Need help to find cause of buzz in a Fender AB763 circuit (new video)

[Planobilly]

Hi,

I just finished the build of a Fender Deluxe Reverb clone. The amp works as expected and sounds fine except one issue.

As the gain is increased the amp buzzes only on the low E string. When I inject a signal below 120hz the same condition occurs. I can not see this buzz on my scope. The sine wave is smooth and normal looking.

This happens with different speaker cabinets and different guitars. It occurs on both channels.

There is the possibility it is buzzing at higher frequencies but if it is I can not hear it or see it.

I have done all the normal things like re-solder grounds, change tubes, hold or move around wires.

I just don't know the best way to troubleshoot the issue. Need some advice.

Schematic



Link to perhaps a better schematic.

http://schematicheaven.net/fenderamps/deluxe_reverb_ab763_schem.pdf

New heater wiring. NOTE there is next to zero hum in the amp now.


Photo of the amp before the heater wire change.


Thanks,

Billy

[KhronX]

When you say "The sine wave is smooth and normal looking", where exactly are you probing the signal?

[Planobilly]

At the output transformer or at the wiper of the volume pot.

I can probe anywhere you like if you have an idea.

Billy

[KhronX]

Since you spoke of injecting a sub-120Hz signal, would it be safe to assume you have some form of signal generator?

[Planobilly]

yes, I have a good signal generator. It is a Owan AG1012. I have a Owan DS7102V scope. Not high end gear but I think it is good enough to deal with tube amps.

20Mhz signal generator  100Mhz scope

[calexanian]

I have a very specific thing in mind. Try it again with the reverb driver tube removed. There is a condition that can occur in those circuits where the grid of that tube goes leaky and when it goes into grid rectification causes a noise the can be heard on the output. It sometimes can be describes as a scratchy buzzy sound during the decay of a note, particularly on a lower note. Apart from that I will have to think about it.

[calexanian]

Oh. and of course. how could I forget. rule number one when working on one of thees fender style circuits. Do all of your first round of troubleshooting on the normal channel with only the first preamp tube and phase inverter tubes installed. along with the power tubes. This eliminates the possibility of the rest of the circuit causing problems. A small amount of 120 hz may be introduced from the un terminated stage of the reverb channel but that can be ignored for this sort of oh shit test.

[KhronX]

I was just thinking of something along those lines - eliminating all the extraneous variables (reverb, tremolo), and diagnosing the bare-bones circuit first 

Oh. and of course. how could I forget. rule number one when working on one of thees fender style circuits. Do all of your first round of troubleshooting on the normal channel with only the first preamp tube and phase inverter tubes installed. along with the power tubes. This eliminates the possibility of the rest of the circuit causing problems. A small amount of 120 hz may be introduced from the un terminated stage of the reverb channel but that can be ignored for this sort of oh shit test.

[Planobilly]

OK guys

Give me a minute to set up.....something happens around 70 years old...not so fast anymore...lol

Billy

[Planobilly]

The issue is not there with  all the tubes pulled. I assume you want to add one tube at a time.

[calexanian]

Yeah. start with just the phase inverter tube and then the input tube on the far end towards the input. Then plug into the normal channel and give it a go.

[Planobilly]

I did it the other way around and the issue came back with V4.

I also have more hum than normal in the reverb circuit that increases with increased gain of the reverb control. The tank is not installed and the issue happens either way.

Perhaps the two issues are related.

There are no other issues with the amp. I assumed the noise reverb circuit was more or less normal...perhaps not.

[Planobilly]

Just so you know, I have plenty of new tubes and have replaced all the tubes a few times. Even tried 5751. Nothing changed.

Be back in 3 min

Billy

And BTW, thanks for the help

EDIT: I sort of get the idea the issue is in V4. If you think I should I can change every component in the V4 A and B circuit. The excessive hum in the reverb has to be coming from V4a recovery circuit I assume although I don't understand just how V4 could cause a low frequency buzz.

I also had changed the b voltage cap ( goes to the reverb transformer)  from 16uf to 33uf which reduced the ripple from 116mv to 65mv which did reduce the reverb hum about 5% to a point it was not to bad. Having said that turning the reverb control up past 4 creates abnormal hum. 

