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 Post subject: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Sun 04, 2018 11:01 pm 
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Does anyone know where I can find the tube voltages for Hallicrafters S-11? None of the documentation i.e. Hallicrafters and John Rider has this information.

This ebay unit came with 6V6 instead of the 6L6. The interstage transformer is not original. The output tube runs very hot using either 6V6 or 6L6. I've restored the unit i.e. re-capped, component replacement, restringing the dial, etc. Seems to play OK but the output tubes are too hot to touch.


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Sun 04, 2018 11:13 pm 
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It is normal for the audio output tubes along with the rectifier to run too hot to touch so that is not a sign that anything is wrong. Check that the cathode bias resistor is correct. You can get the standard operating data (element voltages, cathode resistor) from the tube data sheets and running 6V6 tubes in place of the 6L6 will likely require a different cathode bias resistor to put them at the proper operating point.

Often excessively hot output tubes are a result of leaky control grid coupling resistors or a leaky cathode bypass cap but the SX-11 uses a transformer instead of a tube type phase inverter so there are no coupling caps to leak nor does it use a cathode bypass resistor.

I suspect that your SX-11 is operating as intended. (note that a S-11 becomes a SX-11 by adding the optional 455 Khz. crystal, if your receiver doesn't have one it is a plug-in upgrade).

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 05, 2018 2:55 am 
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Location: San Jose, Ca.
I agree with all that Rodger said.

Another important thing to check is the temp of the output xformer. It should not be hot. If it is, that's a bad sign for its longevity. The typical idle current for a 6L6 is 40 to 90 ma and 30 to 74 ma for a 6V6. The cathode bias res in the S11 is 250 ohms, so make sure it is not less than that. This value leads me to believe that it was designed for class B op, so the bias should be around 25 to 30 volts on the cathodes. Probably a little less with the 6V6's. This will vary depending on the plate voltage.

The interstage xformer should not affect the operation current of the outputs.

Some info needed would be the cathode, plate and screen voltages.

Regards, Larry


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 05, 2018 7:43 pm 
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The voltages for the two 6L6's are:
Plate: 302 and 304 VDC
Screen: 286 and 286 VDC
Control: both 0VDC
Cathode: 12.3 and 13.5 VDC

Each tube has its own cathode resistor at anout 130 ohms each.

I am running into a peculiar problem when power this thing up, I hear noise and a faint station or sometimes no station. If I tap on the output IF can or the 2nd IF can, the radio starts working. I need to resolve this one first. This one is a pain to trace. I hope its a bad ground.


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 05, 2018 8:12 pm 
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Whoa! Stop quickly before you cook something. That output stage has been monkeyed with and needs to be put back with a single cathode resistor of the proper value. From the data you have provided those tubes have about 100 mils of cathode current flowing through them which is far excessive and will cook them along with the output and power transformers in a hurry.

Whoever made the changes to the output stage values hasn't a clue as to what he was doing. The value for the single cathode bias resistor as originally used is fine for a pair of 6L6 tubes but you will need to increase it a bit if you leave the 6V6 tubes in there which are being run at pretty much their maximum voltage rating according to the screen and plate voltage readings you posted.

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 05, 2018 9:03 pm 
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Calculated the cathode current and you're right, it's over a 100ma. Need to find a couple of 250 ohm resistor a 500 ohm with tap, several watts. I'm noting the revision Hallicrafters made in the lower left corner of the schematic.

Well, the cathode resistors are replaced the current has come down to about 75 and 60 ma. The former is still very hot but the latter is cooler using 6L6's. I now have an intermittent problem to resolve. When poweree on, there's a little noise but no station us heard. I have to tap the output IF can or bang on the oscillator tube to get it to play. Sometimes the problem would go away and the radio behaves after each power up. This is a difficult radio to signal trace with the three tube sockets by the tuning capacitor made inaccessible by the tuning coils.

I'm taking a break from this for now and see if I can work on another radio.


