[TMC] TMC GPR-90

[Richard Knoppow]

----- Original Message ----- 
From: <wb9tow at egr.msu.edu>
To: <tmc at mailman.qth.net>
Sent: Sunday, June 30, 2013 7:21 PM
Subject: Re: [TMC] TMC GPR-90


>I want to correct my previous inquiry...I am wondering if
> anyone has modified the GPR90 front end to the circuit of
> the GPR90RX...not the GPR92...73
> Steve w8tow
>
    I suspect this is not a trivial mod. The overload may 
also be coming from the second converter since it occurs 
only above 7 mhz. Both the GPR-90 and SP-600 make the change 
from single to double conversion at 7 mhz.
    TMC used a crystal filter very similar to the patented 
Hamarlund filter.  The Hammarlund patent was issued in 1938 
and was first used in the HQ-120-X (the X meant it had the 
crystal filter).  It would have expired 1955 but receivers 
designed before this like the collins 51J and 75A series 
also had a similar filter so might have been licensed.  This 
is the best of the single crystal filters.  The GSB or MSR 
SSB adaptors have 19khz LC filters with quite steep skirts. 
They are probably the equal of a Collins mechanical filter 
that way.  While the filter is not switchable for CW the 
skirts will reduce interference coming through the crystal 
when both are used plus the product detector probably 
results in less effect from noise.  I have no experience 
with either adaptor but they have very good reputations and 
are used on other receivers with approprated IF frequencies. 
I am pretty sure the MSR can be adjusted to work with the 
500 khz IF of the 51J series, perhaps someone on the list 
has tried this.
    From its own literature TMC evidently chose the grounded 
grid triode because it was very quiet.  The problem is that 
noise is not so much of a problem in the HF communications 
bands as is the ability to handle weak signals in the 
presense of much stronger ones. Many receivers have problems 
with this.  I think the reason the  6BA6 is found in so many 
front ends is that its pretty linear and has good IMD 
performance.  The 6AK5 used in the Collins 51J and some 
other receivers is a sharp cut off pentode which had very 
low noise when it was new but like all sharp cut off tubes 
is not too suitable for use in gain controlled stages.
    From some papers from RCA I've been reading recently AVC 
controlled stages have inherent distortion problems which 
can be minimised by careful design but are always there. 
Mixers and converters have a set of vices all their own. RCA 
tube designers worked asidously on this during the 1930s and 
1940s because set designers complained about the problems 
they had. Originally triode or pentode tubes were used as 
mixers (as in the original HRO) but had too many issues with 
the lack of isolation of the oscillator.  RCA deveoped the 
pentagrid converter (6A7) and the triode-hexode converter 
(6K8) and eventually more advanced tubes like the 6SA7, 
6SB7-Y and eventually the 6BE6 and 6BA7. Each was better 
than its predecessor but none was completely satisfactory.
    Anyway, I am wandering pretty far afield now and will 
stop.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com