4-125A RF Deck-grid tuning parts to match plate tuning bits?

[KI4YAN]

So it's time to start looking for the rest of my RF deck parts, and I've taken stock of what I have. If you see something that is just not jiving with the rest of the parts, please, shout out and let me know. I've been planning this rig for way too long and not doing anything with it...analysis paralysis and all.

I've got a 2x200pf air variable, 0.125" plate spacing for the tuning cap. EF Johnson rates it at 4kV. I could re-assemble this with wider plate spacing, since I don't need a dual-section cap for a single RF final.
I've got a 3x720pf air variable, rated at 500V, for the loading cap.
I have a single socket for a 4-125A/4-250A/4-400A, and a heat sink plate cap to match, and a few 4-125A/6155 tubes.
I've got a box of 40kV doorknob caps, most of them are 480pF.
I've got a microwave oven blower fan, 5 microwave oven transformers, and two big ceramic resistors out of microwaves, one is 1250 ohms, the other 720 ohms, both rated at 500W.
There's a box of 1/4" soft copper tube, a few ceramic standoffs, and a nice big ceramic bandswitch, and I just folded three new 17x12x3 chassis boxes.

I am thinking that for a 2000V plate supply, the parts I have will be OK-but I have nothing for the grid circuit. I looked around and found a National MB40-SL on Ebay for 85$ shipped, which would seem pretty ideal, if expensive. Surely, I can make something out of other parts that's just as good, right? I see the MB40-SL schematics listed in the information I can find here, and I have a picture of what it looks like. Does anyone here use one with good results? Can anyone count turns on the coils and measure the section capacitance of the four-section capacitor?

I'm going through my air-variable caps tonight to see if I have a four-section that looks about right, if I don't, does anyone have know of a handbook or magazine article that shows how one of these gets put together?

[KA2DZT]

When you take a close look at the dual Johnson cap you will see that increasing the spacing will not work out.  Plus you will end up with much less than 1/2 the capacitance due to the increased spacing.  1/4inch spacing is rated at 9KV.

[w4bfs]

you might take a look at the 2 - 813 final that tom K1jj posted here on the forum .... a 813 and a 4-125 are kissing cousins in the hf range and can operate at roughly the same conditions (yes I know different voltages).... his grid drive tank should work fb here...its a starting point

[KI4YAN]

When you take a close look at the dual Johnson cap you will see that increasing the spacing will not work out.  Plus you will end up with much less than 1/2 the capacitance due to the increased spacing.  1/4inch spacing is rated at 9KV.

Sure, but 3/64 spacing should be enough, and by shortening the center spacer from 3" down to 2", it would give enough room to get it in the same frame. Being a single tube, I don't *have* to have the dual sections. I'll look again and see how difficult it would be. One of the ceramic bits for the stator frame appears to have a crack in it that's been glued back together in the past, so I don't feel terrible using this cap for frame parts and plates. I can cut a new shaft in the lathe if I have to.

That said...I'd rather not if it isn't needed. Since it's only rated to 4KV, I think I'll need to shunt-feed the tank so that the tank only sees RF voltage, not the plate supply+RF voltage.

[Opcom]

from KI4YAN
I've got a microwave oven blower fan, 5 microwave oven transformers, and two big ceramic resistors out of microwaves, one is 1250 ohms, the other 720 ohms, both rated at 500W.

I had no idea such nice resistors came from microwaves. What is their function?

I am thinking that for a 2000V plate supply, the parts I have will be OK-but I have nothing for the grid circuit. I looked around and found a National MB40-SL on Ebay for 85$ shipped,

$85 is not to bad if it is in new condition. Even so, average/well used ones can get $60.
I use one with a 4-1000 and it works well although a reduction drive is useful as the tuning can be vary sharp. An additional low impedance 'input' coil (the green coil) will almost surely be needed for driving efficiency on the higher frequencies. It's simple to add around the existing HF coils without modding the MB-40 and is 2 turns, pancake style, 0.32 uH, #14 solid. I experimented with swamping. its main effect was not broadening the tuning but improving stability of the 4-1000 stage whch had been built with only AM Broadcast frequencies experience. Backlash from things like u-joints must be eliminated or minimized. I can not get to it to count turns etc. because it is enclosed deep inside the transmitter but there are some pictures that may help. I did not try to get 160M but in some or other of the articles someone said how they did it.

