Bi-directional Linear amp - news of the weird
Kenneth Scharf
had picked up. The antenna relay needed work and he was replacing the
stupid RCA connector in the input side of the rig with an SO-239 (why
did Heath use those RCA connectors for the antenna in all their ham
rigs?).
Anyway we were talking about how the linear improved the transmit signal
strength, but did nothing for the receiver and he wanted to also build a
small receiver preamp to put inside the linear cabinet, and we both got
the samw brainstorm, at the same time. Why not wire the linear up so it
would work in BOTH directions? I suppose a grounded grid preamp using
tubes with a combined plate dissipation of some 300 watts seems crazy,
and one's first though would be "you'll blow up the receiver!". But
remember that those tubes have a power gain of only 13db and that's
about the same as a 6BA6 rf amp tube. Of course the noise figure of the
572B as a preamp probably isn't quit as good as a 6CW4, but you won't
notice it much on 20 meters. Also at the signal levels we are talking
about the usual cutoff bias for the tubes STILL leaves them in class A
for use as a small signal preamp!
To make a long story short, we wired the relay up so it would do the
required switching and tried it. The linear did boost received signals
by about 13db, just as we expected. Made a difference in the DX contest!
coffelt2
"Kenneth Scharf" <wa2...@bellsouth.net> wrote in message
news:hhdtvs$vlq$1...@news.eternal-september.org...
Now that's really, really scary!
Old Chief Lynn, W7LTQ
Terry
"coffelt2" wrote
>> Anyway we were talking about how the linear improved the transmit signal
>> strength, but did nothing for the receiver and he wanted to also build a
>> small receiver preamp to put inside the linear cabinet, and we both got
>> the samw brainstorm, at the same time. Why not wire the linear up so it
>> would work in BOTH directions? I suppose a grounded grid preamp using
>> tubes with a combined plate dissipation of some 300 watts seems crazy,
Try 1000 watts for the combined plate dissipation.
>> and one's first though would be "you'll blow up the receiver!".
Just wait until it goes unstable and oscillates. Bye-bye receiver!
Kenneth Scharf
> "coffelt2" wrote
>
>>> Anyway we were talking about how the linear improved the transmit signal
>>> strength, but did nothing for the receiver and he wanted to also build a
>>> small receiver preamp to put inside the linear cabinet, and we both got
>>> the samw brainstorm, at the same time. Why not wire the linear up so it
>>> would work in BOTH directions? I suppose a grounded grid preamp using
>>> tubes with a combined plate dissipation of some 300 watts seems crazy,
>
> Try 1000 watts for the combined plate dissipation.
>
>
used two of them. The T160L version of this tube had a 160 watt plate
dissipation. Many SB200's grossly overloaded these tubes and got away
with it.
>>> and one's first though would be "you'll blow up the receiver!".
>
> Just wait until it goes unstable and oscillates. Bye-bye receiver!
>
The odds of a pair of 572B's in grounded grid oscillating were very
slim, at least at HF. VHF parasitics were another story, but the SB200
was well built in this regard. I'm sure the rig was stable even without
an antenna connected. IIRC we thought of putting in a pair of back to
back 1N4007's on the rig side during receive just in case.
Terry
"Kenneth Scharf" wrote
>>
>>>> Anyway we were talking about how the linear improved the transmit
>>>> signal strength, but did nothing for the receiver and he wanted to also
>>>> build a small receiver preamp to put inside the linear cabinet, and we
>>>> both got the samw brainstorm, at the same time. Why not wire the
>>>> linear up so it would work in BOTH directions? I suppose a grounded
>>>> grid preamp using tubes with a combined plate dissipation of some 300
>>>> watts seems crazy,
>>
>> Try 1000 watts for the combined plate dissipation.
>>
>>
> The 572B actually had only 150 watts plate dissipation and the SB200 used
> two of them. The T160L version of this tube had a 160 watt plate
> dissipation. Many SB200's grossly overloaded these tubes and got away
> with it.
Sorry - I misread the OP as an SB220
K7ITM
There's something to be said for an amplifier with high IP3. One way
to get that is to use high-power devices in the amplifier. It's not
to hard to find modern solid-state devices with fairly high power
dissipation and also low noise figure. One of the obvious problems is
that such an amplifier may have the potential to blow up things
following it...
Ulrich Rohde published an article in Ham Radio magazine some thirty
years ago about a way to get a good noise figure and extremely good
IP3 in an amplifier; his example was based on CATV transistors of
modest dissipation. The article seemed to leave quite a bit to the
imagination of the reader, and I've never seen any commercial products
that come close to the performance he suggested should be possible --
not even from the company in New Jersey of which he's a director. I'd
be really happy to find such an amplifier just now... (NF ~ 2dB, TOI >
+70dBm, on the order of a watt bias power, per Rohde).
Cheers,
Tom
Kenneth Scharf
any more power output than the input signal multiplied by the power
gain, so unless a KW station a few hundred feet away fires up you're safe.
> Ulrich Rohde published an article in Ham Radio magazine some thirty
> years ago about a way to get a good noise figure and extremely good
> IP3 in an amplifier; his example was based on CATV transistors of
> modest dissipation. The article seemed to leave quite a bit to the
> imagination of the reader, and I've never seen any commercial products
> that come close to the performance he suggested should be possible --
> not even from the company in New Jersey of which he's a director. I'd
> be really happy to find such an amplifier just now... (NF ~ 2dB, TOI >
> +70dBm, on the order of a watt bias power, per Rohde).
which come close to meeting this description.
>
> Cheers,
> Tom
K7ITM
> K7ITM wrote:
...
> > One of the obvious problems is
> > that such an amplifier may have the potential to blow up things
> > following it...
>
> ONLY IF the amplifier is allowed to become unstable. It can't generate
> any more power output than the input signal multiplied by the power
> gain, so unless a KW station a few hundred feet away fires up you're safe.
Well...that depends on what the following stages can tolerate, the
gain of the amplifier, and the signal environment you're in. I know
that European AM broadcast stations in the 7MHz band can put signals
into the East Coast of the US that deliver 0dBm to the output of some
hams antennas, and from what I hear, listeners at many locations in
Europe have to deal with even larger signals. When the phases of such
signals all line up, the peak voltage can be substantial. It's at
least something to beware of.
Do you happen to have any more definite references to preamps with
very high TOI? I've had trouble finding anything better than about
+56dBm in commercial amplifiers that don't also draw several watts (or
more!) quiescent. What I'm looking for needs to be broadband,
500kHz--30MHz.
Cheers,
Tom