6G3 build buzz

[Bieworm]

Hey

I built the 6G3 with the plexi channel mod, push pull parallel channel switch and PPIMV .
It sounds great!! at all volumes.. but when I do some things it starts to buzz and I cant find it.

it buzzes when I probe the B+ nodes with my DMM. It buzzes when I push the turretboard. It buzzes when I go from tremolo to normal operation. Weird thing is that when I press the tremolo on off switch again.. it stops buzzing. When I touch the turret where the hot side of the tremolo switch is connected (between the 2 .01uf caps) it stops buzzing too. The ground buss has an audible ticking sound when I probe it..
Voltages seem normal enough

There is also noise when tapping the chassis... this is clearly a grounding issue...

What am I looking for here? Could this be a ground loop? I didn't use isolated jacks for the trem switching jack and the inputs. he speaker out is the only one grounded

Link to pics:

https://www.dropbox.com/sh/f7tbsir8tqbn5m5/AAB9B19LA1RNUI239NtZaBm9a?dl=0

[pdf64]

What’s the resistance between any connection on the heater circuit and the chassis?
If there’s no circuit between heater and cathode, then buzz is likely.

[Bieworm]

What’s the resistance between any connection on the heater circuit and the chassis?
If there’s no circuit between heater and cathode, then buzz is likely.

I measure 0.3 Ohm on every heater connection to ground.
I'm doing the isolation of all jack inputs just in case right now...

Maybe this is important too.. when I set the bright channel volume to zero it's instant buzz

[pdf64]

…
I'm doing the isolation of all jack inputs just in case right now...

Maybe this is important too.. when I set the bright channel volume to zero it's instant buzz

That symptom can indicate there’s a problem in the 0V layout arrangement.
So isolating the input jacks may help, but I suggest you take a close look at the whole 0V circuit common design. Try and work out where the high current loops are, and make sure they’re contained, so as to minimise them contaminating early stages in the signal path.

[Bieworm]

As promised I isolated all input jacks  and grounded them properly. This helped a lot. Now I can only get the hum/buzz when I set the bright channel volume to zero OR when I yank the turretboard.
Suspects are:
- ground loop likely at the bright channel volume pot. I will revisit that part thouroughly
- bad filter cap,while new and soldered using alligator clips as heat sink to prevent heat from soldering ruining the components

My guess it's the ground loop. Yanking the turretboard probably disturbs the loop causing buzzing sounds

[pdf64]

Is the vol pot 0V common terminal connected directly to the 0V terminal of the 2nd stage’s cathode resistor?
Rather than eg a buss bar or star point.
Pictures may be beneficial 

[Bieworm]

Is the vol pot 0V common terminal connected directly to the 0V terminal of the 2nd stage’s cathode resistor?
Rather than eg a buss bar or star point.
Pictures may be beneficial 

I'll try to make pictures,  but it's hard to get a clear view under the chassis at the pots.
What I did... I have a ground buss for the preamp on the turretboard which ends up at the 1st input ground. From there it goes to a dedicated ground point near that input.
Simultaneously I ran a separate ground buss over the pots ( not on them) where all pot grounds connect. That buss goes to input ground too...

[pdf64]

…
I have a ground buss for the preamp on the turretboard which ends up at the 1st input ground. From there it goes to a dedicated ground point near that input.
Simultaneously I ran a separate ground buss over the pots ( not on them) where all pot grounds connect. That buss goes to input ground too...

I suspect that arrangement may be contributing to / causing the issue.

[Bieworm]

…
I have a ground buss for the preamp on the turretboard which ends up at the 1st input ground. From there it goes to a dedicated ground point near that input.
Simultaneously I ran a separate ground buss over the pots ( not on them) where all pot grounds connect. That buss goes to input ground too...

I suspect that arrangement may be contributing to / causing the issue.

