Pages: 1 2 3 4 5 [6] 7 8 9 10 11 ... 14   Go Down

Author Topic: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build  (Read 201130 times)

K8AXW

  • Posts: 7391
    • HomeURL
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #75 on: July 25, 2014, 09:21:29 PM »

Martin:  Once again, absolutely beautiful!!  Obviously to anyone who looks at your photos, the work of a perfectionist. 

BTW, the light bulb was my "dummy load" for many years!!  They worked fine for thousands of hams back in the day.

If your light bulb socket is ceramic or porcelain, that is a piece of history as well.
Logged
A Pessimist is Never Disappointed!

G3RZP

  • Member
  • Posts: 2254
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #76 on: July 26, 2014, 12:53:11 AM »

Back in the 1960s even, we had 'Robinson lamps' as 600 ohm loads for professional tx testing. They were a big - 12 inch long, 3 inch diameter festoon lamp with a zigzag filament down the middle. I believe they were 600 ohms at nominal 250 watts. Tune for maximum glow!
Logged

G3EDM

  • Member
  • Posts: 1456
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #77 on: July 26, 2014, 04:27:26 AM »

Elmer alert!

I spent a couple of hours triple-checking all of my wiring visually, and with continuity and resistance tests, against the schematic.

So the moment came and I powered up the transmitter. Nothing. Even the neon power bulb didn't light.

A quick check determined that the 0.5-amp, fast-blow fuse on the Live side of the power circuit had blown. To be more precise, it may have blown right now -- or there is a tiny chance that it was a defective fuse. I had checked that part of the circuit visually, but didn't test the fuse resistance.

The simplest explanation is that there is a wiring error and that the fuse did its job. I just spent about an hour checking everything around the power circuit, including checking the DC resistance of all the transformer primaries and secondaries, and they check out with what the Hammond spec sheet says. I checked the rectifiers: very high resistance in one direction, low resistance in the other direction.

Another explanation: my fuse is wrongly rated. At 117 VAC that would be about 60 watts to blow the fuse. Is it possible that my transformer (which is somewhat beefier than the original ARRL transformer) is drawing 60 watts? Seems rather unlikely to me.

Edited to add: A reminder that this transformer has Universal primaries. There are two 120-volt primaries, which are wired in parallel (and in phase) for operation in North America. That is the key difference compared to the physically smaller transformer used in the original ARRL design.

Finally perhaps I should use a slow-blow fuse. I tend to think that fuses blow for a reason, and that this is not the moment to mess around with fuse values!

I don't have any spare fuses, so I'm off to the hardware store to buy more.

73 de Martin, KB1WSY
« Last Edit: July 26, 2014, 04:35:29 AM by KB1WSY »
Logged

G3EDM

  • Member
  • Posts: 1456
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #78 on: July 26, 2014, 04:43:33 AM »

You are going to laugh because surely my math has gone screwy.

The DC resistance of those paralleled primaries is 9 ohms. (As per the Hammond spec sheet, and verified on my VTVM.)

My house AC measures right around 120 volts (actually, slightly higher).

E = IR therefore I = E/R

So the current through the primary is 120/9 or about 13 amps!

What is wrong with this scenario? Is it something to do with how AC behaves as opposed to DC?

73 de Martin, KB1WSY


Logged

G3EDM

  • Member
  • Posts: 1456
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #79 on: July 26, 2014, 06:10:10 AM »

Unbelievable.

I bought some new 0.5A fuses at the hardware store, inserted a new fuse and: the transmitter powers up fine!

The original fuse must have been defective. What kind of quality control is that?

Was it Sherlock Holmes that said after you have eliminated all the possible reasons, the remaining one, however unlikely, must be true!

Now, time to do some real testing.

73 de Martin, KB1WSY
Logged

N2EY

  • Member
  • Posts: 5698
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #80 on: July 26, 2014, 06:17:04 AM »

The DC resistance of an AC transformer does not indicate what the transformer will draw when connected to AC. What matters is the impedance at the frequency of the AC. That impedance is much higher than 8 ohms, and is determined by the inductance of the windings as well as the load on the secondary, the losses in the core, and the DC resistance.

Ironically (bad pun intended), the primary of an ideal transformer would measure 0 ohms DC resistance and infinite AC impedance, when the secondaries are not connected to anything.

Now to the blown fuse.

If you feel lucky, and have lots of fuses, try using a higher-rated fuse (say 1 to 2 amps). Slow blow if you have it. Here's why:

The HV power supply in that tx uses silicon rectifiers and a capacitor filter. When you first turn on the power, there is an enormous surge as the supply tries to charge the filter capacitors. This surge dies down once they are charged - but it may be enough to blow a half-amp fast-blow fuse.

