[Paul Dorman]

In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example, either
at one of the speaker voice coils, or at the crossover.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use my
regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?

My meter is NOT a True-RMS meter, so I know if the signal is too far off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no signal?

[John Williamson]

On 23/01/2021 08:24, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example, either
at one of the speaker voice coils, or at the crossover.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use my
regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?

My meter is NOT a True-RMS meter, so I know if the signal is too far off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no signal?

Given the figures you quote, 0dB will be the "standard for the period" 1
milliwatt in to a 600 ohm load, which corresponds to 0.775 volts RMS, so
5dB in power above that is the given 1.4 volts RMS. The reference was so
ingrained in audio electronics at the time, nobody even considered using
any other voltage reference without specifying it.

As for specifying the use of a VTVM, the standard input impedance for
one of those was as close as could be got to a purely resistive 10
megohms, and as the circuit may be high impedance with either a
capacitive or inductive component, you need to match that on the meter
you are using for accurate results. (I take it the organ uses valves
(tubes)?) If it is a valve based unit, then all voltages will probably
be specified as measured by a VTVM, and if so, using a moving coil meter
(usually with an input impedance of 20 kilohms per volt) will give
incorrect readings, unless the manual specifies the meter impedance. The
manual should tell you which type of meter to use when prodding round
the circuit.


At the speaker, meter impedance is not critical, though using a VTVM is
still specified as most moving coil meters of the period did not have
either true RMS capability or a level frequency response, as well as
having a variable impedance at the test leads, depending on the range
that was set.

(I grew up working with that generation of gear and its limitations,
goodness, I feel old now.)
--
Tciao for Now!

John.

[Paul Dorman]

On 1/23/2021 2:02 AM, John Williamson wrote:
On 23/01/2021 08:24, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example, either
at one of the speaker voice coils, or at the crossover.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use my
regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?

My meter is NOT a True-RMS meter, so I know if the signal is too far off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no
signal?

Given the figures you quote, 0dB will be the "standard for the period" 1
milliwatt in to a 600 ohm load, which corresponds to 0.775 volts RMS, so
5dB in power above that is the given 1.4 volts RMS. The reference was so
ingrained in audio electronics at the time, nobody even considered using
any other voltage reference without specifying it.

Ok, so that's practically the same as the Radio Frequency/Microwave
standard of dBm, where 0 dBm is 1 milliWatt, but usually into 50 Ohms.

And +30dBm is 1 Watt.



As for specifying the use of a VTVM, the standard input impedance for
one of those was as close as could be got to a purely resistive 10
megohms, and as the circuit may be high impedance with either a
capacitive or inductive component, you need to match that on the meter
you are using for accurate results. (I take it the organ uses valves
(tubes)?)Â* If it is a valve based unit, then all voltages will probably
be specified as measured by a VTVM, and if so, using a moving coil meter
(usually with an input impedance of 20 kilohms per volt) will give
incorrect readings, unless the manual specifies the meter impedance. The
manual should tell you which type of meter to use when prodding round
the circuit.


This service manual is for a Conn transistor organ (no tubes).

It says "Audio VTVM" for type of meter used in the measurement.


At the speaker, meter impedance is not critical, though using a VTVM is
still specified as most moving coil meters of the period did not have
either true RMS capability or a level frequency response, as well as
having a variable impedance at the test leads, depending on the range
that was set.

(I grew up working with that generation of gear and its limitations,
goodness, I feel old now.)

[Paul Dorman]

On 1/23/2021 10:25 AM, Paul Dorman wrote:
On 1/23/2021 2:02 AM, John Williamson wrote:
On 23/01/2021 08:24, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example, either
at one of the speaker voice coils, or at the crossover.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use my
regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?

My meter is NOT a True-RMS meter, so I know if the signal is too far off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no
signal?

Given the figures you quote, 0dB will be the "standard for the period"
1 milliwatt in to a 600 ohm load, which corresponds to 0.775 volts
RMS, so 5dB in power above that is the given 1.4 volts RMS. The
reference was so ingrained in audio electronics at the time, nobody
even considered using any other voltage reference without specifying it.

