SE Voltage Drop Across Tranny

[Amp Kat]

Is it normal for a SE Tranny to drop 20 volts across it from plate to screen ?
This is a typical CLC network power supply single tube 6V6 and the screen is 20 volts higher than the plate under no load or with a preamp voltage load.

100uf /15h choke /100uf pi-filter network .
AC in 315 CT to winding to a 1n4007 FWB rectifier.
Plate connects to first filter/choke and screen to the other side of filter/choke which is at 388 volts or screen potential.
Plate voltage 366
Screen 388
Cathode Biased 470 ohm resistor and the voltage across it is 28 vdc.
SE Tranny is a Mercury FTP O-5

[Enzo]

well, hey KB.

Remember, the voltage drop will be due to current, and the screen node is supplying not only the power tube screen, but also any preamp tubes. SO add up all those tube currents and see what volatge drop you ought to see given the resistance.

If the plate is lower than the screen, what is the DC resistance of the transformer primary? Looks to me like you have about 59ma of current through the power tube plate. What voltage drop from 388 would 59ma cause through that resistance?

[R.G.]

Georg Ohm knew:

V = I * R

[kleuck]

Your anode is dissipating something like 18-19 watts !

[diagrammatiks]

I think you need to check your biasing first...

that tube is biased way to hot.

also you need to know what choke you are using.

I'm seeing something like 400-500 ohms as the dropping resistance.

that's a bit high for a choke but definitely not unheard of.

[g1]

the plate is 20 volts higher than the screen under no load or with a preamp voltage load.

Plate voltage 366
Screen 388

Just so everyone is on the same page, you meant the plate is 20 volts LOWER than the screen, right?

[Enzo]

Yes, and as to biasing, it sounds like a little class A amp, so it will naturally be running at higher currents.

[kg]

i do see a bit of confusion wrt which node is higher in potential and where that potential is being dropped...

[Amp Kat]

Just so everyone is on the same page, you meant the plate is 20 volts LOWER than the screen, right?

Yes thank you I'll edit it and change it. Thanks !

Screen is higher than the plate

Plate Voltage=366
Screen = 388 or 1st cap node voltage.
Choke correction : 90 ma 4 HY DC resistance 100 ohms
SE Tranny resistance 385 ohms

[Amp Kat]

well, hey KB.

Remember, the voltage drop will be due to current, and the screen node is supplying not only the power tube screen, but also any preamp tubes. SO add up all those tube currents and see what volatge drop you ought to see given the resistance.

If the plate is lower than the screen, what is the DC resistance of the transformer primary? Looks to me like you have about 59ma of current through the power tube plate. What voltage drop from 388 would 59ma cause through that resistance?

The primary of the tranny is 385 ohms .

The part that throws me is from the the side of the primary that connects to the 1st node of the cap and choke is 388 volts and through the tranny to the other side of it to the plate is 366 and the screen is at the potential of the 1st node or 388 volts. There are no preamps hooked up just the power tube and the 470 ohm cathode resistor and that's it.

So the voltage at the 1st choke node and the other side of the choke are equal (388) so the only thing in between is the SE tranny which across it's winding drops 20+ volts or 388 at one end and 366 at the other and was wondering if this is normal.

[tubeswell]

Yes thank you I'll edit it and change it. Thanks !

Screen is higher than the plate

Plate Voltage=366
Screen = 388 or 1st cap node voltage.
Choke correction : 90 ma 4 HY DC resistance 100 ohms
SE Tranny resistance 385 ohms

...Cathode Biased 470 ohm resistor and the voltage across it is 28 vdc.

Yikes! 59.6mA tube current and 57.1mA plate current! (Are you absolutely sure its a 6V6? ;-) ) Maybe the Rk isn't 470R, but some lesser value (like 270?)

[Amp Kat]

Yikes! 59.6mA tube current and 57.1mA plate current! (Are you absolutely sure its a 6V6? ;-) ) Maybe the Rk isn't 470R, but some lesser value (like 270?)

That was a 6L6 and yes you are right I originally stated 6V6 and good call on it too Ole chap but I switch back and 4th between the two from tube to tube when I take the preamp away but the drop across the tranny is the same amount from tube to tube.

Since I pulled the preamp I installed a 6L6 because the voltage rises to about 388 but with all of the preamp tubes in the voltage drops to about 350 and the Cathode falls to around 18 to 20 volts so the 6V6 runs at about 13 to 14 watts which is alot but sounds really nice especiall on the distortion channel. Right now though I'm just trying to find out why the drop is so large with virtually nothing connected but the power tube and even with everything connected the drop is still there.

[Bruce / Mission Amps]

... Right now though I'm just trying to find out why the drop is so large with virtually nothing connected but the power tube and even with everything connected the drop is still there.

I think you are answering your own question there Kerry.
Primary on the OT is 385 ohms x .058ma = 22vdc. That would be the drop...
The tiny load of a couple ma of screen current through the choke, of around 75-100 ohms, is just a couple volts.
Use a 470 ohm to 1000 ohm screen resistor on the power tube socket.
And or, put a much bigger R value resistor between the screen end of the choke and the second filter cap to lower the screen node voltage (and preamp dropping resistor feed), to around the same as the power tube plate voltage... otherwise, just forget it and play.

[Amp Kat]

I think you are answering your own question there Kerry.
Primary on the OT is 385 ohms x .058ma = 22vdc. That would be the drop...
The tiny load of a couple ma of screen current through the choke, of around 75-100 ohms, is just a couple volts.
Use a 470 ohm to 1000 ohm screen resistor on the power tube socket.
And or, put a much bigger R value resistor between the screen end of the choke and the second filter cap to lower the screen node voltage (and preamp dropping resistor feed), to around the same as the power tube plate voltage... otherwise, just forget it and play.

Thanks Bruce and that is what RG was trying to tell me I just did't comprehendo ha. It's the exact topology when I have it going that I use but was just wondering because obviously the earlier Fender SE trannys had much less DC resistance so the drops weren't as large. Most of the Princeton 5F2-A /Champ/ AA764 schemos show 350 volts for plate and screen with no screen resistors but used the resistor to filter the screen in place of the choke. Think I'm to hooked on the push-pull not used to a difference in plate and screen variations and SE is a little different but does sound awesome.

I do have a 1K resistor between 2nd and 3rd cap and then the 18k dropper for the pre section just as you said.