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Any Fi X3 300B amp owners out there?

setup1 said:
AFAIK, Don Garber used AC heating on the 300B filaments.
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That was established at the beginning of this thread. There is not enough room for a diode bridge and smoothing capacitors within the X chassis. However, Garber's original build has the entire current for the output tube going through the wiper of the hum pot. This creates noise and hum which worsens over time as the wiper degrades do to the current. The solution is to bypass the two legs of the pot to the wiper with 15ohm resistors. The two resistors carry the current bypassing the wiper of the pot.
hum pot a.JPG
 
dowto1000 said:
Hi Grantray,

How and when did you get Isamu Asano amps??

I never knew these were made, I thought Asano was only a DIYer who had his circuit published, prior to Shishido's DC amp??

The Fi schematic posted in this thread shows no Hum Pot, yet you mention a Hum Pot, in two of your thread's posts.

Are you sure you have a Hum Pot in your amp?? If so, please allow me to mention something .......for you to consider.

Most DHT tubes using hum pots ( only ) have a ( fixable ) weakness in their design.

The interface between the pot's center wiper, and the pot's internal resistive element, acts somewhat like a diode, it partially rectifies the JJ 2A3-40 or 300B's cathode's audio signal, in the tube's return-to-ground path.

Designers / DIYers / people who are aware of this, simply add a SOLDERED pair of resistors, from each end of the pot, to the center ( wiper terminal ).

A typical resistor value may be 15 to 25 Ohms , 2 Watt, high-quality would be preferred.

This does several things, the most notable IMHO, is that you have changed the return to ground from a " diode - like / rectifier - like " wiper-interface, to a added - SOLDERED current - return - path, to ground.

Most who do this, report an improved sonic result, and your large / beefy JJ 2A3-40 or a 300B would benefit from such a soldered connection IMHO. Please look this up, on line.

I personally am a big fan of the two stage DC SET circuits, as you seem to be. Less becomes more. I have written a 32 page paper, tracing the history, and implementations of this specific amp type, ( from 1929 to 2018 ) and would be happy to email it to you, if you would like to read it.

I drew out a diagram, below, of where to add the resistors. Also, I can mail you (4) 15 Ohm, 2 Watt Cornings, for the cost of postage, if you would like to hear this.

Good listening to you !! I DIY build these amps from scratch, just for myself ( use ALTEC VOTT A7-800s ) and have done so, past 12 years.

Dowto1000

.Hello, I would be very interested to read your white paper on DC SET circuits. I’m trying to find a very simple design for a 300B amp build. I was drawn to the Don Garber design.
I really appreciate your consideration.
Brian in Seattle.
Frisbie55@hotmail
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