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Sat 17, 2018 11:09 pm 
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I finally determined why the one 6L6 is running so hot. Theirs a huge resistance difference in the audio output transformer. The hot tube side is 142 ohms vs 482 ohms. I need to report that back to the boss and see if he has a spare transformer. This could explain why there's distortion in the audio.


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 19, 2018 2:35 am 
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Sounds like that audio output xformer was overheated at one time and some windings have shorted together as a result. That is probably a big source of distortion. On sets that don't have a means of balancing the output's current (like this one), it's important to also use tubes that are fairly equally matched to further minimize distortion.

'Play Things Of Past' ( http://www.oldradioparts.com ) has a lot of used audio xformers. You might find one there. Although P/P 6L6 output transformers are common and should be easy to find.

Regards, Larry


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 19, 2018 2:46 am 
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Make sure to address the cathode bias resistor situation or the replacement transformer will also get cooked. Go back to a single cathode resistor for both tubes as intended and check the voltage drop across it to determine cathode current and adjust the value of the resistor accordingly. You can do this with the old transformer still in place so that you don't take a chance on damaging the replacement.

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 19, 2018 9:04 pm 
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rsingl wrote:
Make sure to address the cathode bias resistor situation or the replacement transformer will also get cooked. Go back to a single cathode resistor for both tubes as intended and check the voltage drop across it to determine cathode current and adjust the value of the resistor accordingly. You can do this with the old transformer still in place so that you don't take a chance on damaging the replacement.

Rodger WQ9E


I've addressed it by replacing the two 130 ohm cathode resistors with a pair of 227 ohm. The original single cathode resistor that's still riveted to the chassis was incorrectlly a 130 ohm.

I'm not sure if there's any benefit to.going back to a.single cathode resistor. Hallicrafters did make a revision to change this due to some problem. There is one thing I need to check and that's the interstage transformer. It is not original, held on the chassis with one screw, and is suspect.

Wednesday when I get over this bug, I need to search through the transformer pile.


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Mon 19, 2018 11:06 pm 
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I assume you still have the output cathodes tied together? They should be for this push/pull parallel output stage. With that set up properly the only reason for using two cathode resistors would be if the single one didn't have enough dissipation rating. But if you use two resistors each resistor has to be twice the value.

With the two resistors in parallel you have about 113 ohms of cathode bias resistor which is quite a bit less than typical for a pair of 6L6 tubes at that voltage and it will cook a pair of 6V6 tubes with their lower plate dissipation rating. For a pair of 6V6 tubes operating at the screen and plate voltage level of the S-11 I would expect a cathode resistor of around 270-290 ohms would be about right to provide the desired 19 to 21 volts of cathode bias to obtain the design center idle current of 70 mils (for both tubes combined).

130 was the correct cathode bias resistor value for a pair of 6L6 tubes running class A1 and that stage would have been a current hog idling at around 120 mils.

Are you using 6V6 or 6L6 tubes in it now?

I am not following what revision Hallicrafters made to the circuit. Later receivers moved from the current wasting class A push/pull parallel to far more efficient circuits in Class AB1 or AB2. The same pair of 6L6 tubes that could produce about 15 watts in push/pull class A were good for over 30 watts in P/P AB1 and up to 60 watts in AB2 while the smaller 6V6 tube running in class AB1 P/P could produce the same output as a pair of 6L6 tubes running class A.


Rodger WQ9E


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Tue 20, 2018 3:21 am 
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The 6L6's replaced the 6V6's. The cathode resistors are not in parallel. Each are connected to its own 6L6 pin 8. The only thing they share is a common ground.


