[AMRadio] DX60 modulator chassis ready to go

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Sat Mar 18 19:56:15 EST 2006

I would build a modulator using a pair of KT88 or KT90 tubes,
multi match mod transformer, drive the grids of the KT88 tubes 
with a HIFI amp.

The KT90 pair is good for about 140 watts of audio
in AB1 (no driving power).

809's or 811's in class b would also work.

Build a nice modulator with a multi match mod trans 
and you can modulate anything with it.

A thordarson 300 watt multi match would do well...

You could have fun with this project, make it mike
input (D104 or a low Z mike) with tone controls,
or the easy way of the line level equipment 
(berrenger ultra voice) and a power amp.

A while ago, I found a 25 watt dual channel power amp
made by symetrix for under $100.00 at a trading post
place, its 1 inch high rack mount, along with the 1 inch high
berrenger, it makes for a very small clean audio setup.
25 watts is way more then enough to drive all the modulator grids
I have, the 4x150a's, the 811a's, etc.
You get eq, compression, noise gate, line or mike level
input, and with the twist of a few knobs, can restrict the audio 
down on the low and/or high end, or open it up to
full fidelity.


> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of John Lyles
> Sent: Saturday, March 18, 2006 5:56 PM
> To: amradio at mailman.qth.net
> Subject: [AMRadio] DX60 modulator chassis ready to go
> This modulator is not exactly like the 1952 nor the 1959 ARRL 
> HB 807 modulators. It appears to be a class B modulator, not 
> AB1 or AB2. It has 5 tubes, three are 12AU7 and two are 807. 
> The input audio is conventional mike preamp and audio stage, 
> using both halves of first 12AU7 with a gain pot to the grid 
> of the the second 12AU7. It appears to be connected with both 
> triodes in parallel, driving a UTC S2 single ended to push 
> pull 2:1 driver transformer. The ends of the secondary each 
> drive a grid of the third 12AU7. Plates are hard connected to 
> B+ and they are configured as cathode followers. These drive 
> the grid and screen of triode connected 807s. There is no 
> bias supply at all, the 807s being direct coupled to the 
> cathodes of 12AU7 triode sections, and the grids of those are 
> through the S2 secondary to ground. I suppose some self bias 
> might develop on the 807 grids. Output transformer is a 
> Stancor A3892 30 Watt polypedance. There is an octal socket 
> to bring all the power in
> (filament and B+1 and B+2) and a mike connector, and a volume 
> control. Entire chassis is 9.5 inches long, 4 inches wide, 
> and about 7.5 inches tall to the plate caps on the 807s. It 
> is dirty but there are only a few caps that might need 
> replacing if they are leaky. 
> Has anyone seen this particular design commercially made 
> years ago, or in any articles? It appears quite compact, and 
> with the cathode followers to the 807s, reminds me of the 
> audio lineup of a Gates BC1G, with 807s driving 833s that way. 
> I am asking $80 plus S/H for this thing with all tubes in it. 
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