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 Post subject: Looking for suggestions on my Halli S-38A
PostPosted: May Mon 08, 2017 9:24 pm 
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I have a couple of S-38s that have complete chassis but with roughed up cabinets, and I so decided to mess with one of them by turning it into an AC operated receiver. Yeah, yeah, I have too much time on my hands. And too many parts that are just begging to be used. Anyway, the conversion is nearly complete--it now has a transformer power supply that supplies 6.3vac for the filaments and about 140vdc on the B+ line. The choke is mounted on the underside of the chassis. Tube line-up is now 6SA7, 6SK7, 6SQ7 and 6K6.

So the question is, what's the best use of the empty socket where the 35Z5 used to be? After all, nature abhors a vacuum (and so do I unless it’s inside a tube). I could insert a BFO and noise limiter using a 6SQ7 much like the one incorporated into the original S-38 which was a 6 tube receiver. However, I could insert a tiny solid state BFO and that would still leave the now-empty 35Z5 socket, but for what? RF preselector? Q-multiplier? VR-75 for the LO plate voltage? Just looking for ideas, suggestions, crazy notions, etc.

Tom

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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Tue 09, 2017 2:54 am 
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The empty socket is a Locktal.
You'll have quite a time fitting any Octal tube into it.

My suggestion would be a VR tube to feed the oscillator (need to replace the socket).
I'm sure that will improve the receiver's stability.

- Leigh

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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Tue 09, 2017 5:44 am 
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Thanks Leigh, but no, it is an octal socket. The empty socket you're looking at once held a 35Z5. I don't believe the S-38 series, at least through the S-38B, ever used loktals. The standard tube line-up for the A and the B model was 35Z5, 50L6, 12SQ7, 12SK7 and 12SA7.

Tom


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Tue 09, 2017 6:09 am 
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The octal socket could be easily re-wired to accommodate the VR tube.

Circuit modifications should not be onerous.

- Leigh

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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Tue 09, 2017 3:56 pm 
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My vote is for a RF amp or Q multiplier. A zener will do the same thing as a VR tube in this small space. Just my opinion. The VR glow will be cool if the cabinet is left off.

I did something similar with my S-38A; adding a SS power supply and 12 volt heater circuit. I had a bunch of 12A6s. So used them for a push pull output. Subbed in a 12SN7 for the audio amp and phase splitter. Then used germanium diodes for the detector. Did the push pull because there was an empty tube socket sitting there in the right place. No real improvement in performance though.

The Q multiplier or RF amp might actually improve the radio. Regulating the B+, however your decide, will also be an improvement. The SS power supply is definitely an improvement.


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Thu 11, 2017 5:08 pm 
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tjuga, I like the way you think! I'm inclined to go the solid state route for voltage regulation and the BFO. I like the idea of a tube type Q-multiplier, but need to find an appropriate circuit (and components!).

Tom


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Thu 11, 2017 6:14 pm 
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My vote is this.

The 6K6 tube will not give much power output at the B+ you have.

Unfortunately to keep the lower B+ while producing more output you either need a higher B+ and a resistor to drop the voltage to the radio circuits or you need a different power tube.

A good choice is the 12L6, but that requires changing the other tubes to 12 volt ones and the transformer will need a 12.6Vac winding.

The extra socket could be used for another 12L6 with the first audio tube replaced by a 12SL7 and the detector diode replaced by a 1N5711 diode.

I do have a schematic of a S-38 B showing the mods for push pull using a pair of 25L6 tubes. I can email you it after changing it to 12L6 tubes if interested.

I figured out that for 200Vdc B+ on a 12L6 25L6 or 50L6 plate the secondary needs to be 140Vac. That will produce maximum audio power.

I did this to a stock S-41W and the improvement in audio quality was very noticeable with an external 12" speaker in a proper open back box where it sounded like a console radio.


Last edited by Tube Radio on May Thu 11, 2017 9:35 pm, edited 1 time in total.

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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Thu 11, 2017 7:54 pm 
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Location: Isanti MN
When I get back to my S-38 project, I plan to look into the Heathkit Q multipliers. I haven't done that as yet but understand that they use power from the radio that it is coupled with. I would build the circuit into the S-38 chassis. Think there is some more info on this ARF site or just google it; a bunch of stuff shows up.

Also, to keep the zener regulators quieter, bypass them with an electrolytic, 5mFd or so and a .01mFd ceramic. Seems ridiculous but some zeners make some RF hiss. You can try without first and add bypass caps if hiss is noticed, try the capacitors. I used two 1N5374, 75Volt in series for the 150 volt B+. Not sure, but one 75 volt diode might be good for the oscillator. They are 5W, so, of course, two in series would be good for 10. Just looked, they are $0.46 each.

Share if you do this, would like to learn from your experience. TJ.


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Fri 12, 2017 1:08 am 
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Great thread! Thanks for posting.

