Graph of Gain from Input to PI Output

[jecars]

Hi All,

I'm a very satisfied with my Hoffman Blues Jr conversion (w/octal power tubes). Since the amp was sounding terrific to my ears, I decided to take the time to document the internal voltages... as a baseline just in case I ever needed a future reference.  I used my ancient Kikusui oscilloscope for peak-to-peak voltages, used Excel to calculate the cumulative gain [dB = 20log(Vout/Vin)] and made the attached graph of the signal path based on tube pin voltages.

Hopefully the graph makes sense intuitively, as the input for each component is the output of the previous component. For example, V1A has a gain of 29 dB from the input jack, followed by loss of 23 dB across the volume pot, and so on.

I can't recall ever seeing such information so I decided to share for comment.  It was educational for me to see the rise and fall of signal gain.

 

[PRR]

> I can't recall ever seeing such information

Just such a graph used to be common for mixers.

[jecars]

^^^ I stand corrected, at least for mixers. Thanks, PRR

One item of curiosity for me about FMV Tone Stack: 

* Internet references say that FMV has 15 dB loss, more or less. 

* Looking at my graph, I can see V1B has 18 dB of gain (vs. 29 dB from V1A) which I attribute to FMV load (vs. V1A connected to 1M pot). In addition, there is 6 dB loss across the FMV circuit itself for a total of 17 dB loss.

Would you say that the statements above agree but just say it differently, or is there a better explanation?

[jjasilli]

I propose that 15 is more or less 17.  You can use the Duncan Tonestack Calculator to figure the drop in your tonestack; and/or measure the insertion loss directly.  Also, there may be a necessary approximation: the drop from the bass side of the curve may exceed the drop from the treble side a by a few dB, e.g.

It would be nice to see: actual voltages stated along with dB; how an amp treats gain/loss for more guitar-centric frequencies like 400 or 200 Hz, e.g.  IOW, its not clear (at least to me) that the goal is met to create a baseline for future reference.  I'm thinking that a different circuit that duplicates this performance at 1000 Hz, may not duplicate this amp's performance in the guitar-centric frequency range???

[sluckey]

OK...

[jecars]

^^^ Yes, those are the peak-to-peak voltages, thanks Sluckey.  Vout/Vin = 10^(dB/20), starting with 100 mV (0.1V) as input signal.

[jjasilli]

Thanks for doing the math!  That's helpful. 


I still wonder how lower frequencies would be treated.  Of course tonestack settings would matter.  Maybe vol too???


From:  "Servicing Radio and Television with a Vacuum Tube Voltmeter", Copyright 1951, Sylvania Electric Products Inc.: 

3.3 SIGNAL LEVEL MEASUREMENTS IN AUDIO AMPLIFIERS. . .The entire test should he repeated at a low and high frequency (such as 100 and 10,000 cycles), in addition to the 1000-cycle frequency employed in the foregoing series. This will enable the technician to determine whether amplification is uniform throughout the audio spectrum. If the amplifier has a tone control, separate tests should he made at various settings of that control. A stage-by-stage signal voltage measurement of this kind is a dynamic test of the amplifier and should follow the complete dc and ac voltage measurements described in Sections 3.1 and 3.2 of this Chapter. The signal voltage test will isolate defective stages quickly and will even disclose bad tubes. The whole series of voltage measurements described thus far in this Chapter form the basis of a complete trouble shooting procedure. They may appear unnecessarily laborious, but after a reasonable amount of practice, they may he performed in a routine manner with complete ease.

3.4 TONE CONTROL TESTS
Connect the Polymeter, set to AC VOLTS, across the speaker voice coil or across a substitution load resistor (as described in Section 3.3). Set the amplifier volume control at about 1/2 maximum. Connect an audio oscillator to the amplifier input terminals, as shown in Figure 3-2. If the amplifier is a low-gain unit, dispense with the signal voltage divider and use the oscillator output control to adjust the input signal to a comfortable level. Adjust the maximum input signal to approximately 1 millivolt if the amplifier is high-gain. Set the tone control to its bass position. Starting at about 50 cycles, increase the oscillator frequency in regular steps up to 10,000 cycles, recording the output voltage (as indicated by the Polymeter) at each frequency setting. Keep the input signal voltage constant. Draw a frequency-vs-output voltage curve from the
data obtained. Next, set the tone control to its middle position and repeat the frequency run as before. Finally, set the tone control to its treble position and repeat the frequency run. The three curves obtained will show completely the performance of the amplifier in each position of the tone control. The technician will he able to see at a glance which hand of frequencies is emphasized and which is muted by the tone control, also where cutoff points are located.

