RobRob Blackvibe 6V6 - Bias trim pot wiring?

[gradyclouse]

I am building the Blackvibe 6V6: https://robrobinette.com/RR763_BLACKVIBE.htm

Regarding the 50K bias trim pot, the layout (and my as-built) show one outer leg and the "center" both grounded to the main bias pot, and the resistor wired from the center lug of the main pot to the remaining outside lug of the trim pot.

The schematic doesn't seem to show the same configuration, or am I misreading the schematic?

Could anyone comment on my "as built" image? Correct for the build? Note that my image is from the back of the bias pot, flipped from the view in the layout image.

Thanks!

[EL34]

I don't see any grounds to the bias pot
It's a bias balance system and a bit over complicated,IMO

Rob commented on his bias scheme here in post #5
https://el34world.com/Forum/index.php?topic=24873.0

I would go with a vintage Fender bias personally
Any AB763 older amp has a simple bias pot that would work

[EL34]

This classic Fender bias is a whole lot simpler

[vibrolax]

Hi,

I currently happen to be making my own layout for Rob's RR763V Blackvibe 6V6.
I think your bias circuit as-built corresponds to the schematic.  Other than the orientation angle, your as-built picture corresponds to the layout illustration.

I don't rely on pot body contact with the chassis for the ground connection, I run a ground wire to the phase inverter supply ground.

Oh yeah, I plan on using a simpler bias voltage adjustment circuit as I don't need the balance feature.

Jon


Jon

[EL34]

More infoPost #4 here has info on parts that were changed/substituted
https://el34world.com/Forum/index.php?topic=24873.msg300608#msg300608
I can't remember why the .1/200v was changed
I do stock a .1/250v capBut it does not hurt having a higher voltage cap

[tdvt]

I can't remember why the .1/200v was changed

A bit tangental, but why was a voltage specified for that cap in the first place?

[sluckey]

A bit tangental, but why was a voltage specified for that cap in the first place?

Because Leo was pennywise. A 200v cap was more than sufficient for the lower PI input for the AB763 amps. At the time a 200v cap cost less than a 400v cap.

[cfortner]

Hi,

your first schematic is failsafe, the Fender circuit not.

If one of the pots in the first circuit fails, full negative BIAS-voltage is applied to the valves grids and nothing burns, only no output. Fender -> redplating.

I prefer the failsafe type of BIAS-circuits.

[gradyclouse]

All: Thanks for comments on my prior post linked below. I may have mis-wired bias circuit? I am at step 7 in RobRob's startup procedure  (https://robrobinette.com/Tube_Amp_Startup.htm - checking bias voltages before power tunes go in).

I am getting an expected -32V at pin 5 on v4, but 483V on pin 5 of V3? The corresponding grid leak resistor burned up as well...

Thoughts welcome. Please see RobRob schematic with my markups attached. I also find the bias adjustment circuit confusing on the schematic (and I think there are discrepancies vs. the layout - also pasted here).

Thanks!

Hi,

your first schematic is failsafe, the Fender circuit not.

If one of the pots in the first circuit fails, full negative BIAS-voltage is applied to the valves grids and nothing burns, only no output. Fender -> redplating.

I prefer the failsafe type of BIAS-circuits.

I am building the Blackvibe 6V6: https://robrobinette.com/RR763_BLACKVIBE.htm

Regarding the 50K bias trim pot, the layout (and my as-built) show one outer leg and the "center" both grounded to the main bias pot, and the resistor wired from the center lug of the main pot to the remaining outside lug of the trim pot.

The schematic doesn't seem to show the same configuration, or am I misreading the schematic?

Could anyone comment on my "as built" image? Correct for the build? Note that my image is from the back of the bias pot, flipped from the view in the layout image.

Thanks!

[PRR]

> 483V on pin 5 of V3? The corresponding grid leak resistor burned up as well...

Short, inside the tube/socket or in that coupling cap.

Though I would not expect 483V from that phase inverter....