Magnatone 425 and 12DW7 Questions

[misterc57]

I have a Magnatone 425 I am checking out that plays great and has a perfect sinewave on the output with a tad of clipping when you dial past 8 on the volume pot.

No issues on performance but I have questions on V2 which is supposed to be a 12DW7, but instead has a 12AU7 installed.

Looking up the 12DW7 is states... "The 12DW7 is a dual triode tube that's half 12AU7 and half 12AX7. The 12AU7 side has a gain factor of 20, while the 12AX7 side has a gain factor of 100. The 12DW7 is an uncommon tube that's useful in circuits with limited space."

All other tube V look good compared to the schematic, except V2. Some readings are way off and 69 VDC at the grid pin 2 does not seem right???

Below are the schematic values followed by the reading values;

PIN	schematic V	reading V
1	215	227
2		69
3	110	113
4	H
5	H
6	170	69
7
8	1.7	3.5
9	H

Schematic attached.

Thoughts please?

Also, I was curious what the "circle to ground" symbol means on the schematic, they are in a few places like V1 grids 5 and 8?

Thank you, Mark

V2 is to the right of the two red .22 caps in the picture.

Magnatone_425.pdf














pin 2

[The Dude]

Check the coupling cap (.0047) from pin 6 plate to pin 2 grid. I suspect it may be leaky. You can also just lift one side of the cap and see what happens to pin 2 grid voltage. The "circle to ground" indicates a shielded wire.

[Helmholtz]

Both the 12AU7 and the 12DW7 contain 2 triode sections.
One is wired as a gain stage, the other one as a cathodyne PI.
The cathodyne triode section is identical between the tubes, so plate and cathode voltages are fine.
There's no need to know the grid voltage (that's why it's not specified in the schematic).
The grid voltage is supplied by the 1M resistor. As there's no grid current, the voltages at both ends of the 1M resistor are the same.
But when you connect a meter, the grid voltage will be pulled down.

The gain section before the PI will show different plate and cathode voltages when a 12AU7 is used.
The plate voltage can be corrected by using a larger value cathode resistor.

[Jon Snell]

Regardless of 'fault condition', the control grid of a valve must follow, (withing certain parrameters available on the specification sheet of any valve), the cathode voltage.
For instance, any valve must have a lower grid voltage than its cathode to work in an amplifier. Class C is different but not used in this instance.
If not, the valve may have failed or will fail if left in that condition.

1 Anode at 227v, 2 Grid at 69v and 3 cathode at 113v! That suggests an anode cathode leak or faulty components around the valve.

[Helmholtz]

1 Anode at 227v, 2 Grid at 69v and 3 cathode at 113v! That suggests an anode cathode leak or faulty components around the valve.

My simple eplanation is that the meter has a low input resistance (probably 1M) which pulls down the grid voltage. At the same time the cathode voltage will also drop by about the same amount as cathode voltage follows grid voltage.
Can be verified by connecting a second meter to the cathode.
As plate and cathode voltages look ok, I'm quite sure the stage works fine and the undisturbed grid voltage is around 110V.

[g1]

No issues on performance but I have questions on V2 which is supposed to be a 12DW7, but instead has a 12AU7 installed.

If you use a 12AX7 for V2, instead of your 12AU7, you should find that the pins 6,7,8 section voltages are now correct and the pins 1,2,3 section voltages are off.

[misterc57]

I lifted .0047 from pin 2 and still have the 69V.

So the odd V are a result of the wrong tube being used? The owner loves the sound of the amp so I lean towards not making changes.

[Helmholtz]

I lifted .0047 from pin 2 and still have the 69V.

So the odd V are a result of the wrong tube being used? The owner loves the sound of the amp so I lean towards not making changes.

The 69V at grid are not "real". Actually should be around 110V.
It's not a tube problem but a meter/measuring problem.
A better meter would show a higher voltage there.
There's a reason why schematics don't show grid voltages of PIs and cathode followers.
Please read my post #5 for explanation.
(Maybe you missed that with this circuit the grid is not ground referenced?)

[mozz]

I would still put in the proper tube, 12dw7/7247 and he might even like it better.

[misterc57]

My simple eplanation is that the meter has a low input resistance (probably 1M) which pulls down the grid voltage. At the same time the cathode voltage will also drop by about the same amount as cathode voltage follows grid voltage.
Can be verified by connecting a second meter to the cathode.
As plate and cathode voltages look ok, I'm quite sure the stage works fine and the undisturbed grid voltage is around 110V.

Thank you. I have much to learn.

[misterc57]

I would still put in the proper tube, 12dw7/7247 and he might even like it better.

Thank you. Going to make that suggestion.