LPB-1 Unusual Version / Circuit Comparison?

[DIY Dood]

I have an LPB-1 from 1975. It's a little out of order, so I'm trying to rebuild it.

I found a circuit diagram that I drew back in 1976 of the original circuit. It's significantly (from my narrow viewpoint and limited electronic knowledge) from any other variation I can find of the LPB-1 on-line. I'm wondering about some of the differences and the effect on the output sound, etc. I've done some googling and learned a bit, but my theory is still very weak.

Questions are below the diagrams.

My Circuit:



Common Circuit:



Questions:

- The first difference is the 22K resistor in series in the input. Is that just to provide impedance? Or is it an R/C filter with the .2uF DC blocking capacitor? The resistor doesn't run to ground, so, the former?

- The second difference is the .2uF cap vs. the .1uF I find in the others on-line. That's on both the input and output. So it's blocking DC on the way in but I'm guessing that it's potentially filtering bass too... but with the .2uF, it's not really doing any bass filtering?

- R3 is 5.6K vs. 10K. So that's feeding more voltage to the collector for less possible gain? The common version uses 10K.

- The emitter goes direct to ground instead of through a 390K or similar resistor in the other versions. Is this a way to get more gain and "compensate" for the 5.6K resistor vs. 10K?

- R4 isn't there at all for the usual voltage divider. ??

Thanks,

[Gargaman]

My guesses:

- The first difference is the 22K resistor in series in the input. Is that just to provide impedance? Or is it an R/C filter with the .2uF DC blocking capacitor? The resistor doesn't run to ground, so, the former?

The resistor by itself won't change the impedance; also won't make filter with the cap since there is no path to ground; I guess is more like limiting the current going in.

- The second difference is the .2uF cap vs. the .1uF I find in the others on-line. That's on both the input and output. So it's blocking DC on the way in but I'm guessing that it's potentially filtering bass too... but with the .2uF, it's not really doing any bass filtering?

Input and output capacitor blocks DC and also limit the amount of bass; generally, the greater the capacitor, more bass going in and out; 0.2uF is greater than 0.1uF, so it's filtering less bass.

- R3 is 5.6K vs. 10K. So that's feeding more voltage to the collector for less possible gain? The common version uses 10K.
- The emitter goes direct to ground instead of through a 390K or similar resistor in the other versions. Is this a way to get more gain and "compensate" for the 5.6K resistor vs. 10K?
- R4 isn't there at all for the usual voltage divider. ??

Check for transistor Feedback Resistor Bias (1st) vs Voltage Divider Bias (2nd).

[Rob Strand]

The 22k resistor sets the gain and it does affect the input impedance.

It looks like they have tuned the 22k to have roughly the same input impedance as the classic version.  (After checking with spice your circuit has a lower impedance.)

When you consider the impedance of the guitar the signal gains are about the same as well.   The 22k also contributes here.

The bias point seems to be little different on your circuit although it does come down to the specific gains.    A small effort in choosing the transistor gain could match that as well.

In fact all the above depends on the transistor gain to some degree.

So in simple terms they are two different circuits doing the roughly same job.

FWIW, here's the classic version,

[antonis]

+1 to what Rob said..!!

Both are single BJT boosters with different bias and stage gain configuration..
R_Collector / (R_Emitter + r_e) vs R_Feedback / R_In (roughly)

Also, 390R + re vs 1M feedback resistor take care of beta variations..

[DIY Dood]

Thanks. Interesting... as mine was very early in the manufacturing years. 1975 at the latest. More classic than the classic, I'm guessing.

The .22uF cap was actually two .1uF caps soldered in parallel. Must have been an on-the-fly tweak to decrease bass reduction. Maybe it was competing with the Screaming Bird for sound. I recall those being in the catalog.

Mine also sports a 10K volume POT per my check yesterday, not the 100K of the "Classic".

I'll get to rebuilding it and see what it does.

[Rob Strand]

Mine also sports a 10K volume POT per my check yesterday, not the 100K of the "Classic".

Interesting.   I wonder if it has the 10k resistor + 10k vol put or it's another variant where the 10k pot *is* the 10k resistor, like the Rangemaster.

RG's schematic shows the 430k + 43k divider but it mentions the 1M + 100k (which is in the Beavis Audio schematic posted at the top of the thread).

The were some Maxon boosters in the 70's.  IRC they used a 50k pot at the output.  I think the divider on the base was 470k and 47k and the transistor was a 2SC828 (maybe an 2SC828R but don't quote me on it).   Maxon had a few model sort of paralleling some of the EHX offerings.

