Tom Lees' Test Results

8/3/2019 Tom Lees' Test Results

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I asked Tom Lees, audio engineer for Bass Gear Magazine and Patent Law attorney, to

evaluate the low frequency channel of a Nitewalker Bass Guitar Tube Preamp. I also asked

that he change one of the capacitors for another (orange drop .1uf for a Russian PIO .1uf)and do a comparative analysis. When the orange drop capacitor is in the circuit the preampcreates a modern tone, and when the PIO capacitor is in the circuit the tone changes to a"78 record sound" (something like Count Basie's or Duke Ellington's bass).

Initial Results

This result shows the frequency response of the preamp between 20Hz and 20kHz with a400mVrms input signal. The industry standard for audio preamps is +/- 0.5db between 20Hzand 20kHz, although it isn't unusual for certain musical instrument amplifiers to strive formore response in a certain bandwidth of frequencies. The red line represents the highfrequency channel and the blue line represents the low frequency channel. Notice that thetwo lines are the same until 800Hz.

In discussing audio specification tests the following may be of interest:

The audio specifications tests description by Rane, Inc. is located here:

http://www.rane.com/note145.html. It shows all of the different tests that are used in analyzing audio

gear today. The section on frequency response explains how most manufacturers try to attain a

frequency response of 20 Hz to 20 kHz +/- 0.5 db.I've done some further google research into this matter, and I've found some other facts. This is a

quote from Wikpedia Online:

"Unlike home "hi-fi" amplifiers or public address systems, which are designed to reproduce accurately

the source sound signals with as little harmonic distortion as possible, instrument amplifiers are often

designed to add additional tonal coloration to the original signal or emphasize (or de-emphasize)

certain frequencies. The two exceptions are keyboard amplifiers and "acoustic" instrument amplifiers,

which typically aim for a relatively flat frequency response."

Here is a quote from a new Traynor Bass Amplifier advertisement:

"The Scoop control sweeps from a flat frequency response to a mid-frequency dip at 400 Hz and can be

completely bypassed from a separate front panel on/off switch. The Range control shifts the overall

shape and general tonal characteristic of the master EQ section of the amp while the Resonance knob

Tom Lees' Test Results Thursday, September 15, 20116:37 PM

Tom Lees' Test Results Page 1


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controls the damping factor of the amplifiers output stage, allowing the player to take full advantage of

the tone of his loudspeaker enclosure. "

Here is a Demeter Tube Bass Preamplifier advertisement link, where the frequency response is listed as

simply 10Hz to 40kHz: http://www.demeteramps.com/products/bassamplification/vtbp201s.html

As you can see the prior art is varied, but the industry standard remains as 20 Hz to 20kHz +/- 0.5 db.

This has been going on since the Norgaard (U.S. patent No. 2,270764, filed April 5,1940) days when this

flat response across the entire frequency spectrum was first made possible.

This result shows that the low frequency channel begins to distort at an input voltage of

530mVrms, while the high frequency channel is still not pushed to cut off.

This shows that both channels are pushed to cut of at 630mVrms input voltage.



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This result shows the transient response of the two channels of the preamp at a 250mVrms

input.

Test Results Nitewalker Bass preamp FirstImpressions

After discussing the initial results with Tom I asked him to elaborate and with morefocus on the difference between the use of the two types of .1uf capacitors in the circuit.(orange drop or large physically sized paper in oil)

Frequency Sweep of Nitewalker Bass Guitar Tube PreampThe Black trace is the input, 100 mVrms, swept 20 Hz to 20 kHz. The Red trace is the High

Output Channel, the Blue Trace is the Low Output Channel, all traces normalized to 0db.Biamp down, OD down, Volume of each channel calibrated for unity gain. Note that the lowfrequency response is substantially identical for both channels up to approximately 800 Hz.I repeat: the industry standard for a test of this type is +/- 0.5 db from 20 Hz to 20 kHz.(flat response)



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A Black Input Signal calibrated to 100 mVrms, 200 Hz, Red is the High Channel Output, Blueis the Low Channel Output. Notice that there is no clipping at 100mVrms input voltage.Note a couple of features here. Both outputs are inverting relative to the input signal. Also,although both outputs were dialed to approximately match the input level for unity gain,

the Red trace of the High Output Channel swings relatively higher on the positivetransitions compared to the Blue trace of the Low Output Channel. Correspondingly, theBlue trace of the Low Output Channel swings lower on the negative transitions comparedto the Red trace of the High Output Channel. I believe this occurs because there isrelatively more negative DC on the Low Output Channel compared to High Output Channel.

DC Bias on the Outputs under Steady State Conditions Black Trace is the Input Signal,Red Trace is the High Output Channel, Blue Trace is the Low Output Channel

As seen in the above figure, there is just over -21mVdc on the Low Output Channel andabout -14 mVdc on the High Output Channel. Input is off, both volume knobs are off.Toggling the O/D switch had no effect upon DC bias.

Effects of Grid Leak Biasing Scheme as a function of Input Signal LevelFor the next three graphs, Tom calibrated the Low Output Channel to have unity gain at

100 mVrms Input. During the tests, the Gain knob was not adjusted.



