I am looking at this schematic and it is not entirely clear to me what the resistors R21 and R22 are doing in the filament supply.
My best guess is that they are providing a ground reference for the 6.3V winding.
My other question is whether a center tapped 6.3V winding AC coupled to ground through a 30-50uf capacitor would have lower noise.
This is an earlier version of the schematic; the one I am working from omits the 12AU7 stage and has paralleled EL34 power tubes but is otherwise identical. I don’t have permission to post it whereas the above schematic is publicly available.
Thanks in advance.
My best guess is that they are providing a ground reference for the 6.3V winding.
My other question is whether a center tapped 6.3V winding AC coupled to ground through a 30-50uf capacitor would have lower noise.
This is an earlier version of the schematic; the one I am working from omits the 12AU7 stage and has paralleled EL34 power tubes but is otherwise identical. I don’t have permission to post it whereas the above schematic is publicly available.
Thanks in advance.
Thank you artosalo.
The power transformer has not been ordered yet as it will be a custom order from Edcor. I am inclined to leave the design as-is but curiosity is getting the better of me and I’m considering a center-tapped 6.3v winding and even regulated DC for the filaments. Any pros/cons people feel like voicing?
The power transformer has not been ordered yet as it will be a custom order from Edcor. I am inclined to leave the design as-is but curiosity is getting the better of me and I’m considering a center-tapped 6.3v winding and even regulated DC for the filaments. Any pros/cons people feel like voicing?
An accurately connected filament winding center tap will be better than the pseudo tap with resistors.
Yes, the hum pot is another variation I’m considering. Is the hum pot compensating for the slightly different DCR of each leg of the winding due to winding geometry?
When adjusting the hum pot, you can easily hear a minimum hum point, but this is limited by the discrete contacts
in the pot winding that are available to choose from.
I use DC from a LDO regulator for the small signal tubes and raw AC for the output tubes. The negative of the DC is grounded, the AC is floating. There is no hum.
R21 and R22 are sometimes known as a "Virtual Center Tap." As with many things, there is debate about whether the resistor center tap or an actual center-tapped heater winding is better.
One known "feature" of resistors as a center-tap is that in the event of a catastrophic short, usually between pins 2 and 3 on the tube socket, the resistors will burn up and act as fuses. A center tap will not. This can happen in amps like the Fender Bassman, which, because it is a head, is more likely to become disconnected from a speaker load while someone is playing, resulting in a destructive arc from pin 3 to pin 2.
You can also use a hum-balance pot and/or elevate the heaters to a positive (or, theoretically negative) voltage such that the heater waveform no longer crosses the tube's cathode voltage, which also lowers hum. The latter is a very old trick, found as far back as the 1930s.
One known "feature" of resistors as a center-tap is that in the event of a catastrophic short, usually between pins 2 and 3 on the tube socket, the resistors will burn up and act as fuses. A center tap will not. This can happen in amps like the Fender Bassman, which, because it is a head, is more likely to become disconnected from a speaker load while someone is playing, resulting in a destructive arc from pin 3 to pin 2.
You can also use a hum-balance pot and/or elevate the heaters to a positive (or, theoretically negative) voltage such that the heater waveform no longer crosses the tube's cathode voltage, which also lowers hum. The latter is a very old trick, found as far back as the 1930s.
I take it in the above instance the regulator is being used to keep the small signal heater supply isolated from the output tube heater supply?
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Actually there is a single 6.3V coil on the mains transformer, feeding the heater of the output tubes directly. This AC is rectified to get some 8V DC (with ground reference on the negative of the filter capacitor), and it goes to a voltage regulator MIC29303 to get 6.3V regulated DC for the small signal tubes.
Hum pot is the best choice since it is variable. With a fixed centre tap you are stuck with one option.
