RCA: Hollywood Star, Model 32263

Apparently I can't help myself, I picked up this highly impractical but cool-looking projector with a matching speaker.  It's an RCA that was made under licence in England.  

Being an English amp, it's running EL84s in the power stage, and quite fortunately, it was full of original Mullard valves - all in great condition, with lots of life left.  

RCA Hollywood Star Projector Amplifier – Circuit Overview

The amplifier from the RCA “Hollywood Star” LMI-32263 projector is more interesting than the average projector amp. Its signal path is:

EF86 pentode → volume → triode-connected EF86 → James tone controls → ECC83 driver and phase inverter → fixed-bias push-pull EL84s

The schematic above has been simplified by removing all projector-related components, along with the projector’s mains wiring and switching. It also omits a couple of components that appear to have been factory-fitted. These included a 3.3nF capacitor from the phase-inverter input grid to ground (pin 7, the boost stage) and a 33kΩ grid-stopper. Together, these formed a low-pass filter. I removed the 3.3nF capacitor, and the amplifier sounds better without it. 

EF86 preamplifier

The first EF86 operates as a conventional high-gain pentode. It has a 100 kΩ anode resistor and a fully bypassed 1 kΩ cathode resistor. Its screen is supplied through 390 kΩ and bypassed to the cathode.

The input impedance is approximately 680 kΩ, set by the grid-leak resistor. A 5 nF coupling capacitor feeds the 1 MΩ volume control, trimming some deep bass before the following stages. A 150 pF bright capacitor preserves treble at lower volume settings.

Triode-connected EF86

The second EF86 has its screen connected to its anode, making it operate as a triode. This gives less gain than pentode operation but a much lower output impedance—roughly 15 kΩ—which makes it well suited to driving the passive tone controls.

It uses a 100 kΩ anode resistor and a bypassed 2.2 kΩ cathode resistor. A 10 nF capacitor couples it to the tone network.

James tone controls

The RCA uses passive James bass and treble controls

The main values are:

• 1 MΩ bass and treble controls

• 1 MΩ and 100 kΩ bass resistors

• 500 pF and 5 nF bass capacitors

• 150 pF and 1.5 nF treble capacitors

These values place the broad centre or pivot region at approximately 320–330 Hz. This is not a sharp cutoff: below this region the bass control becomes increasingly effective, while the treble control has progressively more influence above it.

Because the network is passive, it does not truly boost frequencies. Apparent boost simply means less attenuation relative to the middle frequencies. Compared with a conventional guitar-amplifier stack, it should provide smoother and more broadly shelving bass and treble adjustment.

ECC83 driver and phase inverter

The first half of the ECC83 restores signal lost in the passive tone network. It uses a 220 kΩ anode resistor and an unbypassed 3.3 kΩ cathode resistor. Global negative feedback from the output transformer also returns to this cathode.

The second half is a concertina phase inverter. Equal 100 kΩ anode and cathode loads produce two approximately equal signals of opposite phase for the EL84 output valves.

Each EL84 is driven through a 5 nF coupling capacitor, 10 kΩ grid stopper and 470 kΩ grid-leak resistor. The relatively small coupling capacitors also limit excessive deep bass entering the output stage.

EL84 output stage

The output stage uses two fixed-bias EL84s in push-pull. Their cathodes are grounded, with negative bias supplied to the control grids.

The output transformer has several secondary connections, but its turns ratios and intended speaker impedances have not yet been measured. These will need to be established before deciding how best to connect a modern speaker load.

Negative feedback

Feedback is taken from output-transformer secondary pin 4, with pin 3 grounded. It returns to the ECC83 driver cathode through a 100 kΩ resistor.

A 33 pF capacitor in parallel with this resistor mainly affects ultrasonic frequencies and was probably included to maintain feedback-loop stability.

Speaker cab

Single 10" alnico speaker, measures ~11DC resistance - everything points to it being a Goodmans.  The cab is cool, but a tad flimsy.   Sounds pretty nice, great mids and highs, which seems common with these.  

Notes and modifications

• B+ is now approximately 320 V, reduced from around 335 V.

• R28, the shared EL84 cathode-bias resistor, was increased from 130 Ω to 180 Ω because the output valves were running well above a reasonable dissipation level.

• The C10A 150 pF bright capacitor across the volume control was removed.

• R9, the first EF86 cathode resistor, was increased from 1 kΩ to 2.7 kΩ. The original stage had excessive gain and was distorting prematurely; the higher resistance reduced the valve current and improved its bias and input headroom.

• R15 and R16 in the bass-control network were replaced. Both had drifted more than 50% above their specified values, shifting the tone-control response too low and substantially reducing the useful range of the bass control.

• R18, a 220 kΩ anode resistor, was replaced after it had drifted to approximately 275 kΩ.

• An additional power-supply filter stage was installed, consisting of a 150 Ω dropping resistor and 33u filter capacitor. This reduced the B+ voltage and provided additional smoothing.

• An undocumented 3.3 nF capacitor from pin 7 of the 12AX7 phase inverter to ground was removed.  Together with the existing 33 kΩ grid stopper, it formed a strong low-pass filter that substantially reduced the upper-mid and treble response. The 33 kΩ grid stopper was retained.

• The unused magnetic/optical sound selector switch was repurposed as a negative-feedback on/off switch.