Just wondering why you didn't just put a small isolation transfomer in, with one side connected to where the original power cord went, and the other side connected to AC power. Would that work?


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Fri 12, 2017 1:25 am 
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That would, but there are benefits to using 6 volt tubes. One being all heaters are at 6.3Vac which means the heater cathode voltage is low which will reduce the chance for hum pickup.

The other is it allows higher power audio output tubes to be used.


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Fri 12, 2017 6:25 pm 
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gribbled, it basically does have an isolation transformer--it's no longer an AC/DC radio. Since there's plenty of room on the chassis, I installed a small power transformer-type power supply. The transformer's B+ winding is rated 135vac @ 50 ma. With two 'lytics and a choke, B+ was 140 volts. Because the S-38 was designed for about 100 volts on the plates, I added a 5 watt dropping resistor and now B+ is about 115vdc. But that's measuring it with a DMM. If I had an old VOM with 1000 ohms per volt sensitivity like in the good ole days, it would probably read closer to the stated ratings.

tube radio, I know the 6K6 doesn't provide much power output, but its characteristics seemed closer to a 50L6 than the other choices among 6 volt octal audio output tubes. And I don't need much power anyway--it's just a small table radio. :mrgreen:

Tom


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Fri 12, 2017 6:45 pm 
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A 6W6 is basically a 50L6 with a 6-volt heater. The 6W6 might be useful if the 6K6 doesn't produce enough audio.


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Fri 12, 2017 8:10 pm 
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Ed Engelken wrote:
A 6W6 is basically a 50L6 with a 6-volt heater. The 6W6 might be useful if the 6K6 doesn't produce enough audio.

Agreed the 6K6 is going to be mighty puny on 115v B+, especially using the 50L6 output xformer... In addition to 6W6, a 6DG6 or 6Y6 are very similar to 50L6...

EDIT

At 100v B+ the 6K6 produces 1/3 watt, step up to 135v and output jumps to 3/4 watt(I have one in this configuration, performs fairly well)... BUT that's using the correct output transformer, a 50L6 xformer has 3x load vs correct one for 6K6...

http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6k6g


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Sun 14, 2017 3:53 pm 
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Puny output power from the 6K6 is the least of my worries at the moment. My first round with the new 6 volt tubes is a major disappointment. After an alignment and attaching both an antenna and ground, sensitivity is pathetic. This is on the AM band where I can hear a half dozen stations at most when I should be able to hear at least a dozen with, say, a run-of-the-mill AA5 of the same era. Only two in the vicinity of 1500 KHz come in with any volume at all. Not quite sure what to do next.

Question: what's up with R10, the 10 meg resistor from the grid of the 12SQ7 to ground? It's shown on the schematic for an S-38 and for an S-38B. I started out with an S-38A (the one I'm hacking) and an S-38B, and neither had such a resistor. Both were totally stock except for 'lytics that had been replaced long ago. I added R10 to the A model and it makes no difference that I can detect.

Tom


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Sun 14, 2017 10:35 pm 
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Did you make any wiring changes other than filament wiring in the RF and IF stages?


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Mon 15, 2017 12:00 am 
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No changes, Bob. Apart from giving it a "proper" power supply, I just replaced components, locating them right where the old ones were. I was careful to preserve all the wiring in the RF section and bandswitching just as it came from the factory.

Tom


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Mon 15, 2017 1:58 am 
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R10 (for the S38, S38B) = R8 in the S38A. 10M contact bias resistor for the 12SQ7 triode.

Rob


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Tue 16, 2017 2:39 am 
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I guess I'd try the usual troubleshooting methods. Take all of the usual voltage readings to make sure they are correct.

I wonder if you may have somehow lost a ground connection somewhere during the conversion. The old series string would have had DC continuity to B+/B- somewhere. Maybe the change has upset the AVC line? I'd have a look at that.


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Tue 16, 2017 9:30 pm 
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Thanks Bob. Voltage readings are a bit high w/ respect to those of a stock S-38A, but not by much:

6K6 -- plate 125v, screen 102v, cathode 4v
6SQ7 -- plate 71v, grid -0.6v, cathode 0
6SK7 -- plate 102v, screen 102v grid -0.2v
6SA7 -- plate 102v, screen 102v, grid #1 -11.5v

AVC line -- junction of R5 (2.2M), R7 (47K) and R8 (470K): -0.6v no station, -0.75v on the strongest station.

Is the -11.5v on grid #1 of the 6SA7 too far negative? The AC/DC S-38A is supposed to read -8v on grid#1.

Tom


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 Post subject: Re: Looking for suggestions on my Halli S-38A
PostPosted: May Wed 17, 2017 1:39 am 
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Not sure what the bias should be on the converter tube, but I would think that your other voltages should be okay. A little bit higher than specified is to be expected, and is probably better than being too low.

Have you checked to make sure the converter tube is actually oscillating? If it's not, you might still receive powerful local stations.


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