Schematic of Input circuit in 3 Ch Recto.

Schematic of Input circuit in 3 Ch Recto.

 

(Note: The 2.2M load resistor is in parallel to the voicing circuit and gain pot, which are serial to each other.  This has some major ramifications regarding anode resistor values, as the output impedance of V1 will change when the resistor is changed.  When my health improves a little, I will update this article with graphs and more information.)

Nov. 2016: While the objective facts of this article are good, one thing I didn’t address was the harmonic distortion content and I think my initial assessment about the early distortion was wrong.  I absolutely do not recommend this as an improvement to the dirty channels.

As the resistance goes lower, the harmonic distortion increases.  This isn’t clipping, but non-linear tears in the waveform.  If the resistance is lowered to 150k or 100k, it would go a long way toward adding character to the clean channel.  After tests and simulations, I don’t think it will have a major impact on the dirty channels, aside from a further decrease in clarity when the gain control is past 2:00.

As far as the fizzy nature of some of the 3 Channel Rectos, V2b has the biggest impact on it in the preamp and the power supply filtering is another source.  You can test the latter by beginning at Silicon/Bold power options and working down to Tube/Spongy.  As the power decreases, clarity decreases from the midrange and the harmonic overtones increase.

So, this article represents a good idea for improving Channel 1, but other assertions I drew were incorrect.  I apologize.  I’m leaving the article up for critique and transparency.

 


 

Original Article

Some owners or users of Mesa Dual and Triple Rectifiers make complaints about the harshness of the clipping at higher settings on the gain control.  The opinion is that getting super-sustained sound is hindered by grinding distortion (which for me is part of the draw).  Some users also complain about the clean channel not being clean enough.

There is a way to affect both of these issues with the change of a single resistor.  The input stage has a 220k resistor on the anode.  Compared to a 100k resistor found in most vintage amps, 220k presents an increase in voltage gain, while reducing harmonic distortion.  This mod reduces the value of the resistor to 150k.

Almost all iterations of the Pre-Reborn Dual Rectifier and Triple Rectifier have the same input stage, with one main difference.  Early 2 Channel versions do not have the 100 ohm cathode bypass limiting resistor, which is present in the 3 Channel version.  Later 2 Channels have LDRs with resistance at about 100 ohms, which is essentially the same thing as the resistor being present in the 3 Channel version.  The following content applies to any Rectifier with LDRs or resistors from the cathode bypass cap to ground.

Reasoning

In a multi-stage amp, each voltage gain is multiplied by the next.  Guitar amps usually dump a lot of the gain to ground in between the preamp tube stages to get the benefits of amplifying the signal several times, but at a level that is manageable.  As an analogy, imagine 4 or 5 of overdrive pedals turned on, in series, in a signal chain.  Each pedal will be set at one level or another, but they can’t just be cranked up and still have a tone that is applicable to most kinds of music.  Amplifiers do the same sort of thing.

Gain in each stage is important, but if a high percentage of the signal gets dumped afterward, a small reduction in a single stage isn’t going to harm the amplifier.  In a Recto, only a maximum of 26.9% of the first gain stage is amplified by the second stage.  It is further reduced by the “Gain” control setting and whether the frequency is below the treble boost circuits.  When comparing gain reduction from 220k to 150k,  the difference in level after the dump will not be very great (it would be less than 1 dB difference), but it will be noticeable in the sound.  In effect, this difference is like turning the level of the first pedal in the analogy down by just enough to notice a very, very small difference.

The harmonic distortion decreases as the bias is moved to -2 V.  With a 1 V signal on the grid, a 220k resistor, at -1.6 V bias, the 2nd harmonic is about 9.7%.  With a 1 V signal on the grid, a 150k resistor and at -2 V bias, the 2nd harmonic is less than 1%. A non-boosted signal will struggle to overdrive the clean channel and the dirty channels are now going to have fewer upper harmonics being generated and distorted early in the processing.

Bias Points and Headroom

This input stage has a ferrite bead, a large signal gain, and low harmonic distortion.  This circuit is kind of like a hi-fi amplifier stage.  Its job is to get a big, relatively clear, signal pushed out.  However, the bias point set by the cathode resistor could be considered a little bit low at about -1.6 V, which gives 3.2 V peak-to-peak for DC.  AC will be actually be set lower.  I estimate that AC loses close to .1 on the positive side (-1.5 V to clipping) and about .35 overall (2.9 V p-p headroom), due to the cathode bypass limiting resistor.

 

A large enough positive signal will drive the stock stage into clipping and would be compressed a great deal, even when not fully clipped, due to grid current limiting.  This limiting begins at -.9V and introduces an increasing amount of overdrive as it approaches 0V.  The actual “clean” headroom for a signal on the grid is 1.4 VDC and 1.2 VAC, which is fairly easy to exceed, momentarily, with modern humbuckers.  The initial clipping is further amplified, clipped, compressed, and inflected with harmonics in the remaining preamp stages.  Among other considerations, this would produce the fuzzy-wuzzy, harsh sound which is off-putting to some people.

By changing the anode resistor to 150k, it not only reduces the gain and the harmonic distortion by noticeable amounts, it shifts the bias, giving additional headroom.  Incidentally, the negative cycle still has room until cut-off.  The audible result would be softened harsh tones and improved cleans.

