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Author Topic: Building 4-1K amp - parasitics and other problems  (Read 8269 times)
KD7YQM
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Posts: 71




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« on: June 29, 2008, 06:47:23 AM »

Hello Trying to build a HF grounded grid amp using the 4-1000. Everything seems to be going well on 15-40.

Having trouble with 10m. Trying to get the best point for the tap. Seems to be wanting more inductance. So I give it more and the thing goes wild. When I apply drive grid current goes through the roof and parasitic resistors fry. Needless to say I have to pull the plug quick. I'm thinking 90mhz parasitic?

Second problem is at 75m. I seem to be getting good efficiency and it tunes correctly. It's the load capacitance that seems strange. Always wants more. I add inductance and re adjust the loading and it wants to load at max capacitance?

Could post some pics but not sure how to do that.
Well just wanted to hear some thoughts on this.
thanks
Dennis
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WA1RNE
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« Reply #1 on: June 29, 2008, 08:31:59 AM »


 Here's a couple of suggestions:


 For the problem on 10 where you suspect a parasitic oscillation, try disconnecting the drive source, apply filament then plate power.

 While watching for a spike in plate and grid current - and your hand on the plate power switch to quickly shut down - sweep the plate tuning capacitor through its range.

 If you notice a spike in plate and grid current, it is very likely a parasitic is being generated. Quickly switch off the plate supply to minimize any damage.

 Using proper safety procedures, disconnect the plate supply and discharge its filter capacitors.

 What you need to do is increase the inductance of the parasitic choke, by either squeezing its turns together or adding turns, then repeat the above test procedure. If the amplifier has calmed down, try loading the amplifier with drive a little at a time to see how it reacts.


 As for 75 meters, please share some of the amplifier design specifics, i.e. available load capacitance, tank inductance, design plate load resistance, plate voltage, etc.


 ....WA1RNE
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KD7YQM
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Posts: 71




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« Reply #2 on: June 29, 2008, 10:01:06 AM »

Ok just did what you suggested. Filament on plate supply on. Switch in 10m position. Ran tuning cap through it's full range of 10-320pf. No glitch anywhere. This only seems to happen under drive. Plate idling current is 50ma. with the 7volt zener.

The parasitic choke is 3 turns of #12 copper with 3 100ohm 2 watt metal oxide resistors inside the coil.

Here's a description of the output coils. 5 turns of 1/2 inch strap 1 1/2 inch diameter. 4 turns of 1/4 inch tubing 3 inch diameter. 21 turns of #10 wire 3 inch diameter.

The loaded plate voltage is 4k. So that's about 3450 ohms load impedance. The tuning cap is a jennings 10-320 vacuum variable. The loading cap is a 4 section broadcast variable. About 400pf per section. I switch in the 4th section for 75.

The tube socket is eimac don't remember the number. Screen pins have 1/2 inch straps grounded directly to chassis. The grid has a 1 ohm resistor bypassed with .01 for current measurement.

Typical filament choke and transformer with center tap going 20k resistor shorted by keying relay to zener.

That's all I can think of now.
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WB2WIK
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« Reply #3 on: June 29, 2008, 01:48:31 PM »

You did not describe your plate RF choke or method of decoupling, and this is very critical especially in a multi-band amplifier.  For my HB 4-1000 amp I had to experiment with the plate choke quite a bit and the fourth or fifth one was the charm and worked perfectly, making the amp very stable.  In addition to the "big" choke, I had to add a "small" choke in series, and 5-6 mix 43 ferrite beads over one lead of that choke (for more effective VHF decoupling).  My bypass system was .005uF, 7500V ceramic doorknob at the bottom of the main RF choke, then .01uF and .001uF in parallel on the PS side of the smaller (VHF) choke.

I also found stability improved when I added a solid, wide copper flashing ground strap between the RF deck chassis and the front panel to which the plate TUNE and LOAD caps were mounted.  Relying on bolting sheet metal parts together was not sufficient.

Using a vacuum variable as a TUNE cap is a real pain for a multiband HF amp, and it's impossible to "sweep the range" of the cap in any reasonable amount of time while testing.  Too many turns!  I resorted to wide spaced air variable with switched in padders for 80 and 160 meters and eliminated the vacuum cap pretty quickly -- just too much work to change bands.

Without knowing how much inductance you are actually using I can't calculate the Q of the tank, but if it's too high that can lead to instabilities also.

Adding more LOAD capacitance is not "increasing" loading, it's decreasing loading.  Max loading is when the load capacitance is zero.

WB2WIK/6
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WA1RNE
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« Reply #4 on: June 29, 2008, 03:06:08 PM »

The parasitic choke is 3 turns of #12 copper with 3 100ohm 2 watt metal oxide resistors inside the coil.


 >>> Parasitic chokes for HF amplifiers should be made from a low Q conductor at VHF. Copper is considered a high Q conductor at VHF, only behind silver which is about the best.