[DIY Dood]

@ Rob Strand

"I wonder if it has the 10k resistor + 10k vol put or it's another variant where the 10k pot *is* the 10k resistor, like the Rangemaster."

I have a "spare" 10K resistor that I think might have come from one of these. But I'm not sure. I had two of these coupled and I had the 10K off the output of the first one where the POT would have been - before it fed the second one. I had minimal knowledge of electronics at the time (just slightly above that now) and no resources so I just coupled them. I probably just put it in to mimic. But, it might have come from the original circuit.

Are you thinking it would be in series to create a pot with a minimum 10K resistance... transforming it to a 10K to 20K variable?

[Gus]

reminds me of the EM drive

https://tagboardeffects.blogspot.com/2013/01/emerson-custom-guitars-em-drive.html

[DIY Dood]

OK, wired it up P-T-P in the original fashion, because I lack a breadboard. LOL. It's not working.

It's passing audio through and the POT is controlling volume. However, it's actually cutting volume, not boosting it. Test conditions are a single coil guitar into an amp, physically plugged through the circuit or into the amp (no switching).

I assume the caps are OK, i have no tester other than "shorted" but they are passing the audio. I tested all the resistors and values seem good. Battery is running a little low at 8.83V, but I assume that would not be low enough to cut output this way. Polarity is correct per the diagram (negative ground).

I rechecked it all ten times. Seems to match the diagram.. I did some basic voltage measurements without an audio signal, but I'm not sure what I'm looking for. Voltages are in the diagram below.

Ideas?

(Also corrected the Pot to be 10K per the original parts, not 100K).

[antonis]

Base should sit at about 600 - 700 mV..
(as it is, B-E junction looks like 25MΩ resistor..)

Check for Emitter to GND continuity..

[Rob Strand]

Are you thinking it would be in series to create a pot with a minimum 10K resistance... transforming it to a 10K to 20K variable?

The way you have drawn it on your schematic looks fine.   

FWIW, the circuits with a feedback resistor have a lower output impedance than the "classic" circuit so you won't lose much gain with the 10k pot.   However, you do lose output swing.      These things are what they are.  I'm only explaining the mechanics of the circuit.  I'm not saying it's a problem as such.
 
(Re your current issue, antonis is on the right track.)

[DIY Dood]

I've checked the emitter to ground line and I'm 99% sure it's OK, but I'll check again in the morning and update.

[antonis]

I've checked the emitter to ground line and I'm 99% sure it's OK, but I'll check again in the morning and update.

If indeed it's OK, check transistor B-E junction..
(DMM on Diode test setting..)

[DIY Dood]

I'm getting 239 emitter to base.  This is an old T05 package. I don't know for sure if it's a 2N5088 as there are no markings on the package, but it definitely came as stock in the LPB-1.

Bad transistor?

[antonis]

Bad transistor?

Not, if it's Ge BJT..

Otherwise, yes..!! 

[DIY Dood]

I am guessing that the only way to know that would be if I could read printing on the T05 case?  Not other way to know what brand it is?

[Rob Strand]

I am guessing that the only way to know that would be if I could read printing on the T05 case?  Not other way to know what brand it is?

You can use you multimeter like antonis's picture.   Use the diode test range on the multimeter.   A silicon transistor will measure 0.5V to 0.7V and a germanium transistor will measure 0.1 to 0.3V.  You can test either or both of the junctions BE and BC.   It's best to do that with the transistor out of circuit, especially if the circuit isn't working.   On a working (and powered) circuit you can use the *voltage range* on the multimeter and measure the BE voltage (CB usually of no use in this case).

[DIY Dood]

 

You can use you multimeter like antonis's picture.   Use the diode test range on the multimeter.   A silicon transistor will measure 0.5V to 0.7V and a germanium transistor will measure 0.1 to 0.3V.  You can test either or both of the junctions BE and BC.   It's best to do that with the transistor out of circuit, especially if the circuit isn't working.   On a working (and powered) circuit you can use the *voltage range* on the multimeter and measure the BE voltage (CB usually of no use in this case).

Rob: OK, measured both today with the transistor out of circuit. Got 224mv  E > B and 211mv C > B. No connectivity in the other direction.

What does that tell me?

[antonis]

Got 224mv  E > B and 211mv C > B. No connectivity in the other direction.
What does that tell me?

It introduces itself as a healthy Ge p-n-p ..

[DIY Dood]

Got 224mv  E > B and 211mv C > B. No connectivity in the other direction.
What does that tell me?

It introduces itself as a healthy Ge p-n-p ..

OK, then I will have to go back through the other components and the circuit one more time.

What sort of voltage tests can I do with the circuit live other than the power voltages previously tested/posted?