This 1954 article attached explains how to make your own single ended multiband tuner.
Here is a list of every article I found covering the multiband tuner topic.
One is an 813 amp, which may be close to the application.
The file name to the right contains the publication, month, year, pages, etc.

2011-12-29  12:48         1,933,534 3c_6b_tx_QST_Jan_1954_p11-16_112_114_116.pdf
2011-12-29  13:10         1,312,543 3c_6b_tx_QST_Jun_1954_p37-39_118.pdf
2011-12-29  12:48         1,462,559 3c_6b_tx_QST_Oct_1956_p33-38.pdf
2011-12-29  12:37         1,381,428 813_linear_mb40_QST_Jul_1954_p20-22_120.pdf
2011-12-29  12:37           931,619 all_band_neut_QST_Dec_1950_p67.pdf
2011-11-26  19:16           207,298 appnote_NATIONAL_MB40SL.pdf
2011-12-29  01:29         1,311,395 automatic_analog_tuner_QST_Aug_1952_p11-13_112.pdf
2011-12-29  13:21         1,433,165 bandpass_QST_May_1949_p21-26.pdf
2012-05-02  21:52        29,491,962 CQ 05 May 1949.pdf
2011-12-29  01:48         1,011,452 gg_preamp_QST_Oct_1951_p54-55.pdf
2011-12-29  01:19           927,168 mb-40 tips QST_Feb_1956_p45.pdf
2011-12-29  01:27           927,168 mb-40_mods_QST_Feb_1956_p45-1.pdf
2011-12-29  01:26         1,595,763 multiband_6146_QST_May_1953_p33-35_122_124.pdf
2011-12-29  12:41           929,260 on_drilling_rod_QST_Jun_1941_p56.pdf
2012-05-02  21:56        19,817,332 QST 1948-03.pdf
2012-05-02  22:00            69,632 RESONANCE FINDER 20120403.xls
2012-05-02  21:11            30,208 RESONANCE FINDER.xls
2016-09-24  10:02           227,684 se_multiband_tuners.pdf
2011-12-29  12:39         1,031,301 se_multiband_tuners_vernon_chambers_QST_Jul_1954_p23-24.pdf
              21 File(s)     66,033,204 bytes
          

[Opcom]

and..

On the schematic there, I did not label the link coils according to the legend. sorry, i do not remember about it. One is the standard included link on the big set of coils and the other is the small 2 turn pancake coil shown in the pictures. Also, importantly, C1 the 500pF cap was not used in the end, it was an experiment. The 25pF? cap originally in C1's place as the mirror to the grid capacitance was used. With the MB40 as with most other push pull type of grid circuits used for neutralization, the capacitance on each high impedance terminal should be the same, so that C1=grid capacitance as measured with the tube in the socket. Otherwise you end up with two tuning 'peaks', one for each different capacitance. L7 was also not used. Cn is a probe near the anode, about 1/2" outside the 4-1000 chimney and 4" tall, #10 wire. C2 was not used and the bias was sent through the MB-40 as it has provision for that through the included RFC and bypass cap you should find at the junction of C101A and C101B. Avoid anything that would cause one side of the MB-40 to tune differently than the other side.

[Opcom]

hope these help.

[KI4YAN]

Sure did, thanks Opcom.

(and I found one locally for 15 bucks, but it's not the swinging link version-fixed link)

Hopefully this winter I'll have actual time to get something going again this year-just got the notice from the old workmeister that I'll be on overtime every week for the next two months...

I need to update the 6 meter reciever thread-it's working and alignment went well enough-and it's nice and sharp now.

[Opcom]

My experience with it as an input circuit is that the link is fully in all times, so a fixed one is probably perfect. Check and see if someone has changed the number of turns on the link. The stock number of turns is OK if you will install the special input coil for the higher bands as I did. Some hams did change the turns to 4 for higher frequencies as it was mentioned in a couple of articles. I found that trying to drive the LF coil set at higher frequencies (the stock setup) was only making trouble so I left it alone and made the mod with the extra coil for the highs. The match is very good to the ricebox driver.

[KI4YAN]

I was reading that the link coil is for a 300 ohm input, and for 50-75 ohm input you'd want a 3 or 4 turn coil...so maybe that's part of it?

[Opcom]

I do not know how many turns it had when I got it, only that it was hard to drive.