Â*Â* Ok, so that's practically the same as the Radio Frequency/Microwave
standard of dBm, where 0 dBm is 1 milliWatt, but usually into 50 Ohms.

Actually, it's not the same, because the characteristic impedance
is different (50 instead of 600 Ohms), so you'll get different Volts RMS
values for the same Wattage.

But the zero dB point is the same 1 milliWatt.



Â*Â* And +30dBm is 1 Watt.



As for specifying the use of a VTVM, the standard input impedance for
one of those was as close as could be got to a purely resistive 10
megohms, and as the circuit may be high impedance with either a
capacitive or inductive component, you need to match that on the meter
you are using for accurate results. (I take it the organ uses valves
(tubes)?)Â* If it is a valve based unit, then all voltages will
probably be specified as measured by a VTVM, and if so, using a moving
coil meter (usually with an input impedance of 20 kilohms per volt)
will give incorrect readings, unless the manual specifies the meter
impedance. The manual should tell you which type of meter to use when
prodding round the circuit.


Â*Â*Â*Â* This service manual is for a Conn transistor organ (no tubes).

Â*Â*Â*Â* It says "Audio VTVM" for type of meter used in the measurement.


At the speaker, meter impedance is not critical, though using a VTVM
is still specified as most moving coil meters of the period did not
have either true RMS capability or a level frequency response, as well
as having a variable impedance at the test leads, depending on the
range that was set.

(I grew up working with that generation of gear and its limitations,
goodness, I feel old now.)

[[email protected]]

John Williamson wrote:
========================
Paul Dorman wrote:

My meter is NOT a True-RMS meter, so I know if the signal is too far off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.


At the speaker, meter impedance is not critical, though using a VTVM is
still specified as most moving coil meters of the period did not have
either true RMS capability or a level frequency response, as well as
having a variable impedance at the test leads, depending on the range
that was set.


** VTVMs are not "true RMS" - none of them.

Multi-meters with that feature became possible with the advent of special linear ICs called "RMS to DC Converters" - in the early 1980s.
Modern DMMs often have such ICs in them but is still no guarantee of a wide frequency response.

OTOH The vast majority of old fashioned analogue meters DO have wide response - at least the whole audio band within a dB or so.
All the above will accurately read the RMS value of a *sine wave*, allowing for response limits as published in the handbook.

FYI:

I recently bought a Keysight U1232A meter with true RMS and speced to a few kHz.
On test however, it is flat to *over 100kHz* on all ranges.

https://au.mouser.com/ProductDetail/...2BgT2eJg%3D%3D


...... Phil

[Mike Rivers[_2_]]

On 1/23/2021 3:24 AM, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example, either
at one of the speaker voice coils, or at the crossover.

An "audio VTVM" is a voltmeter with a high (usually greater than 1
megohm) input impedance AND has a flat frequency response over the full
audio range of 20 Hz to 20 kHz.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use my
regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?

It depends. Is this an analog VOM, or digital multimeter? And what
frequencies will you need to measure? Most analog meters will have
reasonably good frequency response over the full audio range. On the
other hand, most service-bench DMMs tend to poop out above around 500
Hz. They figure you only need AC volts to measure power line voltage.

My meter is NOT a True-RMS meter, so I know if the signal is too far off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.

That's true. It depends on what you need to measure when you're doing
your troubleshooting or tune-up. The meter is calibrated to read the
correct value for a sine wave within its working frequency range. If you
need to make voltage measurements of organ notes, your meter won't be
accurate - but that may or may not matter, depending on why you're
making that particular measurement.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no signal?

I don't think this is talking about noise floor, but it depends on the
context.

"+5 dB" by itself is a ratio. It tells you how far above a reference
point your measurement is. For example, if you're measuring frequency
response, typically your reference is the voltage at 1 kHz.

