Hammond 2220 Modulation Iron

[kb3ouk]

Steve,
Since the step down of the transformer is too much of a difference to run a lower voltage on the modulators than what is on the final, what if the modulators were run at a higher voltage than the final? I've been wondering about this the last few days.

Oh yes, that can definitely be done and it works as expected - the only thing is, you lose the advantages of a common power supply.

Ok. but there are advantages to having two supplies, too. for one, you can be sure that you wouldn't run out of headroom if you have two supplies, one for audio and another for RF. and if you make the modulator supply's voltage variable, you can control how much positive peaks you have by pushing on more voltage.

[W8ACR]

Don't want to hijack this thread, but now you guys got me wondering if my thinking is clear on this. A couple of posts have mentioned that it is difficult to achieve 100% modulation when the plate voltage of the modulators is lower than the final RF voltage. Take the following example:

Modulator - 811A's at 1300V=optimum plate to plate impedance around 10000 Ohms

RF final - 810 running 2000V@200mA - 400W input, 10000 ohm plate impedance

811A's at 1300V can produce 300W of audio

Using a 1:1 modulation transformer, will the 811A's be able to modulate the RF 100%? I would have answered yes, definitely, since at 400W input, the 810 should only need about 200W of audio to get to 100% modulation and the 811A's can easily do more than 200W. So how do the differing plate voltages come into play here?

My 254W rig runs about 2200-2300V@200mA on the RF final and 1400V on the modulators. IIRC, the taps I'm using provide a turns ratio of about 1.2/1 PRI to SEC stepdown. I seem to have plenty of audio and am apparently modulating very well. For what it's worth, I am running a 50H mod reactor/modified Heising system. I have plate voltage but no plate current on the secondary of the mod transformer.

Comments?

Thanks, Ron W8ACR

[N4LTA]

I think that I should add a modified Heising type choke for this rig to help the modulation transformer.

Large chokes are very hard to find so I am probably going to have to series the Hammond 10H 500 mA chokes.

How many do I really need?


Is there a formula for calculating the choke size?


Pat
N4LTA

[kb3ouk]

the rule of thumb that i saw is 10H for every 1000 ohm of RF final impedance.

[N4LTA]

That means I need 4  - 500 mA 10H chokes - That is about what I was thinking.

Boy, This is going to be heavy, Anyone know of another source for modulation chokes?

Pat

[KC2ZFA]

Dennis (the Iron Menace) w7tfo can help...

Peter

[W7TFO]

Audio iron is in short supply right now.

Power & filament iron is plentiful.

73DG

[N4LTA]

Any power chokes?

Pat

[KE6DF]

That means I need 4  - 500 mA 10H chokes - That is about what I was thinking.

Boy, This is going to be heavy, Anyone know of another source for modulation chokes?

Pat

Pat,

A modulation choke needs a pretty high voltage spec.

The peak voltage at 100% modulation is twice the B+, or 3KV in your case.

To be safe, probably figure 5KV chokes.

I'm not sure which Hammond chokes you are considering, but the ones I found listed on their site were rated at 1000VDC.

This is too low -- I'm not even sure they would work if insulated from ground as the audio voltage across the choke would be pretty high when used as a mod reactor.

Dave

[kb3ouk]

they might work if you insulate them from ground. i was looking at those same chokes, i wish they would put on there what they hi-pot tested them at, if they do such a thing with their chokes.

[N4LTA]

I have used them at 2000 volts in the past sitting on 1" insulators.

There just isn't much currently manufactured iron around. My thought would be to mount all four on a plate and insulate the plate above ground. They are expensive - wholesale $55 each - I hate to blow up four of them.

Pat
N4LTA

[KX5JT]

Can an individual buy transformer parts like the E and I sections, laminate and all that stuff and just assemble their own transformers?  Maybe some kit offerings where one can sorta customize the ratings would be an interesting biz.

[Bill, KD0HG]

Pat:
Start by using all of the copper a mod transformer offers, don't worry about impedance numbers. End to end windings on primary and secondary, don't bother with the taps. Then measure, scope it and tweak it from there. You likely won't find a better setting than using the whole of both windings.

You can use Zeners in the cathode return of the 811s to properly bias them at more than 1200 plate volts.  Very easy to do. OR you can use forward biased regular diodes at .6 volts per each. At 1200 volts on them, zero bias on 811s will prolly be fine.

My ART-13 makes a good 150 watts out on 75, with 1200 volts on both the 811s and 813. My friend, KE0MT gets near 250 watts out of his, with near 2 KV plate voltage and a blower on the toobs.

Bill

[N4LTA]

Thanks Bill. I had forgotton about using zeners. That will be easier than a bias supply.

I may have found a modulation choke which will help matters.

I am passing some time this weekend making a "scale model" to play with.

If it isn't too ugly, I might post a picture.

Pat
N4LTA

[N4LTA]

Decided to build up a "scale model" of the twin 813 rig to experiment with and a Slab said to learn on. Using two 807s at 400 volts to keep the driving impedance low. I am not worried about max performance on this one. Has a Hammond 278 that was out of the box - 400  - 0 - 400 V at 200 Ma - so A choke input supply for 400 volts was in order. I also wanted to experiment to see how much the standard Hammond  chokes swing at low load.

I built a small PC board with two 220 uF/450 volt snap ins, four 1N5408s (two in series in each leg) and a little DC supply in case I need it for relays. The caps were balanced with two  five watt 25 K resistors.

The resistors will pull 8 mA at 400 volts - not enough for a bleeder to bring the choke input supply to the critical value. The choke is a 10H - so if it is 10 H at light load - it should take 40 mA to make things work.

Fired it up and got 497 volts (with 8 mA load) which was a little better than I expected. With this light load, I expected it to be close to the peak  of 565 volts. I then stuck two 10 watt 10 K resistors in series on the output and it settled down to 401 volts nicely. So that makes the total load 28 mA and the choke input filter is within the critical current. That tells me the choke is at least  14 H at light load - which "might" indicate that I can get 40% more inductance out of the Hammond chokes at light load. It does swing somewhat.

So now to the RF section......

Pat
N4LTA

[KE6DF]

I have used them at 2000 volts in the past sitting on 1" insulators.

There just isn't much currently manufactured iron around. My thought would be to mount all four on a plate and insulate the plate above ground. They are expensive - wholesale $55 each - I hate to blow up four of them.

Pat
N4LTA

So where can we ordinary people get these chokes for $55?

They are about twice that much at the on-line retail sites.

Where is the best place to get good deals on Hammond xfmrs and chokes?

[N4LTA]

I am a Hammond dealer

PM me for a AMer discount