Biasing the '63 Vibroverb RI

[alange5]

I'd like to add a bias adjustment pot to my '63 Vibroverb reissue, and I'd like more clarity on how the amp is biased in the first place.

Schematic:
http://www.prowessamplifiers.com/schematics/fender/63_vibroverb_reissue_2x10-63_Vibroverb_reissue_brownface_2x10.html

As I understand it, R58 (1K) and R59 (33K) are the resistors I'll be working with.  My amp is dated 1990.  In '92, Fender began using 3.9K for R58.  I have yet to make this swap.  I've seen folks recommend adding a pot in series with a resistor in place of R59, but I've not seen much discussion on how R58 comes into play.  I don't quite understand how they interact to determine the bias.

I might take the route of "if it aint broke, don't fix it", but the idea of an adjustable bias is appealing.  Any input is much appreciated.

[HotBluePlates]

I'd like to add a bias adjustment pot to my '63 Vibroverb reissue, and I'd like more clarity on how the amp is biased in the first place.
...
As I understand it, R58 (1K) and R59 (33K) are the resistors I'll be working with.  ...  I've seen folks recommend adding a pot in series with a resistor in place of R59, but I've not seen much discussion on how R58 comes into play.  I don't quite understand how they interact to determine the bias. ...

CR5 and C36 are a half-wave rectifier which create a negative d.c. voltage, but with a good deal of ripple on the d.c. R58 and C30 filter that d.c. more to remove the ripple. R59 is a load on this small "power supply" to adjust the voltage, which is then passed to the output tube grids as bias.

You could look at R58 and R59 as a voltage divider. If R58 gets bigger or R59 gets smaller, the negative d.c. voltage is made smaller, so tube bias is less and tubes pass more idle current. If R58 gets smaller or R59 gets bigger, the negative d.c. voltage is made bigger (up to a limit), so tube bias is increased and tubes pass less current.

You could make either R58 or R59 variable by replacing them with a pot wired as a rheostat. However, if you choose the smaller-valued resistor (R58) the adjustment might not be as wide-ranging and you could reduce the filtering of the bias voltage which will inject hum right into your output tubes for amplification. So people usually swap the R59 position for a pot.

But you never want to be able to dial in zero bias, so a resistor is generally in series with the rheostat at R59. I'd chose a value of ~1/2 R59's value, so probably 15kΩ. This allows you to idle the tube hotter than stock (15kΩ in place of 33kΩ for more idle current). I'd also like the option of cooler than stock in case some tubes need that, so I might use a 50kΩ pot in series with the 15kΩ.

Or maybe a 25kΩ pot in series with a 22kΩ to ground. The main idea is to have a resistance for R59 that is adjustable to both more-than-stock and less-than-stock resistance.

Exact values to use are usually a matter of availability and in-circuit testing. While R58 and R59 can be understood as a voltage divider, they usually don't follow the simple math for setting the voltage output of a d.c. divider because the bias circuit is receiving pulsating d.c. and has caps in the circuit. Design calculations become cumbersome in rectifier circuits without some kind of simulation to do the tedious math for you.

[alange5]

great explanation.  Thank you!

[HotBluePlates]

No problem! Hopefully that helps you cement how the circuit works so the recommendations you've read make sense.

[2deaf]

R59 is a load on this small "power supply" to adjust the voltage . . .

Exact values to use are usually a matter of availability and in-circuit testing.

That's what it would come down to.  We have no way of knowing how much the voltage changes with different resistive loads on the bias winding.  When there is a bias tap on the HT winding, it acts a lot like a constant voltage source so that voltage divider calculations can be amazingly accurate.  If that were the case here, changing R59 from 33K to 65K would have a minimal effect on the bias voltage due to the nature of the voltage divider.  But this is a separate winding and there is a pretty good chance that doubling R59 will have a significant effect on the voltage of this winding and on the bias voltage.

I haven't said anything different than what HBP said.  I just expanded on his very few, very potent words.

[redtabby]

Curious if you went ahead and got your bias adjustment worked out?  I have the same amp and have been meaning to do so as well.

Years ago I found a post in another forum where someone reported a reasonable range of bias adjustment with these values:

R58 2.5K
R59 10K and 50K pot in series

Completely unverified by me!

[HotBluePlates]

I have the same amp and have been meaning to do so as well.

Try the values you posted, or try what I posted before (basically original-R58 plus resistor & pot for R59).

All you care about in the end is you have acceptable range above and below the original bias voltage to accommodate different tubes.

[redtabby]

Will try your suggestion first.  Thanks!