[TMC] GPR-90 6BE6

[Rodger Singley]

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


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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.