Hum and Buzz in Cordovox Build

[pdf64]

Yeah that circuit is not elevating the CT so something not working correctly, unless it is not why its there.
BTW the cap is 0.22uf wrong on the schematic.
But, there is no change in the hum when jumping xfmr CT to ground. So that is not the issue apparently. ...

There's a remote possibility the CT is open circuit, what resistance do you measure between each green heater wire and ground?

[Williamblake]

Maybe it would help if you document the unused power transformer windings and anything else present in the amp, too?

[scstill]

There's a remote possibility the CT is open circuit, what resistance do you measure between each green heater wire and ground?

I have not measured the continuity BUT when I measure voltage I get 3.37vdc each leg to CT

Maybe it would help if you document the unused power transformer windings and anything else present in the amp, too?

There is only a 7vct secondary that is not used in the conversion, shrink wrapped and out of the way. In the Original Cordovox this was filament voltage for the 60+ tubes in the Tone Generator.

[scstill]

With the intent to reduce the hum and buzz, I have done a lot of work on this amp since last visit.
Was interesting/rewarding to use Gemini as tool for feedback/guidance.

PreAmp
Rebuilt and rewired the entire Preamp V1 and V2 to be more guitar amp design (see new schematic) and less "chaotic". Added new shielded socket. This did not really help. Note the pic doesn't show it but input wires are separated as much as possible from the vdc wires.

Control Panel
Rewired the entire Control panel to be less "chaotic", replaced all pots with new CTS, replaced jacks with new Switchcraft. Redid the star ground. did not really help.

After these mods there still was noise (hum and buzz).

Biggest improvement
When I added dedicated grid stoppers close to V1 and V2 grids the microphonic signal cables from the control panel got quiet and most the the noise was gone. Previously relied on grid stoppers in control panel for V1 and vol pot for V2.

Squeal on Pots
There still was squeal on tone and vol pots at max setting caused by stray DC, likely do to the 5E3 tone stack getting strange feedback in this high gain multi-chassis layout. When a more simple tone stack with independent tone and vol was implemented (see schematic) the squeal was gone in the tone pot and only slightly present in the Vol pot. This was without guitar, with guitar there was more vol pot squeal higher in Hi channel less in Lo channel.
 - Tried different V2 grid leak 2M made it worse 500k made it "somewhat" better.
 - Tried to improve lead dress but not much improvement (limited room in tiny preamp chassis)
 - Also removed all the original Cordovox NFB with no change

12AY7
Thinking that the high gain 12ax7 was to much for the small chassis (there are 3 gain stages plus PI)
For V1/V2 Tried 12au7 - squeal gone, tried 12at7 - squeal reduced but present; Thinking that 12ay7 might be the answer, ordered this tube.

There still is a bit more hum at max everything but that might be normal for this build. We'll see once the 12ay7 gets here...

BTW - it sounds good.

[Platefire]

Alright, Good Progress! I just had a flash back. I have Pignose G40V which is Basically a Bassman with an extra gain stage. It was always noisy, too gainy and not good tone. I tried several things without any success. I read about others that had the same problem with that amp and solved it by bypassing the extra gain stage. I tried that with just a temporary jumper and bingo! That fixed it! Then I installed a permanent jumper around that gain stage. It's now nice and quiet and has a great tone. If you haven't tried that already, you might just try a temporary jumper over that gain stage to see how you like it.

[scstill]

Have done a lot of rebuilding/redesign and troubleshooting
Still frustrated as I cannot remove the last of the 60hz hum.
Its isolated to the V3 stage but doesn't seem to be related to heater.
All the troubleshooting steps below. Any suggestions appreciated.

Summary of rebuild
   Rebuilt/redesign entire V1/V2 preamp
   Isolated all pots and jacks
   Ensured only two star grounds G1 in lower chassis, G2 in upper chassis
   All signal wire shielded one side ground to G2 star
   After all this work still some noise and squeal

Summary of noise troubleshooting
Squeal (at vol pot max)
   using a 12ay7 (instead of 12ax7) in v1/v2 resolved the squeal
Hum 1 (controlled by MV)
   grounded all three chassis' together resolved
Hum2 (not controlled by MV)
   isolated to V3; removing V1/V2 hum; removing V3/v4 no hum gone; bypass V3 no hum
   Rebuilt/redesign V3/V4 replacing carbon resistors with metal film (supposedly quieter) - still hum
   Tapping - no microphonic wires or components
   Elevating heater to 50vdc- no change; then tried adding hum bal pot - no change hum remained
   Replaced old diodes with UF4007 thinking something could be getting in - no change
   Not B+  - both V3 and V4 use same B+ bypassing V3 hum stayed
   Possibly H field - Tried to shield V3 cathode components - no effect
   Tried to bypass the MV shielded signal (in 14" umbilical with AC/Heater and HT) to V3 grid - no effect
   All V3/V4 components tied close to pins
   Only 12au7 in V3/V4 reduces noise “acceptably”  but this is a coverup not a fix

[Platefire]

The only thing that comes to mind after all the checking that has been done is, chop stick it one more time from head to toe

[TIMBO]

Hey mate,
This is one I did awhile ago
Upstairs, Downstairs Chassis?????
It all came down to earthing the chassis(two) and a grounding associated with separate chassis
So I can imagine three chassis being a grounding headache

[scstill]

I have chopped sticked everything, there are no microphonic/noise results anywhere

Yes all three Chassis' are grounded to each other with a bolted wire.
In reply 42 you grounded the Neg side of PSU that killed the buzz.
https://el34world.com/Forum/index.php?topic=15795.msg157568#msg157568
Is that Power Supply Unit? Isn't neg PS always grounded?
Am I missing the secret in your build that I haven't tried yet?

Thinking the hum is magnetic, I added a temp ferrous metal housing over V3/V4 - did not affect the hum
I am running a 12au7 in v3/v4 which lowers the hum and 12ay7 in V1/V2 which eliminates the squeal.
I bypass v3 which further lowers hum (and signal) but still present.
This is a very high gain amp with three stages before PI and super clean 7591 power tubes.
Maybe I just have to live with this hum,   :-( 
If there is any Good news, I have learned a lot about isolating noise during this troubleshooting
and I have found Gemini to be a valuable resource