Fender Pro Reverb AA1069 Bias Balance Question

[NotWesYet]

Hi Gang,

I am rebuilding a '71 Silverface Pro Reverb.

The layout indicates this should be the proper wiring.

https://i.ibb.co/hgrSLWd/Pro-Rverb-AA1069.png

Previously, this was the wiring.

https://i.ibb.co/FDhXvpQ/Previous-Balance-Bias.png

Is either preferable?

The tech who maintained the amp changed so many thing and many of the values of the resistors and the wiring did not match either the AA1069 or the AB673 circuits. So I am starting over, replacing everything but the transformers following the AA1069 circuit.

Thanks again for everyone's help.

[2deaf]

The "Previous Balance Bias" looks bogus to me.  One side has a constant bias voltage and no grid leak while the other side has the same bias voltage or less.

If you wish to retain the bias balance style, then use the AA1069 version.  If you wish to change to a bias level style, here is a suggestion.

[Latole]

look a AA270 with balance bias pot.
Use second picture to mod .


https://el34world.com/charts/bias_conversions.htm

[Latole]

The "Previous Balance Bias" looks bogus to me.   

 

I agree

[NotWesYet]

Thank you again!

The idea that the previous is "bogus" is what I suspected. Nothing in the amp matches any single schematic and some of the values of the resistors match nothing. Voltage rating on the caps were wrong, as well.

I am going to stick with the AA1069 schematic I have which, other than having a mid control which was not implemented, should be correct.

I have never had the occasion to use the acronym FUBAR, but after this tech worked on it, I think it is apropos.

Thanks again for everyone's help.

[NotWesYet]

The "Previous Balance Bias" looks bogus to me.  One side has a constant bias voltage and no grid leak while the other side has the same bias voltage or less.

If you wish to retain the bias balance style, then use the AA1069 version.  If you wish to change to a bias level style, here is a suggestion.

Thank you.

As I understand, the balance simply adjust the power to each tube in the case of a mismatched set. And the bias level adjust the current.

The 6l6's I have are a new matched set.

As to the level, I found this online some time ago.

"A 6L6GC can handle up to 46ma. Those are at 455v plate readings, the average voltage for a Pro Reverb. The 6L6GC is usually biased between 30 to 35mA."

Also, since much of the amp may have been incorrect, the previous electrolytic caps were three 20uf's@500v and two 100uf and 350v. I just ordered new ones of identical values as those were installed 20 years ago.

Your thoughts would be appreciated.

[shooter]

never had the occasion to use the acronym FUBAR

Boss sent me to check out a system, distilled water cooled 6'X3'X8' cab, I opened it to find EVERY type and size pipe, plastic, brass, iron..., rubber hose's ............
when the Boss answered Fubar was my answer. 
I had a job for a month 

[2deaf]

If you ever do consider a bias level instead of a bias balance, here is an idea for an AA270 Pro Reverb.

[Latole]

The "Previous Balance Bias" looks bogus to me.  One side has a constant bias voltage and no grid leak while the other side has the same bias voltage or less.

If you wish to retain the bias balance style, then use the AA1069 version.  If you wish to change to a bias level style, here is a suggestion.

Thank you.

As I understand, the balance simply adjust the power to each tube in the case of a mismatched set. And the bias level adjust the current.

The 6l6's I have are a new matched set.

As to the level, I found this online some time ago.

"A 6L6GC can handle up to 46ma. Those are at 455v plate readings, the average voltage for a Pro Reverb. The 6L6GC is usually biased between 30 to 35mA."

 
Your thoughts would be appreciated.

My opinion is to bias at 60% power dissipation.

 

[Latole]

Undoubtedly the work of a pseudo-technician, a skillful handyman certainly but who learns to repair amps with the amps of these customers.
To follow the forums, competent amp technicians are very rare.

[NotWesYet]

The "Previous Balance Bias" looks bogus to me.  One side has a constant bias voltage and no grid leak while the other side has the same bias voltage or less.

If you wish to retain the bias balance style, then use the AA1069 version.  If you wish to change to a bias level style, here is a suggestion.

Just one more quick question...ya know the old verify and clarify!

Is diode to be replaced a 1N4007?

I can't find any info indicating any other, but I wanted to be sure.

Thanks again.

[sluckey]

yes, although the present diode is most likely good.

[NotWesYet]

yes, although the present diode is most likely good.

Thank you.