The AM Forum

I recently purchased a Globe King 500.  I wonder (out loud) if adding a 50 Henry mod reactor from a Gates TX would help the fidelity along with touching up the speech input stage.

I would think so, that mod iron needs all the help it can get, but it might open up other issues.

Hi Bob,

A Heising reactor may help. It depends if the existing mod transformer can handle unbalanced DC current well at lower audio frequencies.

The easiest way to find out is to run some low frequency audio sinewaves through the rig.  (50 - 300 Hz area)   See where the lows fall apart.  Then clip lead in the Heising reactor (and 1-2 uF Heising coupling cap) and see if the response improves. You may see a cleaner and lower frequency waveform capability and less current and strain to achieve it.

With Globe mod iron, you may approach -30% negative modulation at say, 70 Hz, but be careful going much lower in frequency and hitting the modulation too hard. You can pop a mod transformer, as you probably know. Keep the tests of short duration.

The objective is to test how the existing system handles power at low frequencies and see if adding the iron improves things.

Check to be sure that the audio is clean coming into the modulator grids. We cannot expect the test to be valid if the speech amp or audio driver is grossly distorting the very low frequencies first.

Maybe someone has some actual Globe mod xfmr experience to advise too.

Let us know what you figure out.

T

Bob,

Please let us know the experimental results!

If you have time to be detailed, perhaps jot down the before and after % modulation datapoints at which distortion occurs from 70Hz every 25Hz or so up to around 200Hz, and maybe a few 100's of Hz above, to where you think it has no meaningful effect.

I apologize if it is a lot to ask but such a test and measurement would be valued by most GK owners. Don't feel as though there's any pressure. I just enjoy the data! heh. data vampire me.

I myself would never put a low frequency sine wave into a globe at anything above about 25% modulaton, and only then if I had a spare mod transformer.
Better transformers have been blown out with less.
When I was young and dumb I blew out the mod iron in my 30K1 doing just that.

Great approach Tom!  Its nice to collect the data first to determine what needs to be done. 

Joe GMS

I'd do the same with the driver transformer.

Bob,

Take a look at the design of the entire audio chain.  If you not a design type, then get your signal generator out and start with a response plot stage by stage.  Set your requirements up front as to what you want the audio chain to achieve.  Based on your requirements you will soon see what needs to be fixed.  The RC coupled stages are easy.  Ultimately you will find that the magnetic's will be the limiting factor, but you don't know how they perform until you do the testing.  The modulators will draw grid current so if you find that you need to change out the driver transformer you will need a step down driver ratio transformer so it can supply the grid current required by the modulators.   An alternate approach is to shunt feed the current driver transformer with a choke to get the DC off the transformer and I think that is what Steve was suggesting.  If it was me, I would just use a good CG or LS driver transformer.  Most mod transformers can be improved by using a Heising choke but without seeing your requirements against the plots its really premature to determine what needs to be done to achieve the requirements you have set.   BC mod iron is typically designed not to have DC through the secondary of the transformer.  A lot of Ham mod iron is designed to have that DC current through it, however typically improvements can be made by removing that DC component.   Then there is negative feedback to put the icing on the cake if your determined to go that far. 

Enjoy the project.

Joe             

After I posted this question, I looked again at the book,. and decided on inserting line level audio (-10 to 0 db)
at the grid of the first 6C5, replace the couplet with a .25/ 600 volt orange drop, and do the same for the second 6C5 as it drives the 6L6.  If their is too much level, I will by pass the first 6C5.  For those who do not have the skematik handy, the modulator is a 6SJ7 mic preamplifier, driving two 6C5's (in sucessive stages) and that driving the
6L6G which drove the 811A's. I am really not concerned with any audio under 100 Hz. I threw out the mod reactor question, because I figure this either has a great or minimal mod transformer from WRL, and might need help.

OK..It sounds like you would not need a reactor. Voice communication is not that demanding. Maybe cleaner audio can be achieved to modify what you have done so far and remove the interstage driver transformer, and build a phase splitter circuit using a 12AX7 to drive the modulators.
Another easier approach is to get a hi-fi tube audio amplifier, 25 watts or so, and feed that into an old output transformer from another dead 6L6 type amplifier, and drive the grids of the modulators that way. The PRIMARY side of that output transformer would connect to the modulators. The center tap would allow the bias D.C. volts to flow to the Globe King 500 modulator tubes.
The secondary would be connected to the external hi-fi amp 8 ohm tap. The external amp should be a retired Public address amp with microphone inputs to properly terminate a quality microphone. No need for high dollar mics, unless you're pushing for a broadcast sound; then it would be a lot of re-work, re-designing of the Globe 500.

Fred

Yes, I have seen that precaution stated here before.

I have used mostly over-rated broadcast iron in my rigs and have never had a problem with sine waves up to -90% negative after lots of testing over the years. I realize it's on the hairy edge.   We do need to run a certain threshold of current through the core to see where it begins to saturate.

However, you are correct to be careful with marginal ham-type iron and tread lightly with the tests.  

I edited my previous comment regarding sine waves to limit it to -30% negative. We don't want anyone blowing their iron....   :'(  

A normal voice test can also work with lower duty cycle and be quite revealing too.

 I would add that the point where a sine wave begins to distort is the point we don't want to exceed. An overdriven  sine wave can turn into a distorted square wave and do damage. But we do have to use some kind of audio waveform to test with, so careful testing must be done at some point.

Bob, sounds like you are on the right path regarding the audio driver mods. Sweep and run tests, follow the good advice on this BB and you will get where you want to go.

T

Amen!

Darrell

agreed! I'd assumed the test at various frequencies would stop when clipping began but it was not stated explicitly.

Hi Bob

If your Globe King 500 has its original driver transformer, you will find that it is a PP transformer as was used in the earlier GK-400 series, wired as SE in the 500 for the 6L6 driver tube. You should find a wire tucked up one side of that transformer.

More good GK-500 information here too http://w1cki.net/Globe_KingSpeechAmp.htm


Al VE3AJM

I have heard some very good sounding globe kings, stock or slightly modified.
I think a few simple mods and the right microphone can make them sound good.
Not like broadcast transmitters, but like a bc610 they sound very cool on the air with a power hum in the background and a big thunk then they go key down.