Question for the experts:
Looking at a Silvertone 1433 schematic, I noted that 6L6GB tubes have cathode bias connected through two 12AX7 heaters. It struck me as a potentially simply way to have DC filament heaters. However, I got curious about all the pros and cons of this approach. I started a list and would like help completing it.
[Edited based on comments as of 2/10/17]
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Toss-ups / Shrugs / Personal Preference:
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* "Fixed bias" in this case means "fixed grid voltage and variable cathode current"
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Cons:
* Power tube configuration must have cathode current of 140-160mA.
[Per PRR: This rules out 2x6V6 and 2xEL84. 4x6V6 or 2x6L6 can be made to work, though not at very high B+ ]
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Connect more of the dots:
Your posted schematic shows 2x 12AX7 heaters, using pins 4 & 5, connected in-series from the 6L6 cathodes to ground. When you series-operate the 12AX7's heater-halves, the heater requires 12.6v at 150mA. We have two such tubes, so the heaters are looking for something like 25.2v at 150mA.
Data sheet of 12.6V/150mA implies that series resistance is 84 ohms per 12AX7.Tube filament resistance is low when cold and increases as temperature increases. So what temp do you want to assume?
Don't overthink this. 19V should light those tubes just fine and also provide a nice safe bias voltage for the output tubes. Probably last a long time. Remember, this is not a high performance amp.
... When you series-operate the 12AX7's heater-halves, the heater requires 12.6v at 150mA. We have two such tubes, so the heaters are looking for something like 25.2v at 150mA....
* However, on the Silvertone schematic in original post, it shows PT cathode nominal voltage of 19V. ...
I'm not certain, but suspect the current on a 12AX7 heater matters more (really, the "heater power," voltage * current, is the key to warming the cathode, but...).I've always thought the current was the more important factor too, but that's based on experience with high power radar transmitting tubes and may not hold for receiving tubes. Never really thought too much about filaments with small tubes. Just kinda take them for granted. But big transmitting tubes were another matter. The last transmitter tube I worked on had filaments rated at 5V and 50A. The filament transformer was on a big variac and we would bring the filament current up slowly to 50 amps watching a panel meter. We never even cared what the actual voltage was.
Power is the main thing.
Bias V Bias I Power from two 5881 (watts)
Note: From 12AX7 data sheet, heaters in series = 12.6 x .150 = 1.89W. Double for two 12AX7 gives 3.78W Based on this, I conclude that cathode @19V is more plausible because power available for 12AX7 heaters is near the maximum power available from the cathode. In other words, balancing power between the tubes would tend to reduce cathode bias --> move to peak of idle power ---> that's all the power that 12AX7 heaters get What do you guys think of this? |