SSS CF Driver Compression Questions

[bcmatt]

I also see that it should not accompany a bypass cap.

It might be true when the idle point is shifted so far towards cutoff but you can try a mildly asymmetric bias and see how that works, even small changes in value have an effect on the headroom. In my case I used a 2k7 resistor with an unremarkable 4.7uF cap. It's a different amp I'm working on but the idea is the same. Mine isn't buzzy at at all, a fuller sound I would say.

https://ampgarage.com/forum/viewtopic.p ... 50#p465050

Good point. I am not searching for a high gain monster. I just need to thicken it up a bit. I played a bit this morning and to get a reasonable thickness I need volume at 9 with Mid boost and PAB both on... and it's still fully in "clean" territory.

[Stephen1966]

Do you think adding resistance in series between the regular cap/resister and ground would avoid the buzzy pitfalls of increasing the regular bypassed resister value?

That's an interesting question and I wanted to think about it, so forgive the delay... and sorry if I haven't properly understood your line of thought here.

Referring to the 100 Ohm tail resistors used in the cathode circuit I think you have an answer with the idea that they can act as a shunt to prevent the NFB from bleeding straight to earth. There's more to it I think because they are used on the second triode and on the "HT4" triodes in the reverb as well; they aren't connected to NFB throughout, though in the reverb they do seem to behave like voltage dividers for signal going forward. The first stage of the reverb is NFB attenuated by the 2k2 resistor. It would be easy to get lost in the weeds here but if we look at what they are and what they can do on the cathode it might answer your question. You basically have a voltage divider and the cathode cap is in parallel with (bypasses) the first leg of the divider, lets call it Rb (1.5k). The parallel Ck and Rb are in series with the 100R tail resistor, call it Rt.

[Edit: sorry! I said lets call the tail Rt and then promptly forgot and continued, calling it Rk! Corrected below.]

The voltage loss at the junction of Rb and Rt is marginal. If the grid voltage is -1V (hypothetically) then the voltage across Rb = 0.9375V, with a drop across Rt = 0.0625V. If we compare the size of the tail resistor alongside the grid leak resistor you have an almost identical voltage going from the top of the grid leak to earth and the top of the bias resistor to its junction at the top of the tail resistor. So, what is the tail resistor doing? It isn't creating a bootstrap effect because for that to work, we would need to connect the bottom leg of the grid leak to the junction of Rb and Rt. So, we can say that it isn't about relieving the input impedance of the triode which isn't heavily loaded anyway. What it does to the bias cap though is provide a stable voltage which is clamped at the junction of Rb and Rt, just slightly above the ground potential. I don't know if you can consider that a virtual earth but, additionally, it does seem to represent a parallel shunt when you use it as a mixing node as he does. The voltage "elevation" at the base of Ck might be a factor but is probably too insignificant to be considered given the size of Rt.

The "buzzy pitfalls" sounds like a great name for a band, and it would work too because if you google "buzzy pitfalls" you've definitely got a gap in the market ripe for development Truth is, I don't really know what you're talking about with this BUT, if it's parasitic oscillation then changes to the cathode circuit are unlikely to be a contributing factor. If we take parasitic oscillation to be the resonance between capacitive and inductive forces then the Miller capacitance in combination with the series inductance in the control grid circuit is effectively dealt with by the grid stoppers. Any inductance in the cathode circuit only reduces loop gain because it is a form of negative feedback. On the other hand, if it's fizz, then fizz is a property of high frequency distortion and can have a contributing factor with the problem of inductive cross-talk in the lead dress and as such, it's controlled by more effective filtering.

I think for it to work as voltage shunt you don't want your tail resistor to lift the ground reference much above the ground potential because you want it to more or less offer the same voltage drop as your grid leak. But, if you bias really cold I don't see how adding resistance (in series) with Rb is going to prevent the signal going into more severe asymmetric clipping, grid clamping (at cutoff) followed by bias excursion.

