[JMS] 90801

Fri Nov 26 17:06:08 EST 2021

Wilson,

You are badly misinformed about the Millen 90800.

Mac and I have had a long history and experience with the 90800.  Our first encounter was back around 1953 when our St. Xavier High School radio Club acquired a used 90800 from Steinburgs in Cincinnati for, as I recall, $25.  We drove it with an 80  meter Command Set vfo and it in turn drove our push-pull 812 final amplifier on all bands up to 10 meters.  I acquired my current 90800 back around 1975 from a friend in in Columbus who had acquired it along with a 1941 Meissner Signal Shjifter and had no idea what either one of them were.  I've had it on the air ever since, using it with a single, Harvey Wells Bandmaster power supply that puts out 480 volts, and I'm currently driving it with a Millen 90711 vfo, the big brother to your 90700 Variarm in the Millen line.  I understand Mac has had a 90800 more recently as well.  Let me respond to your concerns about the 90800.

Millen copied the 90800 from "A Simple Two-Tube Exciter" by Don Mix, ARRL Assistant Technical Editor, in QST, November 1940.  They used a 6L6 for the crystal oscillator because it could generate enough drive for the 807 amplifier up to the fourth harmonic of the crystal.  The "Cathode Coil In Out" switch on the panel above the crystal socket enables the 6L6 to run either as a straight tetrode oscillator for operation on the fundamental frequency of the crystal with the Cathode Coil Out (shorted), 80 meters for example, or to run as a Tri-Tet oscillator for doubling, tripling, or quadrupling the crystal frequency.  My 90800 does a fine job of running straight through and multiplying with FT-243 and older 3/4" pin crystals with no chirp or drift.

Most of the time I drive my rig from a 90711 vfo as I said.  It has a low-impedance link output as does your 90700 Variarm, so I run a length of RG-58 from the vfo to the rig and I couple it to a link on the coil of a parallel-tuned circuit which I use to drive the grid of the 6L6 as shown on the schematic of your 90700.  Despite what the 90800 manual says about operating the 90800 6L6 as a doubler when it is driven by a vfo in this manner, I have no trouble with the 6L6 operating either straight through or as a multiplier with this connection.  I do have to be a little careful about adjusting the input paralel tuned circuit though, when operating straight through.  I do key the 6L6 and 807 together in their cathodes as shown on the 90800 schematic and manual.  So I have no "6L6 problem" with my 90800.

The 807 is indeed "reliably stable."  I have never seen a self-oscillation problem with the 807 as I operate my rig on 80, 40 and 20.  I do not remember our St. X 90800 ever showing a stability problem either on 80 through 10.  I attribute its stability to Don Mix's physical design which separates and partially shields the oscillator plate coil and the amplifier plate coil, and in particular for the shield on the bottom half of the 807 which cuts down on the coupling from the plate tuned circuit to the control grid leads in that lower half of the 807.  That same kind of shielding of the bottom of the 1625s in the Command Set transmitters seems to have helped them avoid oscillation, though they also do include a neutralization capacitor of sorts.

The manual does say you can run the 807 with 90 volts of bias if you want to avoid keying it and just key the oscillator.  I've never tried that, I just use 45 volts of battery bias and key the rig as originally designed.  90 volts should put the 807 into cutoff.  I don't really see why you would want to key just the oscillator and let the 807 idle on cut-off bias.  The rig works very well with both the 6L6 and 807 being keyed together.  But of course it's your choice.

As to the physical design of a skinny chassis with all of the parts "buried" inside. I can only guess that Mix and Millen thought a lot of folks would be building the rig into a larger transmitter in a 19 inch rack.  I can't recall ever having to go inside to change out parts, so the "buried" feature hasn't been a problem for me.  Yes, it can be a little inconvenient changing coils, but I've long since learned to live with that.

