Why are there 2 filament strings? The two output tubes, 50c5, 50hk6 are in series with the FM tuner tube 17c9 all on AC.
The remaining six tubes 20ez7,20ez7, 18x6a, 18w6a, 18cd6,18y6a... are all in series with a 150 ohm resistor right after the rectifier being fed 135vdc, dropping to 120vdc right after that firt 150 ohom resistor.
What is the reasoning for this?

The peanut gallery needs a schematic, please. Otherwise, without seeing the schematic, I suggest that the DC heater supply might be for hum supression. In my really nice DYNACO pre-amplifier, direct current is used for all of the 12AU7 tubes (or maybe they're AX7...I forgot). It is fed through a selenium rectifier, and is filtered by some pretty big condensers. This completely eliminates all hum. This is very necessary for a pre-amplifier, due to the high gain of the circuitry, and due to the amplification that will take place afterward, in the main amplifier. I do not see this as a necessity in your radio, unless it has some hefty pre-amplification for other devices, such as a phonograph or tape recorder. Perhaps this is a fine high fidelity unit. Eliminating all hum would make FM listening all the more enjoyable.

Thomas

Hi

I had an amplifier where the pre amp received it's filament voltage from the output tubes cathode. The pre amp tube generated grid bias as well as received DC to its filament.

Norm

:The peanut gallery needs a schematic, please. Otherwise, without seeing the schematic, I suggest that the DC heater supply might be for hum supression. In my really nice DYNACO pre-amplifier, direct current is used for all of the 12AU7 tubes (or maybe they're AX7...I forgot). It is fed through a selenium rectifier, and is filtered by some pretty big condensers. This completely eliminates all hum. This is very necessary for a pre-amplifier, due to the high gain of the circuitry, and due to the amplification that will take place afterward, in the main amplifier. I do not see this as a necessity in your radio, unless it has some hefty pre-amplification for other devices, such as a phonograph or tape recorder. Perhaps this is a fine high fidelity unit. Eliminating all hum would make FM listening all the more enjoyable.
:
:Thomas

That's very interesting Norm.. the preamp's filament was acting as the cathode (grid bias) resistor in the output?

Usually the output tubes don't use the kind of cathode to plate current that I'd think a filament requires... unless it is very very high power output..no?

Peter

This amplifier used push-pull 6L6's. The pre amp tube was 12AX7. Current was probably less than the 150 ma rating of the 12AX7 but it operated well. This method does take a little longer to heat. Output tubes have to draw current before pre amp starts to heat.

Norm

:That's very interesting Norm.. the preamp's filament was acting as the cathode (grid bias) resistor in the output?
:
:Usually the output tubes don't use the kind of cathode to plate current that I'd think a filament requires... unless it is very very high power output..no?
:

Odd. What happens when you really drive the set? Does the 12AX7 get really bright? I suppose that this might be self compensating, though, because heavier current draw will make the cathode more positive (and the grid more negative), which will cancel out the heavier current draw.

Interestingly, I was able to light a 6L6 to full brightness with moderate output from push-pull 6V6s on a Zenith Cobra consol. This was from the output secondary, though.

T.

:
::That's very interesting Norm.. the preamp's filament was acting as the cathode (grid bias) resistor in the output?
::
::Usually the output tubes don't use the kind of cathode to plate current that I'd think a filament requires... unless it is very very high power output..no?
::

Here's the schemattic Thomas:
Silvertone 4019 am/fm set
...if you go to link here at the bottom of that page is a two part schematic for which I am using.
http://www.audiophool.cjb.net/radio.html

Hmmm....well, the string covers the FM and pre-amplifier stages, though there are no pre-pre-amplifier stages that I see. The string, minus the 150 ohm resistor, adds up to 120 volts. If you want, disconnect the string from the resistor and rectifier, and connect it directly to the line. See if you notice any difference in audio quality (hum).

T.