5E3 Tweed Deluxe model with all 3 knobs, and the resulting channel interactions

Rex said:
We pretty much have that now with the Impedance selections in the In1 block.
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That loads the guitar itself. The loading that FractalAudio refers to happens in between the first and second stages of the preamp due to one of the peculiatities of that particular amp circuit and causes some of the interactive weirdness we all love about the Tweed Deluxe....
 
FractalAudio said:
The output of the unused channel rolls off at 20 dB decade and starts at 20 dB down relative to the used channel. The corner frequency is around 11 Hz. The impact of summing the unused channel with the used channel is negligible.

Here's a plot of the used channel plus the unused channel if they were summed equally:
View attachment 56206

The maximum difference is less than 0.6 dB rapidly diminishing to zero as the frequency increases.

What causes the difference is not summing the out-of-phase signal as it is very small in relation to the in-phase signal. What changes things is that the load on the summing junction changes. With the unused channel volume control at zero there's a 1M load. As the volume is increased that load decreases due to the low plate output impedance. With the unused channel volume at max the load is roughly 60K, quite a difference.

Yes the input stage looks like a (very poor) differential stage but the cathode cap shunts most of the signal to ground before it can be amplified. The finite cathode output impedance of the used channel actually exacerbates this. If it's impedance were zero there would be more signal at the plate but the output impedance creates a voltage divider. Differential stages need a large cathode resistance to work properly.
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That escalated quickly ! :oops:
 
Joe Bfstplk said:
I can also dig out all the formulae from the RCA Radiotron Designer's Handbook to prove it to you with math, too.
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The math. Apologies to you who are about to suffer my handwriting - I don't have a fancy program to graph this stuff.

Analysis ignores the volume pots and tone pot, and simply focuses on the coupling of the two cathodes and the low/low-mid output at the plate of the channel you are not plugged into.

Terms:
Triode #1 is the channel your guitar is plugged into.
Triode #2 is the channel your guitar is not plugged into.

The equivalent circuit at top left shows what is going on when Triode #1's cathode drives Triode #2's cathode in this circuit, with the 820 ohm and 25uF shared cathode resistor/cap, as well as the output impedance of Triode #1's cathode and the input impedance of Triode #2's cathode. The doodle in the lower left shows what "corner frequency" really means - i.e., where the slight drop-off turns the corner and becomes a straight -6dB/8va drop-off. Shown in the doodle is an 11Hz corner frequency. Nearly flat for all frequencies in the audio band, and certainly the triode's cathode resistance is nearly 100% bypassed at guitar frequencies, for the purposes of calculating Triode #1's audio gain.

I show the calculations gain of a common-grid 12AX7 stage, about 62.15 or 35.87dB.

I also show the calculations at 200Hz for the gain lost due to the cathode resistor in parallel with Triode #2's input impedance and the 25uF cap. The impedance of the cathode resistor, cathode cap, and input impedance of Triode #2's cathode all in parallel form the lower part of the voltage divider with the output impedance of Triode #1 as the upper part, and this cuts the amount of signal applied to Triode #2's input by about -25.35dB at 200Hz. Calculations for 20Hz, 60Hz, and 600Hz are similar with differing dB losses, and the cap's impedance dominates the gain loss calculation.
20190703_220626.jpg


I only show the 200Hz gain for the calculations on the above page. I was getting writer's cramp by this point as I don't write very often these days.

The following pic shows this value along with the remaining 20Hz, 60Hz, and 600Hz gains:
20190703_220718.jpg


Gain at 20Hz at the "unused" channel's plate: 26.38dB, only about 8dB less than the gain in the stage you're plugged into.

The below graph is a drawing of the one the nice common-cathode triode calculator drew for me after it did all the calculations for that for me. These type of circuits are common, and calculators for them are as well. I superimposed the first graph onto this one, and subtracted the output of Triode #2 from Triode #1, which shows the cancellation in the lows/low-mids at its maximum, as when the "unused" channel volume is full up. As the common-grid stage is non-inverting while a common-cathode stage is inverting, the resulting mixture is not additive, but subtractive. So, that nice 35dB gain gets a lot subtracted from it at lower frequencies. A far-from-insignificant amount of destructive interference is going on. Mind you, this neglects the mixer circuit formed by the two volume knobs, as well as the tone knob and the caps that make that work, and solely focuses on the plate output of the two triodes. Once you mix these two opposing-phase copies of the guitar signal with the three knobs, you get varying amounts of cancellation down low. As demonstrated in my video above, turning up the "unused" channel trims a significant amount of lows and low mids from the signal....
20190703_220706.jpg


My calculations for the Triode #2 (unused channel) gain, and the various dB losses from the cathode cap to the signal driving Triode #2:
20190703_221120.jpg

For you who like to look at the "scratch paper" as well. I used a handy online RC parallel impedance calculator for the impedance calculations, to save some time.

Sorry it's sideways. My phone sometimes does that to pics.

