[JMS] 90801 (90800)

[Don Buska - The Radio Lab Works]

Another thought on Post WWII projects is that tube selection was 
governed by availability of military surplus.  So perhaps there were 
better tubes that could be used, perhaps 6L6'es were a dine a dozen at 
the time.

73
Don N9OO

On 11/26/21 9:58 AM, Don Buska - The Radio Lab Works wrote:
> Hi Wilson,
>
> I think your title was suppose to state the 90800, as the 90801 used 
> the 5763-6146 line-up.  Sorry can't provide much insight into the 
> 90800, but there are many in service with the group members. From our 
> webpage the 90800 was "based on a transmitter published in QST 
> November 1940 by Don H. Mix, as "A Simple Two-Tube Exciter". It also 
> made its appearance in several editions of the ARRL Handbook for the 
> early 40's.  So yet again, this was essentially a QST project made 
> into a product by the Millen company.
>
> 73
> Don N9OO
>
> On 11/25/21 7:58 PM, Wilson Lamb via James_Millen_Society wrote:
>> 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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