5 dB represents a ratio of 1.78 [How do I know this?
http://www.sengpielaudio.com/calculator-db.htm] so if your reference
signal is 1.00 volts, +5 dB would be 1.78 volts.

Unless you're measuring power, in audio, for practical work, we use dBu,
which uses 0.775 volts as the 0 dBu reference. I won't go into the
explanation of that number here. But, for instance, if you measure 2.5
volts, that would equate to +10.18 dBu.

To fully answer your question, we need to know what you're measuring and
why. What does the manual say?

--
For a good time, call http://mikeriversaudio.wordpress.com

[Paul Dorman]

On 1/23/2021 11:26 AM, Mike Rivers wrote:
On 1/23/2021 3:24 AM, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example,
either at one of the speaker voice coils, or at the crossover.

An "audio VTVM" is a voltmeter with a high (usually greater than 1
megohm) input impedance AND has a flat frequency response over the full
audio range of 20 Hz to 20 kHz.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use
my regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?

It depends. Is this an analog VOM, or digital multimeter? And what
frequencies will you need to measure? Most analog meters will have
reasonably good frequency response over the full audio range. On the
other hand, most service-bench DMMs tend to poop out above around 500
Hz. They figure you only need AC volts to measure power line voltage.

I have a standard Digital multimeter. The test notes go up to A#4,
which is 466 Hz, which might be pushing the frequency response of the meter.

The organ stop tabs used are Diapason 8' and Tibia 8'. I am not
sure about the Fourier Transform of these waveforms, but hopefully they
are close enough to an ideal sinewave, that the measured RMS value
will be close enough.

But the harmonics of a 466 Hz fundamental, can be far beyond
500 Hz, and hence out of the meter's range.



My meter is NOT a True-RMS meter, so I know if the signal is too far
off from an ideal sinusoid, the RMS reading will be off, but I am
hoping it will be close enough.

That's true. It depends on what you need to measure when you're doing
your troubleshooting or tune-up. The meter is calibrated to read the
correct value for a sine wave within its working frequency range. If you
need to make voltage measurements of organ notes, your meter won't be
accurate - but that may or may not matter, depending on why you're
making that particular measurement.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no
signal?

I don't think this is talking about noise floor, but it depends on the
context.

"+5 dB" by itself is a ratio. It tells you how far above a reference
point your measurement is. For example, if you're measuring frequency
response, typically your reference is the voltage at 1 kHz.

5 dB represents a ratio of 1.78 [How do I know this?
http://www.sengpielaudio.com/calculator-db.htm] so if your reference
signal is 1.00 volts, +5 dB would be 1.78 volts.

Unless you're measuring power, in audio, for practical work, we use dBu,
which uses 0.775 volts as the 0 dBu reference. I won't go into the
explanation of that number here. But, for instance, if you measure 2.5
volts, that would equate to +10.18 dBu.

To fully answer your question, we need to know what you're measuring and
why. What does the manual say?


This test procedure appears to be a simple level-setting
adjustment for volume, for the different groups of stop tabs in the organ.

Some measurements are at the voice coil of the 12" speaker, and
some are measured at a crossover network.

I just noticed that the service manual mentions that a model
870 Amphenol is an excellent choice for "measuring audio signals
as low as -50dB". I've never had this model (a bit before my
time!).

[Don Pearce[_3_]]

On Sun, 24 Jan 2021 04:03:59 -0700, Paul Dorman
wrote:

On 1/23/2021 11:26 AM, Mike Rivers wrote:
On 1/23/2021 3:24 AM, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example,
either at one of the speaker voice coils, or at the crossover.

An "audio VTVM" is a voltmeter with a high (usually greater than 1
megohm) input impedance AND has a flat frequency response over the full
audio range of 20 Hz to 20 kHz.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use
my regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?

It depends. Is this an analog VOM, or digital multimeter? And what
frequencies will you need to measure? Most analog meters will have
reasonably good frequency response over the full audio range. On the
other hand, most service-bench DMMs tend to poop out above around 500
Hz. They figure you only need AC volts to measure power line voltage.

