THE EARLIEST FIXED BIAS AC100s - 1965 - THE 100W AMPLIFIER

Black transformers, but no brimistor - June and July 1965
- conforming to the "100 Watt Amplifier" schematic



100 Watt Amp Schematic

Dated 30th May 1965.

Around two hundred and ninety "100W Amplifiers" were made at the Burndept works in Erith in June/July/August of that year, large numbers of which were shipped to the Thomas Organ Co. for distribution in the USA - at the very least in a couple of sea shipments. A small proportion of amps were retained, however, by JMI for sale in the UK and Europe. Most of the amps sent to the USA seem to have accompanied 4x12" cabs - ie. the AC100 SDL.

A schematic was included by Thomas Organ in its repair dossiers, along with a parts list and schematics of the cathode biased AC80/100 and the newer AC100 Mark 2 (the AC100/2 with brimistor).

The fixed bias amps ran much much cooler than the AC80/100s, even if the bias was well on the low side for the EL34s, and delivered a true 100W.


early fixed bias ac100, mains transformer
serial number in the 500s, chassis front
serial number 520
serial number in the 600s (03), chassis top

More of a note than anything: four chassis survive with long triangles of purple/russet sticky tape behind the mains transformer. Chassis nos 11xx, 1201, 1237 and 1384. What is?

The Who, Copenhagen 1965

Repeated from the introductory page, The Who in Copenhagen, 25 September 1965, with six AC100s. The amps are most likely the new fixed bias "100W Amplifier" - one can tell they are not cathode biased by the presence of two screws along the top edge of the backboard. Cathode biased amps had only one.

Serial number unknown (chassis number 11xx) - currently in the USA (owned by plexi69)
early fixed bias ac100, front
Vox AC100 early fixed bias ac100, back
Vox AC100 early fixed bias ac100, top
Vox AC100 early fixed bias ac100, chassis top
early fixed bias ac100, preamp right
early fixed bias ac100, preamp left
early fixed bias ac100, preamp centre
early fixed bias ac100, chassis bottom
early fixed bias ac100, chassis underside left
early fixed bias ac100, chassis underside centre
early fixed bias ac100, chassis underside right
early fixed bias ac100, mains transformer

Probably one of the earliest "100W amplifiers" surviving. The serial plate is gone but the chassis number is 11xx, which suggests closeness in terms of date to the latest cathode bias amps listed on the serial numbers 320-430s page. The latest of those, going by the chassis, is 1093. Transformers are the new black units, there is no brimistor (and no holes on the chassis for the stand-offs required), resistors are by and large the white carbon composites ("carbon comps") made by Erie. Electrolytic capacitors are or were TCCs, and the capacitors in the signal path (in the preamp) are gold WIMA tropyfoils. On the underside - the last four pictures - note the large (2W) plate resistors on the left of the board, laid out at 90 degrees to the other components, and the fixed bias network in the centre of the board. Two new dark brown resistors stand in place of the zener diodes specified in the schematic. The wiring of the output valve bases has also been adjusted to accommodate new resistors (the square white ones) that serve to drop the screen voltage slightly. A great amp.

Serial number 502 - currently in the USA
serial number 502, cab and amp
serial number 502, midax horn
serial number 502, crossover capacitor
serial number 502, Cannon XLR
serial number 502, chassis top
serial number 502, control panel left
serial number 502, control panel centre
serial number 502, control panel right
serial number 502, chassis underside
serial number 502, chassis underside left
serial number 502, chassis underside, valve socket
serial number 502, preamp left
serial number 502, preamp right
serial number 502, bass pot
serial number 502, preamp valve socket
serial number 502, volume and treble pots
serial number 502, grid resistors
serial number 502, preamp cathode bypass capacitors
serial number 502, output terminal block
serial number 502, Burndept chassis number
serial number 502, preamp valve
serial number 502, choke
serial number 502, serial number plate
serial number 502, back panel, Cannon XLR

Chassis no. 1207, serial plate no. 502. Amp and cab have been together since 1967, and probably left the Vox factory together July-ish 1965. One can see in the pictures of the amp above that one of the pots has the stamped code DM = April 1965, and that two of the TCC capacitors have codes WB and WD, respectively February and April 65. Both are in fine condition. Note for instance the original wiring and stapling in the speaker cabinet, and the intactness of the preamp. This amp had no brimistor. The single hole (by the choke) is new. As in the case of the amp with chassis 11xx (above on this page), the choke has the code 66429, doubtless having been supplied by the company that made the black-shrouded mains and output transformers, which are later bear the numbers 66775 and 66776. One of the earliest original amp and cab sets surviving.

