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

The signal path is:

EF86 pentode → volume → triode-connected EF86 → James tone controls → ECC83 driver and phase inverter → cathode biased EL84s

The schematic above has been simplified by removing all projector-related components, along with the projector’s mains wiring and switching.  The schematic is also missing a couple of components that appear to have been factory-fitted.  These included a 330pF capacitor from the phase-inverter input grid to ground (pin 7, the boost stage) and a 33kΩ grid-stopper. 

EF86 preamplifier

The first EF86 operates as a 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 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, and a 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.

12AX7 / 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 bass entering the output stage.

EL84 output stage

The output stage uses two fixed-bias EL84s in push-pull - cathode biased and bypassed.  DC from the cathode is used to elevate the heaters.

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 very high 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.  I had to reglue some plywood that was separating.

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 330pF capacitor from pin 7 of the 12AX7 phase inverter to ground was removed.  The 33 kΩ grid stopper was retained.

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

BE: MKI Tone Bender - Shorty

Yes, it's another MKI.   I designed this layout based on a Zonk to fit a specific enclosure width.  So nothing remarkable, just one for me to use later.

MKI TONE BENDER - SHORTY

It's missing the 33k resistor that normally sits across the 50k pot, cause I like to use an A25k instead.   I sometimes make a few other taste related changes, depending on the transistors, etc. 

 

RUSH: Pep Box

One that I've probably overlooked a little, given its place in the early history of English fuzz pedals - which was likely inspired by the Maestro...  

There was a three-transistor version prior to this called "Fuzzy" which by all accounts is a Maestro.   The later versions dropped the input buffer and made the jump from 3 to 9 volts.

These are gated nasty wonders of the fuzz world.

RUSH PEP BOX - GE VERSION

The 56k resistor is sitting across the 500k volume pot, so you could just drop the pot down to 50k and lose the extra part.  

RUSH PEP BOX - SI VERSION

Later versions transitioned to silicon devices and became more widely recognised in their large red WEM enclosures.  Rush had originally manufactured these units for WEM, before the company reportedly parted ways with him while continuing to use his circuit design.  

Note:  power polarity needs to be reversed on this layout.  i.e. it is a regular negative ground circuit, it's not positive ground.  -V should be positive (top  row).  +GND is a regular ground (bottom row).  

Further reading:  

Link to Stompboxes UK

Link to FSB

Reverb

BE: MKII Tone Bender

One of my projects - just a MKII layout built to suit a particular enclosure and layout style that I've been working with lately. 

The board is just the right size for this enclosure.  Most of the assembly is done on a wiring jig first, before transferring it into the enclosure - would be a nightmare to do it any other way.  

Sounds pretty good.  The OC75 adds some texture without the need to go all out and use three of them.  They're a little expensive for that these days.  The OC84 are pretty decent.    

BE MKII TONE BENDER

Q1: OC84   Q2: OC75  Q3:  OC84

  

VOX: Vintage AC15 Schematic

Ignoring the gigantic and overly complicated trem section - this is one of my favourite sounding AC amps, which is most likely the inspiration for the Matchless DC30 pentode channel.

VINTAGE VOX AC-15 SCHEMATIC

Version:  NO V-1-5 / OA/031

Apparently this schematic is a v3, redrawn by Thomas Organ, with some different component numbering.   I can understand why they redrew it - a lot of people, including me, struggle with some of the original Vox schematics.   It appears that they may have done the same thing for the AC10 (which practically shares the entire output stage as the AC15, with a couple of minor differences)

Below is the original, which isn't the most confusing Vox schematic that I've ever seen, but I still prefer the one above. 

SUMMARY

In the Normal channel, the AC15 is an EF86 voltage amplifier feeding a volume control, which then drives an ECC83 long-tail pair phase inverter and a cathode-biased EL84 push-pull output stage.

It's nice and direct, with limited tone shaping along the way - there is of course the bright switch, and the traditional Vox Top Cut, but apart from that, this is a fairly bare bones signal path.

I'm not going near the trem channel - just too complicated for me, and I'll never build one.  

