What causes this duty-cycle shift?

[LOUDthud]

My experience relative to this thread is general in nature, I'm still waiting on a D'Lite kit to arrive. (Moss!!!)

When a sine wave of sufficient level is capacitor coupled into the grid of a tube such that grid current flows on the top of the waveform, that grid current accumulates on the capacitor increasing the voltage stored by it. This causes a baseline shift such that the "center" of the waveform as seen by the tube, is above the center of the sine wave on a narrower portion. This is inverted when it gets to the plate. If the tube has enough gain, it will clip both sides of the incoming wave on it's output but the result will not be symetrical. This is in addition to the effect of the tube's non-linearity. In extreme cases, this is called blocking distortion.

Any resistance in series with the capacitor slows down and can reduce the effect, but when you are looking at a signal generator, you don't see this dynamic behavior. The interstage attenuators and/or pots reduce the baseline offset but do not eliminate it completely when grid current flows.

[Fischerman]

Thanks loudthud....I kinda figured it was because the entire waveform had been 'shifted' up or down (due to coupling cap charging)...which when combined with the clipping makes it look the way it does. That is...if you were to take a see-through rectangle (bounded on top and bottom by the clipping) and shift it up or down on a sinusoidal wave it would appear that one half of the waveform was 'wider' than the other.

[odourboy]

Somewhat late, but I had my chassis pulled today so I took the opportunity to hook up a scope and take a picture as per 'butwhatif's request.

1 kHz sine wave input with amp on OD just after the onset of preamp clipping. Amp is nonHRM with 220/150/220/150 and 'standard' Kr values. Sorry about the blurry picture.

[Fischerman]

When I scoped mine I had dialed in more clipping but see how your bottom waveform is a little wider than the top? I ended up going to 220k/150k on OD1/OD2 because it makes it easier to test 220k vs. 180k and 150k vs. 120k by just using a parallel resistor (same with the Rk). But I haven't tested that very much yet.

[butwhatif]

I have had to play with some of the values in order to get a more symmetrical OD waveform-mostly the OD input values. My amp has all 1.5K /4.7uf cathodes, with differing plates. Also I have used a load resistor on the output of OD2 (470K currently) before the series resistor which tightens the bass, reduces the bass buzz.

[odourboy]

When I scoped mine I had dialed in more clipping but see how your bottom waveform is a little wider than the top? I ended up going to 220k/150k on OD1/OD2 because it makes it easier to test 220k vs. 180k and 150k vs. 120k by just using a parallel resistor (same with the Rk). But I haven't tested that very much yet.

Yes - it's true that the bottom waveform is more squashed than the top... but isn't that what you'd expect? One half is subjected to saturation while is going into cutoff. Why would you expect them to be symmetrical?

FWIW, I set my OD up so the clipping is primarily occurring in OD2. OD1 is 'relatively' clean.

[Fischerman]

I didn't expect them to be symmetrical...or not. I was scoping it to find out. Originally it was fairly symmetrical but when I switched OD1 from 180k/2.7k to 220k/3.3k it became assymetrical and was just looking for input on an explanation.

My set up is similar...OD1 can be quite clean while OD2 can be quite dirty. But my amp is HRM and PAB is ON.

[odourboy]

My bad. I thought you guys were striving for symmetry.

[ic-racer]

Seems to me like any of these preamp tube biasing results are going to be highly 'tube dependent'

I thought of an analogy: Back in the 60s and 70s, the manufacturing quality of USA 6L6s etc was consistent, so a USA amp builder only needed to specify a given bias voltage (ie -52v) and all was good.

Today, with the extreme variability in 6L6s, a bias scheme like that is a little crazy.

Likewise, I hypothesize that in the 60s and 70s, the manufacturing quality of 12AX7s was consistent, so no need to tweak the bias point. However, today one is faced with a huge range of 12AX7 performance and the two options are A) have a big box of 12AX7s and keep swapping until it sounds right, or B) try to tweak the bias of a given tube until it sounds right.

So I am waiting for someone to post pictures of a "MILLENNIUM AMP" with bias pots for each 12AX7 stage

[bluesfendermanblues]

Gents, this an interesting thread....

However, its seems to end with a cliphanger?

a) Did you end up preferring symmetrical OD
b) and what cathode values did you end up with on OD2 in order to achieve symmetry

I tend to get the described farty bass end on the OD channel, which after a while of listening catch my attention and I feel the sound I'm gettin' is very far from the RF tone I'm striving for. Could this be beause of to assymmetrical clipping.

[Fischerman]

Gents, this an interesting thread....

However, its seems to end with a cliphanger?

a) Did you end up preferring symmetrical OD
b) and what cathode values did you end up with on OD2 in order to achieve symmetry

I tend to get the described farty bass end on the OD channel, which after a while of listening catch my attention and I feel the sound I'm gettin' is very far from the RF tone I'm striving for. Could this be beause of to assymmetrical clipping.

I think the answer is that different people like different amounts of asymmetry. It also makes the attack different so it feels different (too much asymmetry makes the attack too fat and thuddy).

Regarding the farty bass...too much asymmetry can sort of make it seem like that but in my limited experience, the OD entrance is what affects the amount of growl in the lows more than anything.

[bluesfendermanblues]

Fisherman, Point taken.

I'll keep trimming the OD entrance to make it a little less bass heavy.

I've tried Heistl's input filter with a 47nf followed by a 10m to ground into the usual 220K + 100K trimmer. This gives a cuttoff frequency (in connection with a 10nf on CL2) of around 60hz.

Versus the ususal 10nF (from CL2) into 220K + 100K trimmer, which gives a cutoff of 49 hz.

I've also tried the TwoRock version 220k into 22k trimmer with 22k to ground, which should also be around 60 hz.

Maybee, I should cut a little higher - any tips you could pass on ??

[Fischerman]

You could try a larger cap across the 470k...try up to a 500pF but then decrease the trimmer to account for all that extra gain you just added with the bigger cap. Hear how much 'cleaner' the low end is? And there's a bazillion points in between.

[ElectronAvalanche]

Hi there,

thought I might chime in with some scope pics of one of my amps I took some years ago.

Much more gain than in the other scope pics posted in this thread. Oddly enough, the MID control really deforms the lower half of the signal already.

Hope this helps.

Cheers,

Dominik