What causes this duty-cycle shift?

[Fischerman]

In my Skyliner/HRM/BM PI amp I recently swapped the OD2 plate/cathode pair from 120k/1.8k to 150k/2.2k...still using the 5uF bypass. I noticed on my scope (100mV @ 880Hz input) that the top waveform on the OD2 plate got 'wider' and the bottom waveform got 'narrower'. With 120k/1.8k the top and bottom waveform were about the same width...but now with the 150k/2.2k the top waveform is noticably wider than the bottom. I also measured the AC output on the OD2 plate and the signal level dropped a little bit (I expected it increase if anything).

Did the signal level decrease because of the 'unbalanced' duty cycle? Is this because the higher plate resistor increased the output impedance a little? Any comments appreciated. I'll probably end up switching OD1 to 220k/3.3k and then use parallel resistors to try other values...including trying some different CL1 and CL2 values but I wanted to have a better idea of what's going on beforehand.

PS - in my amp when I scope the plate of OD1 and OD2: On OD1 the top is slightly fattened but it's a relatively clean sinusoidal shape...on OD2 the top is heavily clipped (almost square except for the tilted top) and the bottom is also fairly heavily clipped but less so than the top and with smooth corners.

[UltraHookedOnPhonix]

By your description it sounds like the 150K/2.2K combo clipps in a more asymmetrical fashion. I know the more heavily a signal gets clipped, the lower the percieved loudness will be. Maybe you can post some pics of the waveforms?

[Bob-I]

Maybe you can post some pics of the waveforms?

I'd like to start to get a better understanding of these waveforms. I[ve seen the same thing, the asymetrical clipping and I'd like to understand how that connects to plate loads, coupling, OD entrance etc.

[greiswig]

Me three. As well as what the psychoacoustic effects are of symmetrical versus asymmetrical clipping, etc.

[greiswig]

In my Skyliner/HRM/BM PI amp I recently swapped the OD2 plate/cathode pair from 120k/1.8k to 150k/2.2k...still using the 5uF bypass. I noticed on my scope (100mV @ 880Hz input) that the top waveform on the OD2 plate got 'wider' and the bottom waveform got 'narrower'. With 120k/1.8k the top and bottom waveform were about the same width...but now with the 150k/2.2k the top waveform is noticably wider than the bottom. I also measured the AC output on the OD2 plate and the signal level dropped a little bit (I expected it increase if anything).

Have you looked at any of the "Ultimate Tone" series of books? I've got one of them, and in the beginning of the preamp chapter the author talks about plate and cathode resistors, their relationship to gain, P-P voltage, etc.

If you were saying that the top waveform started to square off earlier than the bottom one, I'd say he addresses that. If it just looks like the slopes differ from top to bottom, I have no idea.

[dobbhill]

I found this in my archives, probably 8 years old, but I thought this might be the place to post it.
Never tried it myself.
D

[greiswig]

Well, I took a look at my scope, and tried a cathode resistor in parallel with the stock one on V1b to see if I could detect an effect on the symmetry of a 1kHz sine wave. Although I could see a change in gain, nothing particularly happened to the waveform. This would have been on a 100k plate with either a 3.3k or 1.5k cathode resistor.

According to O'Connor in TUT1, if all other things remain equal, you increase gain by either lowering the cathode R or increasing the plate R. He also suggests that the optimal Q-point of the stage is when the plate voltage is half the supply voltage. So that would determine what your plate resistor should be. Too far in either direction from that Q-point will result in asymmetrical clipping. On these amps, we sit at about 67% of supply voltage. Nevertheless, when I clip that stage, it looks symmetrical to me. (Then again I don't see a lot of O'Connor's London Power amps used by well-known stars, so for all I know he's wrong on this.)

From the file dobbhill posted (SIC):

“Dumble prefers to bias the OD stages the more symmetrically he can in order to reproduce the same kind of distortion that push-pull power amps have. Following this idea yesterday evening I experimented different biasing points by putting a trimmer in place of the cathode resistor (last OD stage) and results are very interesting.

The procedure is as follows:
-attach a voltmeter to measure the cathode voltage
-read the voltage at idle
-play fully distorted and take notice if the voltage goes higher or lower
-adjust the trimmer and repeat until the voltage remains almost stable from idle to full distortion play

This way you should have balanced the output signal from the stage. To me it sounds way better than any usual/unusual cathode/plate resistors combination I ever tried.”

That sounds like it would be difficult to do with a guitar. It also sounds like one of those "if it was this easy everyone would do it" things. Does it really work this way?

On my amp, both the cathode measurements in V1 went a couple tenths of a volt lower as gain was increased. Both V2 cathode voltages rose a few tenths, with the 180/2.7k pair rising the most.

