• 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.

Understanding load on triode and impedance matching

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nigelwright7557

nigelwright7557

Member
Joined 2008
murtonpikesystems.co.uk
TonyTecson said:
you won't learn if you do not build anything...
Click to expand...

I learned pretty quick with valve amps.
My first valve circuit was just a guitar valve pre amp.
I built it up and it didn't work.
So turned it off and grabbed the circuit, got a huge shock !
My tutor laughed and said you have to discharge the power supply capacitors before touching the circuit.
So next time I discharged the cap. Then I touch the circuit and got another shock ! I had forgotten to turn it off.

Its pretty amazing I ever got to 62 !
The first rule of working on valve circuits is safety.
I always add a LED and resistor to my valve power supplies then I know if its safe to touch.
 
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itsikhefez

Member
Joined 2015
Most of the things I've built came in kit form or a framework for determining some of the parts that can change between builders.
Such as the AMB CK2III, Bottlehead Mainline, RJM Sapphire, and lately the Elekit TU-8600 (which got me hooked on SET as it is easily produced the most enjoyable sound out of my headphones).

The tubelab website is indeed extremely informative and has many explanations, really no criticism there at all, only praise. But it does seem like the dedicate headphone amp builders is rather limited or they aren't too active.

Maybe I'm overthinking the problem here but the OPT is the most expensive part (and has a rather long lead time) so thats not something I want to have to order twice... everything else is rather easy to swap or replace.

The various answers in this thread alone have contained the same amount of conflicting information that I have been reading from various sources....
I thought the answer to this would be rather straight-forward, something along the lines of:
This has been built many times before. You want a 5K:Z OPT which should work fine for cans in the 64-300 ohm, with a load resistor of value X to ideally load the triode (and protect from no HP's connected), and a series resistor in the range of Y to reduce noise
 
TonyTecson

TonyTecson

Member
Joined 2003
your experience building tube amps will give you good learnings, just do not get shocked, safety first, do not overthink things, tubes are quite forgiving of load mismatches...you will soon learn, so start doing....take the first step...to expect things will work for you perfectly for the first time is asking too much imho..
 
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itsikhefez

Member
Joined 2015
I found this useful graph of the 45 with 250V operating point.
Power decreases almost linearly with higher load impedance... distortion decreases as well (as most of you have suggested).
Too bad it doesnt go over 10K.
 

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johnsurnamerobinson

Member
Joined 2018
The use of load lines is invaluble when aiming for max power transfer.However as I posted before in your case you need to make sure the o/p transformer has ample iron so as not to saturate when passing anode standing current +max signal.A good transformer manufacture will work this out for you as it depends strongly on core material.BTW a class A push pull output stage is far more forgiving in this regard as standing current is cancelled in the transformer.
 
TonyTecson

TonyTecson

Member
Joined 2003
powering headphones, you will actually see much much less power than the graphs showed...

btw, speakers that are nominally rated at 8 ohms is only 8 ohms at one frequency and goes all over the place with different frequencies, i suspect that headphones are not much different...
 
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johnsurnamerobinson

Member
Joined 2018
More on output transfomers.Care also needs to be taken when including a CUSTOM o/p transformer in the negative feedback loop.Thats a whole new world of pain!!!!

Re your point about headphone amps. I am an old timer and over the years can recall seeing many designs for valve headphone amps in the UK hobby press.Since almost everything ends up on the net these days they should be out there.I am no expert on google you may need something more specific. As I recall most seemed to use small signal triodes with No o/p transformer they were cap coupled. Really sorry I cant provide links.
 
Tubelab_com

Tubelab_com

Member
Joined 2005
tubelab.com
Use 5K:32.
Click to expand...

Without actually testing anything, that would be my best guess too. Unfortunately I have been busy slinging asphalt and ripping one of the heads off my dead lawn mower lately.

I would get the amp working with the 32 ohm transformer using a cheap speaker as a (mismatched) load, then try the headphones. Once working you can play with resistors in PARALLEL with the headphones to adjust the load seen by the tube for the best balance of power and distortion. Something between 33 and 1000 ohms will be the best.

As I stated before, I believe that a 5K to 8 ohm OPT will be marginal at putting enough power into 300 ohm phones without getting the 45 too close to clipping. The 32 ohm OPT will have enough voltage on its output to feed the cans, and enough reserve power to feed a resistor, while keeping the 45 in its happy zone around half a watt or less on peaks.
 
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