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Negative ground PNP Rangemaster Biasing weirdness.

Started by Toney, April 22, 2009, 11:25:53 AM

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Toney

April 22, 2009, 11:25:53 AM

So,
I build a Rangemaster for a friend with a Japanese PNP (2sb77) 80HFE and lowish leakage, but negative ground, so as to play nice with my buddy's other pedals. 
The trannie had been kicking around my parts box for some time. The build was quick as anything but that trannie just refused to settle at 7v as recommended.
I went up and up in resistor values until I had 13k on the base and finally 100k on the emmiter and then finally, I got my 7v... so far out of the recommended values.
I really dont know why, but it seems PNP negative ground just causes some wackiness.
But....it sounds good, so I suppose that's all that matters.
Why those silly resistor values though??

R.G.

#1
April 22, 2009, 11:28:36 AM
How many times have I posted that PNP germanium pedals with negative ground causes problems unpredictably sometimes?  :icon_lol:
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

SonicVI

#2
April 22, 2009, 11:36:11 AM
Try a CV7112 from Small Bear and build an NPN Rangemaster. It's a good sounding tranny and low noise to boot.

Toney

#3
April 22, 2009, 12:32:55 PM

Yeah, they are good.
I am waiting on a batch of finest Soviet cold-war era GT404B NPNs made by willing and happy party members, comrade.
My buddy gets this as a freebie (western opportunist, capitalist pig-dog huck-teh)  ;D

Mugshot

#4
April 22, 2009, 08:47:30 PM
Quote from: R.G. on April 22, 2009, 11:28:36 AM
How many times have I posted that PNP germanium pedals with negative ground causes problems unpredictably sometimes?  :icon_lol:

i got lucky on mine  ;D
i am what i am, so are you.

petemoore

#5
April 23, 2009, 12:10:35 AM
How many times have I posted that PNP germanium pedals with negative ground causes problems unpredictably sometimes?
  I count more times than I'd like to remember, the schematic has appeal.
  The ground scheme is fairly easy to convert to what a NPN/Neg. or PNP/Pos. schematic shows.
 
 
Convention creates following, following creates convention.

JHS

#6
April 23, 2009, 12:48:01 PM
I've just wired a RM with OC44 for neg. ground, just to see what happend. Same sound, no probs at all. Maybe slightly artificial in the overtones. IMHO the trannie specs. of the 2N77 will not match the stock RM circuit perfect, so some R-values had to be altered.

All in all, I can't recommend a virtual neg. ground in such circuits. For a neg. ground RM or FF I would use a NPN-Ge-trannie like the AC127 or similar.

JHS






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