A simple modification to the stock N5X circuit is to add heater elevation. The idea is to reduce a small amount of 50/100Hz hum I was hearing with the power control above 5. This could be due to where my heater wires are running, but chopsticking didn’t make any difference. It could also be due to the filter caps on the power supply circuit. But I thought it wouldn’t hurt to add in elevation.
Usually, this elevated voltage would be supplied from the cathode of the power tube. But when using VVR this approach is complicated by the fact that the HT+/B+ varies, as does the cathode voltage. So our elevated voltage would vary too. This might not matter but to be safe it would be better to take it from a non-varying HT+ voltage.
In my case, I decided to take the power feed from the junction of R26 and C16 – so the same feed as goes to the preamp and which doesn’t scale.
The values of resistors are chosen to give a voltage within 10-50V:
where , and . This gives . Close enough.
To make the changes I needed:
- 5-way tag strip
- 220k resistor
- 56k resistor
- 47uF/100V capacitor
- short stretch of wire
- De-soldered the connections to the 3-way tag strip (the heater feed from the power transformer, the centre tap and the earth connection from the power board)
- Replaced the existing 3-way tag strip with the 5-way
- Connected the wires and components as below.
Checking the DC and AC voltages of the heaters I get 51.8V – so bang on.
Unfortunately, all this made no real difference to the level of hum I get with the power control above 5, so it may be my lead dress or power filter caps.
Here are a couple of before and after shots. 1st the power section as stock:
A close up of the existing tag strip:
A close up of the new tag strip. The red wire runs over to the R26 / C16 junction, under the board to keep the wire out of the way.