Deluxe reverb build has always had an annoying noise floor

[72Blazer]

Noise is subtle but annoying.  I have built quiet fender clones in the past and the noise floor on this amp this not acceptable to me. I have hit those grounds with iron multiple times.  I am ditching the grounding scheme I tried (based on RobRobs site) and going to ground like fender did it. 

NOTE:  I added a post phase inverter master volume to this amp and Sluckey's Trem-o-nator.

One of the symptoms here pertains to the channel 2 volume: the noise floor is more noticeable when the volume is turned all the way down.  But when the volume is turned up like in the middle the noise floor subsides a little/amp quiets down a bit and of course the noise floor is really loud when the the channel volume is cranked.  Channel one does not affect the noise floor either way.  I tried this again and was not repeatable-so disregard.  Well-maybe is repeatable. 

Another observation is channel 1 volume pot is scratchy and I've used De-ox on it/cleaned it twice.  I cleaned tube sockets as well.

The 6v6s are within 2 mA of each other and biased at 18 mV based on the plate voltage of  463v

Perhaps somebody else has experienced this and has some ideas. 

Note:  I did NOT ground the filter caps to the single ground bar. 

Regardless, I don't like the looks of how I did the grounding bar (looks terrible actually) so I'm gonna redo all the grounding-again-like Fender did it.  I purchased brass shim stock and I'm fabricating my own brass plate like the old fender amps had. 

After I'm done I will report back status of noise and provide pictures etc.

Vr
J

[72Blazer]

I have worked on this amp some more.  I checked for leaky capacitors in both channel tone stacks.  I do have one question though.  I have tried two other 250pF caps in channel 2 tonestack and I keep measuring around 22mV of DC.  Is this normal?  All other caps in both channel 1 and 2 do not measure leaks at all.

The next question I have is could the master volume mod I put in this amp be a contributor to the noise floor?  I am very confident I did this mod right using shielded cable.  I buzzed everything out and good to go.  It's function as intended.

Last but not least, I swapped tubes throughout and still get the same noise.

Anyway, I ask these questions before I pretty much gut this thing and install the brass plate and reverify everything again and get rid of this noise floor.

Thanks

Vr
J

[jim]

I assume you are talking about gain noise but there is hum, buzz, hiss, pops and oscillations. Search this forum on "noise floor"-- its a big topic. You will need to post some hi-rez pics because lead dress is a common culprit. Also check this forum here:  https://el34world.com/charts/fenderservice6.htm for hints.  Jim

[AlNewman]

  I have tried two other 250pF caps in channel 2 tonestack and I keep measuring around 22mV of DC.  Is this normal?  All other caps in both channel 1 and 2 do not measure leaks at all.

I'm not sure this makes any sense.  It looks like all caps connected to the tone circuit have the same voltage potential at the front of the volume pot.  I may be wrong though.

[72Blazer]

Thanks for the point out to that "Oddball Amp Noises" post Jim-I had not seen that.

[HotBluePlates]

... I checked for leaky capacitors in both channel tone stacks. ...  I have tried two other 250pF caps in channel 2 tonestack and I keep measuring around 22mV of DC.  Is this normal?  All other caps in both channel 1 and 2 do not measure leaks at all. ...

Are you using 500v silver mica?  Do you have another film cap of any value handy (as in the other ones used in the tone stack)?

   -  500v silver mica have been notorious for leaking & noise issues for the past number of years.  Try switching to a 1000v mica, or use some other type of dielectric for that cap.

   -  If you have any other film cap, try installing it in place of that mica cap.  You're not listening for sound, but measuring for voltage-leak.  I bet the 22mV you've been measuring goes away, which will prove the issue with the mica cap.

[pdf64]

… I keep measuring around 22mV of DC.  Is this normal? …

My understanding / experience is that black fibreboard is somewhat notorious for being an imperfect insulator when used as a high voltage circuit board. I’d expect to find that kinda voltage almost anywhere on such a board that wasn’t tied down to circuit common.