Inputs: 

• Two jacks wired High / Low 

• Each jack feeds the EF86 grid through a 68k series resistor (R5, R6)

EF86:

• Grid leak: 1M to ground (R1)

• Plate load (Ra): 220k (R15)

• Plate supply / filtering: from +315V through 22k (R14) to the EF86 supply node, filtered by 8uF (C3). Plate voltage is marked ~90V on the schematic

• Screen supply: 1M (R20) feeding the screen, with 0.1uF (C8) bypass to ground

• Cathode: 2k2 (R13) bypassed by 25uF (C5)

• Coupling cap to volume: 0.01uF / 10nF (C12)

Volume, Brilliance, Top Cut

• Volume pot (Normal): 500k (R29)

• Brilliance switch: SW3 inserts a 250pF capacitor (C17) as a “bright” bypass around the signal feed, letting extra treble through when switched on

• Tone: Vox “Top Cut” placed after the phase inverter — a 250k pot (R49) with a 0.005uF / 5nF capacitor (C36) working across the two opposite-phase drive signals

Phase inverter (ECC83 / 12AX7)

• Type: Long-tail pair (LTP) using both triodes (V5A, V5B)

• Normal volume feeds V5A grid via 10nF (C27)

• Trem channel volume feeds V5B grid via 10nF (C28)

• Plate loads: 100k each (R44, R48)

• Supply to PI node: from +315V through 22k (R43), filtered by 8uF (C26). PI node is marked ~220V

• Shared cathode / tail: 1k2 (R42) in series with 47k (R38) to ground

• Coupling caps to power stage: 10nF each (C32, C33)

Power stage (2 x EL84 / 6BQ5, push-pull, cathode biased)

• Grid stoppers: 1k5 each (R56, R57)

• Grid leaks: 220k each (R53, R54)

• Screen resistors: 100 ohm each (R62, R63)

• Shared cathode resistor: 130 ohm, 5W (R60)

• Cathode bypass: 50uF (C39)

• Plate supply is marked ~310V

Output transformer

• Secondary taps: 15 ohm, 8 ohm, and common

• Primary: 8k p-p

Rectifier and power supply (main)

• Rectifier: EZ81 (6CA4)

• Filtering: 16uF reservoir (C10) → choke L1 (10–20H) → 16uF smoothing (C15) to the main +315V source

Speakers

• A few variations of single and twin speakers - all 12" 

• Goodmans Axiom, Fane and of course Celestian blues

EL84 OPERATING CONDITIONS

Operating conditions are similar to the EL84 datasheet. 

EF86 OPERATING CONDITIONS

The preamp circuit is almost identical to the EF86 datasheet

VOX: Vintage AC6 Schematic

The Vox AC6 is a departure from the Vox AC2 & AC4, in that it has a 12AX7 preamp, negative feedback, no trem, and for some reason, the tone control is before the first triode - it's a bit of an odd-ball in the Vox lineup from the time. 

The 12AX7 preamp arrangement would give the amp a different flavour to the EF86 versions, but it's still an EL84 driving an alnico speaker in about the same size cab, so similar to its friends in that sense.

Further reading:  VoxAC30.org.uk   

VINTAGE VOX AC6, GUITAR AMPLIFIER SCHEMATIC

SUMMARY

In simple terms, the circuit uses two 12AX7 triodes feeding a volume control, which then drives the EL84. The first triode is cathode-bypassed, the second is unbypassed and receives negative feedback from the output transformer, and the tone control sits ahead of the first triode.

Some of the schematic values are hard to read, so hopefully this is close.  That said, apart from the unusual placement of the tone control, the 12AX7 stages are biased cooler than a typical Fender-style preamp.  

This cooler operating point doesn’t so much reduce gain as it shifts the clipping behaviour, with the stage tending to run out of headroom on one half-cycle first, giving asymmetrical clipping when driven.

The 220 kΩ input resistors are also a little unusual.  As drawn, they don’t just act as isolation resistors — they form a voltage divider with the 1 MΩ grid leak on the first triode.  The result is a small amount of input attenuation, so some signal level is lost before the first gain stage even begins to amplify.

The 12AX7 Preamp

Inputs:  

• Two jacks.  

