"Phil Allison" wrote in message
...
Arfa Daily wrote:

"Phil Allison"

Sounds like their auto bias board is near identical to the one in the
H&K.

Haven't come across the HK one so can't comment, but might well be. This
one
actually says Fender on it, but they might be trading the design around
amongst themselves.

** Or just copying each others efforts.

It also has three preset 'settings' for soft, normal and
hard bias, accessed via a foil switchpad, and a toggle switch for 'club'
and
'arena' modes. Personally, if they must have user bias settings, I would
prefer three preset pots inside, and a three way switch.


** Having users actually tweak valve bias pots is a bad idea - but having
an automatic system with user selectable criteria is fine.


I really don't
believe that output valve balancing is enough of an issue to warrant this
kind of complexity. On amps that have a bias balance preset, you have to
set
it a long way off before you can hear much effect, and even then, it's
usually only a slight increase in background hum level ...


** On many Fender models (egs 135W twin and Super Twin) you can set the
bias balance by monitoring the residual 100Hz hum at the output. But if
that means a setting way off centre the valves are bad.

Read the blurbs I posted, the auto bias system is there to allow the user
to change valves from old to new or from one brand to another and know the
bias will be correct. Going from NOS RCA 6L6GCs to Russian 6L6wxts or
5881s and Chinese made KT66s etc with no need to see a tech is a big
selling feature.

That H&K model even allows the user to change from EL34s to 6L6s.

However, the system cannot cope with faulty valves and may even be damaged
by them OR fail totally like you found. Also, it is sometimes possible to
put an octal valve in the socket out of key or having broken the spigot
install it any position you like. Results are often dramatic.

IME having guitar players fool around putting valves in their amps is full
of traps & hazards - owners are bound to fall into one sooner or later.


.... Phil

Fully agreed

Arfa