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Old 8th June 2009, 01:23 AM   #1
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Default To anyone using 808s

This question is directed towards anyone using 808 tubes. Are you using AC or DC filament supplies?

Most of the schematics show DC supplies and 7.5v transformers don't seem very common besides the one Hammond makes and its rated for 15 amps, way overkill. Antek makes a nice toroid although its rated for 9 volts.

I tried using an LDO regulator with a very low Rds FET which worked well but at 4 amps for each filament thats a huge amount of heat to dissipate along with the bridge rectifier. So if you can run an 845/211 from AC filaments why not an 808?
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Old 8th June 2009, 03:19 AM   #2
kmaier is offline kmaier  United States
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Well, you can use the 808 with AC filaments... just that the hum level is high. The internal construction of the 808 is such that you really can't balance the hum out with an AC filament. The RCA ones I have consist of a helical filament inside a horizontally spiraled cylindrical grid inside a solid cylindrical anode. You're just not going to get a reasonable hum level with AC.

Rather than use a 4-diode bridge rectifier, I prefer a center-tapped winding with a pair of diodes for full-wave rectification. Less voltage loss in the bridge and less heat lost too. This also opens up your options on transformers as Hammond makes 12.6-, 14-.0, 16.0- and 18.0-volt center-tapped filament units. I would suggest you try the 14.0-volt C.T and check your actual operating voltage. As the 808 is a thoriated-tungsten filament, you want to ensure your filament voltage is at 7.5V... a bit higher is okay, lower is not recommended.

Regards, KM
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