[HotBluePlates]

… I keep measuring around 22mV of DC.  Is this normal? …

My understanding / experience is that black fibreboard is somewhat notorious for being an imperfect insulator when used as a high voltage circuit board. I’d expect to find that kinda voltage almost anywhere on such a board that wasn’t tied down to circuit common.

I agree for a 40-60 year old Fender amp.  It has had decades to absorb moisture, react with the carbon dying the board black & increase its conductance.

A new-build implies a new board, and they're usually no problem when new.  The again, it could be leakage from where the filter caps connect to the eyelet board, which is right near the "no-voltage" side of the tone stack caps.

[72Blazer]

I gutted the amp-fabricated my own Fender style brass back plate for mounting the pots/input jacks to.  I'm gonna follow the original grounding scheme.  Cleaning up solder joints on main board while I'm at it.  I ordered a 500pf 1000v as recommended by an earlier post.  Fingers crossed that when I finish the rebuild the noise floor will be fixed.  I'll post an update sometime soon hopefully.

[72Blazer]

I finished wiring the amp back together with the brass plate/original fender grounding scheme.
I initially had the same results/noise floor.  The real fix was adding the artificial center tap 100ohm resistors even though this PT has a center tap.  I’ve still got the center tap wire from PT hooked up.  Being a novice this confuses me as I was expecting you would either use the PT CT or add the 100 ohm resistors.  The PT used is the mojo 761.  Can anyone elaborate on this for me?
I have one more issue to address but I’ll stand by for now to see if someone might can address the center tap question.  Thanks.
Vr
J

[sluckey]

Being a novice this confuses me as I was expecting you would either use the PT CT or add the 100 ohm resistors.  The PT used is the mojo 761.  Can anyone elaborate on this for me?

Do one or the other. No harm to do both but also not necessary.

[72Blazer]

This is how I noticed this quandry:

1.  I disconnected the red/yellow wire which Mojo PT documentation states is the CT
2.  I installed the 2 x 100R-the noise floor diminished 95%
3.  Thinking I was now good-I went to check the bias and had no readings on the 1R at pin 8
4.  Reconnected the red/yellow PT CT wire and was able to take readings again on the 1Rs for the 6V6s

This is repeatable.

Current state of the amp is the PT CT is hooked up as well as the 100Rs.  Amp is quiet.  I got 468Vs and biased the amp at 18mA.

Picture included of how I have 100Rs tied to the heaters and the PT CT connected to that terminal strip as well.

Vr
J

[sluckey]

This is how I noticed this quandry:

1.  I disconnected the red/yellow wire which Mojo PT documentation states is the CT
2.  I installed the 2 x 100R-the noise floor diminished 95%
3.  Thinking I was now good-I went to check the bias and had no readings on the 1R at pin 8
4.  Reconnected the red/yellow PT CT wire and was able to take readings again on the 1Rs for the 6V6s

This is repeatable.

Duh!      The red/yellow wire is the centertap for the HIGH VOLTAGE winding ONLY. It is not a centertap for the green filament wires. It has nothing to do with a centertap for the filaments. You must connect the red/yellow wire to ground.

Now, since the filament winding (green wires) have no centertap, you should install an artificial centertap with two 100Ω resistors.

[72Blazer]

OK-got it now and taking that on the chin.  After your explanation I now painfully see that there is no "K" CT for 6v winding. 

Thanks again.

[sluckey]

OK-got it now and taking that on the chin.  After your explanation I now painfully see that there is no "K" CT for 6v winding. 

Sometimes you arrive at the place you need to be without knowing why or how you got there. It's wired right. All is good.   

[72Blazer]

I don't know if it's my ears getting old but I think it needs more treble.  The tone stacks are IAW original AB763 schematic. How could I get a little more treble-especially in channel 2?

[sluckey]

Brighter speaker. Hearing aid.