 Nichrome or stainless steel wire or ribbon has a much lower Q and could help to reduce the ability significant improvement.

 
Given all of that, you should try to confirm the self resonant frequencies in the plate circuit. An easy way to do this is with a grid dip meter, obviously with the amplifier completely de-energized. Lowering the Q of the parasitic suppressor, and for that matter, any associated wiring that could create a resonant circuit at VHF is one of the best ways to calm things down.


 ...WA1RNE
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WA1RNE
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« Reply #5 on: June 29, 2008, 03:10:24 PM »


 Sorry, the second sentence should read:


 "Nichrome or stainless steel wire or ribbon has a much lower Q and could significantly improve stability."


 ..WA1RNE
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WB2WIK
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« Reply #6 on: June 29, 2008, 05:18:52 PM »

4-1000 has a lot of gain up to about 110 MHz.

Been there, done that, it took several tries to get it right!
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KD7YQM
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« Reply #7 on: June 29, 2008, 06:02:50 PM »

Ok, I have not experienced any instability on any other of the bands, just 10m.

The plate choke is a B&W #800. It is bypassed at the base by a .005 10kv capacitor.

The DC blocking caps are 2 .005 10kv held together by 1/2 inch copper strap.

The vacuum variable has about 10 turns from max to min.

I wish this forum would let you edit and show you info from previous posts while replying.

thanks
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KD7YQM
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« Reply #8 on: June 29, 2008, 06:12:52 PM »

See what I mean? Couldn't remember all the questions so I have to make a second post.

The tune and load cap are mounted to the chassis base along with the tube and it's grounds.

Boy that .01 10kv cap must be pretty big.

Well I can't remember any more questions.
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KD7YQM
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« Reply #9 on: June 29, 2008, 06:21:38 PM »

Oh yea, where is a good place to couple the dipper?

Should it be next to the plate choke? Or somewhere else?

There are a lot of spurious dips in here!
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KD7YQM
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« Reply #10 on: June 29, 2008, 06:31:03 PM »

Steve please post the winding info on the plate choke that worked for you.
thanks
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KA5N
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« Reply #11 on: June 29, 2008, 07:23:26 PM »

Just hold down the shift key when you click the cursor for reply and the reply box will appear over the forum page.  You can move the box around and also go to any thread entries.
Allen
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KD7YQM
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« Reply #12 on: June 29, 2008, 08:11:27 PM »

Thanks Allen for that suggestion.

ok I got it to oscillate without drive. Wasn't quite as violent as with drive but it's still there.
I got it to do it on several band positions.
When I adjusted the tune cap it didn't have too much effect. Just kept oscillating.
I guess it's on the verge of oscillation whenever the plate current goes above the 50ma idle.
I had to re-attach the drive cable. I didn't apply drive but just attached the cable.
Must be something resonating in my input circuit with the cable connected.
At this point I guess it could be any of the possibilities you have mentioned or combinations.
The input circuit is not tuned. Just a coax to a .01 mica.
I found some large #43 ferrite beads. Let me know where to put them.
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W8JI
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« Reply #13 on: June 30, 2008, 03:36:43 AM »

Dennis,

I've build dozens of 4-1000A amplifiers. The problem you experinece is a COMMON problem. As a matter of fact the better the layout the more problems you will have.


The problem is NOT VHF GAIN. That is a myth or folklore. The problem is the size of the internal tube elements and the lead lengths of the grids to the chassis.

What happens is at some frequency the long thin grid leads to the chassis have enough inductance that they resonant as a parallel tuned circuit with the screen and control grid capacitance to form a parallel tuned circuit.

When the anode path from the anode to the tuning capacitor and through that capacitor to the chassis is near or below the grid resonance, you build a TPTG (tuned plate tuned grid) oscillator.

The 4-1000A has a grid resonance in the 80-100 MHz range. You are just getting too close to that on ten meters.

You need to get the shortest possible grid to chassis connection. What socket are you using? The metal air-hose sockets are the worse. The plastic sockets are best if wired correctly.

What suppressor? You need a suppressor that shifts into resistance quickly with increasing frequency. Otherwise you will waste a lot of power in the suppressor.

It is very easy to tame a 4-1000A and have good ten meter efficiency if you understand the problem. I've never had to compromise HF performance to do that.

So what suppressor are you using? How are the grids grounded? What socket?

Those areas are the problem areas. The cathode doesn't mean a thing.

73 Tom




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W8JI
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« Reply #14 on: June 30, 2008, 03:39:55 AM »

You also really should have a tuned input that is a low impedance at the cathode Dennis. It has to be a low-pass pi network or something else shunting the cathode (filament) to chassis with a low impedance at the harmonics of upper ham bands RIGHT at the cathode.

If you don't do that you will probably have efficiency and IMD problems.

73 Tom
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