Thread: mu feedback
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Alex Alex is offline
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Default mu feedback


"flipper" wrote in message
Most likely the motorboating is caused by C6 in the power supply unit.
Increase it to 470...1000uF.


Zowie, a thousand? That pole's bigger than the 'huge' C6 one

I do think the problem is related to the bias string but, as I
mentioned in a previous post, I tried adding a filter to the 'middle'.
That struck me as similar to a traditional B+ preamp filter since
things look sort of 'upside down' with the virtual ground but it made
things worse.

I did some more looking and I'm beginning to think I need to decouple
HT to A ground because B+ will shift just like bias does.

I think my situation is a little different than the typical old style
'battery radio' because they were usually Class A and I'm running
Class AB1.


Firstly increasing C6 from 47uF to 470uF is not about the pole, but about
the impedance. Say motorboating frequency is 10Hz. 47uF cap would have 400
ohm at this frequency and will not do anything. LF plate current variation
will directly be converted to bias modulation, which in turn is applied to
the grid of the first stage. Increasing C6 reduces this effect. Ideally a
regulator or a Zener shall be used for the bias. The Zener should be set to
generate maqqximum bias for the output tubes. others can be fed through
resistive dividers. In this case the resistors in these resistor dividers
should be relatively large (tens of Ks), so they do not draw current away
from the Zener. Even Zener is good to bypass by a large electrolytic.

A separate rectifier for the bias would be perfect. Better not to create
problems in the first place, rather than struggle to overcome them later.

Secondly, you are right, the motorboating can be caused by the feedback to
the anode of 1N5GT via supply rail. In this case I am not sure whether R11
does any good. It only increases the impedance of the supply rail at very
low frequencies. Pure emitter follower Q1 has less output impedance than
R11.