Any interest in seeing some Dumble Tweed Deluxe photos?

[martin manning]

It will. You will see current pulses (as positive voltage) alternately from each tube. If the drive is not symmetric, or the Gm's are not matched, the peaks will differ in height. Placing the current sense resistor from the bottom of Rk to ground is the preferred option, since you can read from ground with one scope probe. As a bonus, you can use it to read idle current without doing any math.

[Charlie Wilson]

Thanks Martin, yeah I got to learn to read a little better. You are talking about looking at the waveform not looking at voltages with a multimeter, Sorry about that.
CW

[Delmar Evans]

Charlie

So if one does not have a scope , what is the best way to set the PI trimmer? Also , you want v3 and v4 matched regardless of what is hitting the phase inverter tube?

[martin manning]

So if one does not have a scope , what is the best way to set the PI trimmer? Also , you want v3 and v4 matched regardless of what is hitting the phase inverter tube?

You can connect a 1Ω from each power tube cathode to a common point, and then connect that point to the cathode resistor. With signal applied, measure DC across the cathodes (pin 8's) with your meter, and adjust to zero volts. Miss-matched power tubes can be balanced if they are not too far out.

[Delmar Evans]

So you are not measuring the voltage at pin 6 of v3 and v4,but setting voltage to zero at pin 8 of v3 and v4 via 1 ohm resistor by adjusting the pi potentiometer. I thought it was the feed from the coupling caps to pin 6

[martin manning]

Measuring the signal at the power tube grids doesn't catch miss-matched tubes. See here: https://ampgarage.com/forum/viewtopic.p ... 89#p416389

[Delmar Evans]

Followed your suggestion. Installed 1 ohm 2 watt 1% on the
Cathodes of both tubes. Tied the ends together with the feed from the cathode resistor

Using a Fluke 88
Measured cathode voltage with no signal. 22.5 vdc
Measured at top of 10 meg 44.3 vdc
Bottom of 10 meg and at pin 7 of the PI. 3.9 vdc

Injected the following hz signals and did full sweeps on the PI
Potentiometer from full counter clockwise to full clockwise
Measured the following dc millivolt readings

40 hz. .004 to .006 mv
100 hz -.003 to -.004 mv
400 hz. -.006 to -.009 mv
1000 hz. -.005 to -.009 mv

So the movement of the potentiometer did nothing on these power tubes. The most movement was -.004 mv
So these tubes must be really close I quess and the trimmer could
Never get to zero.

[Charlie Wilson]

Delmar, with the amp idle, what is the voltage reading across each 1 ohm resistor?
CW

[Delmar Evans]

At idle V3 from pin 8 Across 1 ohm to cathode resistor .044 vdc
At Idle V4 from pin 8 Across 1 ohm to cathode resistor .046 vdc

At idle from v3 pin 8 to v4 pin 8 .002 vdc

Let me know if you want any more readings. Give me a call if needed

[martin manning]

Delmar, re the measurement across the cathodes with the two 1Ω added, it is going to be that subtle. I see you never crossed zero, so it doesn't look like you can get to a balanced condition. Dropping the PI plate load to 100k might do it.

Placing a 1Ω below the cathode resistor and leveling the upper peaks on a scope will be much easier. Charlie, if you are able to try that I'd be interested to hear what you find. I think that would be a better method for setting the PI trimmer on an ODS too. The layout would have to be modified to run all four 1Ω to the ground banana jack, and then ground that jack with a fifth 1Ω.

[martin manning]

Measured cathode voltage with no signal. 22.5 vdc

Which cathode is this? Power stage?

Measured at top of 10 meg 44.3 vdc

This looks like what I'm calling the bottom of the 10Meg, where it connects to the 4k7 and the trimmer.

Bottom of 10 meg and at pin 7 of the PI. 3.9 vdc

This voltage is in reality close to the one above (44.3V), but your meter is loading the grid circuit.

[Delmar Evans]

22.5 volts is at the v3 and v4 power tube cathode

Correct on the location of the 44.3

[Delmar Evans]

Also installed a 100k in the 110k spot. Still never crossed zero, and remained at .002 no matter where the potentiometer was. So I’m guessing without a scope this adjustment cannot be accomplished successfully. Any thoughts ?

[martin manning]

Also installed a 100k in the 110k spot. Still never crossed zero, and remained at .002 no matter where the potentiometer was. So I’m guessing without a scope this adjustment cannot be accomplished successfully. Any thoughts ?

I would measure AC at the PI output (junction of the 220k and 1k5) on each side. You should see something easily measurable there which you can balance with the trimmer. If you still have the 100k plate load installed, try it with that first. As mentioned, that doesn't account for power tube miss-match, but it's the next best thing.

[Delmar Evans]

Well, I see that the output of the phase inverter hits the coupling cap and goes to the pair of 220 K resistors however I see no junction point with a 1.5 K that you speak of at that area unless you want a measurement at pin 6 of v3 and v4 , which is where I started in the first place ! In addition I installed a match set of JJ power tubes