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April 30, 2002

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Part 4:
Part 5:
Part 6:
Part 7:
Part 8:
Part 9:

Part 3: HT Supplies and Regulators

Auxiliary supplies:
Constant (untimed) AC is applied to the secondaries of two more filament transformers, T3 and T4. These are for valves V1 and V3, respectively; both of these valves operate with their cathodes at several hundred volts above ground, and therefore require isolation to prevent cathode-to-heater arcing.

The secondary of T4 is also connected to the "secondary" of T5, an identical 17V, 2A transformer with dual (120V / 240V) primary scrounged from junked handheld 2-way radio battery chargers. T4 steps the 120VAC line voltage down to 17 volts, and T5 then steps this 17 volts up to (nominal) 240 VAC. This is rectified and filtered by D8 and C13 to provide a low-current 220VDC supply, to power the "small-amp emulator" and also provides the B+ for the voltage follower stages for the power amplifier.

Another rectifier/filter consisting of D9 and C14 provides a negative 220V supply. This provides bias for the 0D3 -150 volt regulator, V4 via resistor R20. Capacitor C15 provides a bit of noise reduction on the regulator. While this tube operates much like a zener diode, it's important to point out that only small parallel capacitances are allowed; otherwise the circuit becomes a relaxation oscillator. This -150 regulated voltage supplies the negative grid bias for the voltage followers (and hence the final PA tubes), and also forms the reference for the positive regulators described later.

Schematic, HT Power Suppies

Main B+ Supply:
Timed AC input is applied to the primary of the plate transformer T6, a very substantial piece of iron that I was fortunate enough to horse-trade with my old pal Peter McMullin (thanks Peter!). This is a military-surplus transformer manufactured by Hammond, with multiple primary taps allowing trimming to the output voltage desired. The taps chosen ended up producing 840-0-840 volts RMS at a load of 400 mA. Diode strings D10-13 and D14-17 (a total of eight 1N5408's) are conventional full-wave rectifiers, with plenty of voltage headroom.

A choke-input filter consisting of L1 (Hammond 193N, 3 Hy 0.5A) provides about 750 volts under quiescent conditions, sagging not much lower than about 720 volts under full power. This project has given me a profound appreciation for choke-input filters, despite its high power capability the power-supply hum in the finals is completely inaudible. The capacitor bank consisting of C4-C9 provides the main energy tank, with a total capacitance of 330 uF this gives storage of 93 joules at 750 volts. Resistors R11 and R12 provide a bit of added insurance to equalise the voltage drop across each half of the bank, in the event that internal leakage currents should drift. R14 and R15 are a voltage divider for the metering circuit. R13 is a 15 mA. bleeder resistor, which insures that output voltage will not soar above the 900 volt rating of the filter capacitors during no-load conditions (as could be caused by a circuit fault, or operator indiscretion).

+420 Volt Active Regulator
The main +420V B+ supply for all the preamplifier sections is derived from a pretty straight-forward active regulator consisting of beam pentode V1 (a 17KV6 or similar "deflection tube"), V2 (a generic medium-mu triode such as 6J5 or 6C5), and reference V4. V1 is essentially a voltage follower (common anode amplifier); the voltage at the cathode will be close to the voltage on the control grid. The anode of V2 is connected to this grid, and is biased by plate-load resistor R17.

The output voltage at the cathode of V1 is sensed by the voltage divider consisting of R18 and R19. The bottom end of R19 is connected to the -150 volt reference, and the divided output is applied to the control grid of V2. This forms a DC negative feedback loop which regulates the output voltage within a few volts, over a wide range of output current.

The network consisting of R16 and C16 provide a rudimentary screen supply for the pass regulator V1. Capacitor C10 has the effect of having its capacitance effectively multiplied by the mu of the pass regulator, providing additional hum suppression. Capacitor C11 limits high-frequency response of the circuit, for stability.

+300V Screen Regulator:
Finally, valve V3 forms a simple pass regulator for the screens of the output tubes. Again, it is essentially a voltage follower, referenced to about 3/4 of the voltage from the main regulator by the divider consisting of R24 and R25. Again, the circuit effectively multiplies the capacitance of C12 for further hum reduction. A slight amount of local feedback (empirically found necessary for overall stability) is provided by cathode resistor R26. Similarly, screen bleeder resistor R28 is added as insurance against instability potentially caused by secondary emission of the screen grids. R27 forms the dropping resistor for the metering circuit.

A late-breaking development (August 2002) was to split the screen bleeder resistor into two sections, R28a and R28b to provide a fairly stable 200 volt supply for the first-stage preamp. The other "raw" B+ supply can be expected to sag somewhat with "small-amp emulator" overdrive conditions, and is therefore suitable for later cathode-follower stages and such, but for the critical first stage a bit more stability is desirable. Hence, the old voltage divider trick.

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