Thinking aloud here really

[bumpy]

I have 45 triode monoblocks with 1.8w output. Can I in some way use these to drive headphones?

[Puffin]

Headphone and earphone impedance
Just like speakers, headphones and earphones have a certain resistance to the electric current transmitted by the amplifier: this is impedance. Expressed in ohms, it often ranges between 8 and 600 ohms, depending on the model. As they do for speakers, manufacturers measure headphone impedance at 1 kHz (midrange), but in reality, impedance varies according to frequency (from lows to highs). It can double, triple or quadruple as the transducer’s mechanical resonance frequency is approached, which complicates the amplifier’s task by requiring it to produce more power.

From here :-

https://blog.son-video.com/en/2016/08/understanding-the-impedance-and-sensitivity-of-audio-headphones/

[paulf-2007]

Yamamoto headphone amp also drives speakers, don't know if there is something in the circuit that changes when headphones are connected/disconnected.

[Pharos]

Headphones are much more sensitive than speakers, they do not have to move so much air.

Typically speaker sensitivity ranges from a low 85dB/W to probably 104dB/W.

Headphones have their sensitivity quoted with an I/P of 1mW, so your amps will almost certainly have enough O/P to drive them.

I use a Nait 1 to drive my 106dB sensitivity phones, but with an attenuator box on the amp O/P, which is necessary to avoid overloading the phones, and in so doing, it reduces the ambient noise from the amp which may be significant without attenuation.

[bumpy]

I read somewhere that if you connect a pair of high impedance headphones to valve amps the amps treat it as if you have connected no speakers and the output transformers can burn out :(

[Pharos]

My experience of valves is from around '66 to '71, and with quite a few models.

In my apprenticeship I unplugged the speaker from a Mullard 5/10, and large sparks jumped across the O/P terminals of the transformer, and it fried. I didn't know that a high voltage forms because of the loss of load, and it had to be replaced..

IME it is OK to under-load a valve amp, that is, to draw less current than under a normal load, and from what I know to run them with no load at all, but not to do as I did and unload them when they are running.

You can always do what I have done if you want to maintain a 'normal' load, and make a resistor attenuator network which presents a 'normal' load to the amp, and which provides a correctly lowered O/P level for the headphones.

If you put an 8 or 10 ohm load across the amp O/P, but with a 0.5 ohm in series with it, you could drive the phones from the 0.5 ohm resistor, and it would give approximately a 12dB attenuation, and present the amp with a load of at least 8 ohms, this dependant on your phone impedance.

Of course others may know more than I do, and with more recent experience, so I welcome criticism and possible better advice

[anthonyTD]

As long as you use a dummy load accross the output of the amp, as Dennis has said, then you can use headphones on the output, but bare in mind, the increased noise lever you may get from the amp, in comparison to a specificaly designed headphone amplifier may be significant, depending on the amp in question, you can only try it and see i guess.
My experience of valves is from around '66 to '71, and with quite a few models.

In my apprenticeship I unplugged the speaker from a Mullard 5/10, and large sparks jumped across the O/P terminals of the transformer, and it fried. I didn't know that a high voltage forms because of the loss of load, and it had to be replaced..

IME it is OK to under-load a valve amp, that is, to draw less current than under a normal load, and from what I know to run them with no load at all, but not to do as I did and unload them when they are running.

You can always do what I have done if you want to maintain a 'normal' load, and make a resistor attenuator network which presents a 'normal' load to the amp, and which provides a correctly lowered O/P level for the headphones.

If you put an 8 or 10 ohm load across the amp O/P, but with a 0.5 ohm in series with it, you could drive the phones from the 0.5 ohm resistor, and it would give approximately a 12dB attenuation, and present the amp with a load of at least 8 ohms, this dependant on your phone impedance.

Of course others may know more than I do, and with more recent experience, so I welcome criticism and possible better advice

[bumpy]

Thank you good people

[Pharos]

The inherent ambient noise level at the amp O/P will not be significant with speakers, but with headphones, of probably 30dB higher sensitivity, it may become so as Antony says.

Hence my suggestion of attenuation; it will bring the O/P of the amp down to a suitable level, and in so doing lower the quiescent noise of the amp, so pulling down the dynamic range 'window' of the source from the amp, to a more suitable level for the phones.