I have a standard Digital multimeter. The test notes go up to A#4,
which is 466 Hz, which might be pushing the frequency response of the meter.

The organ stop tabs used are Diapason 8' and Tibia 8'. I am not
sure about the Fourier Transform of these waveforms, but hopefully they
are close enough to an ideal sinewave, that the measured RMS value
will be close enough.

But the harmonics of a 466 Hz fundamental, can be far beyond
500 Hz, and hence out of the meter's range.



My meter is NOT a True-RMS meter, so I know if the signal is too far
off from an ideal sinusoid, the RMS reading will be off, but I am
hoping it will be close enough.

That's true. It depends on what you need to measure when you're doing
your troubleshooting or tune-up. The meter is calibrated to read the
correct value for a sine wave within its working frequency range. If you
need to make voltage measurements of organ notes, your meter won't be
accurate - but that may or may not matter, depending on why you're
making that particular measurement.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no
signal?

I don't think this is talking about noise floor, but it depends on the
context.

"+5 dB" by itself is a ratio. It tells you how far above a reference
point your measurement is. For example, if you're measuring frequency
response, typically your reference is the voltage at 1 kHz.

5 dB represents a ratio of 1.78 [How do I know this?
http://www.sengpielaudio.com/calculator-db.htm] so if your reference
signal is 1.00 volts, +5 dB would be 1.78 volts.

Unless you're measuring power, in audio, for practical work, we use dBu,
which uses 0.775 volts as the 0 dBu reference. I won't go into the
explanation of that number here. But, for instance, if you measure 2.5
volts, that would equate to +10.18 dBu.

To fully answer your question, we need to know what you're measuring and
why. What does the manual say?


This test procedure appears to be a simple level-setting
adjustment for volume, for the different groups of stop tabs in the organ.

Some measurements are at the voice coil of the 12" speaker, and
some are measured at a crossover network.

I just noticed that the service manual mentions that a model
870 Amphenol is an excellent choice for "measuring audio signals
as low as -50dB". I've never had this model (a bit before my
time!).

My multimeter - a Fluke 73III is specced from 45Hz to 1kHz. I've just
tested it with a function generator and it is flat down to 10Hz. At
the top end it drops by 1dB at 9kHz. What you will be measuring up
there is harmonics, not fundamentals. That means the overall accuracy
is compromised by probably no more than 0.1dB. Just use the
multimeter, it's almost certainly just fine for the job.

d

[Mike Rivers[_2_]]

On 1/24/2021 6:03 AM, Paul Dorman wrote:

Â*Â*Â* I have a standard Digital multimeter.Â* The test notes go up to A#4,
which is 466 Hz, which might be pushing the frequency response of the
meter.

Â*Â*Â* The organ stop tabs used are Diapason 8' and Tibia 8'.Â* I am not
sure about the Fourier Transform of these waveforms, but hopefully they
are close enough to an ideal sinewave, that the measured RMS value
will be close enough.

Â*Â*Â* But the harmonics of a 466 Hz fundamental, can be far beyond
500 Hz, and hence out of the meter's range.

This test procedure appears to be a simple level-
setting adjustment for volume, for the different groups of stop
tabs in the organ. Some measurements are at the voice coil of the 12"
speaker, and some are
measured at a crossover network.

I think you're over-thinking this. Measurements with your meter will get
you well into the ballpark. At that point, take on the job of "organ
technician" and use your ears. If some notes stick out, adjust those so
they blend in with the rest of the instrument. Not all organs are voiced
perfectly, so get it close with your meter and then touch it up as needed.

Â*Â*Â*Â* I just noticed that the service manual mentions that a model
870 Amphenol is an excellent choice for "measuring audio signals
as low as -50dB".

That's the kind of multimeter that was the standard bench instrument on
a radio/tv repair shop. Because measurements often must be made at the
grid of a tube, the megohms-input impedance of the VTVM, because of the
tube at the front end, wouldn't load down the signal that's there.

