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


I currently have a 4 x 6L6 Fender in for repair that has uP controlled
bias,
FFS !! What a load of utter nonsense. What on earth was the designer
thinking of, or his supervisor when he approved it ? The micro is
currently
doing absolutely nothing - not even any clock present, although the 3.3
volts is present on the chip. The bias is supposed to be controlled by
four
PWM signals driving transistors, but of course, as there is no clock,
these
drives are missing with the result that the output grids are all at -89
volts ...

The board that this lot is all on is utterly unrepairable, with SM
components the size of gnat's cocks. We're currently trying to persuade
Fender to tell us if this board is available even, let alone its price
...
:-(


** Hughes and Kettner make similar amps:

http://ru.hughes-and-kettner.com/pro...e%2050%20Combo

All the pots are just encoders for the DSP system, so every tiny movement
makes an annoying click. The blurb says " ..an built-in tube
technician..... "

Yep, a tiny SMD board adjusts the idle current and, if out of range, even
biases off the particular valve. The trick circuit allows allows H&K to
use a small toroidal output tranny - cos it keeps bias balance spot on.

Had one of the little horrors on the bench back in July, dead because the
heater fuse had blown. ******* to get open and put back together too.

When running, I found it was riddled with HF oscillations under all load
conditions - open, resistive and speaker load.

The output stage is a Marshall clone, but there was no cap across the
12AX7 phase splitter anodes - I found fitting 100pF fixed the
oscillations.

Beware.


... Phil


Thanks for the tip, Phil. Noted. I'm guessing that this fender is supposed
to keep the bias balance just so as well, as it has low value resistors in
the output cathodes, with the voltage across them fed back to A-D inputs on
the micro as far as I can see. Except of course that there is no feedback
volts a the moment because with - 89v on the grids, the valves are into
really hard cut-off.

I'm at a bit of a loss to see what the supposed advantage is on this
particular design, which uses a perfectly conventional EI cored output
tranny. Unless the valves have gone really badly out of balance - in which
case they probably need replacing anyway - I would defy most users to be
able to hear any difference ... :-)

Arfa