After some discussion on the Hoffman/EL34 forum, it seems that as I am going to replace the filter caps in my Marshall 2204, it’d be a good idea to replace the “bias components”. This basically means that I should replace all components between the power supply and pin 5 (grid) of each EL34. This little circuit includes a couple of electrolytic capacitors, so the same rules apply…if they’re old, replace them. I’m going to replace the resistors in the circuit with metal film resistors. I don’t see any harm in doing so, and it’ll be good practice.
The schematic I’m using appears accurate when compared to photos of my board. That in itself is giving me practice in identifying the right components, and tracing the circuit visually. There’s a cognitive leap from schematic to physical layout that takes getting used to…like translating a UML diagram to actual source code.
You’ll see a section above the power transformer that is sort of labelled ‘BIAS’. Note the labels by some of the resistors, RG, RB1 and RB2. These seem to be variants for the European market. This initially confused me, as the parts on the actual board didn’t map to the schematic. That was until I remembered I’m British so my board has ‘all’ in ‘all but USA’ component values (see the little box above V4/V5 in the heater wiring section?). I was seeing 150k on the schematic, but finding 200k on the board.
Luckily I made this discovery before I ordered all the remaining parts from Hoffman.