EDIT:
I noticed a wrong value in the calculation for Triode #2 gain - 1600uMho should be 1250uMho (I misremembered the value from the sheet instead of looking it up), for a gain of about 48, or 33.62db. Please subtract approximately 2.2dB from the entire curve for Triode #2's graph. Concept is the same, just a little bit milder effect than originally calculated.

Edit #2:
In the calculation of total gain of Triode #2, I forgot to include that the signal at Triode #1's cathode is less than the input signal. This affects the loss when recombining at equal volume control settings. When mixing the volumes per the settings in the video (and many "how to get the best out of your 5E3" instructions found on the web), the Triode #1 volume control is set a fair amount below noon, while Triode #2's is generally above 3 o'clock, which helps make up for the small signal out of Triode #1's cathode and causes a frequency response curve much like my drawing above when combined, as was demonstrated in my video. Mea culpa.
 
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Your math is wrong. Zin for triode 2 is a lot less than 1609 ohms. It's about 600.

Regardless, you're missing the point I was trying to make. Even if you turn the used input down and the unused input way up the net effect is a little low frequency rolloff. You can simulate this with any of the numerous EQs available. Go to the Input EQ tab and turn up the low cut. Or put a lowshelf on there. You can go far beyond the limitations of unintentional unused channel interactions.
 
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FractalAudio said:
Your math is wrong. Zin for triode 2 is a lot less than 1609 ohms. It's about 600.

Regardless, you're missing the point I was trying to make. Even if you turn the used input down and the unused input way up the net effect is a little low frequency rolloff. You can simulate this with any of the numerous EQs available. Go to the Input EQ tab and turn up the low cut. Or put a lowshelf on there. You can go far beyond the limitations of unintentional unused channel interactions.
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I used the formulae commonly associated with grounded-grid triodes for calculating Zin via the cathode, but will admit I was using my phone's calculator and may have mis-typed something, and there's likely some rust hanging out in my process. Where I have found it, I have noted it, in bold type at the bottom of my post. Regardless of the Zin of T#2 being 1609 or 600, the impedance of the cap dominates the gain calculation in that frequency range anyway.

I get your point just fine. Did you get mine? One of the (many) reasons I bought the AxeFX III was that it listed a Tweed Deluxe amp model.

The simulation of this interaction with the Input EQ would only work for one setting, and the EQ would need to be adjusted differently for each setting of the T#2 volume knob, requiring the end user to figure out where the dip occurs and how deep it is to readjust the effect. It would also eliminate the possibility of using this input EQ the way Leon suggests (the famous 800Hz boost) to push the mids harder when distorted for a punchier distorted tone. Yes, I am aware there are filter blocks that could do that also.

My point: simply adding the third knob and a way of selecting which input you are plugged into would make dialing this in a lot easier for folks who just want their 5E3 model to act like an actual 5E3 OOB instead of having to "guesstimate" or do a bunch of math to figure out how to adjust the Input EQ to compensate for the lack of the third knob. This is how Tweed Deluxes have been used since the '50s, i.e., plugging into one or the other input and using all 3 knobs to quickly and easily dial in your sound. There's also the "jumpered" situation, as mentioned up-thread, which does something more akin to the other "jumpered" amps, which is also worth looking into.

You put in the effort on several Marshalls, the '59 Bassman, the HIWATT, and I'm sure I am missing a few, as there are tons of amps to choose from. I am simply asking that some love be given to a seminal tone monster that is a house amp in many studios across the world, including a good number of rather famous studios. They're on tons of famous recordings.

It's not a matter of being "stuck in the past". It's a simple request for an improvement.
 
I feel like a kid standing in Radio Shack in the 80's listening to a know it all dorky teenager arguing with the equally dorky older store manager about electronics and realizing I could care less who's right because all the cool guys are somewhere else, driving cool cars, talking to girls and wishing I was more like that instead of hanging out here at Radio Shack.
 
Robboman said:
I feel like a kid standing in Radio Shack in the 80's listening to a know it all dorky teenager arguing with the equally dorky older store manager about electronics and realizing I could care less who's right because all the cool guys are somewhere else, driving cool cars, talking to girls and wishing I was more like that instead of hanging out here at Radio Shack.
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Then of course many of those dorky teens went on to making millions selling their startups in the 90’s, and then they got much cooler cars and no shortage of girls, huge homes etc....
 
FractalAudio said:
.... if you turn the used input down and the unused input way up the net effect is a little low frequency rolloff. You can simulate this with any of the numerous EQs available. Go to the Input EQ tab and turn up the low cut. ...
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I suspect a pushed active imperfect device like a gain stage add some musical distortion. It's not simply eq. Subtle? Yes. Add to feel? Probably yes!

Joe Bfstplk said:
I used the formulae commonly associated with grounded-grid triodes for calculating Zin via the cathode
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I also suspect the formulas are good enogh to r'n'r in a "static approach". The dinamic behaviour of an amp is (slightly?) different. Only Cliff can answer the question.
 
