AB763 clone reverb hum - Help!

[VintageRob]

Hi everyone - Apologies in advance for the long post.
A number of years ago I built an AB763 Deluxe Reverb clone using Leo’s layout including the brass ground plate, preamp and power amp grounds separated etc. (some minor mods switchable NFB and tweaking the normal channel tonestack). While always happy with the tone, I was never fully satisfied with the noise floor particularly when the reverb level was turned up high as there is a noticeable 100hz hum (mains here is Australia is 240V 50hz)
Having recently retired and having more time on my hands, I have spent the past few weeks searching this, and other forums trying various solutions but to no avail so I’m hoping someone more knowledgeable than I might be able to point me in the right direction.
Basically with all controls to zero (and the reverb return input to V4 shorted to ground) the noise floor is not too bad. The FFT app on my iPhone shows a 100hz peak of around -80db which shows as a peak to peak hum signal of approx. 10-15mv at the speaker output.
With the Reverb level turned up to 10, the 100hz hum level increases to approx. -63dB equating to around 30mv peak to peak at the speaker output.
Not having access to any AB763 clones or Fender Deluxe Reverb amps to compare, I could of course be ‘chasing a ghost’ and maybe some level of hum associated with the reverb is to be expected although I see comments on other builds indicating they have ‘zero hum’. I accept that there will always be some level of background ‘hiss’ introduced with a reverb circuit.

Below is a summary of my diagnosing so far:
1. Unplugging the reverb tank and grounding the reverb return (i.e. V4 grid) does NOT change the level of hum.
2. The amount of hum is controlled by the Reverb control.
3. Lifting the connection from the .033 cap to the Reverb pot eliminates the hum, confirming the V4 recovery stage is the source of the hum.
4. Swapping V4 with 3 other Tung Sol 12AX7’s does NOT reduce hum.
5. Replacing the lead from reverb return RCA socket to grid of V4 with shielded cable does NOT reduce hum.
6. Isolating the Reverb return RCA socket from chassis and connecting the ground to the point where V4 cathode electrolytic grounds to the brass grounding plate does NOT reduce the hum.
7. Doubling the filtering on the B+ node that supplies V4 does NOT reduce the hum.


Apart from being almost 100% certain the hum is being introduced in the Reverb recovery stage I am somewhat frustrated and totally out of ideas.
If anyone has any thoughts or suggestions I would be extremely grateful.
Cheers Rob

[ac427v]

Your troubleshooting has eliminated many of the common causes of reverb hum.
Older tubes in the reverb return circuit can cause hum. I always use a 220 U capacitor instead of a 22 U on the cathode of the reverb return to prevent this issue.

[Lectroid]

I also built a faithfully stock DR ab763 close and it does have a small amount of idle reverb noise.  But nothing you can hear once the volume goes up.  Yours sounds worse.  ac427v's idea of upping up the cap value makes sense to me.  I'm going to try that--(being in pursuit of Perfect Silence like all builders.)

3. Lifting the connection from the .033 cap to the Reverb pot eliminates the hum

[deleted]

[Helmholtz]

100Hz hum is power supply ripple.
A tube can only cause 50Hz heater hum.
What happens, when you disconnect the cathode resistor and its bypass cap from the recovery triode?

Apart from that, I don't trust phone sound spectra, because of the poor frequency response of the built-in microphone. In reality the 50Hz peak might be much larger.
Please post a scope pic of the output hum.