how do I bias these three amps?

[arizona cowboy]

How do I bias these three amps? (where are the bias pot adjustments
specifically?)


Amp #1 -- Fender PA100 (only see a "hum balance" control available from
outside of chassis)

Amp #2 -- Sound City 100 Mark III (circa 1970)

Amp #3 -- Supro Thunderbolt (circa 1965)


I have a good Fluke 87 III multimeter and one of those bias adapters sold on
ebay that the tubes plugs into.

also, what is a good rule of thumb on how to locate the bias adjustment on
an unknown brand of guitar amp that has no external access for the bias
control?

do I trace down the line coming from the control grid pin of one of the
power tubes to find it?


thanks,

azcowboy