I have a 4-stage per channel phono+line preamplifier with separate B+ power supply. Each stage is an SRPP (6DJ8/ECC88) where the upper cathode sits around half B+, where B+ is 280V. The signal GND of L and R is also separated, they are connected to the chassis via a ground loop breaker (diodes + resistor).
Now I have a heater supply that is common for the two channels. From the raw DC 12V there are independent voltage stabilizers with 6.3VDC output for each stage. The negative of the stabilizers is common and goes to the negative of a big reservoir capacitor filtering the 12VDC.
Now I want to elevate the heater negative to 180V. How should I do it? There are several options:
- Divide the B+ at each stage and connect the heater negative there close to each stage
- Divide the B+ of the L or R channel, and connect the negative of the reservoir capacitor there
- Any considerations about the bypass capacitor of the divider center point?
- Don't care about elevation and connect the negative of the reservior capacitor to the negative of the L or R channel B+ supply
- Same as above but connect to the chassis ground
Now I have a heater supply that is common for the two channels. From the raw DC 12V there are independent voltage stabilizers with 6.3VDC output for each stage. The negative of the stabilizers is common and goes to the negative of a big reservoir capacitor filtering the 12VDC.
Now I want to elevate the heater negative to 180V. How should I do it? There are several options:
- Divide the B+ at each stage and connect the heater negative there close to each stage
- Divide the B+ of the L or R channel, and connect the negative of the reservoir capacitor there
- Any considerations about the bypass capacitor of the divider center point?
- Don't care about elevation and connect the negative of the reservior capacitor to the negative of the L or R channel B+ supply
- Same as above but connect to the chassis ground
Surely you want to elevate the heater to about 50V, so it is within spec for both the upper and lower triodes?
This. 10uF bypass cap should be sufficient.
Thanks for the suggestion. Would a divider made of 120k (upper) and 180k (lower) resistors work well, the lower bypassed with a 100uF/250V capacitor?
The heater elevation should be halfway (+70VDC) to the upper cathode's +140VDC potential.
That makes the H-K potential equal (and of opposite polarities) for the two sections of the stage.
That makes the H-K potential equal (and of opposite polarities) for the two sections of the stage.
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I understand the heater should be more positive than the cathode, in order to prevent the heater taking over the role of the cathode (and the cathode becoming anode wrt the heater).
With many tubes, in many circuits, that is not possible. Since the 6DJ8 has a maximum
H-K potential of 200VDC, it is possible in your circuit, with this particular tube.
H-K potential of 200VDC, it is possible in your circuit, with this particular tube.
I use a potential divider from B+ giving about 45VDC to heater.
Then bypass bottom resistor with a 10uf to ground.
Then bypass bottom resistor with a 10uf to ground.
That is not an issue, heater can be negative or positive without problems. In your circuit the heater should float roughly midway between the lower and upper cathodes.
Hmm, I've never seen that limit quoted before. Philips quotes Vhk(max) = 50V [heater positive], or 150V [heater negative], so I would stick within those limits since you might want to tube roll your 6DJ8s at some point. There's no need to stress the insulation more than necessary.
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Given the quite generic title of this thread: Is it better to realize elevation with a divider from B+ than from the cathode of an output tube (if that voltage is appropriate) w.r.t. hum?