BELL & HOWELL: Model 385 Amplifier Schematic

The original schematic for the 385 was killing me - just too many part numbers listed, so I cleaned it up a bit.  I also removed the projector circuits.

I think there are two versions of this amp - this is the 6V6 version, while the other uses EL84s (which is nearly impossible to find a schematic for).

BELL& HOWELL - FILMOSOUND MODEL 385 SCHEMATIC

After the clean-up, I redrew the schematic in LTspice.  Seems to work as expected, so hopefully no mistakes...

The tone control is unique, as it uses a dual pot for the James EQ instead of separate pots for bass and treble.  Bass is cut while treble is turned up, and vice versa - or you could wire it so both bass and treble go up and down at the same time, leaving the midrange alone...  interesting.  

The dual 3meg pot is not easily found these days - in theory, resistor and capacitor values can be scaled to work with a 1meg pot, however, after spending some time on this, I can't get the response to match.   It's roughly the same, but there are obvious differences.  

The preamp could be simplified a little, and you would most certainly want to get rid of the 1meg resistor on the input.  

Hard not to notice all the little caps rolling off some highs from plates of the 12AX7 gain stages.  Something to experiment with and tune to taste.  

LTspice sim of the eq range

Original pages below

 

UNCLE DOUG CONVERSION VIDEOS

BELL & HOWELL: Model 601 Projector Amplifier

Just documenting my latest Bell & Howell conversion. - Model 601 amplifier, with 1950 capacitor dates.   

 

FENDER: Deluxe 5E3

Some notes on the 5E3, the most famous and widely copied variation of the Fender Deluxe, produced from about 1955 - 1960.

POWER

Transformer:  Triad 6452, 380–0–380 V

Rectifier:    5Y3GT

B+ ~360V  

Filtering:    B1 - 16uf,    B2 - 5k / 16uf,    B3 - 22k / 16uf

OUTPUT

OT:   8k P-P,  8ohm secondary.   Triad 108 (pre 60's).   Schumacher 125A1A.   Note the option to add another 8ohm extension speaker, which implies that the amp can handle a 4 ohm load

2 x 6V6

Va ~340 - 350

Cathode biased.  Rk 250hm, 50uf bypass capacitor

Grid stop: 1k5.  Grid leak:  220k

CATHODYNE PHASE INVERTER - 12AX7

First stage, boost:

Ra 100k,  Rk 1k5 ohms / 25uf bypass

22n coupling to grid of inverter

Second stage, invert:   

Ra 56k,   Rk 1k5 in series with 56k 

Rg 1meg to 1k5 / 56k resistor node

100nf coupling to 6V6 from cathode and anode of 12AX7

PREAMP - 12AY7

Ra 100k,  Rk 820 ohms shared / 25uf bypass.   (1,640ohms per cathode)

Grid stoppers 68k (in parallel, for high).   Standard Fender input wiring

Rg 1meg.  100nf coupling caps to volume

A1M VOLUME & TONE

A1M volume and tone pots.   

In tweed circuits, the volume pots are wired in an older style: the wiper (pin 2) is the input, while the outer lug (pin 3) serves as the output. Electrically, this is the same as the modern convention, just reversed in function.

A 500 pF capacitor from pin 3 connects back to the preamp’s coupling capacitor. This acts like a variable bright cap—its effect diminishes as the volume is turned up.

A 4.7 nF capacitor from pin 1 goes to ground, providing a simple treble cut by shunting high frequencies away.

Pin 2 (the wiper) carries the combined signal forward to the grid of the phase inverter (also connected to the output of the volume pot).

One hallmark of these amps is the interaction between the two volume controls. Both pots connect directly to the grid of the first phase inverter stage, so even if only one channel is in use, adjusting the unused channel’s volume will still alter the response of the active channel.

12AY7 x 12AX7

Parameter	12AY7	12AX7
Amplification Factor (µ)	40	100
Plate Resistance (rp)	22.8 kΩ	62.5 kΩ
Transconductance (gm)	1750 µmhos	1600 µmhos
Typical Plate Current	≈ 3 mA @ Va=250 V.  Vg = –2 V	≈ 1.2 mA @ Va=250 V.    Vg = –2 V
Max Plate Voltage	300 V	330 V
Max Plate Dissipation (per triode)	1.5 W	1.2 W
Heater Current	0.3 A @ 6.3 V	0.3 A @ 6.3 V
Typical Bias for ~1 mA	–2 V to –4 V	–1.5 V to –2 V

6V6

Parameter	6V6
Type	Beam power tetrode
Plate Resistance (rp)	~62 kΩ (effective in Class A)
Transconductance (gm)	~4100 µmhos
Typical Plate Current	34–45 mA (idle, Class A)
Max Plate Voltage	350 V (design centre)
Max Plate Dissipation	14 W
Heater Voltage	6.3 V
Heater Current	0.45 A @ 6.3 V
Typical Bias	–12 V to –15 V (Class A)
Typical Load Impedance	8 - 10 kΩ  P-P
Typical Output Power	≈ 14 W (push-pull, Class AB)

5Y3

Parameter	5Y3GT
Type	Full-wave rectifier
Maximum DC Output Current	125 mA
Peak Inverse Voltage	1400 V
Typical DC Output Voltage	~350 V DC (at 100 mA load).
Max Input Capacitance	20 µF (first filter capacitor)
Heater Voltage	5 V
Heater Current	2.0 A
Typical Voltage Drop	≈ 60 V at 125 mA load

FARRINGTON: Hybrid Valve PA head

This four-channel beast is a hybrid valve PA amp, built by a bloke named Jim Farrington in NSW sometime in the late sixties or maybe seventies.  From what I can tell, they mostly stayed local to where he lived, and you don’t see many pop up anywhere.   I picked this up on eBay, and it was sent from his hometown.

Looking at the few photos on Ozvalve Amps, it seems he used the same head and chassis for most builds, swapping out the control panel and designing amps to suit whatever job it was for.

There’s no schematics around, and I’m not about to trace the whole thing — there’s just too much going on.  I might figure out the reverb section though, and maybe keep that as is.

VINTAGE FARRINGTON PA AMPLIFIER

Some photos from the eBay listing below.

SPECS

It's going to be a little vague at this stage, and might stay this way.

• 4 x solid-state preamps:  bass, treble, level, reverb.  Hi and lo impedance inputs 
• Master volume, reverb and tone controls (bass, treble)
• The modulation indicator lamp is not related to signal modulation or trem of any kind
• Reverb:  valve driven, Hammond / Gibbs 1122 tank.  6BM8 to drive it
• 4 x EL34 fixed bias output stage
• I'm guessing some kind of 12AX7 based phase inverter 
• A&R transformers
• Solid-state rectifier
• Line output from the speaker terminals 

PLANS

The plan is to rebuild it, swapping some of the solid-state preamps for valve ones, but I’ll probably keep one solid-state channel just to hang on to a bit of its history.

Channel one:     Guitar effect, probably a boost

Channel two:     Solid state

Channel three:   Fedner preamp with bass and treble controls

Channel four:    Pentode channel of some description 

Master tone controls to be removed.  I'm guessing this amp will need to keep the master volume, as it has a reasonable amount of output power

As each channel has two input jacks, I'm considering a modular approach, with an in and out for each channel.  

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”.