Plexi clone voltages

[DELAMBG]

Hi All,

I just wired up my plexi clone (with 6L6's) and...

I am getting about 240V on the 6L6's plates and screens with:
The amp on standby
with the high voltage (straight off the diodes) disconnected
new diodes.

What gives? Any ideas. The PT is a triode electronics model# 40-18024

[Chuck H]

With no tubes in, what do you get for the filament voltage?

Chuck

[loudthud]

Could be perfectly normal depending on where the standby switch is located and how the bias supply is implemented. Please post a schematic or a link to one. There is no one "plexi" schematic

[DELAMBG]

1967

Hi guys I think you are right. I am using the older standby switch scheme of an SPST not the later DPST so the half wave is making it's way through to the plates.

[DELAMBG]

THE plate voltages on the output tubes is hovering around 505VDC and the bias is in the -40 to -50 volt range. Any ideas to drop it lower? My can caps are only rated for 500v.

[Chuck H]

Is that voltage with the tubes in??? If not then I might not worry about it. The caps will probably handle it even on a regular basis until the amp is switched off standby. At that point the tube current will bring that voltage down quite a bit. Lots of amps run slightly over the cap rating on standby. If this is the voltage with tubes in place then you will need to bring it down. If you don't want to add sag to the power supply then a zener on the 0V line for the rectifier will work. If you would like to add a little sag then a big resistor in the same place is fine.

Chuck

[DELAMBG]

no tubes

Thanks Chuck, that is the voltage with no tubes.

But lets say I did need to bring it down. How would you do it?

[Chuck H]

The easiest way would be to put a resistor on the 0V line for the bridge recto, 22k @ 25W should do. The resistor will drop about 15 volts which is only about 10 watts, but IME a 10 watt R will get too hot so use a 25W to be sure. This will drop about 15 volts and create very little sag. Use the chassis mount aluminum housed resistors sold at Mouser. They're cheap and effective and can take the volts. Don't locate it near the aluminum capacitors.

Chuck

[DELAMBG]

So you mean use the 22k resistor from the ground side of the FWBR to ground?

[Chuck H]

+++

Chuck

[DELAMBG]

I have another question/problem that just arose.
I can't get the bias current down far enough. What should I do?

[Chuck H]

Raise the value of the resistor in series with the VR to ground. Try a 50% increase in value.

Chuck

[Wilder Amplification]

If that's a 100 watt plexi with the full wave bridge voltage doubler circuit (center tap of the HT secondary connected to the series junction of the first two filter caps) you have to kill BOTH sides of either the AC or the DC side (preferably the AC side) with a DPST switch OR kill the DC after the 1st filter caps but BEFORE the HT Fuse with a SPST switch. Using a SPST switch to kill DC between the rectifier positive output and the 1st filter caps only kills 1/2 the doubler circuit.

Assuming a 100 watt plexi with a 100w plexi PT that has its own separate bias winding, for 6L6s I would use the stock 47K and 15K resistors in the bias circuit, but drop the 27K upstream of the bias rectifier to a 15K resistor. This should get your negative bias voltage into the required range for 6L6s.