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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: Gain Stage for Wurlitzer 6420 6L6 tube amp posted by mr9iron on February 18, 2017 at 15:45:52:
One way you can increase gain is to decrease the amount of global feedback by decreasing the value of R3, or to get rid of NFB altogether. This approach will work fine if you are using antique style speakers that do not need much electric damping. Prior to the Hi-Fi era, they did not use global NFB in pentode amplifiers, even in high end products like Magnavox Regency or Belvedere consoles, and these radios sounded fantastic. Of course, if you are using modern compact low efficiency speakers, you will need at least some NFB. You can play with R3 and find the lowest amount of NFB you can live with.
If you want to add an RC gain stage, you can use 6J5, which is a single octal triode with characteristics similar to 6SN7. Use the following part values:
Rk=2.7k
Rp=100K
Coupling capacitor 0.05
Rg=470K, volume control pot with log taper.
B+ can be obtained from the following stage through a dropping resistor, so that plate voltage is about 180V. B+ should be bypassed to ground with 20 uF electrolytic capacitor.
With 6J5 Rk bypassed with 50 uF capacitor, gain will be ~15; without capacitor it will be ~5.
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- RE: Gain Stage for Wurlitzer 6420 6L6 tube amp - sser2 17:03:00 02/18/17 (0)
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