[bcmatt]

I've just taken a big detour revisiting the global Negative Feedback and presence circuit... because they seem to have a big impact on potentially creating some harsh frequencies in the 5kHz ish? region... especially on the pick attack. I've changed the circuit before in this area when I first wanted to add a presence control... and I'm paranoid I've done something wrong or I have a forgotten wire connecting things under the board.

I did swap the 5th Cathode resister to 2.7K (the triode after the step filters) after playing with a 5k pot in series with the 1.5k. I liked the effect when turning it up to almost halfway and getting about 2.5V Cathode Voltage.

I've since swapped V1A and V1B cathode resisters to 2.7K as well. I need to put a hold on this until I get the GNFB and Presence circuits sorted.

Does anyone know what SSS #004 is supposed to use for presence?

[bcmatt]

I'm still working on this amp continually. I took a week off work to painting our main level of our house, and tweaking this amp in the late evenings after our kids are in bed.

I went back to the GNFB on the Hartman layout with no presence this time (for now). I may play with more GNFB arrangements again after I tweak the preamp/tonestack.

I removed the "Jazz" switch and hardwired to "Rock" mode.

I Switched the bright cap on the bright switch to 250pF from the 150pF because the 150 was so subtle I could barely tell if it was on. Maybe about 200pF would be ideal, but I can come back to that.

I fixed my mid boost to better match the #5? schematics with a 390pF cap that gets bypassed on the way to the treble pot. This time I have a 4M7 resister to ground (to keep the switch from popping I assume... because it was before)

Now I am tacking my step Filters to line up with Martin Manning's latest iteration. My Low filter is backward (I'd like to make it get more lows as I go up). Also, my 5th setting (0.01uF) out of 6 on the High Filter seems to drop its highs from the 4th setting
which is (0.0047uF). I'll still go with 6 settings but try the new filter setup giving up either the top or bottom option on both filters. I'll try to track that schematic down now and see all that needs to change for that.

After that I will see how things sound and decide whether to try some other GNFB options, or put the preamp cathode resisters back to 1.5Ks. If I crank the amp for maximum gain, the overdrive that does start to appear is rather nasty/fizzy.

[bcmatt]

I got the Martin Manning Step Filters in, and I have to say how much I love them! Such even useful steps; each giving the appropriate perceivable increase in their respect highs or lows. Such a beautiful implementation. Definitely worth making the update for anyone considering this and wondering if it is worth the trouble. Because I only had six position switches, I gave up the highest high position and the lowest low(least amount of bass) settings.

20240415_180451.jpg

I also brought my dropping resister back up to 20K again since it seemed all my preamp and PI plate voltages were riding higher than 210V with the 14K resister value.

Next I wanted to try the "Mid Accent" switch that bypasses a cap across the master volume. I tried the 0.001 cap at first, but it was just a high end boost. I think this is because the cap leading into the MV on my Hartman layout is 0.02uF compared to the .01uF on the schematic that has the accent switch I was copying. I experimented by clipping in some more caps in parallel and bringing the cap value up to 0.047uF seemed to add a fair amount more mids with the highs. this is overall quite a large volume jump and it really changes the voice of the amp a lot so that that a bunch of other controls need to change (to much more extreme settings) to compensate. I don't think I will keep it as is, but I wonder if there is some version of it that could be useful.

If I were to use this mid accent, I think the amp would also need some sort of Cut Control as well. Is that possible to implement even though I have a cathode follower driver tube in the circuit? I see the John Mayer amp has a cut control, but it doesn't have the CF driver.

I started playing with my preamp cathode resisters again. I clipped in a parallel 2.7K with the current 2.7K that I have in the stage after the filters. I noticed there are some SSS amps that have a 1K compared to the usual 1.5K, so I figured me biasing that tube cooler might have been my most hasty decision. However, after A/B comparing between 2.7K and 1.35K, I'm still not decided and maybe don't want that extra headroom there. I think maybe I do appreciate the thickening up in that position. I may have to put it on a switch and let the 2 choices live there for awhile.