Ok.. so I should connect both volume pot grounds together and cross over to the buss at the conjunction of V1's cathode caps? And the trem speed pots ground to where the trem tube's cathode cap goes to the buss?
Is it all right to daisy chain the inputs' grounds to the 1st input and to the preamp ground lug from there?

[pdf64]

… so I should connect both volume pot grounds together and cross over to the buss at the conjunction of V1's cathode caps?

I suggest the 0V common end of V2A 1k5 cathode resistor.

And the trem speed pots ground to where the trem tube's cathode cap goes to the buss?

That seems good.

Is it all right to daisy chain the inputs' grounds to the 1st input and to the preamp ground lug from there?

That should be good, but the V1 cathode should reference to there also.

The principle is that the grid and cathode both reference the same 0V common point. That should avoid contaminating the grid signal with other current loops.

[Bieworm]

It's all right now. I did the grounding according to your advice. I had to ground the 100k after the speed pot to the star ground to make the trem wiggle more quietly.
But the culprit I found after the changes. Turns out to be a cold solder joint on the grounded side of the 1st preamp filter cap.( at the ground buss connection)... that explains the noise I had when yanking the turretboard....
Thanks for the tips though..

[Bieworm]

With my new surfybear reverb it's a real match!!!

[Bieworm]

by the way, for those who are interested:

the type 2 PPIMV works really great on this amp! This is to use the amp at home only... but it works very nice across its whole range. So I went for the stock type 2 MV , a dual gang A250k pot replacing the 220k load resistors between PI and power tubes, along with the 2M2 safety resistors across the wiper and the bias connection

[Bieworm]

strange thing I noticed when playing YT vids from others who built a 6G3, even real 6G3 amps and Down Brownie,...
all of them have the fizzy OD when dimed. Mine doesn't!!! Not in the least!!!
Could it be the fact that Fender used no grid stoppers on the 6V6 and others use 1k5? I immediately installed 8k2 grid stoppers there, because I figured it would be a good value, just like the 18 watt amps. I also have 1k 3W screen resistors on the 6V6 tubes, for safety reasons.
Let's be clear.. my amp sounds awesome! I wouldn't change anything!

[pdf64]

Beefy screen grid resistors are fine if there’s a suitable HT fuse to protect stuff if the screen grid shorts.
In the absence of a HT fuse, I suggest 1W fusible screen grid resistors for the beam pentodes we use.

[PRR]

> Fender used no grid stoppers on the 6V6 and others use 1k5? I immediately installed 8k2 grid stoppers there,

Beefy screen grid resistors are fine if there’s a suitable HT fuse to protect stuff if the screen grid shorts.

?? I'm confused. Signal grid resistors or screen grid resistors?

[Bieworm]

> Fender used no grid stoppers on the 6V6 and others use 1k5? I immediately installed 8k2 grid stoppers there,

Beefy screen grid resistors are fine if there’s a suitable HT fuse to protect stuff if the screen grid shorts.

?? I'm confused. Signal grid resistors or screen grid resistors?

G2

[Willabe]

> Fender used no grid stoppers on the 6V6 and others use 1k5? I immediately installed 8k2 grid stoppers there,

Beefy screen grid resistors are fine if there’s a suitable HT fuse to protect stuff if the screen grid shorts.

?? I'm confused. Signal grid resistors or screen grid resistors?

G2

G2 = screen grid, G1 = control grid, G3 = suppressor grid/beam forming plates on US power tubes.

Your schematic shows 8K2 on G1. 

[Bieworm]

> Fender used no grid stoppers on the 6V6 and others use 1k5? I immediately installed 8k2 grid stoppers there,

Beefy screen grid resistors are fine if there’s a suitable HT fuse to protect stuff if the screen grid shorts.

?? I'm confused. Signal grid resistors or screen grid resistors?

G2

G2 = screen grid, G1 = control grid, G3 = suppressor grid/beam forming plates on US power tubes.

Your schematic shows 8K2 on G1.

Yes indeed..and 1k on G2.. or 1k5.. not sure about that