• Each jack feeds the first triode's grid through a 220k series resistor

First Triode:

• Grid leak: 1M to ground

• 12AX7 plate load (Ra): 100k

• Cathode resistor (Rk): 4k7

• Bypass cap: 25uF, 50v

• Coupling cap to next triode: 0.01uF (10nF)

Second Triode:

• Grid leak: 470k to ground

• 12AX7 plate load (Ra): 100k

• Cathode resistor (Rk): 4k7

• Negative feedback from the output transformer to the cathode, 47k series resistor

• Coupling cap to volume pot: 0.05uF (50nF)

Volume & Tone control:

• Volume pot: 500k (wiper feeds the EL84 grid via a 4k3 grid stopper)

• Tone: 250k pot with a 0.005uF (5nF) cap in series to ground from the grid of the first triode

Power stage: EL84 (6BQ5) single-ended, cathode biased

• Grid stopper: 4k3

• Grid leak: via the volume pot

• Cathode resistor (Rk): 150 ohms (2W or 3W)

• Cathode bypass:  25uF, 50v

Output transformer:

• Secondary: 3 ohms

• Primary: Probably 5k or 5.2k, which was common with Vox at the time

Rectifier: EZ80 (6V4)

Power transformer:

• HT secondary: 250V-0-250V (as shown)

• Heater winding: 6.3V

• Primary shown with 250v, 230v and 85v

Power supply filtering:

• 32uF reservoir (C1) → 1k (8W) series resistor (R3) → 32uF (C2) main B+ node - likely to be 270v

• From +270V:  47k dropper (R5) → 8uf feeding 12AX7

Speaker: 

• Elac 8" alnico, 3 ohm

VOX: Vintage AC2 Schematic

Another early Vox model is the AC2.  It’s often described as being the same as an AC4, but there is one key difference — the tone control.  

Instead of sitting in the preamp like the AC4, the AC2’s tone control is wired in the power stage, between the EL84 plate and the B+ supply (effectively across the output transformer primary).  

It obviously works, but no one does this anymore - the tone control is sitting at just short of 300 volts DC, which isn’t ideal for a number of reasons.  Curiously, they chose the opposite direction for the AC6, with the tone control before the first triode of the preamp.    

Further AC2 reading:  AC2 on VoxAC30.org.uk 

VINTAGE VOX AC2, GUITAR AMPLIFIER SCHEMATIC

VOX: Vintage AC4 Schematic

Despite the name - and even the familiar look - the original Vox AC4 has very little in common with the modern “AC4” amps wearing the same badge.  The cabinet and the single EL84 output stage are about all they share.

The AC4 started life as the AC2 in 1958.  Fair to say that the AC4 is probably best known due to its modern counterparts, despite being a different beast.

The circuit, layout and feel are very much of their era — the preamp and power stage are almost text box examples from valve datasheets - and if you want the deeper backstory, the Vox Showroom & AC30.org both have an excellent history of this little amp.  

VINTAGE VOX AC-4 GUITAR AMPLIFIER SCHEMATIC

Minor error on the schematic.  The on / off swich for vibrato is the wrong way around.

SUMMARY

It's a classic single-ended amp design, point-to-point construction, valve rectifier.  High gain from the EF86 firing pretty much straight through to the EL84, into an open back cab with an alnico speaker.   

There's very little loss in terms of low-end or with almost no frequency shaping along the way (at least before it hits the output transformer and small alnico speaker).  The tone control just rolls off some treble; no other shaping takes place. 

There is one unusual thing about the preamp - a huge 5.6meg screen resistor is taming the EF86: it starves the screen to drop gm and gain, improves linearity, and when you push it the screen current dynamics produce that soft ‘screen compression’ feel pentodes are famous for.  By comparison, 1meg is the datasheet value when paired with Ra 200k.

The trem wiggles the bias of the EF86 (cathode coupled), which is a fairly typical design from the era, not unlike a Vibro Champ.

The EF86 Preamp

Inputs:  Two jacks.  Each jack feeds the EF86 grid through a 100k series resistor

Grid leak: 1M to ground

EF86 plate load (Ra): 220k

EF86 screen supply: 

• 5.6M (Rg2) from the preamp B+ node

• Screen bypass: 0.1uF (100n) to ground

EF86 cathode:

• Cathode resistor (Rk): 1k5

• Bypass cap: 25uF

• Cathode voltage: ~2.7V

• Trem oscillator connects to the cathode

Coupling cap to volume/tone network: 0.047uF (47nF)

Volume & Tone control:

• Volume pot: 1M (wiper feeds the EL84 grid via a 6.8k grid stopper)

• Tone: 1M pot with a 0.001uF (1nF) cap in series to ground from the signal node (treble-cut)

