Totem-Pole Progress Report

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Postby frank carcia » Tue Sep 07, 2004 10:32 am

Great job Bill! I will listen for Spurs if I hear you. I think you could have used a smaller transformer since the drain voltage is lower. It is nice to not depend on a rice box to get your AM rig on. fc
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Postby ke1gf » Tue Sep 07, 2004 7:39 pm

Frank, your help is greatly appreciated.

If you wouldn't mind keep the measurements quiet.

We'll meet up eventually on the air; please measure and email them to me.

My old final gate drive was pretty bad, the cores would get pretty hot, I had to use 6 FB-43-1020 so the transformer wouldn't saturate out after a few minutes key down... lost a few fets the hard way trying to find the right combo.

I've been playing with switcher CAD and been working on a better configuration.

I had a complete melt-down the other day, lost about 10 FETs in the final, the driver survived just fine though :D

I made a big mistake with grounding on the VFO... switched to 220 on the power tranny, opened the audio up, big YAAAAAY. You know the rest.

Thanks,
-Bill
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Postby frank carcia » Wed Sep 08, 2004 7:07 am

Bill,
The driver Push pull driver in the 160 meter rig runs about 7 volts and I use 2 A637 cores that are the same size but a bit lower perm. The driver takes about 6.5 amps and they are stone cold. I found the L/C ratio of the
tank has a big effect on the gate signal. Things improved when L was reduced and C increased. Also I have no more stability problems that we
all observed at Steve's during the strap test. It sounds like you may have a similar problem. My load cap is about 1800 pf and series tune about 350 pf if I remember.
7 more pieces of green siding and 84 feet of soffit material to go. fc
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Postby ke1gf » Wed Sep 08, 2004 7:47 am

Frank, I had the thing working tits, nice clean sine wave, very little negative cycle ringing. Just a RCH of miller plateu.

But the modules were very hard to balence, I was using 4:1 on my input transformers, I had a pretty bad match, I was dumping in loads of power to get the desired drive and my transmission line was getting quite warm.

Anyway my TP pre-driver is working fine, just working on the Class-E SE driver and input side of things now.

-Bill
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Postby frank carcia » Wed Sep 08, 2004 8:05 am

Bill,
I just wanted to point out that the FETs in your final and mine see the same load Z. I have a 1:2 output transformer while you have two finals
in series per phase. The only difference is you have more leakage inductance due to multiple finals. Tom Vu arrived at similar tank values
and observed the same results. He is using your configuration. I would build a real power splitter if I was running multiple finals to force a drive balance. The configuration you guys use is simple but very hard to balance. I need about 50 to 60 watts of drive and I had to resistive load
the gates at 25 ohms phase to phase to make the driver happy. I also use a 4:1 transformer with a center tap to sources on the secondary. The inductor across the phases helps the drive a bit also. I could eliminate it
with lower resistive load. The q is low though so it covers the whole band
without tuning. Maybe you you could try some resistive loading on the gates to help the balance?? The twisted pair transmission lines to provide
drive are fairly high Z, I would guess around 100 ohms. You may need
a lower Z line or matching. ??
You should not be blowing FETs so there is still a problem somewhere.
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Postby ke1gf » Wed Sep 08, 2004 8:24 am

Thanks for the input frank.

The reason why I lost fets is, RF from the rig got back into the synth.
An RF current path up the coax from the pre-driver ground to the VFO box.

Except for that I haven't lost a pill in this rig when running full strap for about 7 months now.

Just trying to make an improvement.

-Bill
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Postby frank carcia » Wed Sep 08, 2004 8:52 am

OH I see. You might wrap the coax on some ferrite rod to eliminate current on the shield.
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Postby ke1gf » Wed Sep 08, 2004 10:44 am

frank carcia wrote:OH I see. You might wrap the coax on some ferrite rod to eliminate current on the shield.



Yeah, I'm using an impedence matching tranny to 1:4, I removed the connection to ground on the primary side from the Pre-Driver ground, just leaving the connection to the coax and stuck a ferrite bead on the coax at the pre-driver end to choke the currents on the outside of the line.

Checked it with a scope at 1kw... smooth as a baby's butt.

Big problem tho creating a spot note, I have to key the whole driver chain now to beat.

-Bill
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Postby frank carcia » Wed Sep 08, 2004 11:15 am

A BB transformer likes a resistive load so I tune at the gates. still sounds
like you have it under control. I have a mode on the racal that sets the BFO at zero offset so I just look at the digital read outs and match them.
JS way but it works.
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