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 > > --- [EMAIL PROTECTED] 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: [EMAIL PROTECTED] > > [mailto:amradio- > > > > [EMAIL PROTECTED] 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: [EMAIL PROTECTED] > > [mailto:amradio- > > > > >> [EMAIL PROTECTED] On Behalf Of > > [EMAIL PROTECTED] > > > > >> Sent: Wednesday, October 19, 2005 10:46 AM > > > > >> To: amradio@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 <[EMAIL PROTECTED]>: > > > > >> > > > > >> 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. > > > > >> > > > > >> > > > > >> > > > ______________________________________________________________ > > > > >> AMRadio mailing list > > > > >> Home: > > http://mailman.qth.net/mailman/listinfo/amradio > > > > >> Help: http://mailman.qth.net/mmfaq.html > > > > >> Post: mailto:AMRadio@mailman.qth.net > > > > >> AMfone Website: http://www.amfone.net > > > > >> AM List Admin: Brian Sherrod/w5ami > > > > > > > > > > > > > > > > > > ______________________________________________________________ > > > > > AMRadio mailing list > > > > > Home: > > http://mailman.qth.net/mailman/listinfo/amradio > > > > > Help: http://mailman.qth.net/mmfaq.html > > > > > Post: mailto:AMRadio@mailman.qth.net > > > > > AMfone Website: http://www.amfone.net > > > > > AM List Admin: Brian Sherrod/w5ami > > > > > > > > > > > ______________________________________________________________ > > > > AMRadio mailing list > > > > Home: > > http://mailman.qth.net/mailman/listinfo/amradio > > > > Help: http://mailman.qth.net/mmfaq.html > > > > Post: mailto:AMRadio@mailman.qth.net > > > > AMfone Website: http://www.amfone.net > > > > AM List Admin: Brian Sherrod/w5ami > > > > > > > > > > > > ______________________________________________________________ > > > AMRadio mailing list > > > Home: > > http://mailman.qth.net/mailman/listinfo/amradio > > > Help: http://mailman.qth.net/mmfaq.html > > > Post: mailto:AMRadio@mailman.qth.net > > > AMfone Website: http://www.amfone.net > > > AM List Admin: Brian Sherrod/w5ami > > > > > > > > > > > > ______________________________________________________________ > > AMRadio mailing list > > Home: > > http://mailman.qth.net/mailman/listinfo/amradio > > Help: http://mailman.qth.net/mmfaq.html > > Post: mailto:AMRadio@mailman.qth.net > > AMfone Website: http://www.amfone.net > > AM List Admin: Brian Sherrod/w5ami > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami >
