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Super Pro Heater




 
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WU2D
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CW is just a narrower version of AM


« on: January 19, 2007, 08:56:23 AM »

I am working on a Super Pro (my first experience with this radio) and I found an interesting modification in the tuned circuit cage.

It is a 2K, 10 Watt wirewound resistor which has been lovingly affixed to a terminal strip in the center of the Local Oscillaor tuned circuit compartment. There is a lampcord going through a grommetted hole on the side which comes back to another terminal strip near the rear of the receiver which has a fuse mounted on it. A Hole and grommet directly on the rear of the chassis completes the installation. 

This appears to be a some kind of heater which keeps the LO section warm. If you attach 120 VAC to the cord, the resistor dissipates 7 Watts.

Is the Super Pro a drifty beast? Could this be a standard modification which was written up?

Mike WU2D


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n2bc
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« Reply #1 on: January 19, 2007, 09:19:42 AM »

Common mod of the vintage.  Many Hallicrafters had a heater.  Another common mod is to hang an additional filament transformer and keep the HFO hot all the time.  I don't think any of these help much, especially after being spoiled by rock stable 10hz readouts on modern rigs.

Have fun!   73, Bill  N2BC
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WU2D
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CW is just a narrower version of AM


« Reply #2 on: January 21, 2007, 11:07:37 PM »

Well Bill, I built up a power supply after giving it a complete going over starting with the mechanicals and then the electricals. The Super Pro has quite an audio system. I say system because it has a pair of 6F6's in push pull driven by a triode connected 6F6 and a 6J5. The rear panel has a phono input, so I assume that the radio could be used for entertainment and PA service. Needless to say, the audio is very good on AM.

Strangely there is no worrysome drift, at least on 75M.

Current drain is pretty substantial at just under 200 Watts! Is this thing a transmitter?

I used it to check into the OMRN (Old Military Radio Net) last night with a lash up on the bench. This is a CW net on 3570. It did a great job, easily outperforming my BC-348, which I normally use. And this is all before it has had an allignment. The variable selectivity control is very effective. Imagine if your R-390A had a control which allowed you to set the bandwidth from 3 kc to 16 kc with a smooth continuous action. Hammarlund designed this clever ganged IF can coupling affair which somehow controls critical coupling without loss. In other words, as the coupling gets looser and the selectivity increases, the signal stays relatively constant.

The 6BE6 product detector modification and Hang AGC that was put into the radio is FB on SSB but a bit too slow on CW.

Mike WU2D



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Steve - WB3HUZ
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« Reply #3 on: January 22, 2007, 02:12:57 PM »

I'ved used a SP0-200 and 400 over the last ten years or so. Little no drift till you get to the higher freq bands, like 15 or 10 meters. For AM use, it didn't matter but would probably be bothersome for CW.

I still think these are the best sounding receivers for AM. The audio section combined with the continuously variable bandwidth adjustable out to 16 kc, makes for some very nice audio. Enjoy.
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Todd, KA1KAQ
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« Reply #4 on: January 22, 2007, 02:59:15 PM »

Sweet rigs, for sure. Is that just crud on the chassis, or is the resistor cooking the coil?

Hallicrafters used this scheme in a few of their boxes, the SX-115 comes to mind and I think at least the later SX-101s, probably others. I guess it's handy if you're going to be using the rig daily, but not really needed otherwise (or at all). The earlier models (200s) do wander a bit, but mainly due to aging. Not the usual 5 minutes, let's say. But as Steve points out, drift isn't an issue until you get up on the higher bands, especially on AM. Of course, they tend to be a bit numb up there anyway. If you turn the receiver on a half hour or so before you want to operate, it should stabilize just fine.

They'll fill a room with nice audio and then some.
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WU2D
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CW is just a narrower version of AM


« Reply #5 on: January 22, 2007, 04:04:33 PM »

Sweet rigs, for sure. Is that just crud on the chassis, or is the resistor cooking the coil?

Hallicrafters used this scheme in a few of their boxes, the SX-115 comes to mind and I think at least the later SX-101s, probably others. I guess it's handy if you're going to be using the rig daily, but not really needed otherwise (or at all). The earlier models (200s) do wander a bit, but mainly due to aging. Not the usual 5 minutes, let's say. But as Steve points out, drift isn't an issue until you get up on the higher bands, especially on AM. Of course, they tend to be a bit numb up there anyway. If you turn the receiver on a half hour or so before you want to operate, it should stabilize just fine.

They'll fill a room with nice audio and then some.

That  "tan" is from cookin.  MM

I alligned the higher bands and you are right - the radio needs better front end tubes to do justice up there. Hence all of the articles about using hotter tubes and cascode circuits. I would hate to go miniature however. In fact I am thinking of ripping out the very good 6BE6 product detector and replacing the octal socket and wiring up a 6SA7 in the same circuit just so everything stays octal.

I have some ideas for pepping up the front end at the high frequencies with higher gain lower noise tubes. This will require adding some holes and grommets to bring the grid cap wires below chassis. Better bypassing practice pays off too, i'm told.

First RF Stage: from 6K7 (actually a 6S7 is in the socket which is even worse) to a 6SL7 Cascode or 6CA7 no AGC.

Second Stage: from 6K7 to 6SG7 Semi Remote Trans > 4000 with AGC.

Mixer from 6L7 to 6SA7 or 6SB7 (a hot tube with double the trans of the 6SA7 and three times the 6L7!) No AGC

Local Oscillator to 6SL7 using other half as a cathode follower.

These are not novel ideas. Several articles are out there and many folks were doing similar things over the years mostly with miniature tubes or even full solid stating like the QIX-Pros

Mike WU2D
 

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WU2D
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CW is just a narrower version of AM


« Reply #6 on: January 27, 2007, 05:25:44 PM »

Super Pro ON LINE - R390A banished  Shocked (for now).

Yes that is a key - I am doing the AWA OT CW test tonight. The SP will be the big ears.

Mike WU2D


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« Reply #7 on: January 28, 2007, 04:55:57 PM »

Outstanding choice.  One of the Finest Business OM receivers ever for AM use.
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« Reply #8 on: February 02, 2007, 10:28:02 AM »

I alligned the higher bands and you are right - the radio needs better front end tubes to do justice up there. Hence all of the articles about using hotter tubes and cascode circuits. I would hate to go miniature however. In fact I am thinking of ripping out the very good 6BE6 product detector and replacing the octal socket and wiring up a 6SA7 in the same circuit just so everything stays octal.

Mine has 3-4 miniature tubes retrofitted as well. They did a good job by utilizing the original holes for the octal sockets and attached the pheolic sockets directly in that way. They also fed the wires from grid caps down through other existing holes and voila - smaller tubes in old locations. 'JN suggested that I should leave it as-is since performance is probably better, so I have. But at some point when I have more time than I know what to do with, I might investigate doing the same thing. Like that is likely to happen!

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WU2D
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CW is just a narrower version of AM


« Reply #9 on: February 02, 2007, 08:45:03 PM »

Todd,

I am going slow from now on, but I will continue to make improvements. The AM, CW and SSB performance through 20 Meters is very good right now. Sensitivity is down on band 3 and the HFO is microphonic mechanically on that band. And as I said, band 4 is out completely, so I have some troubleshooting to do anyway. I want to try fixing that and then work on a better HFO.

I am thinking of changing out the triode connected 6J7 hartley oscillator and replacing it with a 6SN7 dual triode. I have an octal shield.

I would run half of the tube as a hartley as before and make the other half a cathode follower. This should eliminate pulling and help some on bands 3 and 4. This idea goes way back as well, with the 12AX7 as the tube of choice.

OK I did it - I changed the HFO from a triode connected 6J7 to a 6SN7 with the cathode folllower. It works!  Grin And I got band 4 back! Here is the schematic as I built it. This idea came from an old article in December 1957 CQ Magazine, called Souping the Super Pro by Leonard E. Geisler. The circuit should also work with a 6SL7, 12AX7 or a lower mu tube like I used like a 12AU7.

I alligned everything and all is well. I will check stability tomorrow.

Mike WU2D


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« Reply #10 on: February 04, 2007, 10:52:53 AM »

I obtained a SP-210 about six months ago and also have been playing with it. A previous owner had modified the first RF stage using a 717A which was a "hot" tube at the time and had no AGC on this stage. I found that on stronger signals the radio overloaded and I had to ride the gain control so the signal was not distorted. Finally put back the original 6K7 RF stage.

Another mod I found had a 3.3pf/N750 temperature capacitor added across the oscillator tunning cap. This has helped minimize drift on the higher bands. The heater idea that you found in your radio has been used commerically as mentioned in another post and does help. Another mod I saw added a VR tube for the oscillator circuit, but most of the drift I notice is thermal; after a few hours the radio is pretty solid.

Had a lot of caps that needed replacement and some broken parts in the IF transformers that need repair. Great receiver for SWLing with the HI-FI audio.

73  Harvey
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Harvey
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CW is just a narrower version of AM


« Reply #11 on: February 04, 2007, 06:45:55 PM »

Harvey,

This SP-400-SX is completely happy with strong AM, CW and SSB signals, at least the way it has been modified.

Here is what has been done to this one:

1. Initial clean up of the wiring harness and cap replacement on all screens and the bias.
2. My new HFO using the 6SN7 
3. 150VDC regulator on HFO and BFO - previous work
4. 6BE6 product Detector - previous work
5. Hang AGC mod using 6H6 or Schottkys - both work the same. - previous work
6. Slight audio mods.  Shielded cables in the low level audio paths including the product detector. The driver is degenerated with a 180 Ohm 2 Watt resistor (unbypassed) from the cathode to ground. Even so the audio pot is at 7:00 with all of the gain - I think it needs an audio taper pot or a bigger speaker!

All bands now work but band 4 (20 -40 MHz) is quite deaf with only strong broadcast and some cw on 15 Meters heard.

Stability is good without the heater.

I did have a funny problem on band 3, even after the HFO conversion. The CW and SSB signals were hum modulated. This turned out to be the grid coupling capacitor to the hartley which was located in the coil deck. This was an evil looking sandwich thing made of metal and mica.     
After removing the offender and replacing it with a 4700 pf NPO disk ceramic, it cleaned right up. Then I lost band 1. Sure enough - no output. Same deal - same cap, but in the band 1 coil tank circuit. The extra RF output must have finally killed this mica postage stamp which read 511 pF. I replaced this and Band 1 came back and stayed back. 

The message is that all of the caps in the coil deck are suspect on these old girls.

Mike WU2D
 
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« Reply #12 on: February 05, 2007, 12:07:54 PM »

Just an historical footnote, my SX-101A has a heater resistor in addition to the always on HFO filament.  A company had the rights to the name "damp chaser" for these resistors and Halli used this term generically in the manual.  Included with the manual is an errata sheet which requires the owner to cross out the words "damp chaser" in several places in the manual and replace them with the words heating resistor or some similar verbage.  The previous owner of my SX-101A followed these instructions to the letter.

I have several of the SP-210 family in the shack and I really like the old Super Pros.  The first one I got was the low frequency version which didn't cover AM BCB so I have since added a couple that included this range.  Of course, our "decent" local stations went to a new religious format so the AM hi fi audio is now mostly useful on the SW ranges.  I tried listening to the local AM BCB stations but I started getting this uncontrollable urge to sell all my vintage gear on ebay and give the proceeds to brother somebody so I quickly tuned to 3885 instead and now all I have the urge to do is buy more vintage gear...

Rodger WQ9E
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WU2D
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CW is just a narrower version of AM


« Reply #13 on: February 08, 2007, 08:55:35 PM »

More Super Pro...

Well next comes the Mixer.  Tongue The existing front end is two 6S7 RF amplifiers and the 6L7 mixer. These are fine old valves but there is just no sensitivity much above 14 MHz. I had an initial idea of rewiring for a 6AC7/1852 1st RF without AGC a 6SG7 second RF amp with AGC and a 6SB7 mixer without AGC. 

But I have heard that simply pepping up the front end of these receivers with higher transconductance (lower noise resistance) tubes is dangerous because the mixer overloads quite easily. Broadcast band stations are the typical culprits.

Running the first RF amplifier without AGC can also tend to overload the mixer.

The existing mixer is a 6L7 which is a pentagrid tube, but it is used with the external HFO (now a dual triode 6SN7).

What about a triode mixer? The R390 uses them and they are nothing more than 6C4's. No tube quieter than a triode rght? So for my first experiment, I am simply going to try a single ended triode like a metal 6C5. Remember- I'm staying Octal!

If that does not work, I will try something a little fancier called a Pullen Triode Mixer. This is a cool circuit where one tride is operated linearly and one (the oscillator injection tube) is operated deliberately at lower gain so that it is non-linear. Both tubes are coupled at the cathodes so the isolation and freedom from pulling is excellent. The tube I have my eye on is the 6SC7 dual Triode (common cathode like the 6J6). This is a metal tube and it is used as a phase inverter in audio amplifiers, but the grid plate and in out capacitances are low..

The example diagram for the Pullen Mixer is for the 75A-4 from the glowbugs site http://www.mines.uidaho.edu/~glowbugs/receivers.htm


Mike WU2D


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« Reply #14 on: February 09, 2007, 07:31:34 AM »

The best mixer ever is a 7360 sheet beam balanced modulator.  A close second is a dual triode mixer, and this arrangement has been used many times to address the serious shortcomings in the 75A4 front end.

I strongly recommend that you do NOT use sharp cutoff tubes such as the 6AC7 in the front end, nor should you ever consider putting tubes in the front end that aren't controlled by the AGC.  To do so will mess up the AGC and create all kinds of intermodulation and image issues.

