My friend Bryan Howard, a long-time member here and massive Rickenbacker fan, recently handed me a pretty incredible amp he found at a music store in Virginia. It looks like a 1934 Electro Type 2, but I can't find the exact circuit online and so I'm going to trace it out before I dive in and service the amp. It uses a 5Z3 rectifier, 57 pentode pre, and a dual triode 53 for the next gain stage and phase inverter. It runs a pair of 47 output tubes into a 10" electrodynamic speaker.
The amp was sold as DOA with a bad field coil. Turns out the field coil is good, power transformer and choke too. The output transformer is a sloppily installed universal OPT that looks to date from the 60s or so. That's going to go ... I'll track down a good contemporary OPT for the 47s. Apart from that, a few things have been done. The speaker was reconed at some point in its life and there's a 1950s-60s vintage 5K volume pot right at the input jack, before the 57 grid. Not sure if the volume control was originally there and was just replaced, or if this is a later addition. Hopefully I'll find some pix of this particular amp that will shed light on it. The rectifier socket was also replaced and the power switch is a modern Chinese junker that's going to go too. The goal is to get this thing back to as close as original as possible.
In any case, I'm getting an education on early amplifier design! Some strange stuff we don't often see today:
First thing I noticed after giving the amp a lookover was the rectification stage. Very odd by today's standards. The 5V filament winding for the 5Z3 is center tapped, and there is no jumper from the 5Z3 to the first filter cap as is normal today. Instead the choke is tapped and one of the three leads runs to that 5V center tap. The B+ runs through that center tap to the first filter cap and on through the circuit. Very weird. Did some reading and it's due to the directly heated nature of the rectifier, and the limits of early 30s filter caps. That center tap arrangement filters out the 5VAC riding along with the DC that today is filtered out easily by larger, better caps. This one has 3x4uf caps in it.
This is going to be a very fun project, and I 100 percent will draw out the schematic and publish it here when done for anyone else's future reference.
If there's anything anyone wants to see or know about this amp, let me know. This is a good opportunity to fill in a few technical knowledge holes for these early amps ... there's so little surviving technical info on them out there. And if you have any information on this particular amp/circuit, I would love to see anything you can provide.
For reference here's a schematic I drew up. It's complete apart from voltages which I should have in a few days if everything goes according to plan. Will update at that time.