Tweaking results in an octave lower artifact

[BobW]

After adjusting my 50W ODS w/ the 220k/150K plate resistors, and using temporary series pots to find a sweet spot on the plate values, I noticed a pronounced bottom end on the tone. I didn't measure the frequency but the lower artifact sounds to be an octave lower across a wide range of the fretboard (1/2 fundamental frequency). Further adjustments caused the lower frequency to fade out, adjusting the values back down brought the artifact back in. I have more trials to run to determine the values and where it does and does not occur.

[Structo]

Are you using the pots to dial in the plate voltage as in the dropping string or are you changing the plate load resistance with them?

From what I have been able to glean about the plates and cathode resistors on these amps is that the ratio of 66:1 or was it 67:1 should be followed to keep the tube bias in the design center of operation for the best results.

But I am always open for experimentation for learning purposes.

[ChrisM]

67:1

100/1.5 = 66.67 ~ 67
220/3.3 = 66.67 ~ 67
150/2.2 = 68.18 ~

[BobW]

Tom,
I'm adjusting the Plate load resistors not the dropping string, and was very suprised to find a sweet spot when dialing it in. However, I imagine that all goes out the window with a different tube.

[Structo]

I was also wondering if when you replaced the variable with a fixed, if the tone remained the same?
Since we generally all use the Dale metal films there.

What was your conclusion on that?

[LPSGME]

Tom,
I'm adjusting the Plate load resistors not the dropping string, and was very suprised to find a sweet spot when dialing it in. However, I imagine that all goes out the window with a different tube.

How different are your plate voltages (between each tube half) after you adjusted the plate resistor to the "sweet spot"?

My plate voltages are exactly 300/295 PI; 200 V2; and 186 V1(sounded better than 190) - and there is never more than a 0 - 2 volt difference between each tube's half section.

But there is something that bothers me (on my amp) that I don't think should be the case. Could anyone tell me if it is or not?

When I scope the signal path, the OD clipping at V2b seems odd.

1) The wave does not clip symmetrically as the input signal increases i.e. one side (top on the scope trace) begins clipping sooner; and

2) As the wave form clips, it rounds at the peak (on the rise) but then falls sharply away from the peak as the wave swings negative.

This sharp distorted transition is mirrored on the bottom half of the wave of course (except on the rise), but the sharpness is more extreme on the top of the wave that clips first.

It's not being caused by the tube or the resistors or the caps, as I've changed them.

I tried changing the plate/cathode ratio, to make the clipping more symmetrical, but it required a radically change (which also made the plate voltage on V2b way lower compared to V2a).

Is that sharp distortion normal?

I think it may be what is causing a high end fizz.

[BobW]

I was also wondering if when you replaced the variable with a fixed, if the tone remained the same?
Since we generally all use the Dale metal films there.

What was your conclusion on that?

Tom, Haven't tried it yet, may have time this weekend. Regards.

[Structo]

I don't think your voltages will be the same unless a) the tube is balanced and b) the plate load resistors and cathode resistors are the same for both halves.
Such as 100K/1K5.

[JD0x0]

is it possible you could have a wolf tone thing going on. often desribed as "hashy bass" its a static-y sounding (almost like when you have phlem in your throat and clear it) it 'hovers" over the bass frequencies causing an unpleasant tone.

[LPSGME]

I don't think your voltages will be the same unless a) the tube is balanced and b) the plate load resistors and cathode resistors are the same for both halves.
Such as 100K/1K5.

For the voltages to be consistent on each tube half (with a balanced tube) I don't think that the load resistors need to be the same on both halves; they just need to be in the same ratio.

[Structo]

Not sure about that.
In looking back through my voltage notes, the B side of V1 & V2 is consistently 6-8v higher.

This amp is high plate on both tubes. (220K/150K)
So it is the 150K side that is reading higher all the time.

That was with quite a few different 12ax7's .

[LPSGME]

Not sure about that.
In looking back through my voltage notes, the B side of V1 & V2 is consistently 6-8v higher.

This amp is high plate on both tubes. (220K/150K)
So it is the 150K side that is reading higher all the time.

That was with quite a few different 12ax7's .

8 volts seems like a lot of difference.

I don't know what resistance the tube impose or conducts under, but if you eliminate the tube and just consider the resistors (like a PS dropping string), then the voltage (where they join) would measure the same for any 2 resistors of the same ratio. Adding in the tube doesn't produce that much difference on my amp.

I'm using 220/150 for V1 and 220/180 for V2 and I only get a volt or two difference (if that) between the tube halves (be it V1 or V2).