[AMRadio] Re: need help


Gary Schafer garyschafer at comcast.net
Sat Oct 22 19:35:09 EDT 2005


Brian,

It sounds like your 4-400's may have been in grounded grid?

The -130 volts is easy to get with a little 120 to 120 volt transformer. No
current involved with AB1 operation. You don't need a regulated supply. A
pot across the supply to adjust the voltage works fine.

73
Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of bcarling at cfl.rr.com
> Sent: Saturday, October 22, 2005 3:26 PM
> To: Discussion of AM Radio
> Subject: RE: [AMRadio] Re: need help
> 
> It seems like my homebrew 4-400A pair used a 35 Volt zener in the
> cathode return to bias off the resting current during key up or
> silence between words on SSB.
> 
> I can see why a 130 volt diode would need to dissipate
> considerably more power as heat.
> 
> Of course you could use a bipolar transistor with it's base held
> at the appropriate voltage of  130V DC if it had a good
> heatsink and could handle the 104 watts.
> 
> On 22 Oct 2005 at 12:06, Jim Candela wrote:
> 
> > 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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