• Power switch is shown as part of the tone control assembly

Power stage: EL84 (6BQ5) single-ended, cathode biased

• Grid stopper: 6.8k

• Grid leak: via the volume pot

• Cathode resistor (Rk): 150 ohms (2W)

• Cathode bypass:  25uF

• Cathode voltage: ~8.5V

Output transformer:

• Secondary: 3 ohms

• Primary: 5k / 5.2k as stamped on output transformers 

Rectifier: EZ80 (6V4)

Power transformer:

• HT secondary: 250V-0-250V (as shown)

• Heater winding: 6.3V, 2A (as shown)

• Primary shown with 230V / 115V options via selector plug

Power supply filtering:

• 32uF reservoir (C1) → 1k (5W) series resistor (R3) → 32uF (C2) main B+ node labelled +270V

• From +270V:  22k dropper (R5) → 8uF cap, ~260V node feeding the EF86 & 12AX7

Vibrato oscillator: ECC83 (12AX7)

• Cathode resistor: 3.3k with 25uF bypass (cathode marked ~1.7V)

• Speed pot: 1M

• Coupling caps shown: 0.02uF and 0.01uF, plus 0.01uF to ground in the network

• Footswitch shown to switch vibrato on/off

Speaker: 

• Elac 8" alnico, 3 ohm, sometimes Goodmans

EL84 OPERATING CONDITIONS

Operating conditions are similar to the EL84 datasheet.  I included both the 5.2k and 7k primary impedance conditions, as some modern transformers offer both options (or at least 5k and 8k).   

Many AC4s had transformers stamped with 5000, or 5200, indicating their primary impedance.  Given how common EL84 based amps would have been at the time, a stock transformer from any number of manufacturers would have been readily available.

EF86 OPERATING CONDITIONS

The circuit is very similar to the EF86 datasheet, but that massive screen resistor is hardly textbook.

BELL & HOWELL: 622 Projector Amplifier, 5E3 Tweed Deluxe Conversion

Documenting another Bell & Howell amplifier conversion.

While it has a different model number and a shiny red faceplate, it's the same amp as the 621 and others in the series.  Quite often, variations of the model number has more to do with the projector than the amp. 

BELL & HOWELL 622 PROJECTOR AMPLIFIER

Stock - the infamous rats' nest.

Now with the unnecessary parts removed - although there is still more work to do.

CONVERSION NOTES

Plan A was to strip it back and rework the preamp, but there were too many out-of-spec parts and it wasn’t heading in the right direction. When I powered it up, it ran really poorly and there was a fair bit of resistor noise.

So I moved to Plan B – gutted it and built a 5E3-style amp (Fender Tweed Deluxe). It came together really well.   There’s a lot more space in the chassis once the old boards are gone.  I added a few tag strips for mounting parts – one for the first filter cap, and a couple more for the phase inverter and preamp.

Preamp

I used an ECC35 / 6SL7 for the preamp.  A single triode provides more than enough gain for a Deluxe Tweed–style amp, as the 6SL7 has higher gain than the 12AY7 originally used.  The earlier Deluxes used 6SC7 ocatal tubes for the preamps, which is almost identical to the 6SL7 when biased similarly, although the earlier models were mostly grid-leak biased.  

I added a SPDT on-off-on switch on the cathode to allow for full bypass using a 22u cap, partial with a 680n, and no bypass cap for lower gain.  At one stage, I tried this on the gain stage in the phase inverter - similar result, but I didn't like the way it was laid out in the chassis.  

Phase Inverter

The Deluxe uses a 12AX7 in a cathodyne inverter.  I kept the 6SL7 from the projector here — it's lower gain balances the extra gain in the preamp.  A 6SN7, which has even less gain, is also a reasonable option if you want something even cleaner.  Similar to the preamp, older Deluxes used a combination of 6SC7 and 6SL7 tubes here.

I recently came across the Dumble mods for the 5E3 - one of which involves adding feedback to the phase inverter.  I gave it a try and liked it.   

I used the spare valve socket next to the preamp to mount the preamp bypass caps, and the existing hole in the front panel for the SPDT switch.  

Rear Panel

I swapped the unusual power connector for a standard IEC.  A few minutes with a file and one extra hole was all it took to make it fit.  This model already had a full-size fuse holder, so that was left as is, after confirming that it was the right value fuse.

The speaker outputs were replaced with cliff jacks, as they just manage to cover the holes.  

