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Author Topic: Wire sizes for homebrew amp  (Read 936 times)
N8CHR
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Posts: 50




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« on: November 07, 2017, 09:23:18 AM »

I'm building an amp.It's called the "Junker Amp:.It's using a single 813 tube. Most of you are probably saying, why build this primitive amp, it's what I want and I have what I need to do it. Lot of mechanical work but that pretty much done.
My question is on the wire gauges to use. Suppose I could calculate it but not sure I would get it right. Plate voltage is around 2400 volts. What size, "gauge" wire would I need for this and the wiring for the loading and tuning caps connections?
Eighty meters use two hefty caps at around 5000v. I am not going to include 80 meters, never run it. Also I'm going to interface this to a Heathkit DX 60. Just that.
Thanks  Tom N8CHR
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N3QE
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Posts: 4878




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« Reply #1 on: November 07, 2017, 09:47:02 AM »

I'm building an amp.It's called the "Junker Amp:.It's using a single 813 tube. Most of you are probably saying, why build this primitive amp, it's what I want and I have what I need to do it. Lot of mechanical work but that pretty much done.
My question is on the wire gauges to use. Suppose I could calculate it but not sure I would get it right. Plate voltage is around 2400 volts. What size, "gauge" wire would I need for this and the wiring for the loading and tuning caps connections?
Eighty meters use two hefty caps at around 5000v. I am not going to include 80 meters, never run it. Also I'm going to interface this to a Heathkit DX 60. Just that.
Thanks  Tom N8CHR

Wire gauge is not awfully important for the DC wiring to the plate. The DC currents are going to be under an amp.

Insulation on exposed DC HV wiring, or any DC HV wiring running near other wires or the chassis, is a much more important factor. Hams have long used the inner conductor and dielectric of RG-8 type cables for running plate voltages. I note that RG213 specs often are rated for 5kV. Even niftier if you happen to have it or need a skinnier wire, are the teflon dielectric coaxes.

The wiring to loading and tuning caps are part of the tank circuit, which will have high standing RF currents, and there you have to think not just of DC current rating of the conductor, but also the conductor's inducatance and skin resistance at RF frequencies. The table below, which appeared in the ARRL handbook for many decades, lists coil wire sizes which are just as applicable to any connections made in the tank. Please note that you should not think about just randomly running hookup wires all around the tank, the physical layout of the tank is super important and you want to avoid any "hookup" wire at all because of stray inductance and capacitance associated with it.



Below is a nice picture of a multi-band single 813 amplifier by a ZL. He might have gone a little overkill on conductor diameters but it's a nice job in any event. Note that all the tank interconnects to/from the bandswitch are made with tubing or strap, and the pieces are all cleverly arranged to minimize stray capacitance and inductance.

« Last Edit: November 07, 2017, 09:55:12 AM by N3QE » Logged
W1QJ
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« Reply #2 on: November 07, 2017, 09:49:21 AM »

The two wire gauges you are asking about are miles apart.  The HV wire you need for the plate supply voltage only needs to carry less than an amp of current.  You are more likely to be gripped by the HV you will be able to find.  Suffice to say anything around 24 gauge would be plenty and that is about what you will find in HV wire. With wire size for grounding doorknob caps.  You would want to hard ground one side to the chassis and on the other side you can use something like RG-58 braid.  You are better off with a wider flat type of wire than round.  You should have some RG-58 braid around.  Bigger is better to a point.
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G3RZP
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« Reply #3 on: November 07, 2017, 01:11:06 PM »

 
Quote
Most of you are probably saying, why build this primitive amp, it's what I want and I have what I need to do it.

If you have the parts, then why not? It could be argued that 'it's green, it's recycling'. If you had (heaven forbid!) to buy parts, there might be better choices. Go with it, and GL, OM!
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KM1H
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« Reply #4 on: November 07, 2017, 01:52:36 PM »

That low power ZL amp is serious overkill; it looks like a 2500-3000W Alpha, Acom, etc for CCS duty.

For tank circuit wire use #10-12 for 10-15, #14 for 17-40, and #16 Teflon insulated for 80-160 on separate toroids plus wrap them with Mylar or other HV tape.

