The power amplifier build part 4, power supplies

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Part 4 – Power Supplies


De de de

At first glance, power supplies may seem straightforward: transformers, rectifiers, filter capacitors and you’re done. In this project, that’s true for some of the low-voltage rails, but the bias and especially the screen supply needed considerably more thought.

High Voltage Supply

The high-voltage (HV) supply is built around a 3 kVA UI-core transformer, followed by a full-wave rectifier, an oil-filled capacitor, and bleeder resistors.

Since Swedish homes typically have 3-phase service, I wired the two 230 V primaries in series across 400 V. This helps balance the load between phases and reduces light flicker during key-down. The secondary winding has multiple taps; I selected a tap that provides about 3.2 kV DC under load.







Screen Supply

This was the most time-consuming part of the project. Several designs were tried, and quite a few MOSFETs gave their lives before the regulator was stable. I eventually settled on a series-regulated design.

Why not a shunt supply?

A traditional approach is a shunt-regulated screen supply. Here, the voltage is dropped across a series resistor, and an active device (tube, transistor, or MOSFET) shunts excess current to ground to hold the screen at a stable potential. The great advantage is that if the tube ever drives current backwards into the screen supply, the regulator can safely sink it.

The alternative is a series regulator, where a MOSFET (or similar) sits in series with the screen and adjusts its effective resistance to stabilize the voltage. The drawback is that if current flows from the screen back into the supply, a simple series regulator cannot sink it. The screen voltage will rise until something gives.

Negative screen current explained

Normally, the screen grid current flows from the supply into the tube. But under certain conditions – usually when the plate (anode) voltage falls close to or below the screen voltage – the screen grid can act like a secondary anode. Electrons can be collected by the screen instead of the plate, and when the plate swings positive again, charge can be returned. The result is negative screen current: current flowing from the tube into the supply.

This behavior is especially associated with:

  • High-gm tetrodes and beam tetrodes (e.g. 4CX/4CX-series, 4-1000A, some TV sweep tubes)

  • Tubes operated with low anode voltage compared to screen voltage

  • Situations with large RF swing at the plate

In those cases, a shunt regulator is the safe option.

The GU-81M, however, is a large, relatively slow tube of 1930s design. Its screen structure and operating parameters make it very unlikely to exhibit negative screen resistance under normal HF amplifier conditions. That allowed me to use a series regulator without the heat dissipation penalty of a shunt design, and for safety measures, a bleeder resistor that draws a couple of mA was no placed close to the tubes.

My implementation

  • Early version: MOSFET series pass element, with a stack of 8×100 V zener diodes for reference. This worked, but temperature drift was severe: voltage crept from 800 V up toward 1 kV as the zeners warmed.

  • Later version: Added closed-loop regulation using an op-amp comparator and feedback divider. This stabilized the output, but the first attempt oscillated.

  • Final version: Based on HA5KJ’s regulator design, effectively a hybrid series regulator with shunt-like feedback behavior. By lowering the MOSFET gate resistor from 100 kΩ to 22 kΩ, stability improved and the supply ran quietly, though with a narrower adjustment range.

PCB layout turned out to matter just as much as the schematic. My first board oscillated and destroyed MOSFETs; the second board, closely following my home etched, experimental, single-sided layout, has been reliable.


HA5KJ series screen regulator with active feedback

My version based on HA5KJ. Note R1 is decreased to 22K to get smoother regulation.
Also, i added one more zener series resistor because I´m using higher voltages than HA5KJ










A simple series regulator. It failed due to temperature drift.
A different approach could be to use VR tubes, say 8 OB2 in series for 864 V regulated.
I got a call from a fellow SM amateur, who did just that, and claimed they dont drift the same as zeners do. Might be worth a try.




Experimental ADD-ON board with the HA5KJ regulator. Later made a new proffessional board


Bias Supply

The bias supply is much simpler: a relay-switched zener shunt regulator.

The control grid draws negligible current, but the supply must still be able to sink current in case of RF overdrive. In my design:

  • Standby bias is about –320 V

  • Operating bias is stabilized at –220 V

Low-Voltage Supplies

The remaining supplies are conventional:

  • Unregulated 24 V for stepper motors and vacuum relays

  • Regulated 12 V for logic and control circuits

A 300 VA toroidal transformer with added turns provides the necessary voltages.


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