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This amplifier use direct heating tubes with AC filaments so at the startup some 50-60Hz hum can be ear.
It is possible to increase the output power from 6W to 8W using fixed bias, in
this case change the LL1648 with LL1669A to get an extra 100V secondary.
For the fixed bias a dedicated power supply for the negative grid bias (~ -60V) will necessary and you must increase the value of the 0.39ohm 3w on 300B filaments because the LL1669A have only 6.3V secondaries.
It is possible use LED
bias in the driver stage, I have tested green led with good result and the same
frequency response, only a little bit less voltage gain.
If you use fixed bias for the 300B and led bias in the driver you can exclude all electrolytic capacitors in the signal path.
All the audio transformers used in this project are Lundahl products.
Lundahl is one of the world´s
leading manufacturer of audio transformers used in many professional studio
recording and radio stations so He have a very big experince and all these
products use a custom audio C-core.
I am using a The input stage tube is the D3a in triode
I am using aDACT stepper attenuator instead of the normal ALPS or NOBLE solution, because the the sound is clearer. A good review of these components can be found on the article published in Hi-Fi World.
The input stage tube is the D3a in triode connection.
About passive components types, this project use the best combinations:
About wire forget the teflon ! use only stranded tinned copper wire with pvc isulator and for loudspeakers the same.
A very good quality stranded tinned copper wire could be buy directly fromE-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.
Follows the first measurements on the basic version (no fixed bias and no led bias).
For the 10Kohm Caddock I have used this A1882 heatsink.