If that's not your style, or you don't have or don't want to risk fuses, or a bigger fuse still blows, here's the step-by-step:

1) Instead of replacing the fuse, wire an incandescent bulb across the fuse holder. For a transmitter of that size, anything from 40 to 100 watts should be OK. The idea is that if there's something drastically wrong, the lamp will light rather than blowing another fuse. Once the preliminary checks are done, the lamp is removed and the fuse replaced.

2) IIRC, that transformer has two secondaries: the HV secondary and the heater secondary.

The first test I would do to unsolder one of the HV secondary leads and one of the heater secondary leads, insulate them, power up and see what happens. If the series lamp lights dimly, or not at all, test for voltage at the heater secondary. If you get some voltage (may not be exactly 6.3 volts), you know the transformer is good and the primaries are phased correctly.

The correct phasing for 120 volt service is that the two "starts" are connected together, as are the two "finishes". Otherwise the two primaries oppose each others' inductance and the transformer draws enormous current.

3) If the above works out, power down, reconnect the heater secondary, and power up. The lamp should light dimly - if it lights brightly, there's a short in the heater wiring. Do this test first with the tubes removed, then, if it passes, replace them and see if they light.

4) If all the above works, reconnect the HV secondary and power up again. The lamp should brighten at first, then go dim.

73 de Jim, N2EY
Logged

N2EY

  • Member
  • Posts: 5698
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #81 on: July 26, 2014, 06:18:59 AM »

Sorry I was slow on the keyboard! Glad it is powering up!

You may still need a slightly-larger fuse if the replacements blow.

73 de Jim, N2EY
Logged

G3EDM

  • Member
  • Posts: 1456
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #82 on: July 26, 2014, 07:29:58 AM »

Wow. Everything works.

It loads up (I used a 7100 KHz crystal) and tunes perfectly. Light-bulb dummy load was glowing brightly at the end of the tune process.

When I tune my receiver to the same frequency and key the TX, I hear a pure tone. No clicks but some very slight chirp -- I am assuming that adjusting trimmer capacitor C1 (see schematic at bottom of this post) could reduce the chirp. The chirp is minor, in fact better than some CW that I hear on the air -- and I suppose it might just be overloading my regenerative receiver, so the chirp could be "fake."

Sidetone monitor also works perfectly. It was too loud but I adjusted the under-chassis pot to a more pleasant volume.

Just one major issue that I've identified. I measured the various B+ voltages (with one hand in my pocket!) and they are all considerably higher than in the ARRL schematic. These are the key-down voltages I measured:

--At the output of the power supply, 290 volts (schematic shows 240).

--At the top of R2, 250 volts (schematic shows 200).

--At the top of R5, 260 volts (schematic shows 195).

All of the components in the power supply are rated exactly as per the ARRL instructions. From my neophyte standpoint, I'm guessing that C13 has something to do with it. I deliberately bought an expensive and large Sprague Atom capacitor to ensure the best possible filtering. But it has the same specs as the schematic: 100uF, 450 volts.

Does this over-voltage "matter"? If so, I presume I should put a dropping resistor at the output to the power supply.

I will post a link to Chapter Twelve, with photos of my testing, shortly.



(Reproduced with the permission of the ARRL.)

73 de Martin, KB1WSY
« Last Edit: July 26, 2014, 07:46:19 AM by KB1WSY »
Logged

G3EDM

  • Member
  • Posts: 1456
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #83 on: July 26, 2014, 08:20:45 AM »

Here's Chapter Twelve, "Checking and Testing the Transmitter":

http://tinyurl.com/psnwn25

73 de Martin, KB1WSY
Logged

N2EY

  • Member
  • Posts: 5698
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #84 on: July 26, 2014, 08:26:08 AM »

The voltages are not critical. Line voltage in the USA has crept up over the years, and now is typically around 125 volts in most residences.

You might consider dropping the screen voltage on the 5763 by increasing the value of the series resistor. (R5). This can be done by replacing it or by putting another resistor in series. The oscillator dropping resistor can be similarly increased (R2).

However, there's a better way.

Using a series dropping resistor is simple, but the resulting voltage drop varies enormously with load. A voltage divider gives better regulation. This is not too important for the 5763 screen resistor, but it can help the 6C4 oscillator. The technique can be used for both.

What you need to do is to connect a resistor of appropriate value from the low-voltage end of the series dropping resistor to ground. For the oscillator, this is the end of R2 that is labeled "+200 volts". The value of R2 and the new resistor will need to be calculated. If this is done, the three 15K 1W resistors across the B+ can be removed.

You do NOT want to use a series resistor for the whole power supply!

The chirp is probably in your receiver.

Antenna time!

