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previous: andyfuchs I remembered an old Scott audio tri... -- 4/22/2000 2:34 PM

Re: Ampeg Heater Circuit Questions

4/24/2000 6:17 PM
Doc	Re: Ampeg Heater Circuit Questions Andy: Can you recall which Scott unit used that scheme? All the Fisher & Scott stuff I'm familiar with (have schematics for most) uses fixed bias output stages, so the cathodes are grounded. The typical DC heater supply in those amps is actually the bias supply. A number of 12AX7 preamp tubes have their heaters strung in series, and that string is placed across the filtered full wave bridge bias supply, which is designed for the extra current demand. Actually the high current draw helps with bias supply voltage regulation. For instance, four tubes would get connected across a bias supply of about -44 volts. Yes, the tube heaters were run at reduced voltage for low hum & noise. The only place I've seen a preamp heater used in a cathode biased power stage was in an Eico HF-87. This is a stereo aplifier, but the 12AX7 heater was connected to one channel only. Here, the first preamp was in series with another resistor to make up the full bias voltage drop. (The other channel had a single fixed cathode resistor.) Remember that the 12.6v heater draws 150ma, so the power stage's expected cathode current should be close to that. In a class A, or almost class A, amplifier, average cathode current, and thus preamp series heater current, remains relatively constant. Doc

4/24/2000 6:17 PM

Doc

Re: Ampeg Heater Circuit Questions

Andy:

Can you recall which Scott unit used that scheme? All the Fisher & Scott stuff I'm familiar with (have schematics for most) uses fixed bias output stages, so the cathodes are grounded.

The typical DC heater supply in those amps is actually the bias supply. A number of 12AX7 preamp tubes have their heaters strung in series, and that string is placed across the filtered full wave bridge bias supply, which is designed for the extra current demand. Actually the high current draw helps with bias supply voltage regulation. For instance, four tubes would get connected across a bias supply of about -44 volts. Yes, the tube heaters were run at reduced voltage for low hum & noise.

The only place I've seen a preamp heater used in a cathode biased power stage was in an Eico HF-87. This is a stereo aplifier, but the 12AX7 heater was connected to one channel only. Here, the first preamp was in series with another resistor to make up the full bias voltage drop. (The other channel had a single fixed cathode resistor.) Remember that the 12.6v heater draws 150ma, so the power stage's expected cathode current should be close to that. In a class A, or almost class A, amplifier, average cathode current, and thus preamp series heater current, remains relatively constant.

Doc

Replies:

andy fuchs It may have been an integrated ampl... -- 4/24/2000 6:40 PM