Grounding & heaters

I used the unused 6V6 socket as the star ground point, leaving the heater wires connected there since it’s the first stop from the transformer.

The elevated heater voltage was also left as is - in a chassis this size, it doesn't hurt to have a bit of extra protection from heater hum (heater centre tap connected to the cathode of a 6V6).

 

VALVE DATA: 12AY7, 12AX7, 6SL7, 6SN7

Parameter	12AY7	12AX7	6SL7	6SN7
Amplification Factor (µ)	40	100	70	20
Plate Resistance (rp)	≈ 22.8 kΩ	≈ 62.5 kΩ	≈ 44 kΩ	≈ 7.7 kΩ
Transconductance (gm)	≈ 1750 µmhos	≈ 1600 µmhos	≈ 1600 µmhos	≈ 2600 µmhos
Typical Operating Point (per triode)	Va≈ 250 V, Vg≈ –2 V → Ia≈ 3 mA	Va≈ 250 V, Vg≈ –2 V → Ia≈ 1.2 mA	Va≈ 250 V, Vg≈ –2 V → Ia≈ 2.3 mA	Va≈ 250 V, Vg≈ –8 V → Ia≈ 9 mA
Max Plate Voltage	300 V	330 V	300 V	450 V
Max Plate Dissipation (per triode)	1.5 W	1.2 W	1.2 W	2.5 W
Heater Current	0.3 A @ 6.3 V 0.15 A @ 12.6 V	0.3 A @ 6.3 V 0.15 A @ 12.6 V	0.3 A @ 6.3 V	0.6 A @ 6.3 V
Typical Bias for ~1 mA	≈ –2 V to –4 V	≈ –1.5 V to –2 V	≈ –1.2 V to –1.6 V* see note	≈ –0.8 V to –1.2 V* see note

Notes: Values are per triode section. “Typical” figures are representative datasheet values; exact bias depends on load, B+, and chosen operating point. For 6SN7, designers commonly use ~9 mA at around –8 V (Va≈250 V); a 1 mA bias is atypical and only shown to preserve the row alignment across tubes.

Sources: RCA Receiving Tube Manual RC-30 (1975): data pages for 12AX7A/7025, 12AY7, 6SL7GT, 6SN7GT/GTB. See also Radiotron Designer’s Handbook, 4th ed., triode data summaries; and Morgan Jones, Valve Amplifiers, 4th ed., Appendix “Valve Data”.

COLORSOUND: 1971 Overdriver

The Colorsound Overdriver / Power Boost family went through a few different versions over the years. This layout is for the classic 1971 9V Overdriver version – essentially the 9V successor to the earlier 18V Power Boost, with the same basic three–transistor topology but set up for a standard single-supply pedalboard. It makes a huge, dynamic boost that can run from almost clean to a thick, fuzzy overdrive, with a very effective bass and treble control.

I went with the 1971 9V spec simply because 9V is more convenient, and I had one of the larger Gapco enclosures that suited the longer 0.15" vero format.  This layout also includes a post-gain volume control (master volume).  Original units did not have this, but given how loud the circuit is, most people prefer to keep the master.  If you really want to be vintage-correct you can omit it and wire the output straight from the gain pot.

There is a lot of excellent historical information on the Power Boost / Overdriver on Kit Rae’s site, including a clear explanation of the differences between the 18V Power Boost and the 9V Overdriver, and notes on various reissues.

COLORSOUND 1971 OVERDRIVER – 0.15" VERO LAYOUT

Layout notes: this is a 0.15" stripboard layout sized for a larger vintage-style enclosure. Check transistor pinouts before soldering – the original BC109 devices can be swapped for common modern NPN types if you re-orient the legs correctly. The added volume control is a true output master; if you omit it, take the output directly from the gain pot lug as marked.

COLORSOUND 1971 OVERDRIVER SCHEMATIC

COLORSOUND 1971 OVERDRIVER VIDEO DEMO

Here’s a short demo of this layout in use, showing the range from clean boost through to full overdrive.

SELMER: Buzztone

Sorry it took so long...   apparently this can be tricky to make, so hope it goes well.  Maybe breadboard it first.

SELMER BUZZTONE 0.15" VERO LAYOUT

BELL & HOWELL: Model 621 Projector Amplifier, 1951

Despite saying never again - I now have another Bell & Howell projector amp.   This is a 621 model from 1951, which as far as the amp is concerned, looks almost identical to my 601 from 1949.  I believe the main differences were with the actual projector itself - the 621 being a later and improved model.