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Black Trace Input Signal Compared to Blue Trace Low Output Channel 100 mVrms





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Note the strong effect of grid leak biasing of the tube stage. The grid bias was observed tofluctuate from less than 0.5 Vdc for input signals. However, as the signal level applied bythe Audio Precision system increased, the grid bias increased to approximately -1.5Vdc for larger input signals. This effect results in extreme distortion in the output signal.This also results in asymmetric clipping of the output where the positive peaks are "squared

off". The effects of tube cut-off causing distortion in the output signal can be easilyobserved with input signals of 400 mVrms and even lower.

Frequency response comparison tests

The next 8 results show a test comparison of the low frequency channel when one of thesmall orange drop capacitors is changed to a large physically sized paper in oil capacitor.By using a small orange drop capacitor in the circuit the sound is much improved, especiallywhen playing a lot of notes in rapid succession, and these tests show electrically thedifference in the characteristics between the use of the two types of capacitors in thispreamp circuit.

The Orange Drop test was run several times and the results were overlaid onto the samechart. The results seem fairly consistent above 100 Hz. However, below 100 Hz, the lowend varied considerably, depending upon how long of a period was provided betweentests. Multiple tests in rapid succession caused considerable inconsistency in the resultsgenerated.



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The PIO test was run several times and the results were overlaid onto the same chart.The results seem fairly consistent above 100 Hz. However, below 100 Hz, the low endvaried, depending upon how long of a period was provided between tests. Multiple tests

in rapid succession caused inconsistency in the results generated. However, the results

varied slightly less than with the Orange Drop.

The Orange Drop test was run several times and the results were overlaid onto the samechart. The results seem fairly consistent above 100 Hz. However, below 100 Hz, the low

end varied considerably, depending upon how long of a period was provided between

tests. Multiple tests in rapid succession caused considerable inconsistency in the resultsgenerated.



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The PIO test was run several times and the results were overlaid onto the same chart.The results seem fairly consistent above 100 Hz. However, below 100 Hz, the low end

varied, depending upon how long of a period was provided between tests. Multiple testsin rapid succession caused inconsistency in the results generated. However, the results

varied slightly less than with the Orange Drop.



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It is apparent that the clipping effect at 400mVrms causes less of a fluctuation between30-50Hz.

Frequency Sweep Low Output Channel ORANGE DROP CAPACITORFocusing on the Low Output Channel, a normalized frequency sweep was run from 20 Hz-20kHz, Black is the input signal 100mVrms, Blue is the low channel calibrated to unity gain,

Biamp switch in biamp mode, OD on. Note that compared to Fig. C, the Y axis iszoomed in to a range of +4 dB to -15 dB to better see the response.



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Frequency Sweep Low Output Channel REPLACEMENT CAPACITORNote that compared to the Orange Drop Capacitor, the low end is nominally different.Particularly, the REPLACEMENT CAPACITOR has a slightly damped response compared tothe Orange Drop Capacitor. High Frequency response is similar, varying slightly, which islikely attributed to component tolerance

THD+N vs Input Level Orange Drop CapacitorWhen taking the above-two figures together, you can see that the unit behaves well for

signal levels below 100 mVrms. For signals above 100 mVrms, gain drops and distortion setsin considerably. By the time the input signal hits 500 mVrms, the outputs areapproximately 50% THD+N. Again, this is due to the nature of the effect of grid leak bias.



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Note that the cathode is attached directly to ground and R14 is connected to both gridsand then to ground. (C10 and C11 are not connected to the grids as the schematic seemsto show) This type of bias is known as grid leak bias. Here is another schematic of thesame Nitewalker Bass Guitar Tube Preamp circuit:

The only difference between the high frequency channel and the low frequency channel isthe value and size of the bypass capacitor(s). This circuit was borrowed from a 1960sBogen PA system. I experimented with various capacitors from a 1950s or 60s Conn organin the Bogen circuit to, and these experiments lead to the introduction of Nitewalker BassGuitar Tube Preamps. In discussing grid leak bias with Tom Lees I sent him this email:

Dear Tom,

After a further review of the test results and what we talked about on Thursday Ive come to several

conclusions. I understand that the forward/backward AC pulse at the input is amplified by the anode

circuit which consists of the two .47uf caps, the .1uf cap and the other components. Referring to the

schematic the musical instrument input signal is coupled to RC (resistance-capacitance) filter circuit (C3,



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R14) and to the grid of half of the dual triode electron tube. Resistor R14 maintains the grid at a suitable

negative bias potential. This signal produces voltage variations on the control grid of half of the dual

triode electron tube, which excite corresponding current variations on the anode through the anode

load circuit comprising the resistance R3 and the shunt resistance R4 and the capacitance C6- C10. The

half of a dual triode electron tube possesses an extremely high impedance, so that the voltage

variations of the anode caused by current flow in the anode load circuit have substantially no effect on

the anode current. These AC voltage variations are transmitted through the coupling network comprising

condenser C4, the condenser C5, and resistors R7 and R1 (a potentiometer or variable resistor) to the

output of the device where they appear as large variations in AC current.

