‘79 DR103 mod question!

[Paul Sherman]

Just picked up a beautiful 100 watt Hylight era head; can someone identify this mod with the pot on it?

Thanks!
Paul

B181F1CD-28B7-4A8C-AFDC-A2E3876C6B94.jpeg (315.72 KiB) Viewed 380 times

[mhuss]

Looks like a voltage doubler for the bias - a common and useful mod for older DR103s, which in stock form don't supply enough bias voltage for modern EL34s. The pot is there to adjust the bias to the correct level.

When these amps were made, the original Mullard EL34s Hylight used were very consistent and robust, so they "hard coded" the bias voltage, providing no adjustment.

[Paul Sherman]

Excellent, thank you!