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