yes.... I Know....I'm starting a #183 build!

[martin manning]

Could you not just turn the pot down and reduce the signal going into the following stage? I view the OD trimmer as setting the minimum amount of dirt you get from the drive in OD mode. I think frequency response going into OD1 is mostly about the series resistor feeding the trimmer and and the grid stopper.

Martin
Yes you could, however, the frequency response (knee) is still shifted. So you have to think of 2 things when you choose a volume pot?.Signal loss and Knee. So the higher value pot will add low mids (darker). As far as the grid stoppers go I don't think your really altering the sound that much below 220k. If there is? it's minimal 15.5kHz w/68K (in combo w/Miller capacitance).
Personally, I think the 220k grid stopper on V1b and the 220k OD input resistor are there mainly to help keep the low end tight.So it's really everything together.That's what I hear anyway!
I changed this before you read it in my last post so here it is again.
As far as the OD trimmer goes I usually set it for best frequency response (with the tube) and control the amount of gain with the drive pot. Then control the volume of the OD with the OD Level.But that's me!

BTW.This is why (I think) Dumble sets his trimmer rather consistently sets and puts it in the amp.He's setting the frequency response of the OD.Why else would he alter the pot value on 124 and 123?

Took the time to do some simple analysis around this question, assuming a reference case where a 15n coupling cap, a 100k series resistor, a 250k Drive pot are coming off the plate of OD1, and 100p Miller capacitance into OD2.

In changing the value of the Drive pot to 100k in isolation, I see five things happening:

- The -3 dB point on the low end moves up ~17 Hz (you lose lows, 1-2 dB at 50 Hz, which could be made up by increasing the value of the coupling cap to 22n).
- The AC load on OD1 is increased, lowering the maximum available gain (at the output of the coupling cap) by about 1 dB.
- The AC load line rotation reduces the headroom on OD1, lowering the distortion threshold ("gainyness" in player parlance).
- The maximum gain at the output of OD1 is lowered by the voltage divider effect with the 100k resistor feeding the pot. If a 250k 10% taper pot is set at 25k (noon), a 100k 10% pot would have to be set at 14k3 (so something higher than its 10k noon position), to get the same attenuation going into OD2.
- There is a small increase in OD2 HF response due to the decrease in the series resistance of the top portion of the pot (-3 dB point moves from 11.8 to 13.3 kHz).

Grid stoppers and other series resistances, i.e. the top portion of the OD trimmer, and the 100k feeding it, affect HF roll-off through the Miller capacitance, without affecting low end response of the stage (OD1). In OD, the size of the trimmer will affect the low end coming out of the previous stage (Cl2) the same as as the Drive pot value affects OD1 bass (larger value means more lows). Larger values result in more series resistance in front of OD1 for a given signal level, reducing highs a bit. Dumble seems to have addressed that in more direct ways, including large grid stoppers and snubber caps. That said, I really don't see the OD entrance trimmer setting having much impact on frequency response once its value and the value of the resistor feeding it are chosen because it's set in a narrow range. There is little effect on the highs, and the setting won't affect the lows coming out of Cl2 at all. I'll stand by my comment above: where you set it is mostly about the OD distortion floor.

In the analog world it's truly "everything together" as you say above ;^)

[talbany]

Martin
Taking a look at your analysis.What was your sample frequency?

Tony

[martin manning]

I'm considering the full range. Basically this is looking at the "negative space" from the anode of one stage to the grid of the next, where gain is typically -20 dB. Kind of an interesting concept, no?

Here's what I mean, with the simple hand calc analysis nicely validated using a simulator. The plot is for the "183" 100k (blue) vs 250k (green) Drive pot scenario above, with the pot adjusted in each case to give the same -20 dB mid-band gain from OD1 out to OD2 in.

(Plot line colors edited in)

[talbany]

I'm considering the full range. Basically this is looking at the "negative space" from the anode of one stage to the grid of the next, where gain is typically -20 dB. Kind of an interesting concept, no?

Here's what I mean, with the simple hand calc analysis nicely validated using a simulator. The plot is for the "183" 100k vs 250k Drive pot scenario above, with the pot adjusted in each case to give the same -20 dB mid-band gain from OD1 out to OD2 in.

