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Vacuum Tube Audio Gear
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meurich10
Jan 03, 2025
In Questions & Answers
Skunkie and Forum,
I just built the EL34/12AX7 amp and it sounds really good. Thanks for everything you do to help us with this Hobby Skunkie!!! I am interested in trying to learn more about how to tune it for my preferences. Although with all the work you did it may be the best version as it sits.
I have it set up to switch between two different Schade resistor values and I was wondering if I can disconnect the Schade feedback completely while the amp is running or if I need to power down first. In reviewing the circuit I can't think of a reason I would need to power down unless it would cause a current/voltage spike. I'd like to do this to understand better how the amp sounds under different configurations. It is hard to compare if you have to remember the sound between subsequent power cycles. Does anyone have experience with a mod like this?
I am also planning to add the ability to change out the driver tube for 6-volt tubes like the 6AQ8 and 6DJ8. I plan to add relays and have the configuration controlled by a microprocessor. It would switch the heater wiring and required loads in/out.
any input would be appreciated!
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meurich10
Jun 13, 2023
In Show and Tell
I built my first tube (pre)amp using a couple of 6GU7's I had laying around and a cascode design from https://www.diyaudio.com/community/threads/franks-ultimate-tube-preamp.12042/. The original design was for 12BH7A and a complex power supply but since it was pretty low power, I built a full DC tube preamp using a VDC 5 amp wall PS. For the 305 VDC power, I used a $10 high voltage boost converter from Amazon and a cheap LM317 voltage regulator for 6.3 heater voltage which is heatsinked to the chassis with a piece of bar stock. I did several stages of filtering to clean up the high voltage and had to play around with what the converter liked but it is dead quiet. I found a 22uf capacitor followed by an inductor and some 330uf caps (which I repurposed when I upgraded my Wilsenton R8) was the best combination. The heaters are DC and can be either 6.3 volts for the 6GU7 or ~12.6 for the 12BH7 (I added a switch not shown). I am using this like the color preamp Skunkie built on a class D amp and it greatly improves the sound.
I need to add a power switch on the front and I have a nice LED pushbutton sitting on my bench. I cleaned up the paint job by adding a couple of white pinstripes. I'd like to clean up the wiring/layout of the power supply filtering a little. I was also wondering about adding a bypass capacitor on the bottom tube with a switch but it sounds pretty good as is.
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meurich10
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