"Gareth Magennis" wrote in message ...



"N_Cook" wrote in message ...

On 25/03/2015 18:49, Gareth Magennis wrote:


"N_Cook" wrote in message ...

Valve amp, little used then stored for 3 years now very low level
wanted, undistorted output and constant buzz, minimum wiht middle
setting of A/AB pot on rear.
Good signal out of SEND and bad output from test signal in RET, so PA
problem. Nothing seen weeping nor dodgey solder.
I was hoping S/R bypass switch corrossion but not that.



That A/AB pot on the back is a bit weird from memory. Does it kind of
turn one output tube off?



Cheers,


Gareth.

From the explanation of controls page

"As the TEXTURE control is rotated counterclockwise, the effect of one
half of the 6L6GC power tubes is progressively subtracted from the
circuit, while the gain of the driver tube is slowly increased. The
driver's low-frequency response is also altered along with the gain,
resulting in more even-ordered harmonic distortion from your power amp,
even at lower-than-stage-volume settings. Finally, with the TEXTURE knob
in the fully counterclockwise position, the result is a real
single-ended power amp section that operates and responds exactly like a
true Class A power amp, driven by a real single-ended high-gain tube stage.
This setting still allows the unused power tube(s) to draw idle current,
thus retaining the efficiency of
the standard Class A/B topology. In this mode, power output is also
reduced by as much as 60% versus
maximum rated power."



OK, so it seems to turn one output tube off then, as I thought.

It looks rather drastic on a scope, very top heavy IIRC.


Cheers,


Gareth.





i.e. you will not get a sine wave at any output power, it is purely
distortion.

I think.



Gareth.