I just picked up a Seymour Duncan 84-50 Quadratone amp chassis cheap off ebay - a 1990s 50w (at 5% THD...) all tube amp using a quartet of EL-84s in the ouput stage. I've managed to get the amp working but unfortunately the original reverb spring didn't come with it.

I have a type 4 Accutronics unit but it seems the input and output impedance (both about 200 ohms) are too low to match to the amp.

Does anyone know which model spring would originally have been fitted? Seymour Duncan sent me the schematic for the amp (which was very nice of them) but it doesn't specify what model was used...

Thanks

Chris