[Amps] Alpha 78 problem

[Jim Thomson]

a.. 
I spoke with Ray Heaton about this back when I purchased my first 77Dx.  On 
the Dx/Sx models, the EBS can be defeated by adjusting one potentiometer on 
the bias board.  Plate idling current can be adjusted to reasonable levels 
and the added heat produced when the key line is pulled low is never a 
problem.

By contrast, my Alpha 70V uses a fixed EBS system and when signal strength 
on the Rx end is very strong, artifacts from the EBS can certainly be heard. 
I have two different modifications for this but decided to leave the 
original EBS intact, at least for now.

Paul, W9AC 
a.. 
a.. I thought the alpha's  all used TRI-State  EBS  schemes ??  IE: tube cut off on 
a..  RX.     Hit PTT.. and bias V  drops.. resulting in 10-20 ma idle current.  Then with 
a.. drive applied, bias V is down more... and normal 150-250 ma idle results. 
a.. The ARRL mid 70's EBS  used  BI-state EBS in qst articles.  The BI state I can see 
a.. being a real problem.    I gave up on EBS, and since I use VOX  ssb /cw... no big deal. 
a.. On CW... I crank up the bias V... so idle is way down.... to 10-40 ma... STILL AB-2 ! 
a.. That solves any heat problems.   On ssb,  on Vox, [I use a noise gate, Aphex unit, and 
a.. also a Behringer unit]  still no issue.    I wouldn't use any more bias than u really need 
a.. on ssb  to get good imd.   My adjustable bias is just a series string of 6A10's or 1N5408's 
a.. with a large lytic  across all of them. [5000-40,000 uf @ 50 vdc] 
a..  On the L4B... I use a  simple  SPDT-center off to give me  3 x pos of bias [tapped diodes]  
a..  On the HB amps,  I use a rotary switch. [ on the hb amps I leave the 1st few diodes in the 
a..  circuit, then tap every 2-3 one after that,  coarse adj]  
a..  For fine control,a spst toggle shunts just ONE  diode..[last]    
a.. 
a.. later......... Jim  VE7RF