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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: Amp input pot removal posted by Sidewinder on September 16, 2017 at 11:41:23:
I purchased an unassembled kit on Audiogon for $1100. Thought that was a decent price. In its stock form I wasn't too impressed. Sounded kind of polite. Wasn't able to grab my attention. Eventually I grabbed my soldering iron to try out some ideas. Kevin Carter suggested replacing the 6SN7 as the driver with a tube with more "muscle" such as a 6BX7. That required some "playing around" to find a new value for the cathode bias resistor and rewiring of the input and driver stages. The lower plate impedance of the 6BX7 let me reduce the value of the 2A3 grid leak resistor (that should help keep the output tubes happy). Also, due the increased filament current draw of the BX7 I needed to add a separate filament transformer (note the Hammond in photo). This did allow for biasing the filament voltage of the driver up to 150V. Also added a virtual balanced ground to the input tube filament supply.
Both the input stage and driver stage now have cascode CCS loads and the 6SN7 input stage is biased with two red LEDs in series. The 6SN7 is biased @4.5mA & 120V P/K. The 6BX7 @19.5mA & 120V P/K which is kind of down in the dirt for a BX7. But with only 300V raw B+ and the grid sitting at 120V that only allows for ~50V for the CCS and the signal voltage swing. So there isn't room for more plate voltage.
As I mentioned before this amp and my mono-blocks share outputs on my line stage which are floating transformer outputs. The nature of the mono-blocks require these connections to remain floating. This is why I added Lundahl input transformers to the input. Also made component changes like Blackgate cathode bypass caps and Cardas coupling caps and tantalum signal path resistors.
The amp is a lot more engaging now and fun to listen to.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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Follow Ups
- RE: Amp input pot removal - JKT 09/16/1712:45:24 09/16/17 (1)
- Beautiful build/mods Thanks nt - Sidewinder 12:56:56 09/16/17 (0)
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