My experience is that, while not the MOST sensitive receiver out there, it has MUCH more gain than necessary for work below 15 MHz, and messing with the mixer could create tracking problems as the front end has pretty good selectivity.  It would be an interesting exercise to do the triode mixer mod if your goal is an intellectually challenging project - but if you're looking for improved performance on the low bands it's prolly a waste of time.
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CW is just a narrower version of AM


« Reply #15 on: February 09, 2007, 10:16:11 PM »

The 7360 is a cool tube and everybody from Swan to Collins used them because of their superior carrier supression in balanced modulators and for single balanced mixers! That Squires Sanders used it as a mixer? Neat. Word is that they are rare and cost upwards of 50 bucks each. Don't blame the Swan collectors. Hi

For the gallery, what John is talking about with this Sharp, Remote and semi-Remote pentode stuff is how the tubes characteristics vary based on the tube's internal design. The remote cutoff tubes are known as variable mu tubes - that means that it is very easy to control thier gain with AGC and the control/gain characteristic is very linear. In other words they have a special purpose in communications receivers. The down side is that because they have this characteristic they are generally more noisy and have lower gain compared to sharp cutoff pentodes.   

So why Sharp cutoffs? Not all stages are under the control of AGC. Consider a detector or perhaps the last IF stage before the detector or in an FM receiver like with a limiter; or even a microphone preamplifier.  That is where you just may see a sharp cutoff pentode like a 6AU6 or a 6SJ7. They also make good VFO's and AGC AMPLIFIERS, that is the stage that drives the AGC detector to make the AGC voltage.

Back to the Stuper Pro..

The first and second RF amplifier in my Super pro is a 6S7 tube with a mu of 1750 and a noise resistance of > 10K Ohms. This was actually a soup up from the original 6K7's with a mu's of 1100 and a noise resistance above 12K! This is the father of the 6SK7 by the way.

The 6AC7 is a beast of a different color. It has ungodly gain and is a sharp cutoff tube so it can not be controlled by AGC (easily).  I think it must have been a radar IF tube or something - following a WW2 passive microwave mixer. It has a very high transconductance of 9000!!and a low noise resistance of 716 Ohms.
As John warns - this monster could bring doom. Undecided

And yes I will probably mess something up along the way trying to bring the receiver alive above 15 MHz! That is the trick - getting good performance o 10M without messing up 75M performance.

Mike WU2D






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« Reply #16 on: February 10, 2007, 07:26:56 AM »

Ya don't hafta use a 7360.  There are at least 3 other sheet beam tubes that were used as color TV syncrhrounous demodulators.  Teh 6AR8 is one, not sure of the others.  But as junk TV tubes they're cheap as dirt.

I built a RX largely using the Squires Sanders design.  A 7360 mixer --> Racal crapstal filters --> 3 stages of IF at 1.6 MHz.  It was extraordinarily quiet and had unbelievable dynamic range. 

The only gotchya with these toobs is to use 'em to their fullest you need push-pull LO inputs and IF output.  You can modify the input IF coil by removing the resonating cap and splitting that cap in two, connecting the two caps in series across the IF primary.  Then ground the junction of the caps.  You hafta do some gymnastics with the B+ for the two plates with resistors and a pot to balance thngs out though.  And I doubt you have room for all the extraneous junk needed for a sheet beam mixer in the SP-400.

The 6AC7 was used by the planeload as video IF amps in various IFF and radar units during WWII.
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CW is just a narrower version of AM


« Reply #17 on: February 11, 2007, 11:51:35 AM »

Preliminary Mixer Results

Well I went nuts and stripped the mixer socket and wired in the Pullen circuit using an old 6SC7 dual triode. It was not too bad because there is plenty of room when you pull out the shaft of the noise limiter switch. The whole deal comes out with two screws. This gives you room to work.

I can report that conversion gain is slightly down (perhaps 1 S-unit) compared to the 6J7, but the signal handling is excellent. No signal on the air has touched it - I may need to have WA1GFZ come on the air-Hi.

The new circuit unloads the last preselector tuned circuit and all band trimmers had to be increased a bit for proper tracking. Q has no doubt increased and so has front end selectivity. Best of all, Band 4 is now alive - not that anyone has heard a signal between 20 and 40 MHz of late because of the sunspot cycle!

I did a little test at 27 MHz. I connected the AC RMS meter across the center of the volume pot to ground and adjusted the noise to a 0 dB reference. I then connected a longwire antenna. The noise came up 12 dB.

The better tubes to use with this circuit are the 6ES8, 6BZ7, 6BQ7, 12AT7.

Mike WU2D



* SuperMIX.jpg (34.55 KB, 424x258 - viewed 1583 times.)
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Steve - WB3HUZ
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« Reply #18 on: February 11, 2007, 01:22:25 PM »

Quote
My experience is that, while not the MOST sensitive receiver out there, it has MUCH more gain than necessary for work below 15 MHz


This is true of most older receivers. Just turn down your RF gain once and see how much you can still hear, and lots of noise goes away. A NF of 15-20 is FB for most of the HF range.
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