[PRR]

> lower frequencies ... tonestack settings ... Maybe vol too???

That is a lot of work. Are you volunteering?

What more would we learn from the added data? Some, but enough to justify this approach? Might there be a simpler path for specific typical questions?

And of course Vol knob(s) setting matters! Gain diagram, and these are gain controls.

The knob settings must be specified.

There are two "logical" choices.

1) At "normal happy" playing positions

2) All at some nominal point easy to re-set on the test bench

"Normal happy" is of course situational. You'd set knobs one way for Saturday night headbanger bar rock-out, and a different way for Sunday morning choir accompaniment. Maybe another way in the recording studio.

What are we trying to figure out?

In any "low level" input system, I would look to see if signal level ever gets *lower* than input level. The first stage, which takes the precious input signal, should dominate the overall hiss performance. In a given technology (tubes, transistors), "usually" the hiss level of most stages is similar. So if you take a 100mV guitar with a tube of 1mV hiss, gain it up, then loss it down with fader and tone-stack to 50mV, the next stage has only 50mV of signal and its 1mV of hiss becomes the dominating hiss. jecars' amp shows 2nd stage has twice the signal of the 1st stage, so 1st stage hiss dominates (as we want).

I'd also look how the line lays compared to overload. In most "sane" amplifiers we never want to be close to overload. (Guitar amps is different.)

Both of these are of course signal dependent. If input is reduced to 20mV we are much closer to hiss. If input is raised to 1V we will be closer to overload. Gain knob settings matter. While 100mV input and Vol pot at "noon" does not put V1B close to overload, Vol pot at "10" might, and will if the input signal is much bigger. The existence of *two* gain knobs (Vol and Mas Vol) suggests this is intentional, that the user will trade-off front gain and back gain to control signal levels in V1B V2B.

Part of what we want to know is: can we get ALL the 50 Watts (or whatever) that we paid for, or will some early stage clip first? I have guesstimated levels through the power tubes and OT to the speaker (our real goal). Yes, it is not wise to directly measure signal at power tube plates/ OT primary: huge voltages and the added lead-length on this huge signal may make the amp unstable. But you can get a fine guess from OT turns ratio and level at OT secondary.

[jjasilli]

That is a lot of work. But that's not the point.  The point is whether the 1000Hz chart is reliable to predict gain at guitar frequencies. 

Are you volunteering? No!  I'm underscoring my prior point:  it's not clear that jecars' stated goal is met to create a baseline for future reference.  A different circuit that duplicates this performance at 1000 Hz, may not duplicate this amp's performance in the guitar-centric frequency range.  In fact this amp may not duplicate its own performance in the guitar-centric frequency range.

A lot of the guitar's range is from ~80 - 440 Hz.  The 1000Hz measurement (though standard for the audio industry "at large") has not been demonstrated to be representative of the gain at guitar frequencies.  The quotes from the Sylvania manual prove that this concern/caveat is valid.

[sluckey]

I'm underscoring my prior point:  it's not clear that jecars' stated goal is met to create a baseline for future reference.  A different circuit that duplicates this performance at 1000 Hz, may not duplicate this amp's performance in the guitar-centric frequency range.

I never thought that jecars' graph was meant to apply to anything except that particular amp. Just as I like to document dc voltages in an amp for future reference. His gain chart applies only to that amp. He specified the conditions that resulted in his graph and sometime in the future he can duplicate those conditions to check the gain and compare to his "baseline" graph to see if that particular amp is still performing satisfactorily.

[jecars]

His gain chart applies only to that amp. He specified the conditions that resulted in his graph and sometime in the future he can duplicate those conditions to check the gain and compare to his "baseline" graph to see if that particular amp is still performing satisfactorily.

Yes, and I should have been clearer in my OP. My goal was to document (fingerprint) this particular amp only.  I can duplicate this test set-up easily in the future and note any changes in the pre-amp from input to PI output. Also, as sluckey mentioned above, I have already documented the HT power rail voltages (B+, X, Y & Z) with tubes installed.

