6L6G push-pull with a single 5Y3 rectifier??

[bluto]

I'm working on a couple of old 1950's PA amps that run PP 6L6G's with a single 5Y3 rectifier but am wondering if that is a good way to go for guitar use as from what I've seen similar guitar amps from that era used a 5V4 or 5U4. I could change the rectifier tube but am leery of bumping the B+ too high and/or stressing the PT 5V winding. So far I'm running 6EZ5's (similar to 6V6) in one and it works OK but the plate to cathode voltage is kinda high at 370V. Would very much appreciate any thoughts on what mods I should make if any (rectifier change, 6V6 conversion, leave it alone, etc.)

[pdf64]

6L6G are a big step down from the 5881 / 6L6GC we are used to seeing in guitar amps, probably closer (in terms of its limiting values) to a 6V6GTA than a 6L6GC.
So the 5Y3 is probably fine, we should probably give the designer the benefit of the doubt until the evidence shows otherwise.
A caveat being the line voltage may be somewhat higher that the design was intended for.
What's the heater voltage? That may be telling in this regard.
If >=6.9Vac, then RG's vintage voltage adaptor may be beneficial.
Pete

[bluto]

Thanks! after I posted I went ahead and ordered a variac so I can experiment a bit with plate voltages...the heaters are at about 6.8VAC IIRC on my 120V service. Mostly I'm trying to decide whether or not to stick with the 6V6 conversion since the B+ voltage is high for them but very good for the original 6L6's. If the sound improves with the voltage lowered 10% or so I probably will go ahead and wire up a 12V bucking transformer since it is easy and inexpensive. If I do stick with 6L6's I'd want the amp to be able to handle modern 6L6GC's so that is partly why I was not so sure if a single 5Y3 is up to that.

[EFK]

Sure, the 5Y3 is up to the task of 6L6GC... if you bias them as if they are the 6L6G! What you really need to know here is whether or not the power transformer will be up to the task of supplying a much higher idling current with a changed rectifier and the 6L6GC, because my guess is that it's going to run a whole lot hotter than it is now. Running a PT over-spec combined with 60 year old winding insulation seems to be to be a recipe for a big melt-down. Think about what the current supply limitations are for a 5Y3; this is likely what the amp designer had in mind when the choice of power transformer was made. BTW, the 2A filament draw of the 5Y3 is also likely what the PT 5V winding was spec'd to accommodate, and the @3A draw of a 5U4 may cause a melt-down all on it's own in that respect too.

[bluto]

Thank you, and if I do stick with 6L6's will reinstall the stock cathode bias resistor as is as suggested, 125 ohms IIRC....I'm aware of the 5U4 current draw but when I wasn't awhile back I ran one for many hours in a nearly 70 YO Gibson BR-6F and got away with it...though when I wised up and reinstalled a 5Y3 it actually sounded better and had less hum too. I could use a 5V4 since they pull 2A but that would bump the B+ up and it is already 400V. I should have a variac in a few days and will play with at and see if I can get the NOS 6EZ5/6EY6 tubes (very similar to 6V6) I'm experimenting with to sound better...I was motivated to try them in the first place as they pull more filament current than a 6V6 but less than a 6L6 and should be easy on the PT...I'm hoping this works and if so will install a 12V bucking transformer on the primary side to drop the B+ and filament voltages.