6G16 Vibroverb Complete! Bias range question

[waldner]

My Brown Vibroverb clone is complete and works!  It even sounds pretty good already considering I'm not in the right bias range.  The only problem is with the bias pot maxed I am getting the following:


Plate Voltage - 455
Plate Current - 20
Wattage - 9


See attached photos.  I've read up a little bit and it looks like I need to raise the value of the 'bias range resistor'?  Is the 15K resistor to ground circled in the attached pics the bias range resistor?  Am I correct that raising the value is the right way to get toward a more normal wattage dissipation of 17 to 19 watts for a pair of 6L6GC tubes?  And if so, how much to raise it?


Thanks for any help.

[sluckey]

Plate Current - 20
See attached photos.  I've read up a little bit and it looks like I need to raise the value of the 'bias range resistor'?

That's correct but the resistor you circled is not correct. The bias range resistor is the 1K that's on the bias board. Increase it to 1.5K and see if that gets the bias you want. If still not where you want the bias range, change it again.

[waldner]

Cool.  Thanks Steve

[XYZGnomon]

Down the road I plan to do this amp myself! Thanks so much for posting this!

More pics!

[pdf64]

…The only problem is with the bias pot maxed I am getting the following:

Plate Voltage - 455
Plate Current - 20
Wattage - 9

See attached photos.  I've read up a little bit and it looks like I need to raise the value of the 'bias range resistor'?  …

Unfortunately the bias arrangement doesn’t seem to have been designed by a competent person, rather the adjustment is a bodge.
It would work properly if the control P1 was wired as a pot, and a safety resistor R3 adding, eg 100k. See diagram below.
R2 should be fine at 15k, and the 1k resistor can be left alone.
Just ignore C2, no need to add that.
The bias range of the control should be much greater than Mojo’s nonsense.
See Merlin http://www.valvewizard.co.uk/bias.html

[XYZGnomon]

See Merlin http://www.valvewizard.co.uk/bias.html

Thanks for this link! I am not plugged in to social media at all, and I take every opportunity to learn from those more experienced. I would never have found Valve Wizard on my own.

[Medley]

My Brown Vibroverb clone is complete and works!  It even sounds pretty good already considering I'm not in the right bias range.  The only problem is with the bias pot maxed I am getting the following:


Plate Voltage - 455
Plate Current - 20
Wattage - 9


See attached photos.  I've read up a little bit and it looks like I need to raise the value of the 'bias range resistor'?  Is the 15K resistor to ground circled in the attached pics the bias range resistor?  Am I correct that raising the value is the right way to get toward a more normal wattage dissipation of 17 to 19 watts for a pair of 6L6GC tubes?  And if so, how much to raise it?


Thanks for any help.

I know you've already solved this, Waldner, but the way I dealt with the bias voltage issue in this amp was by replacing the 15k resistor with a 6.8k. My Mojotone Vibroverb now runs at about 18w per 6L6: 449v @ 40(ish)ma.

[pdf64]

I suspect that the numpty at Mojo that drew the layout mixed up a typical Marshall type of bias adjust with a Fender black panel type, and created an arrangement that has very limited range of adjustment