Rounding it out, the power section in this amp is grid biased and only has two bias-check resistors, two screen resistors, two 5881s and OT. It didn't seem worth it to document any voltages here (which requires care) since I could just as easily test these few elements individually.

Incidentally, I did measure output wattage (with 8 ohm dummy load) just after I completed the build.  I got 24 watts at max clean signal. I remember how hot the load got!

[jjasilli]

I think at this point we're going in circles.  E.g., if component values drift over time, gain at guitar frequencies might change enough to noticeably affect tone, even if gain remains the same at 1000 Hz.  In that case, confirming only the 1000Hz readings would be misleading. 


A number of prior threads over time on this Forum have pointed out the usefulness of measuring guitar amps at guitar amp frequencies.  This should be uncontroversial.

[jecars]

confirming only the 1000Hz readings would be misleading. 

jjasili, I will give your suggestion consideration.  My tone generator is my Samsung phone using a free app.  This app can select other frequencies beside 1kHz, so I can easily repeat the measurements the next time I have the amp on the bench. Any suggestions on frequency of most interest? As noted above, maybe 100 Hz (close to low E frequency).  10,000 Hz is high for me, too old I'm afraid.

[sluckey]

We're not going in circles. We're talking about two different things.

You're talking about tone, how something sounds. Very subjective. The only measure of tone is your ears and I guarantee that mine are not calibrated and neither are yours. What I hear and what you hear are totally different and the tone you hear today could very well be totally meaningless tomorrow.

1KHz is a reference frequency that has been used in audio baseline measurements for as long as I can remember. Amps, speakers, mics, etc. Makes no difference whether it's hi-fi, AM broadcast, telephone. It's a standard. That's the frequency I would choose if I were to post signal voltages for an amp. That's exactly what Fender uses in there modern or reissue amps. Take a look...

     http://el34world.com/charts/Schematics/files/fender/Fender_65_deluxe_reverb_manual.pdf

The purpose of documenting signal voltages with a known frequency and amplitude test signal, as well as how the knobs are set, has nothing to do with telling you about tone. The main purpose is for evaluation or troubleshooting. You have a set of known voltages (same principle applies to dc voltages) to use for comparison that you can step through an amplifier 10 years later either to evaluate the amp's performance or to locate a problem in a broken amp.

Using 1KHz as a reference input signal is comparable to specifying line voltage set to 120VAC for all DC measurments. It just sets up a known reference that's easy to use on the bench. Maybe 1KHz ain't the ideal frequency to use for a guitar amp, but I promise you, if your guitar amp can't reproduce 1KHz, you got a sick amp! 

[jjasilli]

We're not going in circles. We're talking about two different things. You're talking about tone, how something sounds. Very subjective.

No, I'm talking about volts, as objectively measured, at different specific frequencies.  And so is Sylvania.  I didn't concoct the concept of thorough measurement, or the "danger" of drawing broad conclusions on a small sampling of data. I'm just the messenger.   


Different voltages may cause changes in audible tone, by altering waveform.   This is objectively true and is well-known.  I know that you know it; and that it is also capable of objective measurement by looking at a waveform, listening, using a distortion analyzer, etc., etc.  An amp that distorts bass notes may be clean at 1000Hz.  This is worth measuring for baseline purposes.

[jjasilli]

@ jecars:

confirming only the 1000Hz readings would be misleading. 

jjasili, I will give your suggestion consideration.  My tone generator is my Samsung phone using a free app.  This app can select other frequencies beside 1kHz, so I can easily repeat the measurements the next time I have the amp on the bench. Any suggestions on frequency of most interest? As noted above, maybe 100 Hz (close to low E frequency).  10,000 Hz is high for me, too old I'm afraid.

The choice is yours.  Your graph is great, and I'm just pointing to some old time stuff, expanding on this topic.


Possible frequencies in addition to the 1000 Hz standard:  400 Hz, a secondary standard, mentioned in prior threads by Hotblueplates, one of our moderators; the open guitar strings, or some of them.  80Hz, near open bass E is good.  The production of bass notes draws more power & may cause interesting voltage swings, especially in the power amp. It would also be good to extend your voltage readings to the speaker, or to a dummyload of the same Ohm rating. With that info you could also measure, or calculate, how many clean watts the amp puts out. (You can use your scope to see when the output wave begins to distort.)  These additional things would round-out your baseline info in useful ways for future reference.