Transistor Pwr Amp Pot

[Winston OBoogie]

OK, the Leach schematic has the same 5k trimmer, but there's a 3K3 in the loop which gives some 400mV in extra voltage regulation at 5k pot setting, rising to several volts extra at trimmer minimum setting.
I get the diodes being on the heatsink for thermal tracking of course.

I would call Marshall Leach's Vbe a "Flacid to Firmish Rubber Diode"
I'm downgrading C.J.'s to 'fairly stiff'.

 

[JohnRoberts]

Marshall Leach
https://groupdiy.com/index.php?topic=42219.msg525059#msg525059
used this type of VBE multiplier.
https://leachlegacy.ece.gatech.edu/lowtim/2ndstage.html
https://leachlegacy.ece.gatech.edu/papers/lowtim/feb76feb77articles.pdf

And I still haven't seen one.

Both link examples are actual Vbe multipliers with diode drops added in series... As already noted the amplifier we are discussing has no resistor in the base-collector leg, only diodes.

Prof Leach (RIP) did some good work (IMO) wrt rise time limited amplifier front ends. If an amplifier is rise time limited it is impossible to slew rate limit it. There is an AES paper from him kicking around the internets describing that but I'm not going after it today.

JR

 

[moamps]

I would call Marshall Leach's Vbe a "Flacid to Firmish Rubber Diode"

I love that name. It’s simple and easy to remember.
IIRC, Cordell uses "bias spreader" name for Vbe multiplier.

 

[moamps]

Both link examples are actual Vbe multipliers with diode drops added in series.

I don't think so. Diode drops are not in series with a Vbe multiplier.

 

[PRR]

JR "talks over" EVERYBODY.

I doubt he has "ignore-list"ed you; he just jumps over and in.

There is no shame in having several similar explanations posted. Each reader reads differently. Some combination of words may turn the trick.

You two, get over yourselves and get a room.

"semi"-rubber diode is WoB making up his own language. This is mostly technical american English, we can do that.

Rubber Diode is recognized by Wikipedia. Since Webster died, Wiki is about as authoritative a consensus as we have.

 

[PRR]

It is a Vj "adder" with a trim on Vj (junction voltage).

Pot causes about 6:1 change of diode current (little change of Q710 current). So about 50mV change per diode, 150mV for the D720 string.

It has the peculiar advantage that it can't be turned way-too-high. Disadvantage is that D720+Q710 drops must be real similar to output stage drops. (Which may vary with part replacement, especially across the decades.)

 

[Winston OBoogie]

Roger that PRR.  I'm  over it, and myself, now.  I probably just need to get out more.
My apologies to John. 

I'm a bit relieved Rubber Diode is wiki'ed, was starting to think I'd made that bit up too. 
'You say Vbe multiplier, I say potato...'

Unless or until C.J. can add to the saga, I'm guessing this plot has thinned for now.

Cheers

 

[Winston OBoogie]

It is a Vj "adder" with a trim on Vj (junction voltage).

Pot causes about 6:1 change of diode current (little change of Q710 current). So about 50mV change per diode, 150mV for the D720 string.

It has the peculiar advantage that it can't be turned way-too-high. Disadvantage is that D720+Q710 drops must be real similar to output stage drops. (Which may vary with part replacement, especially across the decades.)

THANK YOU!  I knew you would know, or figure it out. 

 

[CJ]

thanks for all the help, that youtube vid is awesome!

yes the pot does increase current in the output stage, and it can make things hot,

this is a Carver 2000  receiver, we had to breadboard the circuit to get an edge on fixing this,

 

[Winston OBoogie]

thanks for all the help, that youtube vid is awesome!

yes the pot does increase current in the output stage, and it can make things hot,

this is a Carver 2000  receiver, we had to breadboard the circuit to get an edge on fixing this,

Cool beans C.J.  Breadboarding to fix it is hardcore, glad you sorted it.

 

[Winston OBoogie]

IIRC, Cordell uses "bias spreader" name for Vbe multiplier.

Yes, that's common I think.
Both he and Self also use VAS  to denote the 2nd stage and everyone knows what they mean.  I'm trying to teach myself to use TIS instead though which seems more correct.  I suppose as long as we all understand one another it doesn't matter much.

The term "amperage" makes me cringe though.

 

[JohnRoberts]

I don't think so. Diode drops are not in series with a Vbe multiplier.

In case we haven't beat this one completely to death, the diodes in series with the base-collector resistor change it from a pure or simple Vbe multiplier. In effect it is very similar in action to Vbe multiplier + series diodes, I will concede it is not "exactly" the same as diodes only see the divider string current, not the full collector current.
---

Yes I also found that "rubber diode" wiki page yesterday, that appears to have been created around 2015 using references from 2000s...  Today I also found an old link to a class AB amp discussion published by University of St Andrews (Scotland) https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part2/page3.html, using the rubber diode verbiage (other links to St Andrews examples were broken). 

I have also found the Vbe multiplier called a "rubber zener" in a guitar pedal soft clipping circuit discussion.  So now we have two additional names for a pretty mature circuit topology.

I guess I need to apologize for something...

JR

 

[PRR]

> link to a class AB amp discussion published by University of St Andrews (Scotland)...(other links to St Andrews examples were broken).

Much of the Scots Guide to Electronics can be accessed through
https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/intro/electron.htm

 

[CJ]

thanks for the link!

here is the breadboard we did,  it can break into a radio transmitter, touching certain components shuts this off,

we have unbolted the metal that was bolted to the output devices,

 

[Winston OBoogie]

here is the breadboard we did,  it can break into a radio transmitter, touching certain components shuts this off,

The layout, PCB or breadboard, is the real schematic.  A few stray pF here and there, seemingly small differences in the location of base stoppers etc.  can ruin an otherwise OK design.

I've noticed you've worked on a few amps recently, you'll be the king of repair and diagnostics soon

 

[JohnRoberts]

I am still curious about whether that triple diode was heat sink mounted?



JR

 

[Winston OBoogie]

I am still curious about whether that triple diode was heat sink mounted?

Me too. 
Also curious to know if they thought that by drawing the schematic in an unconventional manner with Vcc & Vee reversed it would temporarily fool folks into thinking this was a cleverer than it really is trick.

 

[moamps]

I am still curious about whether that triple diode was heat sink mounted?

The diode in question is KB362, known as bias diode (about 2V).

 

[moamps]

And here is a picture how it is mounted in one another Carver receiver model.

 

[moamps]

Also curious to know if they thought that by drawing the schematic in an unconventional manner with Vcc & Vee reversed it would temporarily fool folks into thinking this was a cleverer than it really is trick.

It is not so unusual that the right channel is drawn in a "mirror".