[NLRS] Repeater amplifiers for SSB?

[Dr. Gerald N. Johnson]

There may have been some Mirage with SSB bias options, some had a switch 
for SSB/FM, but generally it was simply adding a delay capacitor to the 
RF sensing circuit to hold the relays in transmit during speech gaps 
typical in SSB.

Likely the commercial repeater amps have very little forward bias on the 
bipolar transistors and will amplify only the peaks of SSB which 
distorts it badly. I have an FT-290RII with the clipped on PA that does 
just that. I suspect its from lack of forward bias on the module in the 
PA, I've not dug in to see and haven't found any potential buyers 
willing to work on it.

The typical bias circuits for linear service involve a temperature 
sensing diode close to the PA transistors and since its almost sure that 
the transistors will be silicon RF power transistors, a circuit stolen 
from a Mirage book (available from MFJ's Mirage pages) or a Motorola 
applications note in one of the RF transistor volumes will do the job, 
unless the base is grounded so securely it can't be biased at all. Those 
bias circuits for linears go back to HF linears designed by Ben Lowe, 
K4QF and published in QST several decades ago. He developed them at 
Collins in Texas.

Its rather hard on the idling power, but most active devices, 
transistors or tubes have the best linearity when they are biased to 
dissipate all the power they can idling, then depend on converting power 
to output to keep the disspation from rising when driven. But that could 
be different with different transistors. There should be data sheets for 
the transistors in the repeater amplifiers, though they may be custom 
part numbers that could should whether they can be run linear or simply 
would add too much distortion.

With the going prices for used known linear amps at Radio City, I'd not 
be inclined to go to the work of linearizing a repeater amp.

73, Jerry, K0CQ

On 8/4/2016 4:01 PM, David Palm wrote:
>
>
> Hello all,
>
> I am wondering if old repeater amplifiers, presumably running Class C, can
> be pressed into service for power using SSB modulation.  I presume they're
> good to go for CW and obviously FM.  They're attractive in terms of their
> duty cycle abilities and sometimes can be had at a reasonable price.  So
> speaking broadly, has anybody heard of them being used this way, as-is or
> possibly re-biased to perform differently?
>
> Thanks and 73,
>
> David  W9HQ
> ______________________________________________________________
>