Thanks PB!

[vetmed]

A little while ago you were kind enough to instruct me in the details of fixed bias. I used the bias tap with pot and filter capacitance and it works a treat That it works at all is of course something that always astonishes me Of course not having thought this through I find myself slightly paranoid about the consequences of the bias going out of adjustment Again sincere thanks.

Robert Lees

[Paul Birkeland]

I'm glad it worked out!  (What were you working on again?)

[vetmed]

Fixed bias 46 SE amp. I gathered from research I did that this method seems to be fraught with hazard, you have to keep the grid resistance low. I have around 125K and the amp has been working fine for ~8 hours. I am wondering how often one should check the bias voltage, does it need periodic readjustment? Thanks

Robert Lees

[Paul Birkeland]

Ah, nice! 

The best remedy for that would be to replace the 125K resistors with grid chokes (preferably from Magnequest). 

What you'll want to look at is the voltage present across the 125K resistor.  If you have the + probe on the grid, and the - probe at the negative rail, you will probably read some negative voltage across the resistor.  Where things get nasty is if this voltage wanders around with signal or time, but the low DC resistance of a grid choke will solve that problem nicely.

-PB

[vetmed]

How much inductance is necessary? BTW, the 125K includes the pot and the fixed resistance in the voltage divider. If I measure as you suggest and I get some value that doesn't change am I OK? I'm trying hard to imagine where this voltage would come from, but without any success so far

Robert Lees

[Paul Birkeland]

There's the output of the pot, then the grid leak resistor, the choke replaces the grid leak resistor.

The inductance has to be balanced with the capacitance of the choke, hence the recommendation for going with a grid choke that is well designed.

-PB

[Jim R.]

Robert,

Nice project -- how does the 46 sound? Can you describe it? I've neverr heard one before.

Thanks,

Jim

[vetmed]

Well, there are a lot of variables in terms of trying to compare to another tube, since this is series feed and fixed bias and my 300B Paramounts are not That is my only point of comparison, but having said that it seems that the 46 has a more airy open treble. Ten years or so ago when I was heavily into DIY I built a couple of 45 amps which I never really was too enthusiastic about. One of them was recycled into a 46 amp which I found I preferred over the 45. Hope this was helpful.

Regards
      Robert Lees

[Jim R.]

I realize that it's not a question that can be answered in words very well, but this is helpful as it can get :-).

Thanks,

Jim