved. But you know where to move them. 
 

[Willabe]

Sorry, I have to go do some things. Be back later.   

[Mrsedor]

Now that I see the dog house wiring, just disconnect the B node (B+2) screens cap ground for the C node (B+3) cap ground and now move that B node (B+2) screens cap ground to the A node OT CT (B+1) cap ground.

I just did that. (see pictures)

It doesn't seem to change much, still buzzing idle noise, and lots of noise in the reverb when pushed (but this could be the tank position).

[Mrsedor]

I looked at how it was on the DRRI PCB.

It seems that they separated D, and kept A, B and C negative nodes together.
Should I try this?

[Mrsedor]

If that doesn't help try this;

The K R and bypass cap on the reverb recovery tubes pin 3 and 8, move those 2 parts off the board and onto pin 3 off the verb recovery tube socket and ground them with the recovery tube 220K grid leak R.

Parts shown and where to ground them in pic below. But I didn't show them moved. But you know where to move them.

As for now, it is like this schematic (attached), almost because footswitch is a TRS jack.
No ground around the connectors, I have isolated them. Ground connection is at cathode ground.

When I move the capacitor and the resistor, should I restore the ground of the RCA and Footswitch jack, or could I just restore the ground of footswitch jack with still a ground wire to the isolated RCAs?

Thanks a lot for your help :)

[Mrsedor]

I'm not sure that the layout noted in the OP is ideal, particularly that the negative terminal of the HT C node cap seems to be directly connected to the A node negative.

A negative is connected to the power ground, near PT, along with HT centertap, Biasground, 6V6s cathodes 1Ohm resistors and 10k from Bias pot.
Now B negative is also connected to this point.

C node cap is connected, with D node cap at preamp ground buss, between Mid resistor and V2 V3 cathode.
As the jacks are isolated, the buss bar is passing by them to ground them. So this connection of C and D node is between the vibrato jacks ground and the normal jacks ground. Is it a bad thing?

Thanks !

[Esquirefreak]

I'd re-do that filter cap board just to rule out soldering related issues. The thick jumper wires between the caps grounds are overkill and hard to work with. Google pics on how the original looked or how other builders have done it. The boards have eyelets for a reason.

/Max

[Mrsedor]

I'd re-do that filter cap board just to rule out soldering related issues. The thick jumper wires between the caps grounds are overkill and hard to work with. Google pics on how the original looked or how other builders have done it. The boards have eyelets for a reason.

/Max

Thanks,
They show good continuity, but maybe there is more to it?
I will redo it as well. Do you think it could also be a bad filter cap? They are new though...

[pdf64]

I suggest to wait and see Martin Manning’s design.
Due to the NFB, reverb driver and trem oscillator, the 0V common return paths in these amps are something of a hodge podge.
Lumping all the 0V commons of the board to a bus, as per the OP layout, may not result in a reasonable hum level.

[Mrsedor]

I suggest to wait and see Martin Manning’s design.
Due to the NFB, reverb driver and trem oscillator, the 0V common return paths in these amps are something of a hodge podge.
Lumping all the 0V commons of the board to a bus, as per the OP layout, may not result in a reasonable hum level.

Still now allowed to see Martin's design on ampgarage, but it looks like it's been reposted on Groupdiy.
Hope it's ok I repost it here, If not I'll remove it.
Is it the grounding layout you were referring to?

Thanks a lot.

[pdf64]

Yes.

[Willabe]

You should read this on grounding if you haven't read it yet. It will help you understand Martin's grounding. Read it and re-read it until you understand. I read it several times before it sunk in and so have many here.

http://www.valvewizard.co.uk/Grounding.html

[Willabe]

As for now, it is like this schematic (attached), almost because footswitch is a TRS jack.
No ground around the connectors, I have isolated them. Ground connection is at cathode ground.

When I move the capacitor and the resistor, should I restore the ground of the RCA and Footswitch jack, or could I just restore the ground of footswitch jack with still a ground wire to the isolated RCAs?

No, leave it as you have it now.

I didn't know you added isolated RCA jacks and ran a ground wire back to the K R and bypass cap ground.

[Mrsedor]

Thanks a lot !

I will study both and probably come back with a few questions 

[Mrsedor]

Yes.

Thanks !
I have a few questions regarding this layout:

BC- ground is going to the right corner. Is it connected to chassis near power tubes sockets via a screw? It passes over Speaker tip but I assume it doesn't connect there. Is it the same point as power tube cathodes?

My input jacks are isolated. Is that a problem if they are grounded at preamp ground with the violet wires? or Should I revert to non isolated jacks?

Is the new tremolo resistor 180K? Were does the red wire connect? Is it ok if I leave this part aside for the moment?

Thanks a lot 

[Willabe]

Any grounding scheme can be done with those RCA jacks isolated. Leave them and just run a ground wire over to where ever it needs to go.   

The less chassis grounds connections the better. They sometimes can be a problem.

[pdf64]

…
BC- ground is going to the right corner. Is it connected to chassis near power tubes sockets via a screw? … Is it the same point as power tube cathodes?

See Martin’s layout
The BC- chassis connection point looks to be near V7.
The output valve cathodes look to have their chassis common connection by ghe rectifier socket.

Dunno what the rationale for not all using the same star point is 

It passes over Speaker tip but I assume it doesn't connect there.

Definitely not 

My input jacks are isolated. Is that a problem if they are grounded at preamp ground with the violet wires? or Should I revert to non isolated jacks?

That should be fine. Maybe use the isolated socketed for the speaker, if they’re not already isolated.

Is the new tremolo resistor 180K? Were does the red wire connect? Is it ok if I leave this part aside for the moment?

The new trem resistor on the board has just been relocated from the speed pot, it’s still 100k.
There’s an additional 180k in the footswitch, across the trem switch, to help reduce any pop.
If you’re not running the trem just yet, then no need to bother with them.