Date: Mon, 11 Dec 2017 10:03:29 -0600
From: donroden@hiwaay.net
To: amps@contesting.com
Subject: Re: [Amps] Dynamic linearizer
> ## The dynamic linearizer circuit intrigues me. Did it actually
> work on that sweep tube amp ?
> Do you think the same concept would be of any benefit with a GG triode ??
>
> Jim VE7RF
Sounds like it should be more complicated than a simple series
regulator ( ?? )
Don W4DNR
## I have the schematic buried on the PC some where for the galaxy 2000, which
used
10 x 6HF5 sweep tubes. I will have to dig it out.
## Im having a hard time trying to fathom how screwing with the bias voltage
can
improve linearity. It would have to super fast, and really not be regulated,
but change
its output Vdc, as required.
## What might work, is some kind of super fast sliding bias scheme. IE: amp
biased
for normal class AB operation, when at idle, but no drive applied..... then
decrease
bias V, and increase the idle current a bunch, but only when driven.
## My Denon stereo amp uses the reverse of the above concept, using an
optical isolator.
Its in Class A, but only up to about 8-10 watts. Driven beyond that, bias
slides rapidly into Class AB.
Then its good up to 120 watts out. The idea there was its clean with typ low
power, then
more eff, at high power, without the blast furnace class A issues.
## But with my experiments with playing with adjustable bias on a GG triode,
the results are not
that spectacular. IE: oem Drake L4B amps come with NO bias, with Eimac 3-500Z
tubes, and their
MU= 130, they idle at 220 ma. With higher MU tubes, like MU=200, idle, with
NO bias is a lot less.
## I have found that, even with high idle currents, imd is not reduced very
much beyond a certain point.
8877s can be run zero bias..... up to a certain B+. Much higher, and bias
is then required to keep the
idle power within a reasonable limit, like less than the rated CCS anode diss
of the tube.
## IE: a big heat reduction with increased bias V... and not much degradation
of IMD, maybe 1-2 db.
## On a side note, I have used a small, fast rated 3AGC grid fuse, wired
between Positive terminal of
grid meter..or grid shunt..and the chassis. If grid fuse blows open, no path
for DC grid current, and amp cant
be driven. Power output drops to zero, and swr between xcvr and input of amp
rises to infinity.
Watch out though, a lot of these low current 3agc fuses, like the 200ma, 250ma,
300 ma, will show several
ohms of DC resistance on a good, new fuse. You dont want the Vdrop across
them, so they may not be a good idea
in a lot of applications.
## The RF chokes used on each tube socket on a L4B and SB-220, henry 3-500Z
amp can create similar issues.
The DC resistance on the chokes used on a SB-220 are typ 25 ohms. With 300ma
of total grid current, that’s 150 ma
per tube. .150 amp x 25 ohms = 3.75 vdc on peaks.... so u just added
another 3.75 vdc per tube, when driven to max
pep out. That’s 3.75vdc on top of any normal regulated bias.
## Grid chokes on the L4B measure just .9 ohm. But I removed the chokes
anyway, and also the grid bypass caps.
Then just bond all 3 x grid pins directly to the chassis. I used a string
of 6A10 didoes for bias, then a SPDT-center off
miniature toggle switch to provide for 3 x bias adjustments. Wire a BIG
electrolytic across the extreme ends of the
diode string, and the bias V does not budge, between normal idle
current.....and full bore key down cxr.
## Thats about as good as it gets, that plus a fairly stiff B+ supply.
The xcvr is the weak link, typ with much higher IMD than the typ GG triode
linear. Xcvr makers don’t seem to wamt
to improve TX imd, even though older xcvrs like the yaesu FT- 767 GX has superb
imd, like –40 dbc. It ran on 12 vdc,
and used the same finals as the 200 watt yaesu FT-1000-D. The 767 GX was
a 100 watt out xcvr.
## Look at any marine SSB xcvr, IMD is at least a magnitude better than the
ham version. The marine gear runs on 12 vdc too.
Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|