The input impedance of a modern DMM is pretty high, so you shouldn't
have any problem making measurements either at the speaker terminals or
the input of the crossover network, even if it's a tube amplifier.

In the Amphenol VTVM era, we didn't use dBu or dBV, and 0 "dB"
represented the voltage required to pump 1 milliwatt into a 600 ohm
load, which is the reference voltage for dBu.

[(0.775 V)^2 / 600 Ω = .001 mW]

-50 dBu is about 2.5 mV. If this is a tone wheel organ, you'd find
voltages in that range coming right off the pickups (I think - I don't
know for sure), so unless the adjustment procedure involves mechanical
adjustment of the pickups to adjust the level going into the amplifier
section, you'll be fine with the sensitivity range of your meter.

Happy tweaking, Mr. Organ Technician.



--
For a good time, call http://mikeriversaudio.wordpress.com

[[email protected]]

Mike Rivers wrote:
===============

I think you're over-thinking this. Measurements with your meter will get
you well into the ballpark. At that point, take on the job of "organ
technician" and use your ears. If some notes stick out, adjust those so
they blend in with the rest of the instrument.

** Absolutely.

This Q has been thoroughly over answered.

The OP is being too pedantic for words.

In the Amphenol VTVM era, we didn't use dBu or dBV, and 0 "dB"
represented the voltage required to pump 1 milliwatt into a 600 ohm
load, which is the reference voltage for dBu.

[(0.775 V)^2 / 600 Ω = .001 mW]

** Errr - might be .001 Watts ?


...... Phil

[Mike Rivers[_2_]]

On 1/24/2021 6:00 PM, wrote:
[(0.775 V)^2 / 600 Ω = .001 mW]
** Errr - might be .001 Watts ?

Yup. I had milliwatts on the brain.

--
For a good time, call http://mikeriversaudio.wordpress.com

[geoff]

On 25/01/2021 3:04 am, Mike Rivers wrote:
On 1/24/2021 6:03 AM, Paul Dorman wrote:

Â*Â*Â*Â* I have a standard Digital multimeter.Â* The test notes go up to
A#4, which is 466 Hz, which might be pushing the frequency response of
the meter.

Â*Â*Â*Â* The organ stop tabs used are Diapason 8' and Tibia 8'.Â* I am not
sure about the Fourier Transform of these waveforms, but hopefully they
are close enough to an ideal sinewave, that the measured RMS value
will be close enough.

Â*Â*Â*Â* But the harmonics of a 466 Hz fundamental, can be far beyond
500 Hz, and hence out of the meter's range.

Â*Â*Â*Â*Â*Â* This test procedure appears to be a simple level-
setting adjustment for volume, for the different groups of stop
tabs in the organ.Â* Some measurements are at the voice coil of the 12"
speaker, and some are
measured at a crossover network.

I think you're over-thinking this. Measurements with your meter will get
you well into the ballpark. At that point, take on the job of "organ
technician" and use your ears. If some notes stick out, adjust those so
they blend in with the rest of the instrument. Not all organs are voiced
perfectly, so get it close with your meter and then touch it up as needed.

Â*Â*Â*Â*Â* I just noticed that the service manual mentions that a model
870 Amphenol is an excellent choice for "measuring audio signals
as low as -50dB".

That's the kind of multimeter that was the standard bench instrument on
a radio/tv repair shop. Because measurements often must be made at the
grid of a tube, the megohms-input impedance of the VTVM, because of the
tube at the front end, wouldn't load down the signal that's there.

The input impedance of a modern DMM is pretty high, so you shouldn't
have any problem making measurements either at the speaker terminals or
the input of the crossover network, even if it's a tube amplifier.

In the Amphenol VTVM era, we didn't use dBu or dBV, and 0 "dB"
represented the voltage required to pump 1 milliwatt into a 600 ohm
load, which is the reference voltage for dBu.

