Copyright © Nov 2003. Author: Dmitry Nizhegorodov (firstname.lastname@example.org). My other projects and articles
3. Hum-complimentary DHT stages
4. Composite indirectly-heated driver tube
5. Conclusions and future work
Here we present several designs, all being variations of classical 2-stage SET topology, that reduce or completely eliminate the DHT hum. We preset each ide with SPICE simulation. most of these ideas were implemented with modified Moondog 2A3 SET amplifier and resulted in reduction of amplifier's hum to or below audible levels when tested with very high efficiency speakers (sensitivity is above 103 dB/W/M).
Another direction to go opens up after one can realise that typical SE topology involves signal polarity flips. That is, a plate-loaded driver stage inverts signals injected into its grid or cathode. Thus, we can develop a hum-like signal on driver which can cancel hum generated by the output tube - iff the phase and amplitude match perfectly.
The first design uses AC comping from the taps of the center-tapped transformer of the split-load B+ rectifier circuit. Such circuits are very common in SET amplifiers and the Moondog uses that, too.
Here, best cancellation happens for R14 and R15 equal to ~ 97K
Above, the value of parameter R runs from 80K to 120K by 5K, and as shown, cancellation occurs with R ~ 97K. When cancellation happens the AC voltage on cathodes of 6SN7s is ~ .95V amplitude.
The next design is very similar but uses a low-voltage secondary tap of the amplifier's transformer. Any winding that is floating and provides > 1VAC will be sufficient.
R14 and R15 are ~ 1...2k when cancellation happens.
The winding that feeds the heaters of 6sn7 tube is an excellent candidate to be wired this way.
A very elegant variation of this approach is that the taps are connected directly to the cathodes of the tubes, thus perfectly modeling a DHT, but this has proved to be sensitive to quality of the transformer; in some cases this injects RF noise into the circuit; still such variation is temptive as it is very close to how a DHT driver works. One addition twist is to use the output tube winding. This would elevate the DC level of the cathodes of 6sn7, though. Here is the idea:
Note this wiring bypasses the driver tubes. Yet the cancellation will happen, too, The values of R14 and R15 are ~ 1k.
Each design presented will work "as-is" only when the driver triodes are a perfect match; otherwise some balancing using R14 and R15 is needed.
6B4G PP amp based on Mullard toplogy: a mod to Eico HF-60
Author: Dmitry Nizhegorodov (email@example.com). My other projects and articles