![]() That's a great place for a volume control- after the gain, so that any noise from the gain stage is attenuated along with the signal as you turn down the volume, but before the cathode follower, which allows for low, constant, output impedance.Īs for PT- a 250-0-250 power transformer, with high-speed diodes, and a small choke (as mentioned, 50ma current capacity will be fine) should take care of business. In my preamps, I usually use the 22K (2 watt) resistor for R2- and instead of at the input, I put a 100K volume pot between the two stages- between the first-stage coupling cap and the grid of the cathode follower. Yes, the gain is lower, but IME, still plenty for any amp I've connected one to.Īlso, without the big electrolytic (or whatever) bypass cap, it's probably "cleaner" sounding. I use a 22K or 47K instead of the 220K at R2, and a 1.5K cathode resistor at R3, with no cathode bypass cap. Mark, that's almost what I use for the Dynaco PAS resto/rebuilds I do. These were the days of tubes, so sometimes a failed component would also pass B voltage as well. For C3- if you are looking to make sure you have adequate energy for dynamic Bass, bump up to 47microfarads at 35 volts.Īs for why the high voltages for C1 and C2? Even though you have 300 volts at the plates, there are times when the signal can swing the grid to zero or sometimes to negative potentials- which makes the potentials across C1 or C2 close to 400 volts. It has been a few years since I did any designs around those octals.Īs for cap types- Orange Drops are good for C1 and C2. I would plow through the "Resistance Coupled Amplifier" tables and adjust the resistances accordingly if they show any difference between the 12AU7, 6SN7 abd 6SL7 for the specific tube you are using. The issue is more stability related than redplating. There are some circuits where the 6SL7 and 6SN7 can be used interchangeably- but very few. If you are using the octal because you have them or want to do something different, the 6SN7 is closer to the 12AU7 specs. The 6SL7 is a higher gain factor than the 6SN7, and instability is a potential issue. The heater current is different, and as far as plate disipation, I think 125milliwatts( each) (my tube manual is not handy at the moment) is what the 6SN7 and 6SL7 are rated for, but they are very forgiving if less.
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