811's make great modulators, don't they? I've had 2000 volts on 811A's with a 9v transistor radio battery for bias, just to see how/if they would work. They were very sensitive. Had to reduce the audio drive considerably from the previous 1100 volt/0 bias. No oscillation or any weirdness though. They actually seemed to sound a little cleaner, but that could have been anything - reduced drive - better impedance match - wishful thinking?
Sounds like a good project Geoff. 73', Kent/KX5KW On Tuesday 24 July 2007 11:02:00 am Geoff/W5OMR wrote: > Jim Wilhite wrote: > > Geoff I have never seen a tuned circuit in the screen lead. Have you > > and if you have, I would like to see the diagram for interest's sake. > > > >> question... (and I'm asking because I just don't know > >> pentodes/tetrodes) should the screen circuit be resonant at the > >> operating frequency, as well? > > I just wasn't sure, Jim. I don't believe I have seen a tuned circuit in > the screen lead. Therefore, it's probably just a control voltage. > Helps me understand better. > > I still want to build a 'desktop' AM rig, with a digital VFO like Brett > has in his rigs (beautiful setup - Brett, where are those pictures, > again?) and driver stage. > > I've still only managed to come up with (1) 4-65 (thanks to Kent/KA5MIR) > and do have 3 sockets that were located and secured (thanks to > Marv/KC9VF) and now have a 4-125 on the way, that I purchased from > e-bay. I think I'm gonna change the design up a bit, and use the 4-125 > in the final w a 1500v power supply, and modulate it with a pair of > 811's with about 9v of bias on 'em, or so. I think 0 bias is required > on 811's to bring up to Class B, with around 1100VDC. To that end, I > have the old buzzard 1:1 RCA modulation transformer that has a > screen-winding on it. A small percentage of the modulation from the > final, will be sent to modulate the screen DC voltage, reducing audio > and cross-over distortion. > > The power supply is built, I have a roller inductor, plenty of > air-variable capacitors and I believe a socket (around here somewhere) > for the 4-125 and plenty of 811's. > > This thing is starting to sound like a Globe King sized rig, with a > built-in VFO :-) > > Then again, if I can build the final and modulator on the same deck, I > could put a receiver in the rack as well, and use the RX vfo to act as > the frequency required for the xmtr exciter circuit. > > Oh, the ideas are plentiful, but I'll probably go with a more > 'conventional' design. > > -- > 73 ______________________________________________________________ AMRadio mailing list List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.