[(0.775 V)^2 / 600 Ω = .001 mW]

-50 dBu is about 2.5 mV. If this is a tone wheel organ, you'd find
voltages in that range coming right off the pickups (I think - I don't
know for sure), so unless the adjustment procedure involves mechanical
adjustment of the pickups to adjust the level going into the amplifier
section, you'll be fine with the sensitivity range of your meter.

Happy tweaking, Mr. Organ Technician.




Here are some organ technicians. Or specialists at least , though mostly
Hammond-centric.

Have got useful info and replacement parts off them in the past for my
C3 and L122 ....

http://www.tonewheelgeneral.com/

geoff

And for those suspicious of the link:
view-source:http://www.tonewheelgeneral.com/

[Paul Dorman]

On 1/24/2021 7:04 AM, Mike Rivers wrote:
On 1/24/2021 6:03 AM, Paul Dorman wrote:

Â*Â*Â*Â* I have a standard Digital multimeter.Â* The test notes go up to
A#4, which is 466 Hz, which might be pushing the frequency response of
the meter.

Â*Â*Â*Â* The organ stop tabs used are Diapason 8' and Tibia 8'.Â* I am not
sure about the Fourier Transform of these waveforms, but hopefully they
are close enough to an ideal sinewave, that the measured RMS value
will be close enough.

Â*Â*Â*Â* But the harmonics of a 466 Hz fundamental, can be far beyond
500 Hz, and hence out of the meter's range.

Â*Â*Â*Â*Â*Â* This test procedure appears to be a simple level-
setting adjustment for volume, for the different groups of stop
tabs in the organ.Â* Some measurements are at the voice coil of the 12"
speaker, and some are
measured at a crossover network.

I think you're over-thinking this. Measurements with your meter will get
you well into the ballpark. At that point, take on the job of "organ
technician" and use your ears. If some notes stick out, adjust those so
they blend in with the rest of the instrument. Not all organs are voiced
perfectly, so get it close with your meter and then touch it up as needed.

Â*Â*Â*Â*Â* I just noticed that the service manual mentions that a model
870 Amphenol is an excellent choice for "measuring audio signals
as low as -50dB".

That's the kind of multimeter that was the standard bench instrument on
a radio/tv repair shop. Because measurements often must be made at the
grid of a tube, the megohms-input impedance of the VTVM, because of the
tube at the front end, wouldn't load down the signal that's there.

The input impedance of a modern DMM is pretty high, so you shouldn't
have any problem making measurements either at the speaker terminals or
the input of the crossover network, even if it's a tube amplifier.

In the Amphenol VTVM era, we didn't use dBu or dBV, and 0 "dB"
represented the voltage required to pump 1 milliwatt into a 600 ohm
load, which is the reference voltage for dBu.

[(0.775 V)^2 / 600 Ω = .001 mW]


If we calculate 1 milliWatt into a speaker 8 Ohm load, it will be:

(0.089 Vrms)^2 / 8 Ohms = .001 Watt

And 5dB above that would be .00316 Watts = (0.159 Vrms)^2 / 8 Ohms.

So it's obviously impedance dependent.


-50 dBu is about 2.5 mV. If this is a tone wheel organ, you'd find
voltages in that range coming right off the pickups (I think - I don't
know for sure), so unless the adjustment procedure involves mechanical
adjustment of the pickups to adjust the level going into the amplifier
section, you'll be fine with the sensitivity range of your meter.

Happy tweaking, Mr. Organ Technician.

[geoff]

On 23/01/2021 9:24 pm, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example, either
at one of the speaker voice coils, or at the crossover.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use my
regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?




Thatwas written in the days when the only other option to a VTVM where
moving coil voltmeters, which have a relatively low input
impedance/resistance rating in (k)ohms/volt.

Just use a DMM and Bob's your uncle.

My meter is NOT a True-RMS meter, so I know if the signal is too far
off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no
signal?

A bit of a loose use of dB there, but I would assume it means +5 dBV,
near as dammit 1.8v (unless they actually state something about 'above
noise floor'.

geoff

geoff