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Tue 20, 2018 11:53 am 
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Hallicrafters was obviously experimenting with output stage design during the S-11 production period. The original build used a single 250 ohm cathode resistor (this is how my SX-11 was built) which moves the output stage from pure class A push/pull into the AB push pull region. With the revised values of 250 ohm separate cathode bias resistors the stage is put purely into Class A push pull and the current demand goes up greatly. In Class A the tubes conduct over the full cycle and the plate current remains the same from idle through maximum grid voltage excitation resulting in a lot of heat generation because the tube is always running at full plate current often with no output so the plates are dissipating the entire input power level and the power supply is always at full load. Class A stages have the somewhat odd characteristic of running less hot when they are producing rated output at a high duty cycle.

I don't believe Hallicrafters ever used this setup again because it is very tough on the power transformer, rectifier, choke, output tubes, and output transformer. The very slight difference in distortion between Class A and Class AB!, or AB2 is of no significance in a communications receiver and later Hallicrafters sets did not use push pull class A setups. For a communications receiver the biggest advantage of class A push/pull over other classes of output stage operation is the B+ output voltage won't vary with audio level so regulation of the voltage for the various oscillators is less important. This may be why Hallicrafters tried class A operation in the S-11 receiver. The revised S-11 is the only Hallicrafters communications receiver I am aware of that used a class A push/pull output stage and later models along with the S-12 commercial frequency version of the S-11 all used higher cathode bias moving the tubes out of class A.

Given the fragile nature of older power transformers I would go back to the original single 250 ohm cathode resistor for the S-11 putting the tubes in a less power hungry class AB mode. It will sound virtually the same with much less heat generation.

The only reason for separate cathode resistors in a push/pull stage is as part of a scheme to reduce the significance of slightly mismatched tubes and typically those designs also feature fixed grid bias in addition to cathode bias with a means of individually adjusting the grid bias. When the revision was approved for the S-11 it appears one of the ancestors of the current audiophile tribe had taken control in Hallicrafters engineering division :) Probably it was an attempt to provide better voltage stability but later designs moved away from this probably because the additional heat buildup (with resulting frequency drift) and stress on the power supply negated the benefits of class A operation.

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Fri 23, 2018 1:55 am 
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Rodger, I've read through your posts carefully and have further researched the amplifier classes and their implementation in a push pull circuit. Prior to this I was not familiar with the tweaking that can be done to a push pull circuit to get an amplifier to run in Class A, AB, or B. With that said, Class A operation would be too stressfull and wasteful for this old radio. It had a hard life just based on components that needed to be replaced. I will connect both cathodes to a single resistor. I will see how much current is going through this resistor. I may want to hook up the scope to see how much of the waveform is conducted thru each tube.

Thanks Rodger!


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Fri 23, 2018 2:03 am 
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Lar H wrote:
Sounds like that audio output xformer was overheated at one time and some windings have shorted together as a result. That is probably a big source of distortion. On sets that don't have a means of balancing the output's current (like this one), it's important to also use tubes that are fairly equally matched to further minimize distortion.

'Play Things Of Past' ( http://www.oldradioparts.com ) has a lot of used audio xformers. You might find one there. Although P/P 6L6 output transformers are common and should be easy to find.

Regards, Larry


Yes, that's the only explanation. The interstage transformer is not original. The failure could have started there and then cascaded. Anyway, this is one of these radios that are just full of surprises everytime I thought I turned a corner.


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 Post subject: Re: Hallicrafters S-11 Tube Voltages??
PostPosted: Mar Fri 23, 2018 4:00 pm 
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whirligig wrote:
Rodger, I've read through your posts carefully and have further researched the amplifier classes and their implementation in a push pull circuit. Prior to this I was not familiar with the tweaking that can be done to a push pull circuit to get an amplifier to run in Class A, AB, or B. With that said, Class A operation would be too stressfull and wasteful for this old radio. It had a hard life just based on components that needed to be replaced. I will connect both cathodes to a single resistor. I will see how much current is going through this resistor. I may want to hook up the scope to see how much of the waveform is conducted thru each tube.

Thanks Rodger!


You are welcome and the old tube data sheets will give you some good starting parameters to get it out of pure class A into AB1. The tubes and transformers will be much happier.

Rodger WQ9E


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