Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TexasTone on August 28, 2026, 04:11:43 pm

Title: Cathodyne Phase Splitter Issue
Post by: TexasTone on August 28, 2026, 04:11:43 pm
A local young amateur builder brought me this one, which isn't working right - low output, distortion, etc.

He used the Valve Wizard Cathodyne Phase Inverter as his guide, using a 12AX7, a B+2 Supply Voltage of 366, and 47k load resistors, 1.5k cathode, exactly like the web page (screen shot attached).  He's got a 150k grid stopper on it, fed by a .022 coupling cap.

The voltages I'm getting aren't what they should be (photo attached)
338 V on the plate
4.5 volts on the cathode
0 volts on the grid.

I was expecting more like
235 - 310 on the plate
83 - 125 on the cathode
74 - 76 on the grid

Tracing the wires and components checked out.

Any takers?
Title: Re: Cathodyne Phase Splitter Issue
Post by: Helmholtz on August 28, 2026, 04:39:10 pm
The left side end of R14 (intended as a grid leak) should connect to the grid.
Otherwise there's no grid bias.
Title: Re: Cathodyne Phase Splitter Issue
Post by: Helmholtz on August 28, 2026, 04:40:00 pm
The left side end of R14 should connect to the grid.

Sorry, double post due to connection problem.
Title: Re: Cathodyne Phase Splitter Issue
Post by: TexasTone on August 28, 2026, 05:18:30 pm
The left size of R14 connects to the junction (black dot) of C7 (and the output of the Treble control/tone stack).

R11 from B+3 to C5 and pin 6.
R12 to Ground and C6, R14, R13.
R13 to pin8 and C6, R13, R14.
R14 from C6, R12, R13 to Treble out and C7.
R15 from C7 to pin 7
Title: Re: Cathodyne Phase Splitter Issue
Post by: passaloutre on August 28, 2026, 05:21:56 pm
C7 prevents the grid from its DC reference. In other words the grid needs a DC path to ground.

R14 needs to go to the other side of C7

Can we see the rest of the schematic?
Title: Re: Cathodyne Phase Splitter Issue
Post by: Helmholtz on August 28, 2026, 05:34:11 pm
Quote from: passaloutre on August 28, 2026, 05:21:56 pm
C7 prevents the grid from its DC reference. In other words the grid needs a DC path to ground.

The grid is not referenced to ground with a cathodyne but to the lower end of the bias resistor R13.
The bias voltage is the voltage drop across R13.
If correctly wired, the grid voltage will be around 1V lower than the cathode voltage.
It is not possible to exactly measure the grid voltage with a DMM here, because of the meter's loading effect.
Title: Re: Cathodyne Phase Splitter Issue
Post by: passaloutre on August 28, 2026, 05:36:33 pm
In this case the grid has no reference to the cathode or the ground below it. There is a cap in the way.

Title: Re: Cathodyne Phase Splitter Issue
Post by: Helmholtz on August 28, 2026, 05:46:05 pm
Quote from: passaloutre on August 28, 2026, 05:36:33 pm
In this case the grid has no reference to the cathode or the ground below it. There is a cap in the way.

See my first post.