Martin
Thanks for running the sim. it makes sense and my ears in general concur.
Just a few things to add with your sim plots
1-In order to get roughly the same amount of gain as the 100k you have to set the pot lower in the trace whereby increasing the series resistance rolling the high end. We can of course remedy that with a bright cap (You may have mentioned that just want to explain it)
2. Many of the plots posted in the sim will be amplified again after OD'2's gain stage. Especially with the trimmer since it will get amplified by 2 more gain stages, also adding harmonic content, so any small changes in the plots there can make a noticeable difference at the PI.
3. Grid stoppers and snubbers act as Hi frequency roll-off accept the 220K on CL 2 with 500PF bypass cap over it. Not sure why he did that there but what I hear is a slightly tighter low end in overdrive.Would be cool to see a sim on that vs one without?

As far as the trimmer setting. Set yours by the floor I'll set mine by the frequency response. Agree to disagree.



Thanks again for all your work on the sim.Would still be interested in what your ears tell you as well.

Tony

[erwin_ve]

Martin and Tony: is it fair to say that for 1 gain stage frequency response is "Easy" to simulate/calculate and for multiple gain stages with harmonic distortion and clipping distortion it will quickly become very complex?
Will a plotted spectrum analyse, using the different trimmer settings, reveal the harmonic content?

[martin manning]

Side note, I've left out the snubber caps above, but at 25p they are not doing much in the audible range.

1-In order to get roughly the same amount of gain as the 100k you have to set the pot lower in the trace whereby increasing the series resistance rolling the high end. We can of course remedy that with a bright cap (You may have mentioned that just want to explain it)

I made that adjustment so that the signal level at the output is the same in both cases, and it shows up in the plot. That's why the high end is reduced a bit.

2. Many of the plots posted in the sim will be amplified again after OD'2's gain stage. Especially with the trimmer since it will get amplified by 2 more gain stages, also adding harmonic content, so any small changes in the plots there can make a noticeable difference at the PI.

True, and I believe blending multiple flavors is the secret to a good sauce. The distortion generated within the triodes does not appear in this analysis, though. It's only addressing the frequency response of the coupling network between stages.

3. Grid stoppers and snubbers act as Hi frequency roll-off accept the 220K on CL 2 with 500PF bypass cap over it. Not sure why he did that there but what I hear is a slightly tighter low end in overdrive.Would be cool to see a sim on that vs one without?

That cap creates a treble boost shelf that begins around the corner frequency of RC (1.4 kHz), but it interacts with the surrounding components too. You could say it counteracts the roll-off due to the Miller capacitance. At some point you just throw in the towel and run the whole preamp, with and without that cap. See below. The green trace has the cap removed. Notice there is no effect on the low end, so if you perceive something happening there it's a relative thing. You see that network on the D'lator recover stage, and of course in Marshall preamps both before the volume control and at the grid.

[martin manning]

Martin and Tony: is it fair to say that for 1 gain stage frequency response is "Easy" to simulate/calculate and for multiple gain stages with harmonic distortion and clipping distortion it will quickly become very complex?
Will a plotted spectrum analyse, using the different trimmer settings, reveal the harmonic content?

Yes for one stage or the simple coupling network we're talking about it's relatively easy to calculate the low and high corner frequencies (they are pretty much independent), and that tells you what the frequency response plot looks like. As the number of components increases, or for multiple stages, a simulator is much more convenient. Harmonic content requires spectrum analysis, which is typically done with FFT. Simulators can do that, but I don't know if the tube models are good enough to get really meaningful results.

[talbany]

Martin and Tony: is it fair to say that for 1 gain stage frequency response is "Easy" to simulate/calculate and for multiple gain stages with harmonic distortion and clipping distortion it will quickly become very complex?
Will a plotted spectrum analyse, using the different trimmer settings, reveal the harmonic content?

Yes it's extremely complex!.We should also keep in mind these types of simulations are useful as a baseline for comparing and contrasting the frequency response between 2 similar circuits everything else remaining equal.Given that Cap types/Resistors/ Shielded cable/Tubes/where the amp is set all of which may affect and alter harmonic content.So you would need to plug in all those parameters if you used different component types in order to give you a very accurate plot of the harmonic structure of your specific amp.
However, if you were an amp manufacturer and were building identical amps using the same parts and want to then check the harmonic structure I would build and document and plot a number of amps then find an average come up with a standard and use that as a guideline to spectrum analyzing my builds to get them as close as possible while factoring in parts tolerances . Otherwise, it just becomes a sort of blueprint or general idea.

