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 10w - 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

Said to be a Schumacher transformer, same as old Fenders.   It looks 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.

TONE STACK: James

The James is one of the more common Bass / Treble controls, most often found in hi-fi, or amps that aren't Fender or Marshall...  although they did use them on rare occasions.  Perhaps the most well known example is in Orange amplifiers and some Ampegs.   It was used in the Marshall Artiste, and a blonde Fender Twin (6G8).

It's hard to mention James without Baxandall - same same but different, so it will not be included in this post.  The original Bax is an active control designed for hi-fi - it is incorporated into a gain stage.  The James predates the Bax by a few years and is a passive circuit (1949 for James vs 1952 for Bax).  

It's a simple design that leaves the mids mostly alone and the bass and treble controls work independently of one another (for the most part).  Think of it as two tone control operating in parallel.  Ignore RIN and RL - these are the loads of the stage either side of the tone controls.   

ORANGE, GRAPHIC MKII

link to TSC for frequency response

FILMOSOUND 385

Despite the impossibly difficult to read schematic with dual 3meg pots on a single tone control, this is actually a James eq.    You probably can't find 3m dual gang pots anymore, but you can adjust values to a 1m dual pretty easily - just divide the resistance by 3, and multiply the capacitance by 3.  

Link to TCS

AMPEG B15

FENDER TWIN, 6G8

note: this uses a tapped pot.  i.e. it has fourth lug - good luck finding one of those.

MARSHALL ARTISTE

VOX: AC2 & AC30 Schematics, EL34 based

 The little known EL34 Vox amps from the late fifties, one of which is fixed bias.   https://www.voxac30.org.uk/vox_ac2_30_1957.html. 

VOX AC2, 30 WATT AMPLIFIER.  OS/003 (FIXED BIAS)

VOX AC30, NO2.  OS/007  1960 (CATHODE BIASED)

EL34: individual 470 ohm cathode resistor and 50uf bypass caps.  5k output transformer

VOX: Vintage AC10 / AC15 Amplifier Schematics

The vintage Vox AC10 & AC15 all have the same phase inverter and output stage, with some very minor variations.   Popping these in as a reference for myself - yes, I have simplified some things / made generalisations.   If you're interested in ACs in general - https://www.voxac30.org.uk/. 

POWER SUPPLY

• Power transformer secondaries: 300-0-300, 120mA
• Rectifier:  EZ81, 320V DC output
• Filtering:  16uf - 8H - 16uf (mostly) 
• Phase inverter:  usually 22k, 8uf.  Sometimes 47k
• Preamp:  As above

POWER AMP / OT

• 8k primary, 8 & 15ohm secondaries 
• EL84 Plates: ~305 - 310V
• Cathode bias resistor: 130ohms, 5-8w
• Cathode bypass: 50uf
• Plate and screen resistors:  either one or the other, never both - 100 ohm
• Grid stopper: Usually 1k5, 10k on the 1959 AC15
• Grid leak: 220k
• Coupling caps from phase inverter: 47n/50n on AC10.  Mix of 50n & 10n on AC15
• Cut:  5n & A250k (10n on AC15 1960)

Expect about 10v on the cathode. 

PHASE INVERTER

Vintage Vox amps have two channels, each using a side of the phase inverter as an input.  For a single channel amp, the unused input of the phase inverter must be grounded with a capacitor - 47n - 100n.

• Classic long-tail pair, ECC83
• 100k plate resistors
• Coupling caps to EL84s: 47n/50n on AC10.  Mix of 50n & 10n on AC15
• 1meg, 1k2, 47k combo for grids / cathode (sometimes 1k)
• Coupling cap on input: 10n & 100n
• Volume pots from previous stage: A500k

VOX AC/10 OS/008 1960

VOX AC-10  NO. V-1-3

VOX AC/15  NO. 2  OS/005 1959

VOX AC15  NO. 3  OA/031 1960

VOX AC-15  NO. V-1-5