Tube DIY Asylum

300B filament tranny

65.82.227.116

I will be building the Je Labs stereo 300B amp soon. The schematic calls for a Hammond 166L5 filament transformer rated at 115vac primary and 5vac secondary @ 2amps for the 300B's. I have a Hammond 185C10 dual primary dual secondary transformer I would like to use as an alternative that is rated at 115vac primary and 5vac secondary @ 2.5amp.

I am aware these specs are under full load conditions and the 185C10 has 6 volts on the secondary without a load at 120vac. While researching this I have found that the general rule is to size the transformer at about 50% above the current draw that the tube would require. Most 300b have a filament current of 1.2 to 1.3amps thus the reason for a 5vac 2amp transformer. Knowing this I have to assume that if I use the Hammond 185C10 that I will end up with a higher voltage at the filaments.

Is there a way that I can figure what size resistors to use to drop the voltage closer to 5v prior to getting the amp up and running? Should the resistors be installed close to the transformer or the filaments?

http://www.hammondmfg.com/5cpwr.htm (Open in New Window)

Follow Ups:

are you using a regulated AC supply? - hermanesque 19:33:35 06/02/08 (1)
- RE: are you using a regulated AC supply? - ecir38 15:31:36 06/03/08 (0)
RE: 300B filament tranny - PakProtector 16:14:08 06/02/08 (2)
- RE: 300B filament tranny - ecir38 16:18:39 06/02/08 (1)
  - There must be better biasing topology for 300B.. - cheap-Jack 08:41:00 06/03/08 (0)

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