MB-150 as grid tank? I think so.

[Opcom]

Here's the latest on the Tucker KW. I finally started working on it again. Because the old bandswitching coilset was ruined, I've removed it and the dual capacitor and will have to put something else in there to feed the class-C 4-1000 grid. Since a national MB-150 tank can match a plate like that of an 807 to a 50-600 Ohm line, why not reverse and use it for driving a grid?

It also is simple to use due to no bandswitching. It covers 3.5-30MHz continuously except for 8.5-12MHz. The RF deck's plate tuning covers 3-30MHz continuously, so the only disadvantage is that small 8.5-12MHz section the MB-150 does not cover. I'd rather have a continuous tuner in there but this is as close as I have seen available. I do not plan to change anything on the plate side unless necessary.

Work has started to put the transmitter back together, so here is the schematic as it could be. The book says the grid need 340 peak volts. Does it look like this ought to work? One definite question about the MB-150 - its low impedance coil is to be tapped, This is supposed to go to the 50-600 Ohm load. Since it would be the input from the exciter, how do I determine how many turns of this coil to use? Will this change with frequency (if so, that kind of defeats the purpose..)

[KL7OF]

I am using an MB-150 as a grid tuner on my 2x250th rig....works like a charm except on 10 meters.....

[Opcom]

Thank goodness someone has used one of these!

That is really encouraging. I have to test this on the workbench since the RF deck won't be in the TX. The only way I can think of to test is to fire up the final with filament voltage and grid bias and see how it drives. - so any help to do it right the first time is greatly appreciated.

Please tell me about 10M - -what do you see as the disadvantages or difficulties? Does it resonate poorly, or there is some issue with driving level or the match?

Also, what do you drive it with, how much power, is it about 50 Ohms or something else, and how many turns are you using on the tappable input coil? Hate to be so nosy, but anything will help.

The reason I ask about the Z is because I would like to drive it with a solid state unit like a TS-430 or a SS 100 amp driven by a VFO, rather than a tube transmitter that can match the typical 50-300 ohm loads.

[KC2FXE]

Hello  Opcom:
 I'm useing the mb 40sl as aGrid tank  for my pair 813's  by 813's homebrew and it works fine.
                     John

[KL7OF]

Hi Opcom...I'm using a pair of 807s to drive the 250th rig......The driver is an old HB 5 band rig with a tapped coil output...I'm not sure why it won't go to 10 meters......I've not investigated why it won't go on 10 meters.....I'm in Alaska right now and the rig is at my Tum Tum QTH so I can't tell you how many turns I,m using on the input...sorry..good luk and let us know what you find out....Steve

[Opcom]

Thanks guys for the info and encouragement. I've seen ads for the MB-40 but never was lucky enough to find one. Ok, I am going ahead. If two others have done it with no issues, I'm encouraged. Let the hole drilling begin. Saved from the original grid capacitor setup is a Millen turns reduction dial part. I ought to use it. I already have the insulated shaft section since the frame is below ground @ bias volts. My biggest challenge is to drill the holes in the right places the first time. I always find that most challenging.

The really cool part of using an MB tank is that it will leave me a clear shot front to back for a CRT for a mod monitor. I think a 5" round will fit. if not, a 4" square 4MP1. I'd rather use a P2 or P31 phosphor though for more brilliance.
relative illuminance (de Tektronix):
P1 - 45
P2 - 60
P31 - 100

Anyway, the tank first.

[WA2IXP]

I use the smaller version to drive a single 813. Works ok. The tuning is very touchy on the higher bands- a reduction drive would be recommended. Mine has fixed links. I did have the mb-150 and used it as an output tank. Worked well also. Just experiment with the tap for highest output.
                                                                                    jay-

[Opcom]

I found out that the MB-150 can't apparently be put too darn close to enclosure walls. I guess all coils are like that.. Abyway, i can't use it in the TX because although it looks like there is plenty of room, the larger coil is too close to the wall when the unit is in a position where the tuning shaft is in a decent place on the panel. re-orienting is not working well because of the two positions it will fit right, one has the MB-150's mounting screws blocked by stuff above the deck and the other would make it impossible to attach leads to the unit once it is in place.

Time to look for another solution. What I need is a little pi circuit from a 100W rig that has continuous tuning, if the rotary coil and geared cap would fit. I guess i'll have to make something up otherwise like the traditional tapped coil and variable cap.

Has anyone ever driven a class C grid broadband with something like a 1:6 transformer? I can see that, but it might need to be swamped heavily for linearity sake. And that would get rid of the neutralizing stuff as well.

50W/50 Ohms ==> 70V peak
1:6 xfmr ==> 1800 Ohms, 420V peak (needs 340 by the book)
12 watts drive
38 watts swampage
maybe a 2300 Ohm NI resisance across the xfmr secondary?

goes without saying I don't care about wasting drive power, it is very cheap.

[AB2EZ]

Hi!

Re: broadband transformer

You need to take into account the capacitance on the grid side of the transformer. 1800 Ohms at 30MHz => the total capacitance across the grid-side winding has to be less than 3pF.

Separately, the magnetizing inductance, looking into the secondary, has to be high enough to produce a reactance of more than 1800 Ohms at the lowest frequency of operation.

I think that the natural law of "conservation of aggravation" is going to apply to this problem.

Best

Stu

[Opcom]

well put. I figured there was a reason this was not done, and not shown in the handbooks.

I suppose an air core transformer would suffer similar levels of conservation.

ahh well, I ought not to be too lazy to tune a grid.