02/25/2016 18:05

MILLEN MOPA

 

Here it is my MILLEN 90801 EXCITER-TRANSMITTER just arrived at home, still dusty.

 

 

Here are a couple of details in the inside. The MASTER OSCILLATOR section:

 

And the POWER AMPLIFIER section:

 

Here below the schematic of its prototype, taken from the 1954 ARRL HANDBOOK. The 90801 differs very little from this, but the original schematic was not graphically suitable to publish here. So I took this:

 

The 90801 iPA is neutralized inductively on the 14-28 Mc/s band. The schematic above, instead, has a fixed neutralization network that acts on all bands. In the pictures the single turn link in both the PA and the MO plate tank can be easily identified. The latter was made so that the operator had to bend it manually for the best neutralization effect.

The 90801 has everything inside needed to operate from 3.5 to 7.0 Mc/s in one band, and from 14.0 to 28.0 Mc/s in the other band. The prototype relied on plug-in coils.

The pictures and and the schematic are rather self explanating.

To be evidenced that the mechanical structure of the 90801 is rather easily reproducible nowadays.

The 1954 HANDBOOK states an important detail: on 28 Mc/s band, quadrupling from a 7.0 Mc/s crystals will not deliver enough drive level to the PA, but in case 9.0 Mc/s crystals were available, then it would be fine. The self-bias is critical when the driver does not drive enough, because the negative bias, developed by the PA control grid rectification, which keeps the tube dissipation below its maximum rating, might be insufficient.

In this sense, the 12BY7-6DQ6 MOPA ... is still the best circuit, because the 6DQ6 full drive level can be actually achieved by quadruplication on the 12BY7 plate. The 12BY7, besides, has much higher amplification than the 5763 and for this it is better suitable to be driven itself by an external VFO, like the Heathkit VF-1 or something similar.

The MOPA-sickness began in the '90s, when I accidentally stumblend on a magazine for radio amateurs, dated in the early '70s, in which the 90801 was accurately described. At that time I had practically no money to buy some radios by which carrying on my traffic on the bands. Necessarily the solutions should have been simple, effective, fast to make. Therefore, which better solution to get on the air, than having a MOPA ?

Since that time I've always preferred MOPA as transmitters, up to my last one, which actually has been a bit improved as compared to the two-tubes classical ones, but has some more operational flexibility allowing me surfing the Shortwaves without any limitation.

 

Radioman, Feb. 2016


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