LPB-1 Unusual Version / Circuit Comparison?

[Keppy]

If that's a PNP, then you need to run the circuit on -9v. Your schematic says your battery is backwards.

[DIY Dood]

If that's a PNP, then you need to run the circuit on -9v. Your schematic says your battery is backwards.

I believe it's a NPN. All the diagrams I've found of the LPB-1 show negative ground. I can guarantee the transistor came out of an original LPB-1.

[DIY Dood]

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

If that's a PNP, then you need to run the circuit on -9v. Your schematic says your battery is backwards.

Ding, ding, ding, we have a winner!!!

Unfortunately I focused on the "healthy" part of Antonis' reply, not the PNP part.

Keppy's post stuck it in my face.

I dug through MY old diagrams, vs. the ones on-line, and I found one where I'd labeled B voltage as positive ground.  Rewired and it's working!

On to the tuning part of it. I will do some testing of varied components and come back with some better questions.

Just one question for right now... what transistor do you think it is if it's a PNP in an early LPB-1. Obviously not the universally referenced 2N5088. Is there something else they were known to use back then?

[Rob Strand]

Ding, ding, ding, we have a winner!!!

Unfortunately I focused on the "healthy" part of Antonis' reply, not the PNP part.

Keppy's post stuck it in my face.

Good to see it working.  These things happen.

Just one question for right now... what transistor do you think it is if it's a PNP in an early LPB-1. Obviously not the universally referenced 2N5088. Is there something else they were known to use back then?

It's very hard to guess.  One way is to look at some EHX pedals with germaniums from the same era and see if you can find any part number.   Nothing to prove it will be that anyway.   IIRC there was an EHX parts list at some point it might even be on that.

What package is it?  Does it have any markings at all?

[DIY Dood]

What package is it?  Does it have any markings at all?

T05. Not a mark on it. Perhaps it faded... or there was never anything there. But it's unmarked at this point.

[Rob Strand]

T05. Not a mark on it. Perhaps it faded... or there was never anything there. But it's unmarked at this point.

Nothing obvious comes to mind.   There does exist PNP TO-5 germaniums  but there's nothing linking the possible part numbers to something that Electro-harmonix used in that era.  I actually couldn't find the Electro-harmonix parts cross-reference online.

Maybe someone else can chip in.

[DIY Dood]

Moving ahead... I tried swapping in the standard resistors to a 10K on the feed from the battery and a 440K on the emitter to ground. I did it with a DPST switch so that I could flip back and forth. I found no apparent difference... at least that I could hear. Overall gain the same.

I also tried changing the input and output caps from .22uf to .1uF and the other way to .4uf. I did it with two switches so that I could change input, output, or both. Also didn't seem to make any appreciable difference. I was testing with a single coil (read bright) guitar, so perhaps that's affecting it.  But I was expecting it to have more effect.

It does cut treble a bit. Not a huge amount, but you can hear it when it's set to unity and you flip back and forth. Not a bad sound, but I'd like to see it just a bit more transparent. Any thoughts on how to achieve that in view of the above testing?

[antonis]

I tried swapping in the standard resistors to a 10K on the feed from the battery and a 440K on the emitter to ground.

Presuming 10k instead of 5k6 one, gain practically remains the same 'cause major gain factor is 1M / (22k + plus signal source output impedance)

Also presuming 440R instead or 390R, gain is lowred by less than 1db..
(10k/(440+re) vs 10k/(390+re), where re = 0.025/I_CQ)

[DIY Dood]

Presuming 10k instead of 5k6 one, gain practically remains the same 'cause major gain factor is 1M / (22k + plus signal source output impedance)

Also presuming 440R instead or 390R, gain is lowred by less than 1db..
(10k/(440+re) vs 10k/(390+re), where re = 0.025/I_CQ)

Thanks... as observed.

Any thoughts on the slight treble reduction? I wouldn't think the tested (.1 to .4uF) caps would be in that range. But there's definitely a little of the high edge cut off.

[antonis]

Any thoughts on the slight treble reduction?

C1 (220nF) toghether with Q1 input impedance (in parallel with 1M divided with stage gain) form a HPF of about 70Hz cut-off frequecy (for a h_FE = 200)

For C1 of 100nF, the above cut-off frequency is lowered down to 30Hz and for 400nF ( ) is raised up to 130Hz..
Of cource, the above mentioned -3dB cut-off frequencies are strongly dependent both on particular Q1 h_FE and Collector quiescent current..
That said, HPF cut-off frequencies might be even higher (highly likely) or lower (more unlikely)..

C2 (220nF) also forms a HPF together with 100k Volume pot of  about 7Hz (considering pot fully CW and next effect input impedance much higher than 100k), so 100nF raises it up to 16Hz where 400nF lower it down to 4 Hz..
(even with next effect input impedance 100k, say, max cut-off frequency is only 32Hz - no treble loss..!!)

