FSR AB763 Channel 2 noise in new build

[72Blazer]

Build:  AB763 with the following mods:  Trem-O-Nator and the rob/rob normal channel mod.

All control knobs working, great reverb and tremelo.  I've tried different speakers and guitar cords.  Different power tubes and rectifier tried.  voltages are in spec for tubes.  Re-verified filter cap wiring.  Using Sprague Atoms.

Tube Voltages:

Rectifier: Pin4: 380  Pin6:379

Output (V8)     Output (V7)
Pin3: 465v       Pin 3: 465v
Pin4: 464v       Pin 4: 463v

Pre Amps:
V6 (12AT7)    V5 (12AX7)   V4 (12AX7)   V3 (12AT7)   V2 (12AX7)   V1 (12AX7)
Pin 1: 245v    Pin 1:  303v  Pin 1: 280v    Pin 1: 460v    Pin 1: 275v  Pin 1: 242v
Pin 3: 100v    Pin 3: 3.15v  Pin 3: 3.98v   Pin 3: 10.2v   Pin 3: 2.2v   Pin 3: 2.0v
Pin 6: 245v    Pin 6: 445v   Pin 6: 280v    Pin 6: 460v    Pin 6: 280v   Pin 6: 291v
Pin 8: 101v    Pin 8: N/A     Pin 8: 3.9v    Pin 8:  10.4v  Pin 8: 2.18v  Pin 8: 0v

Noise/Humm in channel 2 only

Channel 2:
nothing plugged in and volume turned down:  silent
nothing plugged in and volume pegged:  hum (increases proportinate to how much volume turned up
guitar plugged in and volume turned down:  silent
guitar plugged in and volume almost pegged: loud humm and great guitar volume/tone-BUT when the volume is completely pegged, there is a loud obnoxious squeal when treble turned up/approaching max...

Channel one
nothing plugged in and volume turned down:  silent
nothing plugged in and volume pegged:  silent
guitar plugged in and volume turned down:  silent
guitar plugged in and volume pegged: slight noise (normal I'd say for pegged volume) and great guitar volume/tone.

Anybody have any thoughts on source of noise?

Vr
J.

[Willabe]

It would be very helpful to see some clear up close shoots of the inside of the chassis.

[72Blazer]

Here is a pic

Any particular area of rhe chassis you need a more pics of?

[Willabe]

Can you go closer please?

[72Blazer]

sure-and while I'm at it-any particular area?

[Willabe]

No, the whole amp.

Have you tried flipping the power tube plate leads? Or you can flip the PI leads that go to the power tube grids.

You can disconnect the -FB loop wire just to test the amp. If the squeal goes away, then put the -FB wire back in place and flip the plate leads or the PI leads.   

[72Blazer]

Thanks for the suggestion.  I flipped the plate leads as instructed and still have same symptoms.  I returned them back again and same symptoms. 

A little more clarification:  I can trigger that horrendous squeal with the volume or the treble pot when both are nearly pegged.

Lastly, I am not sure if this could be a contributor, but this OPT is 2 Ohms and I do not have a cab or speaker combination to get two ohms-just a couple of 8 ohm speakers which I've got both plugged in using both speaker output jacks.

[Raybob]

Channel 1 works fine, it's not the OT musmatch. i would check all pot values, trace all wiring and components to volume and tone controls of channel 2.

[Willabe]

How to double check your build;

https://el34world.com/Forum/index.php?topic=17701.0

[Willabe]

Sounds like the amps unstable, could be a number of possibility's.

I'd do like Raybob wrote. And then if you don't find anything wrong, I'd look at the lead dress. Take a wooden chop stick and start poking around, moving wires, etc.

Pay attention to any grid wires, they are the most sensitive.

The reverb tank output is less than the guitar PUP's input to the amp. So check the verb return wiring. I'd probably pull the verb driver tube, 12AT7 and unplug the verb tank and cables. Also pull the trem tube. Then see if you can still get the amp to squeal. If stopped the squeal, then it's probably in the circuitry you disabled. If it still squeals, then it's probably not in the verb driver, tank, cables or trem.   