73 es congrats de Jim, N2EY
« Last Edit: July 26, 2014, 08:41:03 AM by N2EY »
Logged

G3RZP

  • Member
  • Posts: 2254
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #85 on: July 26, 2014, 08:32:48 AM »

The probability is that the 100mFd cap hadn't had any volts on it for a long time, and drew a fairly large surge current the first time you powered the tx. As it reformed, it would improve.

As Jim says, the chirp maybe in the receiver, especially if the signal is strong. Regenerative  receivers are well known for 'pulling' with strong signals.
Logged

G3EDM

  • Member
  • Posts: 1456
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #86 on: July 26, 2014, 08:47:40 AM »

The voltages are not critical. Line voltage in the USA has crept up over the years, and now is typically around 125 volts in most residences.

I just looked up the specs in the "RCA Receiving Tube Manual" that I scored at the ARRL/Hartford flea market.

For the 5763: Maximum rated Screen voltage for amplifier, Class C Telegraphy is 250V. Voltage that I measured: 260V, which is probably within the fudge zone on my VTVM anyway. Also, very reassuringly, that's not just the "maximum" rating; the "typical operation up to 30MHz" is also given a screen voltage of 250V.

For 6C4: Maximum rated Plate voltage for RF oscillator, Class C Telegraphy, is 300 volts, and so is the "typical" suggested voltage. Voltage that I measured: 250 volts. So we are well within the specs of the tube.

I would have thought we can leave the 5763 dropper resistor alone. In fact, with the higher voltage, I must be getting greater power input and my peanut-sized transmitter needs all the help it can get.

As for the 6C4, it's clearly safe to run it at 250 volts. The only question would be whether the oscillator would be more stable at a lower voltage. My gut feeling is to just leave it as-is and only modify the dropping resistor if there are problems with the oscillator in future.

I have no reason to be "gentle" with these tubes as they are very easily available and still at low prices.

Edited to add: BTW, with the telegraph key *open* the voltages are very high (360 at the power supply, for instance). I assume this is irrelevant because these are unloaded voltages.

The probability is that the 100mFd cap hadn't had any volts on it for a long time, and drew a fairly large surge current the first time you powered the tx. As it reformed, it would improve.

Peter, I like that explanation very much. The capacitor was bought about 18 months ago, and who knows how long it had been sitting in the warehouse at Mouser. After power down, it is now providing a good display of sparks when shorted with the chicken stick!

73 de Martin, KB1WSY
« Last Edit: July 26, 2014, 08:58:38 AM by KB1WSY »
Logged

G3EDM

  • Member
  • Posts: 1456
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #87 on: July 26, 2014, 09:07:26 AM »

Quick newbie question.

When rotating the Tune control, I got not just one dip (in the middle of the dial) but also a second one, close to the point of minimum capacitance.

Presumably that's a harmonic, which would be in the 20m band?

Peter note: I am indeed building a halfwave filter for 40m, either in one of those paint cans or in a smaller coffee can.

73 de Martin, KB1WSY
Logged

N2EY

  • Member
  • Posts: 5698
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #88 on: July 26, 2014, 10:36:26 AM »

Some more comments:

1) If you are using a VTVM to measure transmitter voltages, it may be reading high because of RF. (This is why I use a VOM for transmitter testing.)

2) I would lower the screen voltage of the 5763 slightly, to get it within ratings key-down. The difference in output power will be nil.

3) Incandescent lamp dummy loads can be a source of chirp in simple transmitters. This is because the impedance of a lamp varies enormously from cold to hot. That varying load when you press the key may affect things enough to cause chirp on the dummy load which is not there on an antenna or resistor dummy load.

4) The second dip is a harmonic. You can prove this by listening to the harmonic on your receiver as you tune.

5) The pi network probably has enough harmonic rejection as-is.

73 de Jim, N2EY
Logged

G3RZP

  • Member
  • Posts: 2254
RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #89 on: July 26, 2014, 10:57:53 AM »

It's not a good idea to short the electrolytic with the chicken stick - the current can damage it. You would be better off with a 470 ohm 5 watt resistor in series.

Jim, you might get 43dB of second harmonic rejection in the pi network alone, but doing the sums, I  doubt it, especially if it's feeding an antenna rather than an antenna tuner.

Tung-Sol list the 5763 as 350 volts max on the plate for ICAS and 300 volts for CCS, with 15mA of screen current at 250 volts and 5mA grid current through a 25kohm grid leak. So you're well within those ratings, and anyway, you're not running Continuous Commercial Service,

VTVMs and DVMs are, with RF around, capable of lying like an HR Manager!
Logged
Pages: 1 2 3 4 5 [6] 7 8 9 10 11 ... 14   Go Up