Bypass condensers C6-C10 are connected from ground to a point between resistors R3- R4 and R1, sothat high frequency AC currents flow from the anode of the dual triode through resistor R1 and

condensers C6-C10. This capacitance value is chosen for maximum response in the desired frequency

range. Additionally, there is a capacitive reactance present here that helps to shunt some of the

undesirable frequencies from the signal.The frequencies to which this all pertains are from 97.999hz, which correlates to a G natural below the

top line of the 8vb bass clef at concert pitch on a musical score or open top string on a bass guitar, and

below. The bottom note on a normally tuned 4-string bass guitar is an E with a harmonic frequency

41.204hz.It may be that you would need to test your grid leak bias theory further by changing capacitors C3,C6,

C10 and C4 to the modern type, as Bogen originally used or some others of the same type. Ive

attached photos of the Bogen P.A. amp. preamp circuit and a schematic of the MX60A. Ive done some

research on the web about grid leak bias in Bogen amps, and one thing that I found says:

I guess you've already discovered the "grid leak" bias (no cathode resistor) doesn't work well withguitar - I think they used it to save parts. Low noise for mike use, but can't take much input. Some like it

for harp though... Others characterize the Bogen grid leak scheme as ratty sounding and other such things. Here is

another Quote:

Grid leak bias, with the exception of some older transmitting tube designs, is not used for power output

stages. For all practical purposes all the bias schemes break down to cathode ("self) bias and fixed bias

(several types). Unfortunately the internet has "standardized" the resistor that connects a control grid to

ground or bias source as a "grid leak" - this does have historical precedence and is used in RDH4. But

later on in the 1960s the term "grid return" was used and I think this term produces less confusion

between a "grid leak" resistor in an amplifier that doesn't use "grid leak bias."Now an earlier reply mentioned getting rid of the grid leak bias - this is the bias on the input triodes

which have the cathodes tied directly to ground. As to whether you really want to get rid of it or not is a

matter of choice. I've converted several Bogens to various circuits - often just messing around to see

what it sounded like. But I've got two primary amps - a 1967 BF Vibrolux Reverb for electric guitar (and

acoustic with a magnetic pickup for some particulur gigs) and BogenM60 - genuine NASA surplus with

8417 outs - that I converted to an "acoustic guitar amp/general small PA system" making it into a

"combo" with a sealed box containing a 12" guitar speaker and a tweeter. While low-Z inputs are

transformer coupled hi-Z ones directly hit the grid leak bias and I think it sounds fine!I would work on the rest of the amp (may I suggest you put voltage doubler on the bias supply and add

individual bias controls for the 7868s - these babies are getting harder to find) and then take a two week

test drive with the grid leak bias. You can even modify one input for cathode bias to give more variety.

I remember bypassing the preamp section as a teenager by connecting the mics by the screws in the

back of the Bogen P.A. head. The preamp seemed to be hard to control and would feed back more. The

volume control still worked the way we used it, and it was still loud enough for what we were using it for.

When I tried to make a preamp for my bass this was the first thing that came to my mind. I remembered

bypassing the preamp in that old P.A. head! When I got a sound that I liked from the same circuit I knew

I had something really big. Try converting the preamp to a cathode bias scheme and retesting it if youd

like. I imagine you will find more noise and less of a limit on input voltage range before clipping hard

(when the grid goes more negative than the cathode). In my experience the input voltages are always

below 400 mvac regardless.Yours truly,

Cliff



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Harmonic spectrum test results of Low Frequency Channel

1vrms input signal results shown above



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600 mvrms input result shown

400 mvrms input results shown

THD + N Test results

THD+N 250 mVrms 100Hz with overdrive switch in normal position



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THD+N 250 mVrms 100Hz with overdrive switch in overdrive position. Comparing the two tellsyou that the overdrive will lessen the distortion and noise resultant. Lowering the inputvoltage lowers this resultant distortion and noise even more, as shown in the following testresult:

100mVrms input shows much cleaner signal!

Transient Response Comparison with and without Orange drop

Capacitor in the Circuit



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The PIO capacitor is shown to pass a higher amount of AC to the output with a 250 mvrmsinput signal. This is significant.

CONCLUSIONS

It would be interesting to compare these results with results of the same preamp circuitwith a cathode bias arrangement to see if the noise and distortion levels were obsessive inthe 50mvrms to 100mvrms input voltage (ac) range. How high would the input voltage bebefore cut off? Would there be more noise in the circuit?

It would also be interesting to see transient response results with this same 50 mvrms to100 mvrms input voltage range to see if there are any changes. It may be that the PIOcapacitor is passing more noise and distortion to the output than the orange drop capacitor

at a 250 mvrms input voltage. Tom answered this question for me, and the answer is no. TheRussian PIO capacitor just puts out a higher voltage in the transient response test, nomatter what the input voltage is.



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This result shows that there is no level change between the use of the two types ofcapacitors in the circuit. Note the zero crossing at 400mvrms which correlates to the cut offpoint shown here.



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This result shows that THD+N is about the same when either type of capacitor is being used.

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Tom Lees' Test Results