From this point the black-shrouded transformers have code numbers stamped on their underside (66775 for the mains, and 66776 for the output)

Serial number low 500s (serial plate not original to this amp) - currently in the USA
serial number in the 500s, front
serial number in the 500s, top
serial number in the 500s, top
serial number in the 500s, back
serial number in the 500s, back close
serial number in the 500s, serial number plate
serial number in the 500s, serial number plate
serial number in the 500s, control panel centre
serial number in the 500s, control panel left
serial number in the 500s, control panel right
serial number in the 500s, preamp
serial number in the 500s, preamp left
serial number in the 500s, preamp centre
serial number in the 500s, preamp right
serial number in the 500s, Burndept chassis number
serial number in the 500s, chassis front
serial number in the 500s, replaced choke
serial number in the 500s, mains transformer
serial number in the 500s, output transformer
serial number in the 500s, EL34s
serial number in the 500s, preamp valves
serial number in the 500s, preamp valves
serial number in the 500s, chassis bottom
serial number in the 500s, chassis underside right
serial number in the 500s, chassis underside right
serial number in the 500s, chassis underside centre
serial number in the 500s, chassis underside left
serial number in the 500s, chassis underside left, detail
serial number in the 500s, mains transformer underside
serial number in the 500s, rectifier diodes
serial number in the 500s, box underside
serial number in the 500s, box underside, grille
serial number in the 500s, box underside, grille
serial number in the 500s, box underside, grille
serial number in the 500s, back panel right
serial number in the 500s, back panel left

Chassis number 1237. This was originally a fixed bias AC100 - black shrouded transformers, "flying" 330K grid resistors in the preamp (among other signs of fixed bias arrangement), and a chassis number consonant with a serial number in the lowish 500s. At some stage, probably in the 1990s, the power section of the amp was converted to cathode bias. Despite the changes, a very nice amp. The box is likely to be original to the amp - note the two fixing points along the top edge of the back board. The box of an amp with serial no. 500s_01 would only have had one.

ROUND AMPHENOL CONNECTORS CERTAINLY FROM THIS POINT

Serial number 520 - currently in the UK
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520

Chassis number 1201. A few replaced caps in the preamp, and some new zener diodes in the power section, but otherwise good. The manufacturer's code 66775 is stamped on the underside of the mains transformer. The output transformer is unstamped. As in a number of early fixed bias amps, the feeds from the output transformer are curled round the edge of the chassis rather than being passed through the hole in the preamp. Note the Mullard BY100 ("top hat") rectifier in the bias circuit. Both XLR and mains connector have been replaced on the back panel. The screw holes that remain indicate that the speaker output, certainly, was a rectangular XLR. Further inspection will reveal whether the mains connector was an Amphenol (held by three screws) or a rectangular Cannon LNE-11 (held by two). The cover, without a legend, is original.

serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520
serial number 520

A Cannon XLR speaker connector installed. Removing the IEC connector showed that an Amphenol was originally there (a replacement now fitted). The pots have the code "DM" = April 1965; and the Daly filter capacitor "WD" = April 1965 too. Note the suppressor cap (Radiospares) for the mains switch, visible behind the panel fuseholder - installed by Vox in amps intended for the US market.

Serial number 526 - currently in the USA
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526
serial number 526

Chassis number 1236. Two XLR speaker connectors on the back panel: one a rectangular Cannon, the other an Amphenol. Remaining screw holes show that the mains connector was an Amphenol. Pictures registered here.

