Single-ended 6v6 diy; over-conducting

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Schematic here!

I've got this home made amp that has been over-conducting and I'd like some general help for remedying this (main issue) as well as any insight as to tweaking the circuit for best operation. This is a sort of champ/garnet herzog mash-up. The preamp slot is meant to accept either 12AX7 or 12AU7 (or anything in between) depending on application. The main idea is for the 12AX7 to serve as herzog duty, or the 12AU7 for taming the circuit and allowing for a broader spectrum use as a preamp. (i.e. leslie preamp).

With the preamp tube out of circuit, the 6V6 conducts at 23mA (cathode+screen). Adding a 12AU7 yields 38mA. Assuming 305vdc plate voltage. The power transformer was slightly under-loaded, hence the zeners. Also the heaters are running slightly hot at 6.8vac. Any help would be appreciated.

The Champ 5E1 and 5F1 had 22K feedback from the speaker to your pin8. I guess you want that 'no feedback' sound but maybe that's why it's working harder. Maybe the 5K primary on the output transformer is a bit low. Your master volume is REALLY final! Another volume at the grid input to the 6V6 might make it more versatile; personally I would have left out the tone control and added another volume control, added a 'bright' switch across the first volme pot, and some kind of tone control at the output transformer, either on the primary or on the secondary like part of a crossover.
I assume you want it a bit raunchy and are getting that?
Keep in mind I'm a guitar guy, not an EE.
The feedback line shouldn;t affect the DC current of the power tube, there is no direct connection, a cap isolates the power tube from the previous stage.

Make yourself a chart. B+ at the first cap, and X, Y, and Z with no tubes, then with power tube only then with both tubes. Don;t assume 305 or any other volts, you need to KNOW.

The 5k ohms of the OT primary is impedance, not resistance.
IF the unit has been built exactly to that schematic then having the preamp tube in or out will not make a rodents hindquarters difference to the 6V6 running current.
I suspect that you have a wiring problem, check the cathode wiring on both the 6V6 and preamp tube and their 0V return points.
Don't worry about the 6.8V on the heaters.
Thanks everyone!


No tubes:
B+ 386
X,Y,Z: 370

6V6 only:
B+ 333
X,Y,Z&preamp plates: 315-310

Both tubes:
B+ 330
X: 312, 305 at 6V6 plate
Y: 305
Z: 255, plate 1: 40, plate 2: 80
Z: 280, plate 1: 130, plate 2: 190

The current through 6V6 cathode/screen is 38mA (12AU7) and 36mA (12AX7).

Ian: the cathodes appear to be wired correctly, all the bypass caps (neg) all have good continuity to ground.

DF96: I don't have a reference for what oscillation should sound like, but I think it is present: When the gain is turned up the circuit doesn't sound happy (squelch?) can also hear the signal vibrating through the circuitry/chassis.

What is your recommendation for grid-stoppers? 1.5k to both 6v6 input grid and 2nd triode? Do you have any suggestions for improving the supply rail?

Thanks everyone!
If you lack a scope to see oscillation, here is an alternative.

Google "RF probe" and see the results. There are variations, but they all boil down to a cap, a diode, and a resistor. A very simple thing to make and use in front of a meter. It allows a basic voltmeter to read levels of RF. If a seemingly silent amp measures several volts of RF, then indeed it must be oscillating.

What this little circuit is would be the "detector" stage of an AM radio.
OK, groud the scope probe to the amp, and probe the speaker output or the plate of the output tube, or for that matter, any plate in there. AC coupled of course. Since this seems to happen even in the absence of signal, you would expect the signal path to remain a flat line at zero. If you have what appears to be a wide lit band across your scope, turn up the sweep speed and see if it resolves into a very high freq waveform.

At sweep speeds suited to audio, the RF will just look like a band of light instead of a line.
If you have X10 or X100 probes you can set the scope input to AC and look at the 6V6 anode directly. Chech for oscillation at ultrasonic frequencies.
If you are "sus" about your or the scopes safety doing that then you should be able to see any ultrasonic oscillation (greatly reduced) on the secondary of the output transformer.
If you really want to see what is happening then lift the 0V side of the 6V6 cathode bias resistor and bypass cap and put a 10 Ohm resistor in there and look across that resistor with the scope.
As a first step, I would just put the grid stop resitor in and see if your problem goes away.
Most amps benefit from grid stoppers. Guitar amps, with their peculiar layout and long wires, definitely need them and often use stoppers which are so high in value that they act as (fixed) tone controls. The exact value is not too critical, but something around 5k for the output and 10-20k for the earlier stages - wired at the grid tag.

Put the scope on the output stage grid. If there is a signal here when there should not be, what is its frequency? (Roughly - subsonic, 60Hz, 50kHz?) If possible, use a 10:1 probe as that will add less capacitance. At the same time continue monitoring the output cathode voltage. If it changes when you connect the scope probe then this is a classic symptom of oscillation.
A grid stopper does two things:
1. It, in conjunction with Miller capacitance, rolls off the HF gain of the stage so stray capacitance can't create unwanted feedback loops (typically over several stages).
2. It adds losses to the grid circuit, so anything which looks like a VHF/UHF resonator is heavily damped so the stage (on its own) can't turn into an oscillator.

In order for the second thing to work the resistor needs to be right at the grid tag.
Thanks for explaining, very helpful!

I found this very short (yet concise) link about common gain stages and what you typically see immediately surrounding a tube. Check it out here;

Questions keep flooding in...
How does Fender end up with 1.5K as a common grid stopper value?
What are your thoughts about the low plate voltage for the 12AU7s (40-80V)?
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