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Amplifier stuff

I am happy to share details and insight into vintage amplifier restoration. My present focus is documenting amplifiers of Australian origin and sympathetically restoring them - often as a guitar and/or bass amplifier, but sometimes as original. Inherent safety and circuit protection are typically improved or added, and performance is improved where appropriate. This website has sections on
  • Amplifier restorations (including additional material on Williamson amps and Selmer organ)
  • Supporting sound and test equipment
  • Self-split push-pull including Parry Cathamplifier
  • Valve reliability
  • Some other valve related goodies
  • And some articles I've put together
Linked files sometimes get updated, so please refresh your cache (F5) if you want the latest file version.
Information on self-split push-pull circuits, valve reliability articles, other valve related goodies, and articles I've put together.

I have amplifier restoration details on the following:

Audio Engineers 30W PA amplifier circa 1959. Microphone input to 6AU6 pentode stage. 6AU6 pentode stage mixer for Radio and Gram inputs. 12AU7 long-tail PI to KT66 PP common cathode bias, with 18 to 300 ohm speaker tappings. Rack mount chassis.

Australian 5W combo amplifier circa late 1940's. Unknown manufacturer. Microphone input to 6J7 pentode stage and 6V6 SE output stage with Rola output transformer and 8" speaker.

Australian 15W amplifier circa 1952. Unknown manufacturer - probably from Melbourne. Microphone and Pickup inputs via separate front panel 2501 microphone sockets. Switched microphone to 6J5. Summed MIC and PU to 6AU6. 6J7 split-load/concertina PI. 6L6G PP 15W. Ferguson OP58 10k OT. Trimax TP1895 PT with 5V4G rectifier.

Australian 6GW8 amplifier circa late 1960's. Unknown manufacturer - probably a kit of parts. Two 2-pin input sockets. Two 4-pin speaker sockets. Zephyr Mic step-up transformer. EF86 gain stage. 6GW8 triode gain stage and cathodyne PI. 6GW8 pentode, cathode bias PP output stage. Ferguson PF170 PT, and A&R OT2632 OT.

7W Stereo Amplifier circa 1959-1960 - Made by Australian Sound & T.V. for Royal Melbourne Technical College. 6BM8 PP UL 9k output stage; cathodyne PI with direct coupled triode preamp; 12A?7 input preamp with baxandal bass/treble tone controls and volume and balance controls; 12A?7 gain recovery.

Victorian school PA System circa mid 1960's - Made by Australian Sound & T.V.. Combination radio, record-player, microphone input, monitor speaker, PA amplifier in free-standing steel cabinet. Features high quality construction, Woden MT101 mic transformer and 12AX7 preamp, EF86-12AU7-quad 6CA7 PP amplifier with A&E 2096 PT, and 2596 OT, and 6AL3 plate-ground catch diodes for PP stage. Essential school additions include a Bell pot and EF86 relaxation oscillator bell generator module with rate and decay pots, plus Garrard TA MkII for playing the national anthem.

Australian Sound Systems A13 amplifier circa 1946-7. Australian Sound Systems from Melbourne closed in 1947. Three input (MIC, PU, Radio) channel PA amplifier. 6J7 anode load RC tone. Voltage divider PI common cathode biased 6SN7. 6V6G cathode biased PP.

AWA 1G7053 35W PA Amplifier. 6J7 mic preamp. 6J7 P.U. preamp. 2x 6J7 Barnes mixer and PI. 807 fixed biased PP with PP choke input grid feed; 2x 5V4G rectifier with back-bias; Circa late 1940's.

AWA 1G8241 12W PA Amplifier. 6J7 mic preamp. 6J7 P.U. preamp. 2x 6J7 Barnes mixer and PI. 6V6 cathode biased PP; 2x 6X5GT rectifier; mains and 12V battery powering; Circa late 1940's.

AWA PA30BZ circa mid 1960's. Two MIC input channels, plus phono and radio, 30W. KT88 individual cathode biased UL output stage, with optical limiter.

AWA PA746 circa 1965. 7W ss PA amplifier with 2N217, transformer coupled 2N270 driver stage and 2N301 PP output stage.

AWA PA774 circa mid 1960's. MIC input 12AX7 triode stage, with PU mixer 12AX7 triode stage, 2x 6BM8 cathode biased PP with floating paraphase PI, 8W.
Details on restoration of 2nd amp. Details on restoration of 3rd amp.

AWA PA806 circa late 1950's. MIC input 12AV6 triode stage, with PU mixer 12AX7 triode stage, 12AX7 stage with output winding feedback, 6V6GT cathode biased SE, 5W.

AWA PA825 15W Amplifier. 6AU6 mic preamp. 6AV6 Phono-Radio preamp. 6AU6 mixer. 12AU7 self-balancing, floating paraphase PI with unbypassed cathode biases. KT66 cathode biased PP; separate feedback winding to PI stage cathode; 5V4G rectifier; Circa 1956.

AWA PA827 70W Amplifier. 6AU6 mic preamp. 6AV6 Phono-Radio preamp. 6AU6 mixer. 12AU7 self-balancing, floating paraphase PI with unbypassed cathode biases. Quad KT66 cathode biased PP; separate feedback winding to PI stage cathode. 5R4GY rectifier; 6AQ5 regulated screen voltage. Separate filament transformer; elevated dc voltage heater humdinger pot. Circa mid 1950's.

AWA PA829 AC/DC 20W Amplifier. 3x 6AU6 or EF86 mic inputs, 6AU6 mixer, 12AX7 self-balancing floating paraphase PI with unbypassed cathode biasing, 2x KT66 cathode biased PP. V5123 vibrator for 12VDC. 2x 6X4 rectifier. Circa late 1950's.

AWA PA872 20W Amplifier. 12AX7 mic input, 12AX7 mixer, 12AX7 self-balancing floating paraphase PI with unbypassed cathode biasing, 2x KT66 cathode biased PP. 5V4G rectifier. Circa 1964.

AWA PA1001BY. Multi-input channel 100W PA amplifier with tuning unit. EF86 mic preamp. 12AX7 preamp and mixer. 12AX7 PI. 12AU7 bootstrapped driver. McIntosh unity-coupled PP output stage with screen-to-cathode capacitors for each of the quad 6L6GC. Circa early 1970's.

AWA PA1002BY. Three input channel 50W PA amplifier. McIntosh unity-coupled PP output stage with screen-to-cathode capacitors, and bootstrapped drivers. Circa early 1960's.

AWA PA1003. 2x 12AX7, floating paraphase PI, cathode biased 7027A PP, 25W, using 12AX7 heaters for cathode bias. SS diode doubler power supply.

AWA PA1005 S.N.CK3336. AWA PA1005 S.N.CZ1345. 2x 12AX7, floating paraphase PI, cathode biased quad 6AQ5 PP, 12.5W. A little Green Terror!

The Parry 'Cathamplifier'. Details about this brief flowering of aussie ingenuity.

Eminar 70W Bass. 3x 12AX7, 2x EL34 fixed bias PP.

Eminar 200W PA. 4x 12AX7, 6x 6CA7 fixed bias PP.

ENCEL X1212 stereo amplifier. Sold by ENCEL (Melbourne), but made in Japan circa late 1964. Stereo 6GW8 PP.

Ferrograph 2A/N amplifier. Part and design details on 1955 amplifier only, including schematic. 3x EF86, 12AT7, EL84 SE with Partridge T1428 OPT.

FI-SONIC 30WGR. 30W guitar head circa 1966-7. 4x 12AX7, 6AV6, 6BM8, 2x 6L6GC, 6V4. 2 normal and 2 brilliant inputs. Bass/treble fender style tone stack. Neon/LDR tremolo. Reverb for 300 ohm input pan. LTP PI with output feedback to cathode tap. Cathode biased and bypassed PP. A&R 5547 PT and 2783 OT.

McPherson Twin-Channel 60W Instrument Amplifier - 12AX7, 6AN8, 2x KT88, 5AS4 - 1964.

McPherson Mk IV 55W guitar amplifier - 3x 12AX7 cathodyne PI, 2x 6CA7 fixed bias PP - 1965.

Philips 952. Three input (2x MIC, 1x PU) channel PA amplifier. 807 PP, grounded cathode, grid driver Tx. Circa early 1950's.

Philips 961. 2x 6AD8, 6N8, 6SN7 cathodyne PI, 2x 6CA7 fixed bias PP, 100W. Circa late 1950's. Modified: 12AX7 replacing 6AD8s, 6N8 - WIP.

Philips 976B 8W PA amp. Two input (MIC, PU) channel PA amplifier. 2x 12AX7 for MIC gain, mixer, tone and PI stages; 6M5 PP cathode biased; 6V4 rectifier; circa 1958.

Philips 979. Five input (4x MIC, 1x PU) channel PA amplifier. 2 MIC inputs to 2x EF86. 2 MIC inputs to 12AX7. 6N8 mixer. Half 12AX7 tone make-up, with feedback to part-cathode. Half 12AX7 cathodyne PI. 6CA7 fixed bias PP, with spark-gap protection. 2x 5U4G Doubler HT. 6V4 for negative bias supply. Circa 1955-6.

Philips 982. Two input (MIC and PU) channel 25W PA amplifier. MIC inputs to 12AX7 triode. 12AX7 triode mixer. 12AX7 triode tone make-up, with feedback to part-cathode. 12AX7 triode cathodyne PI. EL34 cathode bias PP. GZ32. Circa 1963.

Philips 1137. 6N8, 12AU7 cathodyne PI, 2x 6M5 cathode biased PP, 8W. Circa early 1960's.

Philips EV4437A. 2x 12AX7, concertina PI, sextet 6CM5 fixed bias PP, 100-120W, with neon and incandescent driven LDR limiter. Circa 1969.

Philips LBH1015. 2x 12AX7 concertina PI, quad 6CM5 fixed bias PP, 60-80W.
Details at Mat Janssen's website http://members.chello.nl/m.janssen36/Amplifiers-with-valves.htm. Circa late 1960's. Information notes.

Playmaster No.11 Amplifier. EF86, 12AX7, 2x 6BQ5 cathode biased PP, 16W. Circa mid-late 1950's.
Modified to original Vox AC-15 with 2007 Heritage additions and Vintage 30.

Playmaster Control Unit No.6 Preamplifier. 2x EF86. Circa mid-late 1950's.

GE Granville RS8V1 stereogram power amp - 6DX8 push-pull, with triode preamp and triode split-load PI. Circa 1969.

Radio & Hobbies Aug 1954, 12W 'Good quality at Low Cost' Amplifier. Kit chassis. 2x 6AU6, 2x 6BW6, GZ34. A&R OT-896 OPT. Restored with 12dB feedback and unconditional stability.

Radio Television & Hobbies Mar & Apr 1958, 35W Public Address Amplifier. Kit chassis. 4x EF86, 12AX7, 2x EL34, GZ34. Ferguson PF1335 PT and OPM12 OPT.

Radio Television & Hobbies July 1960, 100W Public Address Amplifier. Professional kit. EF86, 2x 6CG7, 2x KT88. Ferguson PF1546 PT. Being restored to use 12AU7.

STC 28-SU-204 35W valve amplifier. CL5244-2 step-up mic transformer in to EF86. 7027A PP. 3-valve radio. Ferguson PF172 PT and OP383 OT. Circa 1961.

STC 28-SU-236 20W valve PA amplifier. EF86 MIC gain stage. EF86 mixer stage. 6L6 PP. Ferguson PF163 PT and 8kPP OT. Circa 1964.

Steane's Sound Systems M265. 2x 6C6, 6SN7 paraphase PI, 2x 6V6 cathode biased PP, 10W - circa 1950.

Steane's Sound Systems S304. Two input (MIC, PU) channel PA amplifier. 2x 6N8 MIC amps, 6SN7 mixer. 6V6 PP cathode biased. 5V4G rectifier. circa 1952.

Steane's Sound Systems combo PA amp and speakers. Two input (MIC, PU) channel PA amplifier. 2x 12AX7 for MIC gain, mixer, tone and PI stages; 6M5 PP cathode biased; 6V4 rectifier; rebadged Philips 976B amplifier with twin open-backed Rola 8M speaker boxes acting as a carrying case. circa 1958.

Trimax A71 and A71AX Programme Line Amplifier. 1970'ish line amplifier with transformer isolated input and output, 600 ohm load 0.25W, with +34dB gain.

VASE PA100. 12AX7 PI, 2x KT88 fixed-bias PP, 100W. Circa 1966. Modified: added 12AX7.

Williamson Amplifier. Williamson amplifiers including an Australian circa early 1950's amplifier & Control Unit Preamplifier; an AWA badged amp; an R&H (1948) kit amp; an Audio Engineers amp; a Goodsell GW-18; a Rogers amp; a UK Miller Organs amplifier for Mark IV organ. Also a compendium of articles and references to the Williamson amplifier.

Prototype amplifiers: Jeffrey direct coupled input with 12AU7 PP 1.5W fixed bias output stage using 12Vdc plugpack. Jeffrey direct coupled input with 6CM5 PP UL 20W fixed bias output stage using 12Vdc plugpack.

I have details on the following equipment:

Morley Rotating Sound Expression (RSE) Pedal. An early oilcan in pedal format - circa 1975.
Morley Vol and Wah/ Vol variants.
MXR Flanger (1978 Grey box).
Locobox Spaceship FL-01.
SAFTPAK 240:240V Isolation Transformer 900VA.
PYE Portable Wheatstone Bridge 7440/G (Four Decade Pattern) - circa 1950, including manual. Restored, trimmed, Hamon divider setup.
Leeds & Northrup Kelvin Bridge 4288 - Kelvin Double Bridge - circa 1970, manual needed.
Hartmann & Braun - Pontavi Th 2 Milliohmeter - Thomson (Kelvin) Bridge - including translated manual.
Gambrell Bros Co. Ltd. London 4x decade resistance box - 1 to 9,999 ohm, 0.1% Manganin elements, circa 1930's. Restored, trimmed, non-inductive.
General Radio 783A Audio Power Load and meter - circa mid 1940's; including manual.
Cambridge Decade Capacitor box. 350Vdc Wkg. 4x decade capacitor box, 0.1nF to 1,000nF. <1% tolerance. TMC polystyrene capacitors. Likely 1960's - no manual.
DIY Inductance Bridge based on AT5 tuning coil and chassis. Measurement range from 14uH to 120mH.
AWA Valve RC Oscillator Type 3A57150 - need manual.
HP 3325A synthesiser/function generator - including manual. Repair notes on a HP3325B
AWA audio oscillator G231. - including links to two manual revs.
TMC Low distortion oscillator. Typ 0.06% distortion level; circa 1967 - need manual and info on TMC.
Wavetek Model 111 Oscillator - including manual.
Wavetek Model 154 Function Generator - including manual.
AWA 3A56068 distortion and noise meter. Includes 25Hz-25kHz active wein-bridge notch filter - circa 1964, including service manual.
AWA F240 Distortion Meter. Includes manual 10Hz-30kHz passive or active notch filter - circa 1976, including manual.
NEC 58B Impulse Counter & Noise Measuring Set. Signal line level inputs with impedance and balanced and transformer isolated options. Flat from 10Hz to >96kHz with +20dB or -10dB gain/attn, with three filter options - circa 1971.
GW GOS-622B 20MHz Analog Oscilloscope - including ops manual - need schematic and service manual.
HP 54201A Digitizing Oscilloscope - including manual.
TEK 2337 Analog 100MHz Oscilloscope - service and repair details..
USB soundcard modifications for PC oscilloscope.
EA construction project Aug 1998: Front-end for PC soundcard. Plus notes.
HP 3400A voltmeter - including manual.
HIOKI 8801 Memory Hi Corder - notes and schematic of input module.
Thurlby 1503 digital multimeter - including manuals.
Systron Donner Model 7004 digital multimeter - including manual.
Systron Donner Model 7205 digital multimeter - including manual.
Keithley 197 autoranging microvolt digital multimeter - including manual.
HP 3497A Data Acquisition with 5+ digit DCV - including manual.
Car cigarette lighter socket voltmeter.
B&K Dynamatic VTVM Model 375 - including manual.
B&K Random Noise Voltmeter Type 2417 - need manual.
HP 403B AC Voltmeter restoration.
HP 5065A Rubidium Vapor Frequency Standard - circa 1969 (early model), repair notes, including all three heater assemblies.
HP 738BR Voltmeter Calibrator - circa 1975, repair notes.
Fluke 801BR/AG Differential DC Voltmeter - circa 1966, repair notes. 500Vdc reference with 5 decade Kelvin Varley attenuator, and null detector with Airpax chopper amplifier.
Weston Standard Cell unsaturated - Munitions Supply Laboratories, SA, 1943.
H.Tinsley Tuning Fork type 3086 - 25Hz with contacts and excitation electromagnet.
Systron-Donner 6250A Frequency Counter - manual.
HP 3560A dynamic signal analyser plus sensors for Philips Electron Microscope pre-installation survey - including manual.
E.A.KEMP P/L Counter-Timer Model 4DS10 with Mullard MX147 and Philips ZP1481 geiger tube sensors - circa 1966, no manual.
Hickok 1890M In-circuit transistor tester (large file!) - including manual.
Paton Electrical VCT-2 Valve Tester. Includes restoration notes, operating manual and schematics.
BPL Megohmmeter RM 175-LZ MK II - circa 1966, need manual.
BPL A.C./D.C. Breakdown & Insulation Tester RM215-L - circa 1970, need manual
AVO A.C./D.C. Breakdown & Insulation Tester RM215-L2 - including manual.
Megger BM81-T insulation resistance and DMM - including schematic.
HP 716B Klystron Power supply - circa 1964, restoration notes.
BWD Model 210 Stabilised Valve Power Supply 2x 500Vdc regulated outputs (300mA, or 180+150mA); 1x 250V regulated 50mA; 2x 6V3 5A. Separate voltage and current meters. 1966.
BWD Model 255 Stabilised Transistor Power Supply - including manual.
Solartron Vari-Pack SRS153S restoration notes.
Power supply with regulated 250-350Vdc 100mA output and fixed -150Vdc 40mA output. From Melb Uni Physiology Dept, circa 1959. 6AS7 pass with EF86 control. 2 panel meters.
12VDC car converter for mains VAC 5W.
12VDC car converter for 220VAC 75W.
Alesis 1622 mixer.
Guarda KM1200S operating manual.
Behringer Eurorack UB2442FX Pro - power supply schematic (the bit that fails).
SoundTech A600 Operation and Service Manual and schematics.
SEAS D014 passive crossover network 3-way 600Hz 3kHz 8 ohm circa 1974
For 2x 25-TV-EW, 15/11-TV-2XM, H87, H8014. Used by Interdyn in SEAS 60.
SEAS D036 passive crossover network 3-way 800Hz 4kHz 8 ohm circa 1974
For 25F-EWX, 10FM, H-086. Used by Interdyn in P33 or S33 but with own Interdyn crossover.
TOCORD Brochure for SHF SP speaker cable.
Partridge PA Manual from circa 1938.
Partridge Amplifier Circuits from circa 1938.
Partridge Transformer Price List from circa 1938.
The Partridge Manual from Oct. 1947. An updated replacement to the two earlier manuals.
Partridge Transformer Price List from Jan 1948, Appendix 1 to The Patridge Manual.
Beacon Radio 1958 catalogue from NZ, including transformers and chokes for Williamson amplifier.
Ferguson Transformer individual part datasheets from 1958-65 for Ferguson OP series datasheets, Ferguson OPM series datasheets, Ferguson PF series datasheets, Ferguson PF extra series datasheets, Ferguson PVD series datasheets, Ferguson MT series datasheets, Ferguson TRS & TRD series datasheets, Ferguson Filter Choke datasheets.
Trimax Price List and some Technical sheets from 1970.
VANE 7100 and VP708 Engine Analyser - including manual and some schematics.
Wurlitzer Organ Model 4100BD Reverb Unit from 1964.
Selmer Concert Organ Information

Information on self-split push-pull circuits, valve reliability articles, and other valve related goodies.

Last update: 26 May 2024 Email Tim for comments / details.

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