K&K Audio / Lundahl Transformers

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K&K Audio / Lundahl Transformers

PP 300B all DHT amp project [pic]

216.152.34.45

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112A first stage, 71A second, 300B finals.
The 112A and 71A filaments are in a series string using a 45 volt A supply which makes for fixed bias. Low value power resistors in the series circuit sets the operating points. No cathode bypassing and associated LF losses.
Building tube amps again after > 20 years of fixing old radios.

Jeff Goldsmith

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Re: PP 300B all DHT amp project [pic] - Rafal 22:45:59 06/14/05 (3)
How is the hum?
In series connection of the 112A and 71A filaments sounds interesting, could you post more information or a schematic.
Regards
Rafal
- Re: PP 300B all DHT amp project [pic] - Jeff Goldsmith 00:32:11 06/15/05 (2)
  Rafal, the filament biased amplifier stages is quite a nice little amp on it's own, but really shines as a PP stage driver.
  The plate current of the 71A adds to the filament string current at the 112A, so a 470 or 510 ohm resistor is needed to shunt the 71A plate current past the 112A filament so it gets 250mA.
  
  I'll have to work up a diagram for the A supply, but it's only a high voltage op amp OPA544 'amplifying' a fixed TO-92 voltage regulator to +45 volts.
  
  Jeff Goldsmith
  - Re: PP 300B all DHT amp project [pic] - Rafal 09:16:03 06/15/05 (1)
    Interesting I will have to try it. No bypass capacitors needed?
    Rafal
    - Re: PP 300B all DHT amp project [pic] - Jeff Goldsmith 12:06:40 06/15/05 (0)
      Give the little 112A-171A filament bias amp a try, Rafal. It's simple enough that someone with a good collection of parts should be able to come up with most of it. You should like the performance this circuit will give. With the 71A plate voltage rating being 180 volts, add 40-45 for the A supply so the B supply needs to be 225 volts.
      One caveat: Not all 112A and 71A tubes will get along in series. The filament current of some brands is well over 250mA. Best to stick to the 'name brands', such as Cunningham, RCA, Sylvania or Tung-Sol.
      No bypass capacitors, only the resistors in the filament circuit. The plate currents are small compared to the filament series circuit current so the filament current essentially does the bypassing.
      If you use a voltage-regulated +45 supply, the Z at the 71A filament will be the Zo of the A supply, the Z at the 112A filament will be around 30 ohms.
      Current sourcing the A supply would give you 140 ohms Z at the 71A filament, you might notice a little loss of gain compared to a voltage basis A supply but I have not worked in this direction yet.
      Very low hum from this amp, the largest contributor is my using the default CT on the filament transformer. Shielding the input tube got rid of the most significant source of hum pick-up.
      
      Jeff Goldsmith

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