Since we don't just hear that circuit we also need to consider other sections that may generate or alter harmonic content like Output Tubes/Output Transformers/Output Impedance settings/Phase inverter type and balance/Amount of GNFB/Setting of the Mstr vol/Phase shifts/PS cap values/any cathode followers or added gain stages and even a drop in wall voltage can and will alter the harmonic structure. This is especially true with a hi-gain amp.

Tony

[martin manning]

We should also keep in mind these types of simulations are useful as a baseline for comparing and contrasting the frequency response between 2 similar circuits everything else remaining equal.

+1! Even if a model is not accurate it may be very useful in determining the difference in response when something is changed.

Given that Cap types/Resistors/ Shielded cable/Tubes/where the amp is set all of which may affect and alter harmonic content.So you would need to plug in all those parameters if you used different component types in order to give you a very accurate plot of the harmonic structure of your specific amp.

Some of those things are easy to model, a bit of parasitic capacitance for a shielded cable, for example. Many of the standard component models have the main parasitic properties included. For caps, the ESR and the ESL, but not the details of dielectric behavior.

Since we don't just hear that circuit we also need to consider other sections that may generate or alter harmonic content like Output Tubes/Output Transformers/Output Impedance settings/Phase inverter type and balance/Amount of GNFB/Setting of the Mstr vol/Phase shifts/PS cap values/any cathode followers or added gain stages and even a drop in wall voltage can and will alter the harmonic structure.

I would say that we do hear the circuit, but the typical schematic diagram is an incomplete representation of the actual circuit, where all these little details make some subtle contribution. Another way of saying the same thing, I think...

[talbany]

Martin
If I am not mistaken wouldn't also things like background noise EMF, any oscillations and added Capacitance from things like circuit board material and design. Power Transformer regulation. If this were the case then you have to factor in construction methods, layout,design and circuit constants as well. Fragile Harmonics.

These are obviously minor but put together?
Thanks for the follow up!
Tony

[martin manning]

If I am not mistaken wouldn't also things like background noise EMF, any oscillations and added Capacitance from things like circuit board material and design. Power Transformer regulation. If this were the case then you have to factor in construction methods, layout,design and circuit constants as well. Fragile Harmonics. These are obviously minor but put together?

A transformer model could include series resistance, interwinding capacitance etc., but background noise is not part of the amp. In theory a really complete model would reproduce oscillations, if the parasitics were well represented. Capacitance between hookup wire leads is real, as is capacitance between components, which varies from one hand-wired specimen to the next (not so much with PCB construction), so that would be hard to get at.

We are kind of hijacking Marcus' thread here, but this is good discussion.

[Reeltarded]

you two get a room...

[norburybrook]

you two get a room...

I'm getting the popcorn ready Miles...…….

Martin and Tony don't worry about the thread hijack it's all good stuff and I beleive it's all relevant to this build as it seeks to address the points I've made since firing her up.

I think my pots will be here today in the post so I'll get those in and report back. I made the footswitch yesterday. I made the mistake of bright LED's .....I'm sure the european space station will see them on every orbit!!! One thing I noticed is the PAB doesn't seem to be actually boosting much, more like a bass cut if anything.

could I have wired it wrong or is this how the #183 PAB is?

M

[talbany]

you two get a room...

Or a Lab!

These are the things I would like to see harmonically modeled.


Chassis? Alum vs Steel= (RC time constants)
Noise?= (Capicator orientation)
Stray Capacitance=PC vs Turrett construction
Capicator Dielectric?

Possibly answer allot of myths and opinions floating around amp building communities with regard to harmonic content?

Pass the popcorn I am done too. Agree Good discussion!

Tony

[martin manning]

One thing I noticed is the PAB doesn't seem to be actually boosting much, more like a bass cut if anything. Could I have wired it wrong or is this how the #183 PAB is?

183's PAB is like the others, so I'd say you have a wiring error. Check the resistance from the top of the bass pot to its wiper, and the bottom of the treble pot to the top of the bass pot, with and without PAB engaged (Rock mode).