My schematic does show the coupling capacitor from the 6L6 plate to the 807 grid, C6, as a 20 mmfd variable.  The left-hand side of the capacitor symbol is curved and there is an arrow on the top.  I think the manual meant for you to adjust the variable capacitor for appropriate grid drive rather than actually changing out the capacitor.  I keep a milliamp meter in series with the external 807 grid bias battery so that I can see how much grid drive I'm getting.  If its too high I just detune the 6L6 a bit.

I do have a spare 90800 that someone kindly sent to me after I published my ER article about it in the May 1995 issue.  It's serial number 3172, It comes with Sylvania 6L6G and 807 tubes, and from the lack of scuffs on the wire terminal strip, it looks as though it has been at most lightly used.  It has one 160 meter coil in good condition and one that has been smashed, but could possibly be recovered.  It has two 80 meter coils, one twenty meter coil, two 15 meter coils, and two 10 meter coils, all in their original coil boxes and wrappings.  In my experience, you can easily make coils for 40 and 20.  I've used Plexiglas for a base and I've found that older banana plugs with the outer spring stripped off make good coil pins that fit into the Millen sockets.  It has one cathode coil that I think is for 40 meter crystals.  You could easily make additional cathode coils as I have for my 90800.  Use a five pin tube base or coil form and wind the coil so that it resonates with the internal capacitor across the coil, C1, 100 mmfd, at about 1.5 times the crystal frequency.  I added some extra capacitance across my 80 meter coil inside the tube base coil form.  Best of all, it comes in the original box, a bit worn and torn but still there.  I want to pass it along to you since I'm sure you will do good things with it and it's a bit more complete than the rig Mac gave you.  I will get it in the mail in the next few days.  I'll also throw in a copy of the schematic and a copy of my ER article.

So, I think you will find that the 90800 is a less quirky and better rig than you originally thought it might be.

73, Jim

________________________________
From: james_millen_society-bounces at mailman.qth.net <james_millen_society-bounces at mailman.qth.net> on behalf of Wilson Lamb via James_Millen_Society <james_millen_society at mailman.qth.net>
Sent: Thursday, November 25, 2021 6:58 PM
To: James_Millen_Society at mailman.qth.net <James_Millen_Society at mailman.qth.net>
Subject: [JMS] 90801

Do we have any actual operators of the 807 exciter?
I have one now and am planning to drive it with my Variarm.
Does anyone know where to find a genuine Millen schematic?
I have one from the web, but it's not stamped, so I don't know if it's accurate.
It's been interesting to start looking at the exciter, which seems strangely designed, to me.
Why a 6L6 first stage?  It's not a great osc tube and is WAY too big for driving an 807!
Even going down to a 6V6 seems more reasonable.
And how strange the rig throws away bands by not being able to run the 6L6 straight through.
If one has a Variarm, 90700 version, one cannot operate 80m, because of the 6L6 problem.
Looking at the schematic, it seems I can jump3 and 5 on the 6L6 socket, with the B+ disconnected from the tank coil, and plug a B&W coil into the xtal socket.
I don't have the coupling unit for the Variarm, so I could feed it to the link on the B&W coil and be all set.
My Variarm puts out a solid 2W on 80m, which should be more than enough to drive the 807.
And is the 807 reliably stable?  They are notorious oscillators and the 90800 has no neutralization or even a parasitic suppressor in the plate!
I want to run the 807 with fixed cutoff bias, as recommended in the manual, but with osc keying, so it has to be stable key up.
And why in the world would anyone bury all those components in a skinny chassis, with very poor access?  Any serious work around the osc socket will require removal of components!  Using a removable top plate on the chassis would solve this problem.

Speaking of design, my unit has a variable coupling cap, but it's shown as fixed on my schematic.
The manual talks about changing it if more drive is needed, so I expect mine is a retrofit?

I don't have a lot of time for this stuff, but will plug along until it's working reliably, so I can use it in CX.

Any operational experience or voltage measurements would be appreciated.

BTW, the Variarm is quite stable and has a fine note, very usable.
I've talked to SC, IL, MA, NY, and PA, mostly with good reports and I have good reports, amazing really, from NH and VT on the Reverse Beacon Network, amazing!

Wilson
W4BOH

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