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In Reply to: Re: DUD 120 ampmods posted by Gingertube on March 6, 2007 at 19:17:27:
Some more thoughts.
You will notice that the cascodes are "Self Biased" relying on the 100 Ohm resistors plus internal impedance of the upper tubes to set the voltage across them. The more usual arrangement for a cascode is for the grids of the upper triodes to be wired to a voltage divider with the bottom arm bypassed to 0V. This arrangement has an advatage that you can also then add a resistor between the high voltage rail and the anode of the bottom triode to get more current through it and hence maximise gm and therefore the gain.With the existing self biased arrangement, if you increase those 100R upper triode cathode resistors to get the Vak voltages better then you may find that you need to bypass those resistors with large value electrolytics to keep the gain up.
Follow Ups:
Thanks for answers ! I will go on and experiment. I have a parallellconnected ECC 40 followed by a Baxandall tonestack and a 1 meg. pot. in front of my DUD 120 . Are using the amp as a guitaramp. It has a very clean, warm jazztone inspite of wrong balance in the cascodesplitter . The gain is a bit low so I´m thinking of splitting up that ECC 40 into two separate triodes with the Bax. tonestack between the triodes . Until now I have no feedback at all but guess I can have some after the change into two triodes in front of the DUD 120 . Stewen
Hi again! Now I´ve changed those 100 ohm resistors into two trimpots 1K. With about 450 ohms on the pots the cascodestages balance with 88 V over each ECC 88 . The amp sounds great with 40 mA through each of the quad 807 powertubes. Anodevoltage is 450 V. Screenresistors 120 ohm/ 2W . Triodemode. Funny that tubes of the same age like myself , 62, can sound better than many modern tubeamps in my lokal musicstore !
Stewen
you said:
Funny that tubes of the same age like myself , 62, can sound better than many modern tubeamps in my lokal musicstore !Not really surprising -
While many of the modern tube makers use resurrected equipment from yesteryear what has been lost is the experience of the people who used that equipment to turn out superior tubes.As for the tube amps in your local music store - a lot of the newer tube amps are comming out of China and they seem to be just building stuff to designs from the 1960s - we are not seeing any really new ideas which is a pity. There is a lot of scope for using modern circuit methods and ideas learned from solid state design in tube amps..
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