TZ-20 Amp Output Circuit

This morning I was showing off photos of my TZ-20 amplifier when one of my friends commented that it certainly was wireless, just look at the one rotatory one switch with no wires attached to it. This reminded me that I had one last update to post on this project.

After WWII almost every transmitter project (except the simplest starter sets) had a pi network output stage. This was not as common in the 30s. Most transmitters then had link coupled output followed be some sort of antenna matching network. Many times the matching "network" was a variable capacitor either in series or parallel with the output link. A series capacitor worked best for low impedance loads while a parallel capacitor was used to feed into high impedance loads. Today's norm is a 50 ohm antenna antenna/coax system requiring a series capacitor.

SW3, a rotatory switch, controls course loading. C5 is the fine loading adjustment. With SW3 I can switch out the series capacitor all together or add in additional capacitance to give me a total range from about 20pF to 900pF. Tune up becomes very much like a pi network "dip and load". With SW3 and C5 set for minimum capacitance the plate current is dipped using C4. After dipping, the loading capacitance is increased using C5, C6 and C7 into the range of operating plate current and then the current redipped using C4. C4, C5, C6 and C7 interact so this process must be repeated until the last C4 adjustment puts the plate current where it needs to be for full output...in my case about 150 mA. 500 VDC on the TZ-20 plates at 150 mA gives me 75 watts input.

AWA 2012 Linc Cundall OT CW Contest

This afternoon I wrapped up my participation in this year's Linc Cundall OT CW Contest. Obviously I'm not a major contester. During the 48 hour contest I netted nine contacts. Not many contacts/points but I did enjoy putting my latest homebrew transmitter on the air.

This year I used my new "Taylor 75", a 1934 style oscillator-buffer deck driving two Taylor TZ-20s in push-pull. The receiver was a National NC-101X. Everyone that commented indicated that the transmitter sounded fine. Keying the buffer stage helped. With the crystal oscillator running continuously (on transmit) there was no sign of chirp.

I went ahead and created a QSL card to send out.

On the Air

Today I got on both 80 and 40 using my new 75 watt transmitter. This afternoon I had a nice QSO on 40 with KC9RXI, Tom, in West Salem, WI and this evening on 80 with NS7E, Art, in New Prague, MN. Both Art and Tom reported no problem with my signal.

Tremendous!

It Works!

After a couple of days scratching my head and asking around trying to figure out why my TZ-20 amplifier wasn't working up to expectations I finally did what I should have done first. I checked the wiring.

One TZ-20 wasn't connected to the final tank coil. I was running with only half an amplifier and a poorly designed one at that. It worked but not well. Once I fixed the wiring error power levels jumped to the 50-60 watt range. Neat, but now I needed to deal with typical amplifier problems.

TPTG feedback -The first problem I had was that the amplifier took off on it's own (without any drive) when the input/grid circuit was peaked for maximum output. This was solved by removing C3, the TZ-20 grid tuning capacitor, from the circuit. C3 in the amplifier and C8 in the final tank circuit of my 1934 transmitter interacted anyway so this eliminated one pesky control. Problem solved but now the drive level was way down...one solution led to another problem.

Drive level - With no tuned circuit on the grids of my TZ-20s they were not getting the drive needed to get anywhere close to full output. This one was solved with a new coil for L1/L2. More turns for L2 got the power transfer up to the point that now I'm easily driving the amplifier to 75 watts input.

Bias - This all started with me wondering if I has the bias set right. The advantage of the TZ-20s is that they do not need an external bias supply to protect them when the drive goes away. With no drive (and no bias) plate current drops to almost zero. For the TZ-20 a simple resistor from the grid to ground is all that's needed to produce needed bias....but what value? It turns out that the 1939 Taylor tube manual has a formula for calculating recommended Class C CW grid bias for a given plate voltage, in my case 500V. Bias came out approximately 20 volts. I'm driving the grids to about 30mA ... 20V/30mA = 666 ohms. I had a 750 ohm resistor, close enough.

One question that had to be dealt with was how best to couple the output to the antenna. It turns out that, for my 40 turn tank coil, a four turn link gives me maximum output into my 50 ohm dummy load. I am set up, though, so that I can add a loading capacitor in series with the link to cover real world antenna conditions.

Now I have an amplifier for my 1934 transmitter that takes about 6 watts of drive to run 75 watts input (45-50 watts output) on 80. With a couple of new coils I should be able to put it on 40.

Look for me during the AWA Linc Cundall OT CW contest next week.

TZ-20 Operating Specs?

I've gotten my TZ-20 RF amp up and running with my three tube 1934 transmitter but its not working like I expected.

I'm running with 500V on the push-pull plates with the recommended 2K bias resistor. Key down I'm seeing grid current (total for the two tubes) of about 40mA and total plate current of 40-50mA. Looking at the TZ-20 specs and tubes curves from the 1939 Taylor Tube manual I expected to see 50-75 watts input with 35-50 watts output, instead I'm seeing 20-25 watts input and only 8-10 watts output. I do have three TZ-20s. Any two of them behave the same. What an I doing wrong...or is it working as designed?

