Schematic check for B&H 179

[angelodp]

A few questions. As I look at the juncture of the Power Tubes cathode resistor, there should be a dot connecting the resistor to the cathodes...yes? That would be an omission on this schematic.

Is this design what is described below? A Hight Mega Ohm Resistor Performance type design?



High Mega-Ohm Resistor Performance for Low Voltage Applications
May 25th, 2016|Tech Info
Resistors are common devices required for today’s electronics. It is so prevalent in design, we often tend to think they do not require close attention to their inherent properties. While many applications can be designed using performance assumptions gained from years and years of experience, this is not the case for all resistors, all of the time.

When using very high value resistors, there are a number of common resistor behaviors that may not apply, or in some case are backwards of traditional thinking. One such counter-intuitive relationship is Voltage Coefficient of Resistance (VCR). VCR is the relationship between the voltage seen by the resistive element and the corresponding resistance measured by a given device. One of the most important parameters of precise high-ohmic resistors is the VCR. The goal of resistor manufacturers is to try to create a resistor device that has consistent stated value of resistance, but does not have varying resistance values across broad voltage ranges.

The Voltage Coefficient is the change in resistance with applied voltage over a specific voltage range. We measure the stability of the resistor with respect to changes in voltage. A resistor with a VCR of 100 ppm/V will change 0.1% over a 10 Volt change and 1% over a 100 Volt change.

The rate of change in resistance value per 1 volt in the prescribed voltage range is expressed by the following formula:

VCR (ppm/V) = (Ro -R)/Ro X 1/( Vo -V) X 10e6 R:
Measured resistance (Ω) at base voltage
Ro: Measured resistance (Ω) at upper voltage
V: Base voltage
Vo: Upper voltage

[Stevem]

Yes the schematic is flawed, the two cathodes should be connected to that cathode resistor otherwise that 19.5 volts will not be produced and nor would that stage have any output.

Also note that this 19.5 volts is being used to bias up the filiment winding center tap for a reduced level of hum entering the audio stream from the preamp section.

It’s also kinda odd that they use the letter A to signify a resistor!

[angelodp]

Ok thanks, yep I see that center tap of the heaters going to that junction.

I am not familiar with other schematics that have so many Mega-ohm resistors, can you point me towards any guitar amps that were designed with a preponderance of high Mega-ohm R's

[sluckey]

I am not familiar with other schematics that have so many Mega-ohm resistors, can you point me towards any guitar amps that were designed with a preponderance of high Mega-ohm R's

In 1945 M represented 1,000, not 1,000,000. Think roman numeral M. Does that ease your mind?

[romberg]

[n 1945 M represented 1,000, not 1,000,000. Think roman numeral M.

Thank you! In two short sentences you cleared up all the confusion I had while restoring and old 1944 AM radio. I kept thinking that either someone at the factory or the engineering department was out to lunch as the values I found in the radio were orders of magnitudes different from the schematic (oddly by powers of ten). It never crossed my mind that the M was not mega and that the Romans were involved!

Mike

P.S. I can feel my past Latin teachers shake their heads in shame. Again...

[angelodp]

Aha, of course, Roman M 1000. So this schematic has M for 1000 and Meg for 1,000,000

Thanks

[thetragichero]

old schematics are wild. i think my favorite is μμF

[sluckey]

old schematics are wild. i think my favorite is μμF

We called those "Mickey Mike".