Unlike the last amp, where I rebuilt everything on tag boards following the original schematic, apart from removing non-audio related components, I did as little as possible to this one.  Surprisingly, it didn’t take as long as I expected.  Many people gut these and start fresh, often going down the tweed fender path.

It's in the 12w - 15w output range, using a pair of cathode biased 6V6.   Controls are simple: volume and tone.  The tone control is best described as a bass cut, with a few tricks to it.

POWER

110v on the primary side, I'm in Australia, so it needs a stepdown transformer.  There's also a weird 91v tap, which I think has somehting to do with the projector.  It also had the infamous death cap across it, which I removed.

6.3v heaters with centre tap, elevated via the 6V6 cathodes 

5v heaters for rectifier

B+ is around 350V

OUTPUT TRANSFORMER

It looks a little small for an output transformer, but it doesn't seem to be lacking in any areas.  

Primary - Yellow, Black (CT), Red.  420 Ω plate to plate 

Secondary - Black (GND), green 8Ω, blue 16Ω

• 16 Ω tap → voltage ratio ≈ 23 : 1 (15.55 V ÷ 0.675 V)
• 8 Ω tap → voltage ratio ≈ 33 : 1 (15.55 V ÷ 0.474 V)
• Reflected primary impedance comes out at ≈ 8.5 kΩ from either tap

621 TONE CONTROL

Probably the most interesting part of the amp is the tone control.  It's quite different to the better known model 385 Filmosound.  Not even a close comparison, as the 385 is a James style equaliser with a very hard to find 3meg dual-gang pot.

Instead of the familiar FMV or James/Bax stack that sits between two stages, this amp has its entire tone circuit right on the cathode of the ECC35 triode - it's primarily a bass cut.  A single 50 kΩ pot and three capacitors juggle the amount of local feedback inside that valve and the amount of global negative feedback (NFB) returned from the output transformer. 

Keep in mind that this is from LTspice, so it's not going to be spot-on, but it will give an idea of what it does.  All three capacitors interact with one another and the negative feedback.

Decreasing the value of C27 shifts the frequency where the bass starts rolling-off down.  I changed mine from 50n down to 33n, as the 50n cap in this amp was dead, and why not...  If you make it really small - like 1n, you actually get a mid-range boost as part of the sweep (according to LTspice).   I'd say don't take my word for it, clip some caps in and see what they sound like.

C26 affects the overall bass response, though it operates in a slightly unusual way. Best to leave it as is.

C28 is a bit harder to describe. It also influences the bass roll-off, particularly around a dip in the response—admittedly, that’s not the best explanation, but it’s a tricky one to summarise.

 

VALVE LINE-UP

6J7 pentode for the preamp - RCA

ECC35 for a gain stage and phase inverter - Mullard

6V6 for the power amp - Radiotron Australia

5Z4 rectifier - Mullard

Before I got started - how about we just call it "an efficient use of space" and leave it at that.  This is why a lot of people just gut them and start fresh.  You literally can't fit a soldering iron down in some areas.

Clean-out underway – removing all non-audio components and doing a bit of rewiring to make things easier to work on.

It came with a friend (on the right) — a widow-maker without a power transformer.  It's useful for parts, but not really suitable as a guitar amp.  One of the electrolytic caps had exploded at some point, so the internals were a bit of a mess.

BELL & HOWELL MODEL 621 SCHEMATIC & LAYOUT

Full user manual can be found here.

BELL& HOWELL 621 AMP SCHEMATIC - TRIMMED DOWN

Trimmed back to the essentials.  I removed some of the weird feedback around the preamp pentode, as it doesn't really add anything useful.  I tried to match the numbering to the original schematic as much as possible.

6J7 PENTODE INPUT STAGE

If you only make one modification, shield the wire running to the cap of the 6J7.

The cap connects to the grid — effectively the input of the pentode, so any noise it picks up gets amplified straight away.   Keep in mind the whole amp was originally housed inside a cast iron projector chassis, fully shielded.  They were never meant to be pulled out and run standalone.

  

My notes are pretty bad - but you can kind of see what I chopped out.  The only addition I made was the 1meg resistor on the input to ground.  

Components shaded in red have been removed.