For bandswitch taps keep them as short as possible especially for the high bands and #14 is fine and fits most switch contacts.

Keep coils at least one diameter and at right angles to each other from anything metal. Toroids can be mounted on aluminum with just a piece of 1/8" or so of fiberglass sheet and nylon or fiberglass bolts/screws. Plus make excellent solder and mechanical contact with all RF wiring.

Carl
Building amps for over 50 years.
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N2EY
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« Reply #5 on: November 07, 2017, 02:07:23 PM »

I'm building an amp.It's called the "Junker Amp:.It's using a single 813 tube.

Do you mean the one by W1ICP, in QST for October 1970?

If so....it's a pretty standard design. I don't know why plate voltage metering was included, though. Since you aren't going to use it on 80/75, some simplification is possible.

The one thing that's a bit odd by modern standards is the lack of a tuned input circuit. The input Z will be all over the place, so "modern" rigs may have issues driving it. A DX-60 will probably be fine, and will have enough RF to easily drive it.

Note that if you intend to run it AM linear, it can't be pushed nearly so hard as on SSB. On CW, it will do 500 watts input OK.

73 es GL de Jim, N2EY

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KM1H
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« Reply #6 on: November 07, 2017, 05:50:14 PM »

Quote
Note that if you intend to run it AM linear, it can't be pushed nearly so hard as on SSB. On CW, it will do 500 watts input OK.

Since the DX-60 is controlled carrier AM and the carbon plate 813 is way underrated in plate dissipation it should still be good for about a 100W carrier peak or 400W PEP OUTPUT. But with only 2400V it will be a bit less.
The DX-60 will also need to be reduced in output to 15W or less unless some of the drive is consumed in a resistive grid load.
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W9WQA
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Posts: 278




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« Reply #7 on: November 11, 2017, 01:56:34 AM »

i run 2 813's grounded grid with 3300v from a sb1000 supply. i use the tuner in my 450d or icom 7300 to tune into the amp. in g g the match seems easy and both/either radio seems to like it.
its home brew over many years of "refinements" but i dont yet get good efficiency above 40 meters. i do well on 160 to 40. using braid as mentioned.
i added a 3ag 2 a fuse and on rare occasions it goes like a firecracker!
i like the 813's, have 6 spares. the amp has 3 sockets but it wasnt too good with 3 tubes!
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W9IQ
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Posts: 1707




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« Reply #8 on: November 11, 2017, 03:20:44 AM »

Tom,

Is your lack of proper capitalization and punctuation due to a disability, apathy, ESL or a poor education? I see this from a few other posters and I am curious as to the reason for the anomaly. No offense intended.

- Glenn W9IQ
« Last Edit: November 11, 2017, 03:23:15 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KM1H
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Posts: 2465




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« Reply #9 on: November 11, 2017, 09:15:07 AM »

Quote
Is your lack of proper capitalization and punctuation due to a disability, apathy, ESL or a poor education? I see this from a few other posters and I am curious as to the reason for the anomaly. No offense intended.

- Glenn W9IQ

Do you have to ruin another thread with your insulting arrogance?

Do you do all those things when texting or on CW?

Ive had another troll nag me for lack of punctuation which I consider a waste of time when the intent is very obvious. At least he knows enough to shut up these days.
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W9IQ
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Posts: 1707




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« Reply #10 on: November 11, 2017, 04:12:07 PM »

Karl,

Happy Veteran's Day! Thank for your service.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9WQA
Member

Posts: 278




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« Reply #11 on: November 11, 2017, 10:46:52 PM »

Quote
Is your lack of proper capitalization and punctuation due to a disability, apathy, ESL or a poor education? I see this from a few other posters and I am curious as to the reason for the anomaly. No offense intended.

- Glenn W9IQ
all of the above. please dont read my stuff if you are troubled,tom

Do you have to ruin another thread with your insulting arrogance?

Do you do all those things when texting or on CW?

Ive had another troll nag me for lack of punctuation which I consider a waste of time when the intent is very obvious. At least he knows enough to shut up these days.


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