EDIT: I mis read what you said. Here is what I did . With the Inverter in  then V1 then V2 then V3 Then V4 and that's where the issue came back.

[Planobilly]

Success

I still don't know what was defective. I changed the plate resistor and the cathode resistor and the cathode cap and the .0033 coupling cap on V4a all at the same time. That fixed the problem.

Sometimes the "shotgun" approach is just the fasted way to get there.

There should be something to learn in all this....which of those components could cause hum in the reverb and  low frequency buzzing and why?

Anyway, thanks to you guys for helping.


Cheers,

Billy

[Planobilly]

Everything ran fine for about two hours.

Now I have a buzz only when I play on the low E string. Plays normally at low volume and very light attack on the string.

I assume there is some instability in the amp somewhere. It is not in the speaker..same with three different cabinets and guitars. I have replaced all tubes and all the caps in the tone stack.

I can not see this buzz/noise on my scope. The signal looks normal at all the plates and the output transformer.

I am not sure what to do next? Ideas?

Thanks,

Billy

[KhronX]

What's the timebase setting on your scope when trying to see this buzz? And what's the ground clip hooked up to?

[Planobilly]

Sorry KhronX,

What's the timebase setting on your scope when trying to see this buzz? And what's the ground clip hooked up to?

Sorry KhronX,

I had to go go do some stuff with my wife. I will try to make a good video of the issue showing the scope and settings.

For the moment here is a video you can listen to that perhaps is of a good enough audio quality to hear the fault. The little orange amp is not the amp/speaker you hear. The speaker you hear in the video is a Weber 12" 30 watt.

https://youtu.be/DPG2LviNojU

Thanks,

Billy

[KhronX]

I don't suppose you've tried any other speakers, have you?

If you indeed can't see the buzzing on the scope, and it only makes that sound on louder, low-pitched notes, the only thing that's left is a dud speaker.

Might be worth testing that speaker with other amps too. If it reacts the same way, you've got your culprit right there 

[xygor]

It sounds to me like a resonance in the reverb (the mechanical part, not the electronics).  Maybe swapping the input and output will have an effect, but that would not be conclusive if it didn't.  Slowly sweep the frequency from a sine wave audio source and search for resonance.  Or tap on the reverb and see what frequencies it produces.  Retune to avoid the frequencies.  Some musicians are experimenting with alternate tunings and scales anyway. (just kidding on the retuning part)

[Planobilly]

Yes guys I have tried several sets of speaker cabinets without any change.  Well...I did try a Weber 12" Neodymium 65 watt speaker which did reduce the issue slightly due I assume to it having a different frequency response.

The condition occurs with channel one disconnected (V1A and B), with V3 A and B disconnected and V5 A and B disconnected.

The signal path is set up now where the signal only goes through V2 A and B then through V4 B and then to the inverter V6 and out through the power tubes. I did this to eliminate as much of the amp circuits as possible The fault occurs with this set up.

I hooked up a audio probe and connected the amp output to a large 8 ohm resistor (8 ohm 120 watts I use for a resistive load on small amps)

It is getting a bit late here and I am tired. I did not want to turn things up loud tonight as I live in a townhouse and don't want to disturb the neighbors.

I will get back into this in the morning. 

Thanks,

Billy

[floobydust]

That sounds like instability in the amplifier. On a scope you probably can see "the fuzzies" or RF bursts on the signal peaks.

I see you have a floating ground scheme but eventually the chassis needs to be connected to that ground for shielding. Where is your ground connection to the chassis? With a long ground bus like that, RF can get in and cause problems.

Mesa Boogie puts a "radiator" wire from plate of the output tube nearby the preamp, to make a feedback kind of growl at high volumes. Keep your plate wires far away from the input tubes circuit; The plates have 100's volts of swing and any stray coupling capacitance causes drama and can sound like what you are hearing.
The cathodes of the 6L6's are highest current and need to connect directly to your PSU ground- not your ground bus.

If it is amplifier instability alone, a 150-220pF cap plate-plate on the 12AT7 driver will stop that. Try adding that.

For good practice I put capacitors (100pF) to GND on the input tubes' grid, to block incoming RF usually AM radio from getting into the input stage. The 68k resistors are supposed to do that but I prefer shunt capacitance.

P.S You've used some top quality parts. what make of resistors are you using?