From: apogosso@tpg.com.au   
      
   "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.   
      
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