Serial number unknown - currently in the UK
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s

Chassis number 1204. The Amphenol mains connector remains. Possibly originally two Cannon XLR speaker connectors; only one remains. The preamp is intact. Mains transformer is original, though its top cover has been removed. The output transformer may be from a later AC100 (to be determined), as there are solder tags on its underside rather than flying leads. Date code the original filter cap. is "WD" = April 1965.

serial number low 500s

The remaining original Mullard yellow-print EL34 has the code B5D4 = Blackburn Factory, 1965, last week of April. The other three valves are GE branded Mullards.

serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s
serial number low 500s

Pics shortly after the arrival of the amp in the UK. Note the Werth "Surgistor" under the lip of the control panel by the voltage selector. The amp seems to have been repaired at a Thomas Organ repair shop, where a replacement slightly later AC100 output transformer was added, and some new components here and there. The shroud was taken off the mains transformer. Very probably the repairman followed the AC100 Mk2 schematic (rather than that of the "100W Amplifier"), supplying the surgistor as a stand-in for a brimistor. An AC50 has also been spotted (thanks to Tom for the info) with a Werth Surgistor added in the same position.

Additional note: the treble and bass pots (1M) have the date codes "DM" = April 1965. See the pic above. The volume pot (500K), which turned out to be defective and had to be removed, has the code "AM" = January 1965. Pic below.

serial number low 500s
Serial number 531 - currently in the UK
serial number 531, front
serial number 531, top
serial number 531, back
serial number 531, Amphenol power connector
serial number 531, serial number plate
serial number 531, Cannon speaker XLR
serial number 531, box side
serial number 531, box side
serial number 531, handle
serial number 531, chassis front
serial number 531, current output valves
serial number 531, chassis edge
serial number 531, chassis no valves
serial number 531, chassis and choke
serial number 531, chassis right
serial number 531, control panel left
serial number 531, control panel centre
serial number 531, control panel right
serial number 531, Burndept chassis number
serial number 531, preamp
serial number 531, preamp left
serial number 531, preamp centre
serial number 531, preamp left
serial number 531, grid resistors
serial number 531, chassis fuseholder
serial number 531, input jacks
serial number 531, bass pot
serial number 531, treble and bass pots
serial number 531, chassis underside right
serial number 531, plate resistors
serial number 531, rectifier diodes
serial number 531, bias circuit
serial number 531, underside tag board
serial number 531, EL34 sockets
serial number 531, EL34 sockets
serial number 531, Amphenol power connector
serial number 531, output valve sockets
serial number 531, original output valves
serial number 531, original output valves, detail
serial number 531, original output valves, detail
serial number 531, original output valves, detail
serial number 531, original output valves, detail

Chassis no. 1219. Serial no. 531. Another early "100W amplifier", ie. without a brimistor (note the absence of holes in the chassis for the standoffs). Evidently close in terms of components and arrangement to the amps above - note for instance the choke, and the large grey/blue Radio Spares paper capacitor across the power tagboard on the underside of the chassis, which has been treated (zinc passivated), a practice later dropped.

Three matching yellow-print Mullard EL34s came with the amp, all XF2s - the latest date code is B5E5 - last week of May 1965. The code of the non-matching GE valve is B5h6 - August 1965, which probably indicates that an EL34 blew fairly early on and was replaced in late '65. Date codes of the original capacitors that remain are VK and WD, respectively December 1964 and April 1965. The amp probably left the factory in June of that year.

Serial number 553 - currently in the USA
serial number 553
serial number 553
serial number 553

A wonderful paisley ensemble - serial no. 553 with one open-backed and one closed-backed Super Beatle cab. Chassis no. 1230. The amp has its original cover (without a VOX logo, as in the case of serial number 392).

serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553
serial number 553

Thanks to Brian for the pics.

Serial number 570 - currently in the USA

No pictures available at present: grey panel, existence reported on a VoxTalks thread.

Serial number 584 - currently in the USA (collection: Andy Sandler)
serial number 584
serial number 584
serial number 584
serial number 584

In the left hand picture, serial number 584 sitting on a 2 x 15" bass cab. Second from left, with its original 4 x 12 cab - a fine thing. Second from right, a back view, showing inputs and white warning plaque. Far right, AC100 number 854, also with its original 4 x 12" cab.

Serial number 587 - currently in the USA
serial number 587
serial number 587

Good condition. Original transformers in place. Control knobs changed, and a standby switch added, but otherwise all good. Thanks to Gary for the pics.

