[AMRadio] Modulator design needed


John Coleman ARS WA5BXO wa5bxo2005 at pctechref.com
Sun Mar 26 22:01:37 EST 2006


Are you using the stock driver circuit with the driver XFMR on the 32V3?

John,
WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Brett gazdzinski
Sent: Sunday, March 26, 2006 8:42 PM
To: 'Discussion of AM Radio'
Subject: RE: [AMRadio] Modulator design needed

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: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] 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: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] 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: amradio-bounces at mailman.qth.net 
> > [mailto:amradio-bounces at mailman.qth.net] 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
> > 
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