Alternately, boosting a stage with a 150k anode resistor will cause the stage to distort on both sides more symmetrically (though still slightly offset) .  The low frequency content will be slightly reduced in volume, because the cathode bypass-limiting resistor has less of an influence toward flattening the frequency response.  There’s less compression, overall, and the improved symmetry for producing distortion will produce fewer overtones, though 3rd Harmonic would be the dominant overtone when pushed to extremes.

Limitations of Tools

For this post, I’m using Merlin Blencowe’s Load Line Plotter spreadsheet.  Two limitations of my analyses are the inability to succinctly graph for the cathode bypass resistance of 100 ohms and showing the AC load line.  This doesn’t present a major problem, but it is incomplete.  To ease the reader in having less images to click on or attempt to compare, the half-boost change from the 100 ohm resistors will be reflected in red on the DC analysis.  I have also provided a hand drawn AC load line, but the lack of detailed markings on the spreadsheet makes it an estimate, only.

The other limitation is the unknown value of the series resistance on the grid from the ferrite bead.  My research has found the highest values to be around 1-2 kohms.  Since Mesa could request custom ferrite beads, the value could be any amount and will be ignored, as the mod does not change HF roll-off from the input to the grid.  On the graphs, the HF roll-off will be inaccurate and should be ignored.

Get A Load Of This
Fig. 2 Load line for 220k resistor.

Fig. 2 DC load line for 220k resistor.

Parameters and table for 220k anode resistor.

Fig. 1 Parameters and table for 220k anode resistor.

Figure 1 shows the operation characteristics of a triode gain stage.  The component values are entered at the top and the tables show frequency information, bypassed cathode information, and unbypassed cathode information.  This example is the stock input stage.  Note: the total gain for the unbypassed frequencies are unaffected by the limiting resistor, except for the frequency knee, and do not need to be recalculated.

According to Fig 1, the half boost frequency is 95.1.  Above this, the 12AX7 (ECC83) has an increasing gain and shown as 76.2 V (37.6 dB).  However, the 100 ohm cathode bypass limiting reduces AC to 51.3 V (34 dB)*.  The frequencies below 95.1 have gain of 45 V (33.1 dB).

Fig. 3 12AT7 with 220k anode resistor.

Fig. 3 12AT7 with 220k anode resistor.

The load line for 220k is shown in Fig 2.  As can be seen, it rests well short of the 2V value indicated on the schematic and lies somewhere close to -1.6V.

Fig 3 is a comparison of the load line using a 12AT7; this response is already listed in Fig 1 on the ECC81 line.

Fig. 4 Characteristics and chart for 150k anode resistor.

Fig. 4 Characteristics and chart for 150k anode resistor.

Fig 4 Shows the tables for a 150k anode resistor.  The half-boost frequency will be 97.9.  Above this, the graph shows up to a 69.2 V (36.8 dB) gain.  However, the 100 ohm cathode bypass limiting reduces the AC gain to 48 V (33 dB).  For frequencies below 97.9 Hz, the signal has up to 36.5 (31.2 dB) gain.

Fig. 5 150k anode load line.

Fig. 5 150k anode load line for DC.

Fig 5 shows the load line for 150k anode resistance.  The bias now sits directly on the -2V mark.  As previously stated, the positive cycle has increased headroom.  The negative cycle has reduced headroom, but still has enough room for the incoming signal.  A large enough signal will be clipped on one side and cut-off on the other almost evenly.  The negative cut-off is also more gradual than the positive clipping and produces a subjectively better sound (it’s why the “cold clipping” stages are used to remedy the previous stages in a high gain amp).  Overall, the DC headroom before full overdrive/distortion of the stage is 4 V peak-to-peak and has “clean” headroom of 2 V p-p.  The 100 ohm limiting resistance reduces overall headroom for AC.  I estimate it providing 3.8 V p-p and “clean” headroom of 1.9 V p-p.  These voltages are greater by about .9 V and .4 V, respectively, than stock.

Good Faith and Holding Horses
150k AC Load Line

Hand drawn 150k AC load line.*

220k AC Load Line

Hand drawn 220k AC load line.*

I’ve attempted to provide plenty of information to back up my assertions and all measures were taken to not overstate or overstep anything I couldn’t show.  The information has been slightly simplified and presented in good faith for this complicated subject.  If you find errors, please let me know and I will correct the article.

If a person is madly in love with their Rectifier, this mod may not be for them.  For people who want to tame it a little, or like to tinker, this mod provides a clear way to make an arguable improvement to the amplifier.  If it doesn’t satisfy a person, it is easily reversible.  Additionally, a 470k resistor may be placed in parallel to the 220k anode resistor to get the same result.  If an fx loop return or other control space is freed up from a separate mod or removal, a switch could be installed to change between stock and modded, but popping from DC might be an issue.


*Boost limiting gain reduction: A= µRa/Ra + ra + Rk(µ+1).

  • Rk= Rk1||Rk2

*AC load line: Iac=Iq+(Vq/Ra(ac))

  • RaAC=Ra||Rg
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  1. […] Input Anode Resistor… on Dual Rectifier Cold Clipping… […]

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