Smilzo said:
I suspect a pushed active imperfect device like a gain stage add some musical distortion. It's not simply eq. Subtle? Yes. Add to feel? Probably yes!
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Totally. Much easier to have the amp model do all the calculations and add the other (missing) knob act like the one on the amp than to have to poke around in the input EQ section and dial in frequency, gain reduction, and Q to get there. Plus, dialing that all in gives a static snapshot of the amp's character and requires twiddling 3 knobs with a fair amount of know-how to do what a simple 10 degree twist of that other channel's knob achieves. Sounds more like using the input EQ is the limitation.

Smilzo said:
I also suspect the formulas are good enogh to r'n'r in a "static approach". The dinamic behaviour of an amp is (slightly?) different. Only Cliff can answer the question.
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Maybe.

The "incorrect math" he picked at above was immaterial to the gain or frequency response of the cathode coupled circuit, since the cap's impedance is about 20x lower at 200Hz, rendering the 820 ohm cathode resistor and the input impedance of the other cathode moot at the frequencies in the low/low-mid. It was, however, a point he could use to cast doubt on my analysis.

The elephant in the room is the fact that any audible sound came through at all, with the bright channel volume at zero and the unused channel's volume up. It followed the characteristics put forth in my statements - lower volume and rolled off above the low mids, and out of phase with the other channel. When combined with the other channel, it destructively interferes with the signal to drop lows and mids variably depending on the relative mix.

There are also loading effects in this mix, per FractalAudio's statement, which add their particular flavor of wierdness into the 3-knob mix.

Can we get past this and get this on the wish list "to-do" pile?
 
Joe Bfstplk said:
Can we get past this and get this on the wish list "to-do" pile?
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I think the way it works is that one man, and one man alone; Cliff, is the sole guy who will or won't make this happen, so there really isn't any "we", nor any official list order etc. You've explained what you wanted, you explained why you wanted it, and you know Cliff personally has read over things and gave you his thoughts on the matter.

All that is left is to wait and see if he chooses to implement it or not, which is totally up to him, as far as the if and when, but at the very least, you got your idea to the big boss and know he is aware of it.
 
The overall direction for Fractal Audio has been to move away from an approximation of tube amps and toward perfecting the very specific anomalies found in a particular amp. For example, don't we now have an axe edit screen that shows just the actual controls used in the amp. In order to follow the path already established, it would stand to reason that those controls alone would yield very accurate results. I remember it being mentioned that the interaction between the dual volume controls on the marshall's weren't quite right and very shortly afterward, it was addressed. There may be very complex programming reasons why the tweed deluxe volume pot interaction hasn't been dealt with yet. Maybe it just hasn't been a priority because it sounds pretty damn good just the way it is, or maybe other things have been deamed more important. What I do know is that Cliff is a perfectionist, this is his baby, and it has never stopped getting better. Patience grasshopper, you've planted a seed that I think we would all like to see grow.
 
guitarmike said:
The overall direction for Fractal Audio has been to move away from an approximation of tube amps and toward perfecting the very specific anomalies found in a particular amp. For example, don't we now have an axe edit screen that shows just the actual controls used in the amp. In order to follow the path already established, it would stand to reason that those controls alone would yield very accurate results. I remember it being mentioned that the interaction between the dual volume controls on the marshall's weren't quite right and very shortly afterward, it was addressed.
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That has been my observation as well.

guitarmike said:
Patience grasshopper, you've planted a seed that I think we would all like to see grow.
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I have been more than patient, I think. 90% of this thread was discussing minutae of vacuum tube circuitry, actually. Hopefully the outcome will be a good one.
 
Be more patient. I think it speaks volumes that the owner is concerned enough to even care what we think or want and then to correspond with us concerning such matters. Seriously, I can't think of any other company whose owner is actually willing to talk to us about our concerns let alone, have a history of implementing them.
 
guitarmike said:
Be more patient. I think it speaks volumes that the owner is concerned enough to even care what we think or want and then to correspond with us concerning such matters. Seriously, I can't think of any other company whose owner is actually willing to talk to us about our concerns let alone, have a history of implementing them.
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Ok, so 350 days have passed. Is that patient enough?

I would still like to see the third knob appear on the Tweed Deluxe, as well as the Jumpered and Mic channel models added.

I doubt I am the only Tweed Deluxe or Neil Young fan out there....
 
I see your all knob 5E3 and raise you an all knob plus 6SC7 (dual triode with shared cathode metal cased) tube 5C3 model, which is my favorite amp in real life, microphonic tubes and all.
 
johnnypig said:
I see your all knob 5E3 and raise you an all knob plus 6SC7 (dual triode with shared cathode metal cased) tube 5C3 model, which is my favorite amp in real life, microphonic tubes and all.
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I also have a wish in for the '53 Les Paul script-logo-grille with the 6SJ7 octal pentode preamps. It uses a third 6V6 to modulate bias and screens on the output tubes to do tremolo. Great sounding little amp.
https://forum.fractalaudio.com/threads/wish-list-new-amps.137944/post-1859940
 
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