On 12/3/25 12:19, John Robertson via Jukebox-list wrote:
Hi Mauro,
Fred Hammond used to tell me that the transformers they made were derated upwards of 50%. You may want to give the techs a call and see if the smaller transformer (125E) that fit better will handle the output of a Class A (AB?-Jay?) pair of 6L6Gs.
Push-pull is going to be class AB. Class A power stages are single-ended, hot, and horribly inefficient with arguably very slightly higher fidelity in some cases.
The output is conditional on the B+, not the tube bulb style (maximum heat dissipation issues in the metal tube), if the original amp was 310VDC at the plate - that would make it around 18W output based on my GE Tube book from 1964 if operated as Class AB(1), 47W if operated at AB(2) which I really doubt, and I don't know how to tell anyway.
It's going to be AB1. AB2 is more complex as it drives the grids positive and the drivers need to be designed to handle this. Also keep in mind that the 6L6 (metal) and 6L6G are rated for substantially lower plate voltage and power output than the 6L6GC. This amplifier was designed for the lower voltage tubes as 6L6GC didn't exist yet.
My RCA book lists the 6L6G, run Push-Pull at 270VDC in Class A, is rated at 18.5W @ 134ma, P-P 360VDC Class AB(1) is rated at 26.5W.
I suspect the stock Wurlitzer amplifier is rated somewhere around 20 watts output. The 1760JB transformer looks much closer in physical size to Mauro's original and is rated at 40 watts with 6K ohms plate impedance, a good match to 6L6s in push-pull. It's going to be a better electrical fit with plenty of headroom in terms of power if he can shoehorn it in there. The 125E will probably work as Hammond makes good goods that are very conservatively rated. I'm a bit concerned about the DC bias rating of the 125E at 60mA in this application.