She's home my little Bauer 707

Discussion in 'Amplitude Modulation' started by KD6CXW, Sep 1, 2019.

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  1. W7TFO

    W7TFO Premium Subscriber QRZ Page

    Early second generation...Early 'tapered' meters, brown two-tone paint. The first ones were green.

    N6YW likes this.
  2. K5UJ

    K5UJ Ham Member QRZ Page

    Thanks for the photos. I believe you have the version that drops to 500 w. for night operation. Basically a pair of 1.5 K 200 w. w.w. resistors in series to the final (modulators stay at 3 KV) to get the final B+ down to around 2200 v.
  3. KD6CXW

    KD6CXW Ham Member QRZ Page

    replaced some HV wires,the plate choke, added band switch and 20-1500pf vacuum cap.
    Was wondering what type,size,rating for the safety choke in case blocking capacitor shorts ?
  4. K5UJ

    K5UJ Ham Member QRZ Page

    The safety choke isn't extremely critical in terms of specifications. Any small pie wound choke with around 2 or 3 mH will do. It only has to short dc to ground long enough to blow a fuse. It can sacrifice itself (but then you have to replace it) or maybe not if the fuse blows or breaker trips fast enough.

    Of a bit more concern is the National plate choke you have. I don't know a great deal about that choke so others more familiar with it may need to comment, but you need a choke that can handle the modulated current draw at full strap. If you only plan to operate the rig below 20 meters, you can eliminate choke series resonances above 8 Mc. as a concern.

    In broadcast service, the rigs usually used high inductance solenoidal chokes with values over 1 mH and low amounts of bypass C (500 to 1000 pF) to improve the high frequency audio response. The stock choke while okay for medium wave, may very well have hot spots on HF and won't be suitable. Ham plate chokes have lower inductance, maybe only 200 µH but they are mainly used in leenyars with a lot of bypass C, 2000 or more pF. When you get up there in that range of bypass capacitance, the total together with your pi network bypass C, will affect your high frequency audio response. I learned this the hard way. If the national choke can handle your current you can use it with 500 - 1000 pF bypass C, then follow it with another choke of higher inductance that works in your frequency range, something that's 1 to 2 mH and another bypass cap of say 300 pF. This gets the RF off the power supply line without a huge amount of total capacitance. The multi-stage choke trick was used by Collins, EFJ in their QRO rigs, and also the T368.

    Put the solenoidal choke as the first one at the tube anodes to take the brunt of the RF voltage and follow it with the high inductance choke, usually a pie wound to clean up any residual RF. The reason for subjecting the solenoidal choke to the full RF voltage is that it can dissipate heat. Pie wound chokes don't dissipate heat all that well. I found that out the hard way too, hi hi.

    If the National choke isn't rated for more than an amp, then it might be advisable to search for another one, or make one yourself using AWG 22 or 20 enamel copper wire. There are choke calculators on line and you can buy tubing to wind on at eBay:

    Check for series resonances with the choke in the circuit using a GDO. Of course you can also just try the choke and see if it burns up. Usually they give off a warning stench that gives you time to kill the carrier but oh boy they can funk up the shack so be ready to get it off the rig and outside : )
    Last edited: Sep 8, 2019
  5. K4KYV

    K4KYV Subscriber QRZ Page

    A phenomenon I have had with pie-wound chokes is not that they open up or burn out, but the magnetic attraction (probably combined with temperature) between sections (most likely during a current spike caused by an arc-over in the final), can be enough that the pie sections all bunch together into one solid lump of wire. It didn't seem to affect anything and I noticed it only when inspecting the final for something unrelated, but I replaced the choke anyway. After this happened a couple more times and I was using up my spare chokes, I installed an overload relay.
  6. AC0OB

    AC0OB Subscriber QRZ Page

    That is a nice Bauer and appers to be in good shape.

    BC Choke.jpg

    This may be overkill, but a 2.5 mH choke @ 1 Amp should be sufficient:

  7. WZ5Q

    WZ5Q Ham Member QRZ Page

    Congrats on the fine Transmitter John!
    Gotta love that Iron.

    You want some overkill Pheel? ;)

    In the proofing stages of re-configuring my AF-67 to use an 809 as an RF Final being driven to 8x cutoff by a 6146B, the plate pi choke I was using for the 6146 kind of did what Don mentioned and made a ball-O-wire along with some extra crunchy results. The smallest thing I had on hand to replace it was a 2.5mH Pi Choke from my RA-1000 parts bin. If you look real close, you just might be able to see it shoehorned into the bottom of the chassis. So far it has survived, tee hee.



    Last edited: Sep 8, 2019
    AC0OB, W2VW, AD5HR and 1 other person like this.
  8. KD6CXW

    KD6CXW Ham Member QRZ Page

    were would one acquire ?
  9. K5UJ

    K5UJ Ham Member QRZ Page

    Not exactly what you are looking for but has an assortment; some are somewhat reasonably priced. I've used the Ohmite Z4 in series with the dual pie 600 µH 3 amp choke they have on that page. The Z4 is rated at 1 amp (knowing Ohmite that's probably a bit conservative) and I move some air over mine. I measured its inductance and it was something like 260 µH. Before you go off and spend money it might be worth it to check the Bauer choke that was in the rig. Let's say you only plan to operate the rig on 75 m. Rig up a test setup. You need an oscilloscope, some clip leads, a resistor, 50, 100 ohms, 200, something in that range and a signal source that will sweep from around 3000 kc up to 5000. Something like a signal generator or swr analyzer. put the signal source into one of the vertical inputs on the oscilloscope using the necessary cabling or clip leads and adapters and adjust the vert. v. / division range to get an RF ribbon trace two or three divisions high. Now insert the choke between the vertical input and the signal source with the resistor to ground on the side of the choke closest to the scope. The resistor provides a load. If the choke works on 75 m. you should see little or no RF on the trace with the choke in series with the signal generator while you sweep it up through 3500 to 4000 kc. The choke may very well have resonances somewhere above 2000 kc in the low HF but if it is at 2500 or 3000 or 5000 you are probably safe trying it on 75 m. Earlier I wrote that the broadcast band choke was probably a problem for ham bands, but it pays to verify and collect facts rather than trusting some guy on the internet.

    Sharp fellows might be reading this and think: Wait, those dual stage plate chokes were described as being used in rigs that don't have modulation reactors. Wouldn't the modulation reactor help keep RF out of the power supply? I don't have an answer for that. My dual choke idea that I tried was just an attempt at achieving the same degree of RF attenuation on HF that I thought was being had in the bc band with the higher L of the stock choke with minimal bypass C.
    Last edited: Sep 11, 2019
  10. KD6CXW

    KD6CXW Ham Member QRZ Page

    Thanks for testing procedure , I plan on testing the Bauer choke now and modding if needed,got a GDO on its way as well. will be running on 160,75,40 meters , may try to put on 20 as well, we'll see how the Mods go ?
    Last edited: Sep 11, 2019

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