• 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.

ECL82 PP "reference" amp

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#21
One way to make up the gain is to put a input transformer up front. This doubles the effective gain of the LTP, and probably performs better to boot.
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I used a microphone transformer with a step down of 9:1. This works very well, and helps to keep the triode from any chance of overloading. Anything greater than 4:1 would be ideal.
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If I could buy or build such a good microphone transformer, than I would try it too, but it is not easy to find one here.

Tyimo
 
#22
Just always looking for an excuse to try out those Edcors.
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Having thought about those second Edcors a little more, I have some doubts about their suitability. They are designed for SS application with 600ohm drive, this brings into question the amount of inductance and hence there bass response.

I cheap and dirty experiment could be made with a 120V:24V toroidal - might just work. I currently use a toroidal as a phase splitting driver, and though not totally happy they work well enough. When I did my initial tests, I found that with an easy load they had good response out to 70khz at least and bass was never an issue (down to 10hz).

The only way to find out is to test some of what you have.

Take a look at the way I have my input transformer configured on earlier post.



I've had the opportunity to meet, hear, and party with Wilson... he is a really inspiring person.
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Did you know that he passed away last year. A great loss to the world. Your very lucky to have met him.

Shoog
 
#24
Shoog said:
I have some doubts about their suitability. They are designed for SS application with 600ohm drive, this brings into question the amount of inductance and hence there bass response.
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There are some mic trafos in there too...

Did you know that he passed away last year. A great loss to the world. Your very lucky to have met him.
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🙁

no i hadn't heard that. It makes me sad.

dave
 
#27
The answer to that can be found over at the Tubecad article on plate to plate feedback. It took me a few reads and a lot of head scratching in order to understand what was going on (its not obvious), but understand what going on is important to get the best out of the circuit.
Also there are two ways of implementing it. One uses the feedback resistor as the load resistor of the driver. The other has a load resistor and a feedback resistor. The first seems better, but places sever limitations on your choice of feedback resistor, The later allows you to maximise the feedback effect without forcing design restraint on the circuit.
I prefere the load and feedback resistor combined.

There is also the option of sticking in a range of resistors from 50K to 140K and seeing which one you think sounds best. Of course this will only work with the seperate load resistor.


one thing to consider is that the driver valve needs to have a very high rp which makes a pentode an attractive option (think 6AU6.
Shoog
 
#30
ECL82 PP

Tyimo:
Too much cook spoiled the soup! You better stay on your idea, I think among the circuit U posted, the Mullard one is superior, it similar to Williamson design, it uses a pentode like Ef86 in the front end, pentode provide very high gain, and the triode of ecl82 for driver, these gives good balance on pos. and neg. wave form. for any details, find Williamson design in British library.
 
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