Serial number in 600s - currently in the USA
serial number in the 600s (06), front
serial number in the 600s (06), front
serial number in the 600s (06), front
serial number in the 600s (06), front
serial number in the 600s (06), front
serial number in the 600s (06), front
serial number in the 600s (06), front

Serial number very probably in the 600s. Note the plain metal fixings for the handle, and the presence of Amphenol connectors for both mains and XLR. Excellent condition.

Serial number in the 600s? - currently in the USA
serial number in the 600s (01), front
serial number in the 600s (01), back
serial number in the 600s (01), amp
serial number in the 600s (01), amp top
serial number in the 600s (01), serial number plate

Advertised on Gbase in late 2009/early 2010. Inverting the mandatory blurry picture of the serial number plate in Photoshop seems to give 6xx. Some sort of standby circuit was installed by the owner (fairly easily achieved unobtrusively) - a sensible precaution as this, if memory serves, was someone's gigging amp. The cabinet is a later Vox Sound cabinet from the 1970s, with handles let into the sides, and tell-tale yellow paint on the screws.

Serial number in the 600s? - currently in the USA
serial number in the 600s (05), front
serial number in the 600s (05), back
serial number in the 600s (05), top

The two screws on the top edge of the back panel indicate a fixed bias amplifier; and the white warning plaque one somewhere in the 440 - 730 range. The round amphenol mains connector remains, but the speaker XLRs have been replaced.

Serial number in the 600s? - currently in Italy
serial number in the 600s (04), front
serial number in the 600s (04), front close
serial number in the 600s (04), side of 2x15 cab
serial number in the 600s (04), back
serial number in the 600s (04), top of amp
serial number in the 600s (04), control panel
serial number in the 600s (04), control panel, right

A nice bass rig. No serial number plate, but a white plaque, so as above, somewhere between c. 440 - 730. Back panel connectors replaced, but all else looks good.

Serial number 660 - currently in South Africa
serial number 660
serial number 660
serial number 660
serial number 660

Sold on an auction site in South Africa in 2014.

Serial number 663 - currently in Japan
serial number 663
serial number 663
serial number 663
serial number 663
serial number 663
Serial number 671 - plate, warning plaque and one 18 inch speaker only, currently in France
serial number 671, serial number plate
serial number 671, bass bin front
serial number 671, bass bin side
serial number 671, bass bin side
serial number 671, bass bin grille
serial number 671, bass bin horn
serial number 671, bass bin jack sockets
serial number 671, bass bin interior
serial number 671, bass bin horn
serial number 671, bass bin horn
serial number 671, bass bin 18 inch Celestion driver
serial number 671, bass bin 18 inch Celestion driver
serial number 671, bass bin crossover

The strange remants of AC100 serial no. 671 - just the serial number plate, warning plaque and one 18 inch speaker, presumably from a Foundation Bass cabinet - therefore a bass amp - entombed in an odd, though rather handsome, PA-style cabinet. The amp, according to the owner, is long gone.

Serial number in the high 600s / low 700s - currently in the UK
Vox AC100 serial number in the 600s (02), front
Vox AC100 serial number in the 600s (02), top
serial number in the 600s (02), top
serial number in the 600s (02), back
Vox AC100 serial number in the 600s (02), preamp
serial number in the 600s (02), control panel
serial number in the 600s (02), chassis top
serial number in the 600s (02), chassis, output valves
serial number in the 600s (02), Burndept chassis number
serial number in the 600s (02), chassis underside
serial number in the 600s (02), chassis underside centre
serial number in the 600s (02), chassis underside left
serial number in the 600s (02), chassis underside right
serial number in the 600s (02), chassis underside, valve socket

Chassis number 1348, so probably a serial number in the high 600s, perhaps low 700s. No brimistor. Note that the Vox logo is composed of separate letters "V O X". One of the three original filter capacitors in the preamp has the code VL = November 1964, a while therefore before this amp was built; another has WA = January 1965. The 470ohm resistor on the output block has the code WD = April 1965; and the choke has the code EW = May 1965. May 1965 is also the date of one of the original Mullard yellow-print ECC82s (the other dates from late '64). The back board is a replacement. The output transformer is the original turned sideways - and has the code stamped in white on the side - see the pictures below.

serial number in the 600s (02), front
serial number in the 600s (02), front grille, logo
serial number in the 600s (02), box top
serial number in the 600s (02), chassis top
serial number in the 600s (02), choke
serial number in the 600s (02), output transformer, side
serial number in the 600s (02), chassis front
serial number in the 600s (02), chassis underside right
serial number in the 600s (02), preamp
serial number in the 600s (02), preamp left
serial number in the 600s (02), preamp right

Now restored to full working order with (as far as possible) new old stock components.

Serial number in the high 600s / low 700s - currently in the USA
serial number in the 600s (03), front
serial number in the 600s (03), front
serial number in the 600s (03), top
serial number in the 600s (03), top detail
serial number in the 600s (03), back
serial number in the 600s (03), control panel left
serial number in the 600s (03), control panel right
serial number in the 600s (03), box underside
serial number in the 600s (03), chassis top
serial number in the 600s (03), chassis underside
serial number in the 600s (03), choke
serial number in the 600s (03), preamp
serial number in the 600s (03), preamp
serial number in the 600s (03), Burndept chassis number
serial number in the 600s (03), Burndept chassis number
serial number in the 600s (03), preamp left
serial number in the 600s (03), preamp right
serial number in the 600s (03), preamp right

Chassis number 1384. Sold on ebay in July 2011, and offered again a couple of months later (with new pictures). Nice cosmetic condition. Changes: a replaced mains transformer (and nicely rewired power section), a new fuseholder, some renewed components in the preamp, and the provision of a switch (for changing impedance?) on the back board by the speaker output socket. Although holes are present in the upper chassis, there is no brimistor - the holes in any case are not in the right positions - see below. Note that the warning plaque on the back board is still white.

Serial number in the high 600s / low 700s - currently in the USA
serial number in the 700s (01), chassis front
serial number in the 700s (01), chassis top
serial number in the 700s (01), choke
serial number in the 700s (01), output valves
serial number in the 700s (01), chassis from side
serial number in the 700s (01), chassis from side
serial number in the 700s (01), preamp

Chassis no. 14xx. No brimistor. The code on the Woden choke is "EW" = May 1965. The amp has been nicely restored. Its black control plate is from another amp - the original (grey) is in the UK, having been sold on ebay. The box and its back panel fittings were all acquired separately.

Serial number in the 700s - currently in Japan?
serial number in the 700s (02), chassis

A single image, originally posted on the Banks Amps website in Japan. The text says no brimistor. Note the small tagboard in the preamp for the 330K grid resistors, the large grey Radio Spares 0.22uf in the bias network (underside of amp), the original 1W Erie plate resistors and green TCC caps. The black-shrouded transformers appear to have no stamped numbers on their undersides. XLR sockets for power and speaker.

Serial number 724 - currently in the UK
serial number 724, front
serial number 724, front
serial number 724, top
serial number 724, back
serial number 724, top
serial number 724, back panel
serial number 724, serial number plate
serial number 724, preamp
serial number 724, front
serial number 724, chassis top
serial number 724, chassis front
serial number 724, chassis, output transformer
serial number 724, back panel
serial number 724, serial number plate
serial number 724, Burndept chassis number
serial number 724, preamp left
serial number 724, preamp centre
serial number 724, preamp right
serial number 724, control pots
serial number 724, volume pot
serial number 724, chassis underside
serial number 724, chassis underside, power section
serial number 724, chassis underside, power section

Chassis no. 1303. New mains socket, but still the original XLR speaker connector. An impedance switch fitted on the back panel. The warning plaque is still white. Note that the new style red plaque had come in by serial no. 748.

A bit of a tangle in the power section - main filter caps have been replaced by pairs of axial caps in parallel. The output transformer is an old replacement. The choke has the code EW = May 1965. The preamp is fairly intact.

Pot codes - see below - "EM" = May 1965. Photos of the codes photographed using a a mirror have been reversed (ie. put the right way round) in Photoshop.

New photos - Sept. 2014

serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front
serial number 724, front

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