Elecraft KX3 On Order

It's winter here and I'm already thinking about spring!

After 10 years I'm retiring my faithful K1 for a brand new Elecraft KX3. See http://www.elecraft.com/KX3/kx3.htm for the details.

I'm excited. The KX3 It covers all amateur bands, 160-6 meters, operates phone, data and CW, and is DSP-based. It will allow me to do digital/PSK31 from the picnic table without an attached computer and the the receiver looks like it should be great.

The first thing on my list was the ATU. With it I can easily load into my favorite end fed half wave antenna. I ordered the KX3 assembled to make certain everything was aligned right and that it worked straight out of the box. The internal charger will make it easier to do really spur of the moment operating. Finally, why the mic? The rig supports CW, phone and data. This will allow me to try a little QRP phone operating, especially if demoing the rig.

I decided to wait on the roofing filter. I'll see how it works as is. I'm not a big contester or pileup operator so I tend to move away from the QRM but sometimes it comes to you. I can always spend the $130 for the filter later.


---------------------Order Summary-------------------------

Item Subtotal: $1269.80
Shipping Cost: $15.68
GRAND TOTAL: $1285.48
NOTE:Customer declared exemption from 93.20 sales tax.

Shipping Method: UPS Ground [55901]
Shipping Weight: 5.50



-----------------Individual Item Breakdown-----------------

Item Ref. Price ea. Qty. Description

KX3-F $999.95 1 KX3-F 160-6 M Transceiver (Assm)
KXAT3 $149.95 1 KXAT3 ATU for the KX3
KXBC3 $59.95 1 Int. NiMH Charger/Clk for KX3
MH3 $59.95 1 MH3 Hand Mic. for KX3

------------------------End of Order-----------------------

Unfortunately the estimated ship date is almost two months away. I'll have to wait.

AWA Bruce Kelley Memorial CW QSO Party

The last two weekends I operated in the AWA Bruce Kelley 1929 QSO Party. This is always a great time to get an early rig on the air and work others doing the same.

On the first weekend I operated as K5CF from Poteau, OK using Vance's type TNT transmitter running about 6 watts using a single type 10 tube. While I heard W0LS, KB0RON, N8YE and KB0MM fairly well I could never get a response to my calls. Power out looked OK so Vance's Gap Titan DX vertical on 80 may have been the problem.

This past weekend I was back in Rochester, MN using my Hartley transmitter to make ten contacts. This event allows any receiver to be used. This year I used my 1941 simple superhet. I have only a fairly low end fed wire antenna and my contacts reflected this. Seven were right here in MN, two were in OH and one "DX" contact was in SC.

What might I change? The antenna is always an issue. I have a 29' base loaded vertical this winter but the recent snow seems to have detuned it. I'll need to check it out before the LC CW Contest in January.

More TZ-20 Amp

I've finished the wiring. Now I need to cobble together a power supply and I'll be ready to test it.

My deadline is about two months away, the January AWA Linc Cundall CW Contest. Two years ago I did ok with my 1934 transmitter running barefoot, this time I'd like to add 75 watt "boots".

A Strain Relief for an End Fed Wire

Once a friend asked about how to anchor an end fed antenna ( such as an EFHW ) at the rig end. A lightweight QRP rig attached to a 67' long wire slung over a high tree limb won't stay put without help. I solved the problem with a "split bolt connector" and some nylon twine.

I found split bolt connectors in the electrical department of my local big box hardware store. The nut, with it's captured cross-piece, screws onto the slotted bolt.

I cut off a couple of feet of nylon twine and looped each end. One loop goes into the split bolt.

Looping the twine around a convenient object (in this case a porch railing) and securing it gives a solid anchor for an end fed wire.

On the Air with my Hartley

The AWA 1929/Bruce Kelley CW Party is coming up the first two weekends in December and the weather has started to cool off here in Minnesota. I've put the K1 back into it's winter operating position (a top the 75A4) and started to warm up the classics.

Right now I'm running my Hartley paired with my 1940 Simple Superhet. They make a good pair but my signal reports remind me of one of the Hartley's main limitations. On the 18th I worked Lynn, NG9D. Lynn commented that my signal had a fast QSY to it. Last night I worked Dave, WA9DOF. Dave reported that he wouldn't have known the transmitter was a vintage rig if I had not told him. The difference was the wind. Last night we had no wind here while on the 18th the wind was 10-15 mph. The antenna blowing in the wind shifts the load slightly that it presents to the transmitter. This in turn pulls the frequency slightly. Lightly loading the rig when tuning up helps (see my Nov 16, 2009 Blog entry) but wind can still be a factor. Pulling my wire antenna tighter or a completely new antenna might be needed. I'll see what I can do.