I still have the 2.7K cathode resisters on both sides of V1 as well, so I need to A/B them with their stock values as well.

A lot of these decisions will depend on me playing with the choices and some different volumes. It's hard to make judgements in the basement in the evening after all the kids are in bed.

I haven't revisited the GNFB yet either...

[bcmatt]

I realized that I had tweaked the wrong cathode resister when I thought I was doing the stage after the filters.
Wires to V4 cathode components were hidden under the board, so I was actually lowering the headroom for the reverb I think. I had just put its value back to 1.5k when my soldering iron tip broke.... so I'm done till at least Friday when new ones arrive (if I have spares somewhere, they are lost). The thing is, I do feel like that accidental mod might have been one of the more positive ones I tried... ironic.
But I think I'd like to try experimenting next with removing the cathode bypass cap on the filter recovery stage and putting on a pot in series with a 1k resister. I guess it would be a "stage 3 bias/headroom control". But I do wonder about maximum headroom potentially being really important right after the filters...
I still have 2 front panel controls to add, so I'm trying out candidates. The jazz option got the axe, so this is its potential replacement.
Since I pulled the presence, and my mid accent is too extreme, I'm still wondering about a cut control.
As for v1 cathodes, I'm leaving them at 2.7k for now since that gets them around 2V like the vyperdoc schematic shows. (I'd love to see more voltage readings from SSS amps)

[bcmatt]

Well, I did a bit more tweaking and I think I should button this up and play it for a little while in some different settings.

Where it's at compared to the original MHartman layout (aside from the physical chassis layout being different and having the separate CF Driver and Bias power supply):

20240418_201146.jpg

-Updated to the latest Manning Step Filters. (but 6 positions on each- but finally correct).
-GNFB switch on the back (used to be the ground switch on the old Traynor) that can bumps the Feedback Resister from 2.7K up to 4.9K (we'll see what I use more)
-V1 Cathode resisters are 2.7K instead of 1.5K )(because this gets the cathode voltage closer to someone's schematic that actually has suggested voltages on it).
-Both V4 Cathodes are also 2.7K, but I also have a front panel switch now that can switch back to the stock value of 1.5K on both as well. (we'll see what I use more)
-Bright switch has a 250pF cap instead of 150pF.. it can still be subtle depending on amp settings
-Other front panel switched are: Mid Boost, Deep, and PAB. The PAB I use the least, but it's still potentially useable
-I eliminated the "Jazz" possibility and removed that switch
-I have no FET input or circuit
-My 3-position LNFB switch is on the front panel where the FET input would be because my V1 is right there underneath it.
-My last knob on the right is currently doing nothing but I am still considering if a cut control could be helpful for taking the edge off slightly when needed (The High Step Filters work pretty well for this though)
- The 2.2K dropping Resister is currently a couple large cement resisters adding up to 20K. I should order a more appropriate single replacement.
- I'm also currently running only 2x6L6s into an 8 ohm load instead of the 4x6l6s into a 4 ohm load.

All in all, I think I achieved my goal of creating a smoother more compressed clean amp with good sustain. Maybe it was less about the actual circuit changes and more about the hours playing and getting more familiar with the control. Maybe my objectivity was thrown out the window, but I feel like it went from the least compressed amp in the history of the world to a very controlled and even sounding amp with a nice round attack to the notes. I do feel like it is compressing much more than I should expect a clean amp to. It's not too hard to dial a bit of bite back in, but I am curious how this will feel when I try it on stage. I think there still can be some surprising jumps in loudness if pedals aren't dialed in well, but I think that will have to remain to some degree because it has remained a very clean amp all the way through.

[bcmatt]

I just installed a slightly less comical looking 20k 5 watt dropping resister instead of these 2 in series.

20240426_115410.jpg

I'm looking forward to running this all through its paces in a variety of settings to see if it's hype-worthy yet.