Swapping rectifiers 5e8a

[Kindablue]

Was wondering what anyone thought in terms of safely using either 5r4gyb/a rectifiers in place of 5u4gbs in a 5e8a low power twin. Would that change the character of the amp much?

Reason for doing this is my plate voltages came out a little high (480 unloaded) and was hoping to drop them a bit. I think I read something about the internal resistances of them needing to be closely matched or you could have problems?

With tubes loaded I have 6l6s biased to around 39-40ma with still 420dcv on the plates. So thats a bit still over 70% plate dissipation and I'm still over voltage (normal is apparently 400). Maybe I'm just bring nit picky.

Also, I'm using a 50k trim pot as a variable resistor : adjustable bias goes 2.2k>diode> 50k pot> 10k tail resistor. I'm getting the values I mentioned above only when the trim pot is almost maxed out. Would like to have more leeway i.e., bias at 40 or a tad higher if wanted and not be maxing the pot out. Any suggestions?

[pdf64]

What VAC do the 6.3 and 5V heater supplies measure?
Is the 40mA anode or cathode current?
What resistance is the choke you’ve used?
Is the 50k pot wired as a potential divider or a variable resistor? Almost certainly a variable resistor I suspect.
40mA x 420V = 16.8W.
I suggest not to go much higher than 18W idle anode dissipation for 6L6GC.
https://el34world.com/charts/Schematics ... _schem.pdf

[Stevem]

I don't think you will find much of a V+ difference with that swap at idle conditions anyway!
The 5u4 is said to drop 50 volts with a 275 ma load, and the other recto drops 63 volts with a 250 ma load.

Maybe try a 5V4 since that with a175ma load is said to drop 25 volts.
This is what my 1954 Gibson GA77 uses with two 6L6gc tubes for 25 watts of clean output.

Or you might get a pair of Tung Sol 7581A tubes, and since they are 35 watt tubes you could bias them hotter to drag down the V+!

[martin manning]

With tubes loaded I have 6l6s biased to around 39-40ma with still 420dcv on the plates. So thats a bit still over 70% plate dissipation and I'm still over voltage (normal is apparently 400). Maybe I'm just bring nit picky.

Probably. Note Fender's '57 Twin schematic shows 460V on the first filter cap. Checking the line voltage and the filament voltages is a good idea to see where you are with those. I would then adjust the bias accordingly, and then then look at the preamp voltages, bringing those into line with the vintage schematic by adjusting the first dropping resistor.

Also, I'm using a 50k trim pot as a variable resistor : adjustable bias goes 2.2k>diode> 50k pot> 10k tail resistor.

If you want to shift the idle current range higher, increase the 2k2 or decrease the 10k.

[Kindablue]

Ok...ordered a set of 7581a tubes and first will try subbing in a 3.3k for the 2.2k and see that leaves me with biasing capacity. You guys are awesome.

[Kindablue]

This is the choke I used:

Choke weber wo14684 (5-8H @ 200ma, 125 ohms)

I measured tube current first off of 1 ohm resistors off of pin 8, then using a bias probe.

[Kindablue]

"Probably. Note Fender's '57 Twin schematic shows 460V on the first filter cap"

Do you think that's 460v with tubes in or unloaded?

[martin manning]

Tubes in.