[dobbhill]

Thanks for taking the time to experiment. Did you look at the waveform at V2a's plate, or just v2b? The authors of that file I posted had comments that I found interesting.

[greiswig]

Thanks for taking the time to experiment. Did you look at the waveform at V2a's plate, or just v2b? The authors of that file I posted had comments that I found interesting.

Just V1b, actually. That was where I already had a piggybacked resistor in place because of some other tweaks. Does this only apply to the OD channel? Why?

[greiswig]

Apologies if this has become a complete thread hijacking.

So I started using my scope after the V2b stage, and I do notice some asymmetrical clipping there. Not sure where that is coming from, since this

I tried putting a pot across the 1.8k cathode resistor (v2b) and adjusted it until I had stable voltage there in either a quiet state or with heavy distortion on a fixed input signal of 800Hz. It is tricky, because once you adjust the pot, the quiescent voltage has changed, too. So you have to try several times.

For that 120k plate resistor, the "magic cathode resistor value" (hereafter MCRV) on my amp was 1.32k. To my ear, changing that value did not noticeably sweeten the overdrive.

For V2a, the MCRV seemed to be a 1.5k cathode with the 180k plate. Here is where this may become relevant to the OP's question: by tuning this resistor, most of the asymmetrical clipping and ALL of the narrower sine on one swing go away. You can watch it happen on the scope as you tune it. So if you're really concerned about that, there is at least one way to deal with it.

HAD didn't use 180k plate resistors AFAIK, but extrapolating from other values I'd expect to see about a 3k on this to maintain the HAD ratio, which close to the 2.7k used. But maybe the plate:cathode R ratio isn't magic at all in isolation...or maybe 180k isn't Dumblesque. Or maybe this method of determining the sweet spot by looking at distorted versus quiescent voltage at the cathode is hooey.

How does it sound? Fine. But I wonder to what extent symmetrical clipping, etc. is desirable in a guitar amp, anyway.

[butwhatif]

What kind of waveform anomalies are we talking-- here's what I roughly get w/ a 1KZ sine , and presence on 6. This amp is all 100k with the exception of od1 which is 180k . All 1.5 k cath w/ bypass 4.7 uf. Can anyone else chime in w/ scope pix of their results?

[dogears]

The Bluesmaster Dumble I had here at my house had a 180K V2a with 2.7k cathode resistor.

Also, the 1997 Dumble HRM that many have based there builds on had a 180K/2.7K V2a setup as well. I have pics.

On the other hand, I have not seen 180K/2.7K on a non HRM amp.

[Fischerman]

Sorry I missed most of this! Thanks for the responses.

Something I found interesting in that attachment:

There are two other things that confirm my idea that only the ODS second overdrive stage is actually clipping while the first one only predistorts the signal with almost no overdrive:

- the output from the recovery is heavily reduced, the trimmer there is almost universally set to very low (less than one-third), so the signal reaching the first stage cannot clip it unless you push the front end a lot (volume above 6-7)

- the two overdrive stages are biased in the middle of the available swing, so their clipping will be very symmetrical (very different from a typical JCM800 100K/10K second stage for instance)

In scoping my amp this is what I'm seeing. I can certainly get OD1 clipped if I want but even with PAB ON I have to crank the volume to get OD1 to be heavily clipped.

I tried 220k/3k3 on OD1 and that put the 'balance' back to the top/bottom waveforms (I'm now at 220k/3k3 and 150k/2k2 for both Clean and OD sections). Then I tried some resistors in parallel to try different combinations on all 4 stages. There were differences but there is a sort of farty/fuzzy thing going on that until fixed/removed, nothing else matters...and none of this fixed it. All I'm trying to do is reduce or eliminate this slight farty/fuzzy thing (and FWIW, I hear this same farty/fuzzy thing on a LOT of clips so maybe it's just an ODS thing that I'll never resolve). Sometimes I think it sounds pretty good but after a short time I get fixated on that farty/fuzzy thing and soon it's all I hear.

I'll give that Rk tuning a shot.

[greiswig]

(SNIP)...but there is a sort of farty/fuzzy thing going on that until fixed/removed, nothing else matters...and none of this fixed it. All I'm trying to do is reduce or eliminate this slight farty/fuzzy thing (and FWIW, I hear this same farty/fuzzy thing on a LOT of clips so maybe it's just an ODS thing that I'll never resolve). Sometimes I think it sounds pretty good but after a short time I get fixated on that farty/fuzzy thing and soon it's all I hear.

I'll give that Rk tuning a shot.

Fischerman, is what you're hearing similar to the clip I posted in the thread ""Torn speaker" distortion sound on OD...thoughts?"

[Fischerman]

I need to listen on another computer...I'm only getting one side on this one. But it sounds sort of similar with loud signals...not quite as bad as your clip.