I have KT88's as modulators in my 32V3, regulated screens (VR tubes), and don't have any problems. I have feedback from the sec of the mod transformer to a stage before the phase splitter, and had to chop off the high frequencies because of the phase shift in the mod iron.
I don't have any fancy bypass stuff, and no instability up to 800 volts on them. I always thought if you were going to amplify audio, you should try to use low distortion tubes made for audio if you could. I like triodes, or AB1 using low distortion tubes. Brett N2DTS > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of John > Coleman ARS WA5BXO > Sent: Sunday, March 26, 2006 9:03 PM > To: 'Discussion of AM Radio' > Subject: RE: [AMRadio] Modulator design needed > > I haven't actually built much in the way of audio with 6146 however > OTIS, K5SWK, has done a lot with the tube I am sure it was > all standard > tetrode connections. He contends that the screen supply needs to be > well regulated and the control grid leak resistor, if AB1, needs to be > kept low, in the 100K or less region. He says that the circuits are > prone to RF oscillation and need carbon resistors in series with the > control grid and plates. I've seen him attach a wire to a plate and > play with low capacitance gimmick feed back to and opposite > grid. It's > sort of neutralizing, although sometimes, depending on the circuit > layout, the gimmick needs to come back to its on grid. It's > a trial and > error thing. I think keeping the grid resistors low helps in this as > well. At any rate he has built some awesome sounding amplifiers with > 6146s at 600V and up where stability is really important. > > I am currently assembling a 50 watt per channel KT88 amp. Two per > channel with a 5000 ohm plate load XFMR. Plate supply is 450V and the > regulated screen supply at 320 volts. Standard tetrode > connection. Also > am using a little inverse feed back from the 16 ohm out back to the > cathode resistor of the triode before the phase splitter. I'm having > some stability trouble with or with out the feedback. At high input > levels I was getting a HF parasitic burp and I am currently > trying some > of Otis's tricks. I think I've about got it settled down. I think > bypassing the screens and filaments with .01 ceramics has > made the most > deference. I'd found in the past that the filament lines can be the > transport path for RF parasitic oscillations and that electrolytic > capacitors don't bypass HF very good. > > John, WA5BXO > > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Brett gazdzinski > Sent: Sunday, March 26, 2006 7:09 PM > To: 'Mike Dorworth, K4XM'; 'Discussion of AM Radio' > Subject: RE: [AMRadio] Modulator design needed > > I never had much luck getting 6146's sounding good as modulators, > I think it takes a fair amount of effort. > > > The KT88 or KT90 on the other hand works great in AB1. > > The KT90 is reasonable in price, and can run up to 800 volts or more, > 170 watts out I think in AB1 at low distortion. > > Brett > N2DTS > > > > > > -----Original Message----- > > From: [EMAIL PROTECTED] > > [mailto:[EMAIL PROTECTED] On Behalf Of Mike > > Dorworth, K4XM > > Sent: Sunday, March 26, 2006 5:05 PM > > To: Discussion of AM Radio > > Subject: Re: [AMRadio] Modulator design needed > > > > 6146.... AB-1, 500 volts, 75 watts, 600 Volts, 95 watts, 750 > > Volts, 120 > > watts.. > > > > I thought the original question was to modulate a Johnson 6N2 > > which uses a > > 5894. Usually one uses the same Plate supply for the final as > > the modulator. > > The 6146 gives way too much power for the voltage rating the > > 6N2 would use, > > 450-600 Volts. The AB-1 807 or some of the hotter 6L6 > > versions such as 6L6GC > > seem more in the same voltage line to give the power needed. > > > > The Viking II could do the whole job with a Ceramic 4 pole, > 2 position > > switch as shown in Bill Orr's 14 Edition Handbook (his First) > > It was mounted > > on the rear apron along with a 3 lug terminal strip. > > > > In the older 12th Edition (Editors and Engineers) there is > > shown a phase > > inverter driving 6V6's and also one driving pair 813. A 6C4 > > in the former > > and a pair of 6SJ7's in the later case. On the AM Forum > > Archives there is a > > pair of 4-400's driven in AB-1 by a pair of 2E26. Google > > N9FOY and click on > > modulator. Circuits from 6AQ5, 6V6, 807, 6146, 813, 4-400 all > > published and > > none using driver transformers shows this is an OK way to go > > for most any > > power level. > > > > Mike > > > > ______________________________________________________________ > > AMRadio mailing list > > Home: http://mailman.qth.net/mailman/listinfo/amradio > > Help: http://mailman.qth.net/mmfaq.html > > Post: mailto:[email protected] > > AMfone Website: http://www.amfone.net > > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb > > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb > > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb >
