Nice amp from 1970 (I’m sure it is now out of waranty )
Bulgin power cord …
Let’s have a look under the hood …
Diode rectification …
WTF ??? OK, this amp *needs* a service !
PT has previously been replaced on this amp, but voltages are too low (300V instead of 390V at the diodes)
Layout … work in progress – Based on excellent work from http://www.amparchives.com
Cleaning process …
Oxidized contacts … to be cleaned !
F&T reservoir cap, new Shinrock / IGPW (Marshall 18W) power transformer
“2022 Lead” mod (2 capacitors added : 4.7nF & 500pF, for a brighter sound on one channel, easily removable …) – Wiring cleaned.
8Ω output added (blue wire) – This amp has now 8Ω + 16Ω outputs
After PT replacement, voltages are as on the schematics.
IGPW / Shinrock Technical Data
Power: 103 VA
Primary 0/230/240 Volt
Sec. 1: 290/0/290 Volt(AC) @ 0.12A (DC)
Sec. 2: 0/5/6,3 Volt @ 2/1A ( AC) (if Marshall 18W EZ81 rectifier used – unused in case of Marshall PA 20)
Sec. 3: 3,15/0/3,15 Volt 3A (AC)
I used a 150R resistor for EL84 bias. Vk # 12V, Ik # 36mA (measured with a bias system). I could not be able to measure Ig2 : 2mA ??? (no resistor …)
Va = 370V
Vg2 = 360V
Pa # 12.6W (if Ig2 is 2mA) quote hot !
Considering relative high anode and screen grid voltage, I chose 6P14P-EV tubes (EL84M equivalent, designed for VA = 400V) …
With Marshall 2045 cabinet, how sexy !