[TMC] GPR-90 6BE6

Mon Aug 29 18:04:19 EDT 2016

    I think the GPR-92 was designed with a very wide pass band for use 
with multiplex adapters. My favorite variable band pass system is the 
Hammarlund variable mutual inductance type used in the old Super-Pro and 
in a modified form in the SP-600.  The Super-Pro version has an ideal 
pass band change obtained by mechanical movement of one of the 
transformer coils. The SP-600 version, also used in the RCA AR-88, uses 
a switched tertiary coupling coil. The advantage of this system is that 
the expansion of the IF band pass is symmetrical while the capacitance 
coupled arrangement used by Hallicrafters in its later receivers is not 
so that the center frequency of the band pass varies with the bandwidth. 
Both the TMC and Hammarlund SSB adapters use low frequency LC filters. 
The HC-10 has the advantage of a more elaborate AVC system which can be 
coupled to the receiver plus a good notch filter and limiter. I have 
never seen an actual graph of the TMC passband so can't compare the two 
but suspect they are similar. The advantage of mechanical filters or 
crystal lattice filters is that they can get the Q necessary for steep 
skirts at higher frequencies than a LC network.
     The GPR-90 has three IF transformers of the conventional kind. 
They are probably set at about critical coupling and intended to have a 
compromise bandwidth for AM. The crystal filter used by TMC is the same 
basic design as the Hammarlund filter introduced on the HQ-120-X and 
used in later Super-Pro receivers. This is the best of the single 
crystal filters, it has constant center frequency and gain. A similar 
design was adopted by Collins.
     My guess is that the GPR-90 was intended to be a fairly low cost 
general purpose receiver adaptable to diversity and SSB with external 
equipment.  It was about half the cost of an SP-600.  It is almost but 
not quite a world class receiver.
     Another possible replacement for the 6BE6 is the General Electric 
5 Star type GL=5750   These are ruggedized, long life tubes with 
generally more closely controlled characteristics and often lower 
microphonics than the prototypes.

On 8/29/2016 2:36 PM, Rodger Singley wrote:
> Dave,
>
> I think it is probably the crystal but C32 and C33 are also suspect given the symptoms as are the coil and resistor R25.  Given the expense and wait time of getting a proper crystal made I would check R25 and replace the two capacitors and reflow the solder connections for the coil before thinking about replacing the crystal.
>
> On another note there is a substitute for the 6BE6 which tends to live longer in some applications and it is the 7036.  The 7036 is a special high reliability variant developed for the early computer industry but the downside is it is taller so the regular 6BE6 shield will have to be replaced with a tall version.  I picked up a bunch of 7036 tubes at a hamfest and have used them to replace problematic 6BE6 tubes and none of these replacements have failed.
>
> I have a GPR-90 and GPR-92 and they are both impressive receivers although the GPR-92 has a very odd IF bandpass characteristic with extremely broad skirts.  I have GSB-1 adapters paired with both receivers although I prefer the Hammarlund HC-10 I used with the GPR-90 prior to acquiring a GSB-1.  The TMC adapter looks great with the TMC gear and is great for crowded conditions but the bandwidth is so narrow that it is narrower than I like when the conditions are only moderately bad.  The Hammarlund provides a choice of bandwidths that can be tailored to the situation so it is more versatile.
>
> My favorite of the vintage selectivity systems was that used by Hallicrafters in their SX-96 and other later receivers (SX-100, 101, 115, etc.) in that family.  The Hallicrafters system provided 5 steps from 500 hz to 5 Khz. and is designed to also receive AM on either upper or lower sideband.  The beauty of the Hallicrafters system, compared to the Hammarlund HQ-170/180 system (also used in their HC-10), is that upper or lower sideband is chosen by using either high or low side injection for the final conversion oscillator so that the IF response isn’t changed like it is with the Hammarlund system when switching sidebands.  With a properly tuned Hallicrafters selectable sideband receiver when tuned properly on AM either sideband can be switch selected with identical frequency response.  Theoretically the Hammarlund system can also do this but because of component tolerance and drift the response is never identical since several constants are shifted in the tuned IF strip to create the proper sideband response.  Heathkit basically copied the Hallicrafters system for their RX-1 Mohawk receiver.  I picked up some parts from a SX-101 receiver to create my own “Hallicrafters style” SSB adapter to use with other receivers.
>
> Rodger WQ9E
>
>
> Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
>
> From: Richard Knoppow<mailto:1oldlens1 at ix.netcom.com>
> Sent: Monday, August 29, 2016 4:15 PM
> To: tmc at mailman.qth.net<mailto:tmc at mailman.qth.net>
> Subject: Re: [TMC] GPR-90 6BE6
>
>     Intermittent _could_ be the crystal but also could be a lot of other
> things including bad solder joints or dirty contacts somewhere. Have you
> treated the tube socket with Deoxit, apply some and wiggle the tube
> around.
>     For some reason I have always been intrigued with the GPR-90 since I
> saw the first one at Henry Radio when they came out. Was never able to
> find one I could afford. I think TMC made a mistake when they advertised
> them because the ads suggested the dial calibration was not very good. I
> think Collins specs made it difficult for anyone else, I remember
> hearing those who could not afford Collins saying "if I had wanted a
> frequency meter I would have bought one".  I understand the calibration
> of the GPR-90 is actually very good.  It is certainly one of the sexiest
> looking receivers ever made.
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL