[AMRadio] Modulator design needed


John Coleman ARS WA5BXO wa5bxo2005 at pctechref.com
Sun Mar 26 21:02:56 EST 2006


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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