• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dynaco SCA35 and headphones

I am using a modified Dyanco SCA35 amp as my main amp in my system enjoying the EL34 as an output tube with the original output transformers. However, I would like to add a modern Audiotechnica M50x headphones to my listening experience and these headphones have an impedance of 38ohms.
Aside from the obvious input jack for these phones, what other mod would I need to make these two components happy?
Would another output transformer switchable between the speakers and the phones work better than a resistor configuration? If so, what would the specs of the tranny need to be to keep them both happy?
And if not a tranny, then what resistor configuration would work?
Please advise.
Thanks in advance.
Thank you for your reply. If you had more details about the configuration, would you be more likely to advise how to implement my question? There is in my question, not only an opportunity to learn something about transformers and headphones, but also to educate interested people on safe practices when modifying amplifiers. Interested?
The dynaco manual on page 6 says to use 100 ohm resistors in series with a headphone. The extra hole in the SCA 35 chassis on the back is for a headphone jack and this is a sanctioned use of the amplifier by dynaco. I suggest you download a copy of the manual and follow their instructions. As others have said be careful!
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Here is the page of the Dyna manual showing how to install a standard headphone jack in the
pre-drilled hole on the back panel. It's very simple. Just use a series 100R resistor on each channel.

Don't forget that the speakers (if used) will continue playing unless you install the optional switch.


  • hfe_dynaco_sca-35.pdf
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Depending on the headphones, a 100 Ohm resistor might be a prescription for you to have to get an appointment with a . . .
Ear, Nose, and Throat specialist.
Good luck!

Not only that, you might have to have the vacuum tube amplifier repaired.

Have you ever heard a stereo system that is not putting out any sound.
Then you turn the volume all the way up.
Oh, the speaker switch was set to B speakers, and the speakers are on the A outputs.
Then you turn on the A outputs, and jump to turn the volume down.
I hope the output transformers were not arcing, and I hope the output tubes were not arcing when you had no load.

A 100 Ohm resistor is a very bad load for a vacuum tube amplifier.

Your Mileage May Vary
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Isn't the tricky part here finding a way to mute the speakers simultaneously to adding a load to the OPTs to simulate their presence so the impedance for the output tubes is correct and feedback works the same?
Seems to be more non-trivial than using a drilled hole? Or is this over-thinking?
Integrated amps of the era got by with just series resistors, or sometimes an additional shunt resistor, and ignored potential stability issues with no loading. Most didn't even have output Zobels, although this would have been safer and best practice, especially with full pentode outputs. Still recommended today; use 0u1F and 10R, close enough.

There was a time (1970s? 1980s?) when an attempt was made to standardize source resistance to the headphone, so headphone manufacturers could design to a common standard, but this went kaput. Wasn't it something like 200 Ohms? Something like that.

All good fortune,
Well, thank you all for this important and helpful discussion.

In keeping with the issues present, would the value of the loading resistor in the amp would somehow have to correlate to the input impedance of the headphones? They are not single digit impedance like speakers would have been back then. In fact they are modern double digit phones (32ohms?).

And what about something (hypothetically) larger like professional 600ohm studio headphones? Certainly there would be a serious impedance mismatch if the output impedance was 8ohms and the input impedance of the phones was 600ohms.

In his case they are not, but practically, is there not some way of figuring out the necessary loading resistor for high impedance phones?

Thanks in advance.
A simple PI (3 resistor) network will get the you the proper headphone output. Valve amps will get damaged if there's no speaker load and they are driven to clipping with a signal. You don't need 8R just something of 100R or less will prevent silly voltages occurring. Secondly headphones are designed to be sourced from 100R whether they are 32R or 600R. So that makes the second resistor to ground 100R. Finally the resistor between needs to be high enough not to blow your headphones or ears. I would start with say 2k2.
I am using a modified Dyanco SCA35 amp as my main amp in my system enjoying the EL34 as an output tube with the original output transformers.
EL34?? Is there a typo that you use EL84/6BQ5 tubes instead as illustrated in the schematic?