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Next problem: EL34s draw more heater current. Solution: replaced stock 22AWG heater wire with 18AWG wire.

OT primary impedance measures 3k8 - perfect for EL34. Problem: stock octal socket (left) was missing Pin 1, which is need for EL34 suppressor grid. Replacement Belton socket on the right.

The initial build. - ultra-fast diodes added to rectifier. - adjustable fixed bias - current-sense resistors added for bias measurements - tube-driven effects loop - other stuff.

The stock amp had a coupling cap between (each) input jack and grid of first gain stage. Initially, I kept that in there. Used shielded wire to board-mounted cap, and added a 33k grid stopper on pin 7 of the tube socket. My wiring colour code for tube sockets: - brown for unshielded grid...

PS. this will compete with Friedman Vintage but will not cost 3k. It'll be closer to 2204 than 1987, however.

Eyelet board cleared and cleaned. New components/circuits loaded onto the right side of the board, including new caps for negative bias supply, new screen resistors, phase inverter plate resistors and coupling caps. Bottom left: a PCB holding a dropping resistor and filter cap for the preamp...

If you have a chance, pick up a YBA-4. Thank me later....

The stock amp uses 7027A tubes. The octal tube sockets didn't even have terminals for pin1. But the primary impedance of the gigantic output transformer measured 3.5-3.8k, so it's probably taken from the Traynor YBA. Perfect for EL34s...

Obvious first steps: replace original filter caps and two-prong cord. But who wants to deal with a permanently attached power cord? New filter caps. Two-prong power cord and accessory plug removed. New fused power receptacle in place of the accessory plug. Yes, you can fit a square peg...

YVM = Yorkville Voice Master YVR = airport code for Vancouver, which is where I am located. Bought this amp on a road trip. 1967. Four identical inputs for 'voice' (mics). Who want see me turn it into JCM800-ish guitar amp?

stock amp has 2 dropping resistors in series (for more resistance). Looks like Carl bypassed one of the 2 series Rs to lower the resistance (increase voltages at PI node).

Hammond replacements

are those all chassis connections? If so, abort and look at other grounding schemes: TUT3, Aiken, and Valve Wizard (which are all very similar).

Isn't it nice to think we are god-like. Good for our egos. Personally, I think god is more like a dolphin.

12AWG aluminum chassis is just a really sturdy tinfoil hat.

Yes, that's about right. Less without a head cab. I have plexiglass covers and cheap feet for it in lieu of a cab :)

Not yet. I am the king of terrible clips tho!

Usually 2-3 weeks.

No, I use Seeed to manufacture the PCB only. I do the assembly myself.