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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET

Post Subject: They are not LPads but they are bitch.Posted by Romy the Cat on: 6/24/2015

Well, they technically not Lpads but juts regular attenuators even though they connected in “protective mode”. You can go with stabilized impedance to maintain the high pass filter over the coupling capacitor to work properly but I do not feel it is necessary. When you make the real amps you will have the attenuator ruining let say between 35K and 36K to set the real tube bias properly. You will not slide the attenuator from 0 to 50K. The range will be buffered by two resistors up and down and in context of 100K biasing resistor the impedance shift that your coupling cap will see will be near negligible and might manifest a 2-3Hz difference in the slop. Above I told you that I tested the slope with analyzer and I did not detect any meaningful crossover shift while I changed bias.

Now, the type of the resistor is the real bitch question. You do not want rotary switch with great resistors but you rather want a 10-20 turns attenuator. However this is bias and everything you do with bias matters sonically. So, find the best you can get 20 turns attenuators. At the time I did it I had 3 or 4 different types and all of them were as good as fixed resistor. I chose the one that I liked best at that time, I do not remember what it was, I think it was some kind of large 20 turns Vishay for $12. It was however over 12 years back and today you might find better parts. Be advised that quality of soldering and wires and even the layout of the bias wiring in the amp’s chasses is very auditable for whatever reason. I actually was very surprised how sensitive and demanding my 6C33C where for biasing line. I was able to hear the sound change when I loved the proximity of bias wired to mass ground. I am not kidding.

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