[DIY Dood]

Any thoughts on the slight treble reduction?

C1 (220nF) toghether with Q1 input impedance (in parallel with 1M divided with stage gain) form a HPF of about 70Hz cut-off frequecy (for a h_FE = 200)

For C1 of 100nF, the above cut-off frequency is lowered down to 30Hz and for 400nF ( ) is raised up to 130Hz..
Of cource, the above mentioned -3dB cut-off frequencies are strongly dependent both on particular Q1 h_FE and Collector quiescent current..
That said, HPF cut-off frequencies might be even higher (highly likely) or lower (more unlikely)..

C2 (220nF) also forms a HPF together with 100k Volume pot of  about 7Hz (considering pot fully CW and next effect input impedance much higher than 100k), so 100nF raises it up to 16Hz where 400nF lower it down to 4 Hz..
(even with next effect input impedance 100k, say, max cut-off frequency is only 32Hz - no treble loss..!!)

Antonis: If I understand what you are saying:
- the C1 side is, at the most, giving me a slight bass filter at 110 or 220nf. So that's not a treble loss.
- the C2 side is, at the most, cutting at 7K, so that's not a loss in the usual guitar range (unless harmonics are the issue)

One factor I will add: The volume pot is not fully CW, it's backed off to equity volume for the back to back testing. Would that lowered pot change the filter and cause the treble loss?

Also: You mention "quiescent current." Does that imply that a higher voltage battery or PS would reduce the effect? I am running at 8.8 off the battery right now (it's a little worn down).


Thanks,

[antonis]

- the C2 side is, at the most, cutting at 7K, so that's not a loss in the usual guitar range (unless harmonics are the issue)

7 Hz (not kHz)..
(no loss at all..)

The volume pot is not fully CW, it's backed off to equity volume for the back to back testing. Would that lowered pot change the filter and cause the treble loss?

No..
For next stage input impedance much higher than 100k, HPF cut-off frequency is always 1/2π*C2*(pot body resistance)

Also: You mention "quiescent current." Does that imply that a higher voltage battery or PS would reduce the effect?

It depends on various factors related to bias point..
Collector quiescent current sets intrinsic Emitter resistor value (0.025/I_CQ) so the higher the current the lower the resistor value..
(maintaining same Collector resistor for higher supply..)

That's good for stage open-loop gain but bad for input impedance (h_FE X re)..
(lower input impedance means higher HPF cut-off frequency -> bass loss ..)

Anyway, for particular circuit and 200mV lower supply, don't even bother..

P.S.
By re-reading your queries, I'm getting confused a bit..
Both IN & OUT filters are High-Pass so none of them has nothing to do with Treble loss..
(they cut Bass..)

The only item which could affect treble loss should be a high value Miller cap (feedback cap between Collector & Base..)

[DIY Dood]

The only item which could affect treble loss should be a high value Miller cap (feedback cap between Collector & Base..)

What sort of values should I experiment with to explore that? Also, in series with R1, the 1M resistor, or parallel?

[Rob Strand]

What sort of values should I experiment with to explore that? Also, in series with R1, the 1M resistor, or parallel?

The 22k input resistor loads down the pickups and you can loose some highs.    If you make the 22k resistor higher it will reduce the loading but it also reduces the gain.    A limitation of that circuit is the gain and input impedance are linked.

So one way around that is to increase all the resistors.   For example input resistor 47k, feedback resistor 1M changed to 2.2M, 5.6k changed to 10k.   They are only rough values as the germanium transistor leakage stuffs up the proportionality.    You might have to play with emitter resistor or collector base-resistor to get the collector voltage to be what it was before.  Also the low valued 10k volume pot means you don't want to make the collector resistor too high.   So for the parts you have you are kind of boxed-in in terms of juggling the input impedance.   By raising the resistances all the caps need to be halved - more or less.

On the classic circuit there's two components which contribute to loading:  The divider on the base and the 390R in the emitter.  The two loading components are in parallel.  The 390R contributes a loading impedance =  transistor gain * 390.   For a silicon version with a high gain transistor, say hFE = 600, 390 * 600 = 234k which is quite high.  In this case the base divider contributes more to loading 43k//430k = 39k.   So for a classic circuit with a high gain silicon transistor it is possible to raise the input impedance by using higher value resistors on the divider on the base.   A germanium version of the classic circuit isn't so easy to increase the impedance as the low gain  of the germanium transistor limits how high you can make the input impedance.  If the germanium transistor gain is 100 then 100*390 = 39k  so the transistor and the base divider contribute equally.   So even if you removed the base divider loading entirely you can't raise the input impedance above 39k.

(I've glossed over a few details but they are the main points and the main bottle-necks.)

[antonis]

Also, in series with R1, the 1M resistor, or parallel?

1M resistor is a feedback BIAS one..
(meaning there is DC flowing from Collector to Base..)

I let considering the effect of a series cap up to you..