Re: Ping Prof Turner: Amplifier questions
- From: Patrick Turner <info@xxxxxxxxxxxxxxxxxx>
- Date: Sat, 08 Oct 2005 11:48:40 GMT
>
snip for space,
>
> That was my first cut at it except I went the extra step to shunt
> zener regulate the -50 to -22 for extra stability.
>
> >I would have a -150V supply so Rk could be as high a value as possible
> >to provide the CF with a high ohmic load to minimise the work the CF has to do.
>
> -50V is about all I can conveniently get with the available power
> components. It's voltage tripled off an 18V tranny I'm using for the
> filaments (CCS, so no turn on filament surge and quiet DC).
But you could easily multiply the voltage further to make -150V just to sling a
large value resistor carrying
the low current of a mA or two.
Or hunt around for another old tranny.....
I didn't let the unavailability of parts ruin my applications of
ideas; I took steps to scrounge up whatever I could find
for breadboarding.....
>
>
> >A CCS between -50V and the SL7 cathode would be very good.
> >With 1/2 an SL7, all you'd need is about 1.5mA at idle.
>
> I was thinking about that but I'm not sure what I'd gain for the extra
> complexity.
>
> Other than the modest Rk taking a bit more juice on the positive side,
> what problem is it? Distortion?
Larger Rk, which is the RL for the SL7 means less distortion.
I split hairs again, but even though the CF distortion is low,
we shouldn't encorage it by cheating with lowering the cathode load....
>
>
> As it stands, with the DC bias (not the self bias), Rk is 33k at about
> 1.15ma (cathode is at -12.1) and draws .84ma to 1.45ma for a 1 watt
> OPT output. 'Full bore' is .45ma to 1.88ma plus 2.42ma going into the
> 6V6 grid.
I would of course use a 6SN7 as the driver, to get less thd since the Rout
of the SN7 as a CF would be lower perhaps.
With 1.88mA and 2.42 mA flowing in the SL7, perhaps this tube is *also*
driven into grid current; check the load lines....
A 12AT7 would be an even better driver because Ra is low
and µ is 3 times the SN7 abd about equal to the SL7, and the CF Rout
will definately be lower than the SL7, and no risk of grid current in the CF,
so the drive is all that much cleaner...
6AQ8 is also another nice tube to use, 6.3V heaters, but otherwise
almost identical to 12AT7.
>
>
> Meaning 12SL7 idle is 1.15ma with peak 4.3ma. With plate at 251 (91V
> dropping zener from 330V B+ and -12.1 cathode) that's a plate peak of
> 1.08 Watt (average comes to .38W).
>
> >For most of the easy class A action of the SL7 the load in its cathode should
> >be
> >the same as what you'd use if you had the load in the anode circuit,
>
> What is the reason for that? I mean, you'd use a high value in the
> anode because you're looking for gain. CF=1.
>
> >so having only about 35V across a resistor in a cathode circuit isn't doing it
> >right.
> >The typical class A load for SL7 is about 150k with say 1mA,
> >so the drop on the load would be 150V, hence the idea that you should have a
> >-150V supply.
>
> Well, the ma=~1 is how I selected the 33k. Could use 39k.
OK.
you are not pushing too many volts into the 6V6.
If it was a 6550 for something with real balls, then you'd need to think
more about drive.
Best to overdesign the drive and to have a higher than needed current
ability because it usually translates to better sound dynamics.
>
>
> <space snip>
>
> >> Yes. It isn't the drop vs signal I was talking about here, it's line
> >> voltage variation. Mains power to you. I.E. if it's adjusted when
> >> mains is a nominal 120V then fixed bias seems to be seriously out of
> >> whack when mains is 105 because B+ is no longer 330 but 288.
> >
> >Triodes with fixed bias are prone to becoming over biased
> >when the line volts go high.
> >
> >The 6V6 has an Ra = about 4k, so if the B+ changes 52 v, then you get an idle
> >current change
> >of about 12.5mA,
>
> That's a pretty darn significant change for a nominal 34ma.
>
> > so you should set the 6V6 up for the 330V condition.
> >When in fact the B+ drops a bit, so to will the negative bias voltage, thus
> >turning on the tube a bit more
> >when B+ is low.
> >Hence there is never a need to regulate the negative fixed bias voltage; one
> >wants it to wander around
> >with the B+ voltage.
>
> Well, there's my problem. I'm trying to make things 'too good' because
> I'm regulating the -50 and then zener regulating the -DC bias to boot
> because I'm concerned about injecting hum onto the grid.
Ordinary RC filtering should prevent hum from the B+ or B-.
>
>
> High voltage caps aren't cheap but zeners and transistors are so it's
> easier to regulate than throw a ton of uFs at it. Hmm. Maybe I'll look
> into a SS cap multiplier arrangement so it's heavily filtered but not
> regulated.
A few 100 uF won't break your bank.
>
>
> My gut tells me that just letting things drift isn't optimal unless
> things are ratioed properly. I need to do some thinking on that.
Letting things drift *is* ok if there is just some drift
and no LF instabilities or tendency for LF bounce.
with varying signal levels.
One instance of nil regulation and lots of direct coupling is the Loftin White
amplifier
which came into being well before WW2 and the existance of large cheap caps
or zener diodes.
Keep your iron hot...
Patrick Turner.
.
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