[72Blazer]

Channel 1 works fine, it's not the OT musmatch. i would check all pot values, trace all wiring and components to volume and tone controls of channel 2.

Verified all pot values are correct.

Sounds like the amps unstable, could be a number of possibility's.

I'd do like Raybob wrote. And then if you don't find anything wrong, I'd look at the lead dress. Take a wooden chop stick and start poking around, moving wires, etc.

Poked around and verified no looses connections,  Found a microphonic ECC83 and replaced it.  Traced all wires again.  I did find a 330Ohm resistor where a 220Ohm should have been and replaced it.

Pay attention to any grid wires, they are the most sensitive.

A little sensitive yes. 

The reverb tank output is less than the guitar PUP's input to the amp. So check the verb return wiring. I'd probably pull the verb driver tube, 12AT7 and unplug the verb tank and cables. Also pull the trem tube. Then see if you can still get the amp to squeal. If stopped the squeal, then it's probably in the circuitry you disabled. If it still squeals, then it's probably not in the verb driver, tank, cables or trem.   

I did all of the above (per Willabe)and I can still repeat the horrible loud squeal in channel two when both the volume and trble pot are nearly maxed out.  Both the volume and treble pots have to close or completley pegged in order for this to happen. 

There is also a humm in channel 2 that increases when the volume of channel two is increased.  I also verified my filter cap wiring as the original FSR schematic calls for.  This circuit was my first attempt at the tremonator mod and the RobRob AB763 "Lead Channel Mod".  The tremelo is awesome as is the reverb for that matter.  I verified all component values and layout of the RobRob mod. 

I still cannot find my mistake (s).  Happy to post more pics and fresh tube pin voltages if need.

thanks
Vr
J.

[Willabe]

I see you tried different rectifier and power tubes.

Try different preamp tube for channel 2 and PI tubes. And the reverb return tube that feeds the PI. 
 

[Willabe]

Try this, I don't trust these wires the way they are dressed as they are now;

The brown and yellow twisted pair, going to the reverb pot, move them away from channel 2's input #2 jack.

They should be lifted above the wires running the length of the eyelet board, not laying on them. They should cross over them at a right angle/90 degrees. Then they should lay down on the chassis going into the corner of the chassis, then run that twisted pair/brn/yel down in the corner of the chassis over to the verb pot and then run up the face side of the chassis to the verb pot.

They should not be anywhere near an input jack. And they should not run along/parallel those wires running the length of the eyelet board. 

Lets see if that helps, if not there's a couple other wire lead dress things to try and clean up. The PI wires and the standby switch wires.   

[72Blazer]

I'll do all this and get back to you.  Thanks

[sluckey]

Is this a steel chassis?

[joesatch]

i'm new to this but are all those soldered grounds normal? 

[72Blazer]

The chassis is steel (magnetic) but has a chrome plating.  I had taken a dremmel stone bit and grinded all the spots down where you see the grounds.  I did confirm good grounds with my multi-meter.  I used star washers for all the pots as well.

Vr
J.

[72Blazer]

I took more tube voltage readings. 
V1      V2     V3     V4     V5     V6
267     300    503   301   404   279         Pin 1
2.3      2.3     11.3   2.4    4.1    116       Pin 3
305     309    503    302   N/A   117        Pin 6
2.4      2.4     117     2.4   2.4    117       Pin 8

V7       V8     
-70      -70     Pin 1
508     507     Pin 3
506     509     Pin 4
-70      -70     Pin 5
506     505     Pin 6

V9
Pin 2     5.9
Pin 4     398
Pin 6     397
Pin 9      1.4

[72Blazer]

I moved the wiring as Willabe suggested and little to no change/effect.

[Willabe]

What/which wiring? Got a picture?

Did you try different preamp,verb return/PI tubes? It could be a slightly micro phonic tube that only acts up when the amp gets loud enough. 

[72Blazer]

Made major headway after I found my stoopid mistake.  I had installed a choke rated for 6v6s.  After I installed the correct choke for 6L6s, the hum is gone.

For the obnoxious squeal, V2 was the culprit.  Replaced that and now I can't induce that anymore.  Thanks Willabe and thanks to all who may have pondered this thread.

Vr
J.