"Brenda Ann" wrote in message
...

"maxhifi" wrote in message
news:U5fsi.22399$_d2.19727@pd7urf3no...
change it to a bridge rectifier across the whole 250V winding, and
everything should work fine!


*repeatedly kicks self in arse for not thinking of that in the first
place.... * It did clear up almost every issue I was having. Next thing I
think I'll do is add negative feedback.



you can also delete the capacitor feeding the grid of the second half of the
12AT7 - the one on the plate of the first half is already blocking DC.

You can also delete the capacitor feeding the grid of
the second half of the 12AT7 -- the one on the plate
of the first half is already blocking DC.

I was going to suggest that, then realized it was part of the biasing. Which
you have altered in the following schematic.

I'm curious though, why anyone would design the circuit as it is, when
electrolytic caps are relatively cheap.

"William Sommerwerck" wrote in message
...
You can also delete the capacitor feeding the grid of
the second half of the 12AT7 -- the one on the plate
of the first half is already blocking DC.

I was going to suggest that, then realized it was part of the biasing.
Which
you have altered in the following schematic.


Sorry for an ignorant question, but can you please explain how it works? As
drawn, I can't understand how the tube is biased properly, in either
section - won't the grid at zero volts, and the cathode at zero volts set
the tube to an undesierable operating point, or is there something here I am
missing?

I'm curious though, why anyone would design the circuit as it is, when
electrolytic caps are relatively cheap.

"maxhifi" wrote in message
news:8ezsi.26512$fJ5.13616@pd7urf1no...
"William Sommerwerck" wrote in message
...
You can also delete the capacitor feeding the grid of
the second half of the 12AT7 -- the one on the plate
of the first half is already blocking DC.

I was going to suggest that, then realized it was part of the biasing.
Which
you have altered in the following schematic.


Sorry for an ignorant question, but can you please explain how it works?
As drawn, I can't understand how the tube is biased properly, in either
section - won't the grid at zero volts, and the cathode at zero volts set
the tube to an undesierable operating point, or is there something here I
am missing?


Just clarify my question - I do understand grid leak biasing, but how is it
possible with the compnent values given in the original schematic? In an AA5
the grid resistor is in the M ohms

I'm curious though, why anyone would design the circuit as it is, when
electrolytic caps are relatively cheap.

"Timothy Twerp was most astounded
When what he thought was -- wasn't grounded.
In fact, one could say he was really transfixed
With his thumb on pin 3 of a live 6L6."

Carl Kohler, ca 1958