[AMRadio] Re: need help


Jim Candela jcandela at prodigy.net
Sat Oct 22 15:06:34 EDT 2005


Brian,

    This would eliminate 1 of 3 supplies needed. If
the zener could hold the filament CT at 130 volts,
then the plate would need to increase to 2630, and
screen to 880 to restore the DC operating point as
before.

    The zener would need to handle the combined 4-400
plate and screen currents, which could be upwards of
800ma for two 4-400's at max CW output. That zener
would dissipate 104 watts! (.8 X 130). I don't think
this is a very practical approach, but it is for
biasing hi mu triodes in GG service where the zener
wattage need is much lower.

    I have a Fisher stereo tube hi-fi amplifier that
uses 7591's in P-P for about 30 watts / channel. This
Fisher uses a weird combination of cathode bias on the
output tubes to provide fixed bias. They use two of
the 12AX7's with series connected filaments in the low
level speech amp as a output tube cathode resistor.
This provides 24 volts bias, and this is all bypassed
for audio. Then they divide the 24 volts down with a
divider to in effect provide a output tube quiescent
current adjustment. This is neat because there is NO
hum from the low level 12AX7's because their filaments
are pure DC driven.

    There is ONE big problem with this approach. Since
the 7591's are in AB1 mode, the cathode current
increases with audio level. This increases the bias
voltage as well making the 12AX7 filaments "pump" with
audio peaks. The compromise here is to heavily bypass
the filaments with a large capacitor, and to not crank
the music too loud. 

    I fought with this for a while, and not wanting to
leave good enough alone, I added a active shunt
regulator across the filament "cathode resistor" and
made the turn on point just about 10% higher then what
the cathode bias was running at. The result was pretty
amazing since the overload point of this amplifier
went up about 25% from before when using a sustained
sine wave drive.

    The cathode bias situation with the 4-400 AB1 grid
driven amplifier is workable, but I think the first
glance simplicity of this idea is offset by a new set
of issues similar to what I went through with my
Fisher audio amplifier.

Regards,
Jim Candela
WD5JKO

--- bcarling at cfl.rr.com wrote:

> What about using a 130 volt 5 Watt zener diode in
> the cathode?
> 
> On 21 Oct 2005 at 21:48, Gary Schafer wrote:
> 
> > I just looked up the 4-400 and I see it requires
> more bias than I had
> > remembered for AB1 operation. It requires -130
> volts with 2500 on the plate
> > and 750 on the screen.
> > So a 50 ohm resistor with 100 watts would only
> provide around 100 volts peak
> > drive voltage. Not quit enough.
> > 
> > A 100 ohm resistor should provide around 140 volts
> peak with 100 watts. That
> > should work with little drive to spare.
> > Probably the easiest would be a 200 ohm resistor
> (close to the 170 ohm
> > resistor). A 4:1 balun should match it close to 50
> ohms.
> > A 200 ohm resistor and 100 watts should provide
> close to 200 volts peak
> > drive voltage.
> > 
> > I did this setup (balun and resistor) with a pair
> of grid driven 1625,s
> > driven by my 20A. It worked well. Voltages were
> less of course!
> > Stray capacitance will change things a bit.
> > 
> > 73
> > Gary  K4FMX
> > 
> > 
> > > -----Original Message-----
> > > From: amradio-bounces at mailman.qth.net
> [mailto:amradio-
> > > bounces at mailman.qth.net] On Behalf Of ne1s
> > > Sent: Thursday, October 20, 2005 12:30 PM
> > > To: Discussion of AM Radio
> > > Subject: [AMRadio] Re: need help
> > > 
> > > With a 50 ohm grid load on 4-400s, I think
> you'll find you'll get very
> > > little amplification from the stage - the 40400
> grid(s) want(s) to see
> > > more
> > > voltage. I went throught this exercise one (on
> paper), so went with a 1:16
> > > balun into a 800 ohm non-inductive resistor
> network in the actual design.
> > > Problem was, I couldn't make it broadband enough
> to cover more than 3
> > > consecutive bands at a time, and finally
> resorted to a T network on the
> > > input, loaded with about 2000 ohms worth of
> resistors.
> > > 
> > >  -Larry/NE1S
> > > 
> > > Gary Schafer writes:
> > > 
> > > > The input impedance should be very near what
> the value of the resistor
> > > is.
> > > > In this case 170 ohms. The 4-400's will most
> likely be run in AB1 so no
> > > grid
> > > > current.
> > > > I would put in a 50 ohm resistor instead. You
> should get enough drive
> > > with
> > > > it. Figure what the bias voltage will be on
> the tubes. Then figure what
> > > the
> > > > peak voltage will be from the exciter at 50
> ohms. If the peak voltage
> > > will
> > > > be greater than the bias voltage on the tubes
> then you have enough
> > > drive.
> > > >
> > > > 73
> > > > Gary  K4FMX
> > > >
> > > >> -----Original Message-----
> > > >> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-
> > > >> bounces at mailman.qth.net] On Behalf Of
> esieb at sympatico.ca
> > > >> Sent: Wednesday, October 19, 2005 10:46 AM
> > > >> To: amradio at mailman.qth.net
> > > >> Subject: RE: [AMRadio] need help
> > > >>
> > > >> I'd have used an MFJ 259B to actually measure
> the input Z.
> > > Alternatively,
> > > >> you  can always use a  small tuner to "tune"
> the input.  I do that
> > > anyway
> > > >> with my Drake L4B, (use a small MFJ mobile
> tuner with meter).
> > > >>
> > > >> 4-400's, eh?  Nice amp!  If your plate
> voltage is high enough, you
> > > ought
> > > >> to
> > > >> get serious power out of that baby!
> > > >>
> > > >> 73, Ed, VA3ES
> > > >>
> > > >>
> > > >> From: Edward B Richards <zuu6k at juno.com>:
> > > >>
> > > >> I want to drive a linear amplifier with a
> rice box that requires a 50
> > > ohm
> > > >> load.
> > > >> The linear amp uses an input to a 170 ohm, 80
> watt swamping resistor to
> > > >> ground, then
> > > >> through a .001 mfd capacitor, then through a
> VHF parasitic suppressor
> > > >> consisting of  4 turns of wire around a 47
> ohm resistor, to the grids
> > > of
> > > >> a pair of parallel connected 4-400A tubes.
> > > >>
> > > >> What I need to know is the impedance of the
> input. Is it close to 50
> > > ohms
> > > >> or do I need to use a matching network
> between the rice box and the
> > > >> linear amplifier.
> > > >>
> > > >>
> > > >>
>
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