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 Post subject: Question: headphones and protecting the OPT transformer
PostPosted: Sep Tue 21, 2021 1:51 am 
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Two-stage audio section of a radio, fairly low power, with, say, a 6K6. A jack for headphones is to be inserted after the first audio tube in such a way that plugging in the 'phones cuts out the AF signal to the grid of the output tube. How necessary is it to supply a low-ohms resistor across the secondary of the AF output transformer to protect it? The OPT is of unknown origin other than it likely came from a vintage consumer radio and so I don’t know what its characteristics really are. I plan to use it with an 8-ohm spkr. Given that limited information, what approximate value of resistor across the secondary would be best? Can such a resistor be selected that could be in place full time yet cause no appreciable reduction in the usable volume from the speaker? Or must I find a ‘phone jack that would switch in the protective resistor across the transformer secondary when the AF signal is shunted from the output tube’s grid to the ‘phones?

Thanks!

Tom


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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Tue 21, 2021 2:20 am 
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If you are switching the grid of the output tube and leaving the speaker connected, no resistor is necessary.

The real question is can the first audio tube supply enough power to drive the headphones? It depends on the tube type and the circuit. You will have to try it and see.

What advantage is there to connecting the headphones at this location rather than at the output as is usually done? If you are trying to save power by turning off the output tube (disconnecting B+ and heater power) then it might be a good plan. If you are leaving the output tube running, I can't see a reason for it.

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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Tue 21, 2021 2:22 am 
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Be careful with the bias on the output tube...

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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Tue 21, 2021 11:57 am 
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Should there be a need to use a headphone jack to connect headphones and disconnect the speaker, a neon bulb across the transformer primary would avoid arcing between the tube socket pins if, perchance, both speaker and phones are out of circuit.

Arcing could also damage the primary winding.

Nandu.

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Last edited by vu2nan on Sep Tue 21, 2021 12:21 pm, edited 6 times in total.

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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Tue 21, 2021 12:03 pm 
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allthumbs wrote:
Two-stage audio section of a radio, fairly low power, with, say, a 6K6. A jack for headphones is to be inserted after the first audio tube in such a way that plugging in the 'phones cuts out the AF signal to the grid of the output tube.

Are you planning to put the headphones before or after the coupling capacitor?

FWIW, I'd do it conventionally and put the headphone jack in the voice coil circuit.


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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Tue 21, 2021 7:58 pm 
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Thanks to all of you for the comments and advice. This is the general sort of scheme I was thinking of. It's from a project in the 1949 ARRL Handbook where the 'phones are in the plate cct of the first audio stage. I will be experimenting.

Tom

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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Wed 22, 2021 2:54 am 
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I would pick the headphone your going to use and go from there . If your only going to wear them every now and then and not for very long at a time it won't mater what headphones you use ,if your going to use them a lot use a modern pair that are real comfortable would be best .When that article was written 600 ohm and 2000 ohm and up was the standard and most headphones felt like a vice on your head .A modern pair with with low impeadance would be what i would be going for and connected to the output transfomer .

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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Wed 22, 2021 3:09 am 
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The 6SQ7 is one of the tubes that might have trouble driving headphones. You would have to try it with yours. A 6SR7 would certainly work but would require circuit changes and give lower gain. Either way, high impedance headphones are required; you can use low impedance phones with an impedance matching transformer.

The way that output stage is shown is a recipe for disaster. It shows the plate of the 6F6 connected to an external speaker. If that speaker were disconnected, the screen of the tube would have to dissipate WAY more power than it could handle and the tube would be destroyed. Make sure that can't happen.

I still can't see a reason to connect the headphone jack that way; does the article give an explanation? Unless things were handled carefully, the extra wiring for the jack could be a source of hum when using the speaker.

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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Wed 22, 2021 5:01 pm 
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There have been a few times when I’ve used an amp out of a console as stand alone or have built my own amp, and in each case I have used a 100 ohm resistor in parallel with the speaker to insure that there is always some load on the output transformer should a speaker connection get knocked loose or a wire broken. While some power is lost to the resistor, it is a very small percentage. I honestly didn’t do any calculations or tests to see if 100 ohms would be enough load to protect the output transformer, I just used what I had on hand.


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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Wed 22, 2021 8:23 pm 
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I think what I'll do is forget the old-time 2000 ohm 'phones. The ones I have aren't that sensitive anyway. Modern low impedance ones will be superior in every way. Now it's just a matter of selecting the right jack to cut out the speaker when plugging in the 'phones.

Thanks again, all.

Tom


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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Wed 22, 2021 11:49 pm 
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The Heathkit AR-3 receiver used a 10 ohm 1W resistor across the secondary of the output transformer. It was connected at all times whether the speaker was on or off. The speaker is probably the usual 3.2 ohm impedance.

Connecting the phones to the secondary of the output transformer will work fine with either high or low impedance phones.

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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Thu 23, 2021 12:13 am 
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You could add a small MOV across the output transformer primary. 150 to 250V rated might be good.

I'm with the put the headphones on the audio output stage output idea. If needed, some attenuation at the phone jack might be good, to get the volume control off the CCW stop. It would be wise to limit how loud it will play through the phones, as well.

Ted


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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Thu 23, 2021 1:18 am 
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I've seen variations of that circuit in both homebrew projects and commerical equipment from the 30s and 40s. Btw, I assume there was an OPT connected to the 6F6 plate, not just a 'speaker'.

2K Z headphones were standard back then. You wouldn't want connect 2K phones to a very low Z (say, 8 ohms) OPT secondary. So, in 1940 connecting them to the driver stage made sense. But not today. While I prefer the old technology, I have to admit those old high Z headphones are uncomfortable and sound like crap. As suggested, use your comfy modern low Z phones and connect them to the OPT secondary. If there's too much bass (ie, 'hum'), you can put a resistor in series with the phones to form a high pass filter.

I like the way that circuit used separate diodes for detection and for the AVC.

Rob


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 Post subject: Re: Question: headphones and protecting the OPT transformer
PostPosted: Sep Sun 26, 2021 4:53 pm 
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Jim Mueller wrote:
The Heathkit AR-3 receiver used a 10 ohm 1W resistor across the secondary of the output transformer. It was connected at all times whether the speaker was on or off. The speaker is probably the usual 3.2 ohm impedance.

Connecting the phones to the secondary of the output transformer will work fine with either high or low impedance phones.


Back in the 1960's I read a suggestion that tube public-address amplifiers have a permanent load of about 25% put across the transformer secondary, since people often forgot to plug in the speakers. It did not cause a noticeable loss of volume. 10 to 20 ohms would be fine.


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