Fender Twin Amp High Mains Current draw in Low Power mode

[nevetslab]

I think that D204 is an additional measure to snubber the voltage spike (and maybe ringing) across S203 (hi/lo power switch) caused by a sudden reduction/inversal of the current through the choke when switching from high to low power. It's always some circuit inductance which strikes an arc between switching contacts by back EMF/ flyback voltage.
To be sure I would need to observe switching transients with a DSO.

(And don't expect too much from someone who has a Rep Power of 5 )

Well, next time a Twin Amp comes in for service, I'll have to take a look at this circuit to see what all they've done. No doubt this network (which isn't on the REV A schematic, only on the REV D schematic), or THE TWIN, which also has the full & quarter power switch...it too lacks that snubber diode network....was added to counteract switching power modes while the amp is in Operate (not Standby). I see D204 on the REV B schematic, but not on the REV D schematic.

The Twin doesn't use a diode (like CR15) to couple the Low Power Voltage to the O/T CenterTap. They're just hard-switching between the two supply voltages.

TheTwin Schematic & Parts List.pdf

[Leo_Gnardo]

And don't expect too much from someone who has a Rep Power of 5

Vastly under rated! I've pitched in a couple of reputation good mentions for you, and others should too. Hit that "sheriff's badge" link folks & let's get Helmholtz up to 25 or so where he belongs.

[The Dude]

I agree, but will add this. I don't pay any attention to Rep Power. Anybody who hangs out here, even a little bit, knows of Helmholtz and his electronic prowess. Definitely an asset to the forum!

[Helmholtz]

The Twin doesn't use a diode (like CR15) to couple the Low Power Voltage to the O/T CenterTap. They're just hard-switching between the two supply voltages.

I can't comment on necessity or effectiveness of snubber networks in these amps without having actually measured one of them with and without snubbers. But I understand that the "Twin" is a prior design.

[g1]

I see D204 on the REV B schematic, but not on the REV D schematic.

The schematic I posted (post #14) with D204 (rev.B) is for the newer Pro-tube version of Twin-amp. It came out around 2000.
The amp you have been discussing is called the '94 Twin-amp.
So the Pro-tube rev.B is newer than the '94 rev.D
Sorry for the confusion.

[Helmholtz]

I'd be especially interested in seeing how fast the screen supply voltage settles to its lower value after switching from high to low power. Right after switching, the screen voltage will be almost twice the reduced plate voltage and the charge stored in the screen filter caps as well as the energy stored in the choke will tend to keep the voltage high for some time. This should cause a high screen current/dissipation surge.
I think at least some of the diodes are intended to mitigate this effect.