BMP Stage #1 input question.

[Rain Dog]

I am messing with GGG's   Big Muff Pi circuit and I get some confusing (to me) results. I have the first stage built on a bread board. My stage 1 circuit is minus C15 and R26, I have the signal generator signal going into the R1 39K resistor. My circuit ends at C3. C1 is .22uf but all other components are exactly as shown on the schematic.

I thought the purpose of Stage1 was to amplify the input signal? Is it, or is it strictly for impedance matching? I ask this because my signal after C3 is smaller than the original signal and looks a bit noisy.

I have a signal at @ 440Hz and @ 250mA peak to peak that I am injecting prior to the (R1)39K resistor. When I bring up a trace just after R1 the signal is nearly gone. Q1 amplifies this tiny signal but the result is still much smaller than the original injected signal. If I inject the signal after R1 then I get a huge gain.

Just for fun I put a 0-250K pot in for R2 and ran the pot through it's full range. The output from Q1 measured after C3 seemed unchanged. Not what I expected.

I haven't plugged my bass into the circuit yet. I wouldn't expect the results to be different but maybe?? Should I put a 39K resistor in series with my signal generator and then adjust the amplitude so I have 250mA after the probe resistor and before R1? I don't really see what that would change.

BTW - My signal generator is a "vintage" all tube audio signal generator that I bought from an old HAM radio guy.

I only work on this stuff late at night after the kids go to bed, so I'm probably not at my sharpest. Any insights would be appreciated.

-RD-

[Rain Dog]

Did I forget to say PLEASE??

OK, Please...

[aron]

It definitely should be boosting. In fact, Jack has an article that uses only one of the stages.

[Rain Dog]

It definitely should be boosting. In fact, Jack has an article that uses only one of the stages.

Could you post the link? Any other clues as to what I might be doing wrong?

[Rain Dog]

I find this LPB2 Booster circuit, which is very similar to stage1. http://www.generalguitargadgets.com/v2/diagrams/lpb2_sc.gif

I see that it does not have the 39K input resistor.

[Rain Dog]

I think I solved it myself. I switched the 5088 transistor with hfe @ 350 for a 5089 with hfe @ 780. Got gain now! I also put a 250k trimpot in parallel with R1 so I can adjust and fine tune the input.

I think I'm starting to get a clue with this DIY thing.  

[Eric H]

I think I solved it myself. I switched the 5088 transistor with hfe @ 350 for a 5089 with hfe @ 780. Got gain now! I also put a 250k trimpot in parallel with R1 so I can adjust and fine tune the input.

I think I'm starting to get a clue with this DIY thing.  

You should have had plenty of gain with the 5088.
I believe it was the trimpot you added --that tube-based generator probably wants to drive a high impedance, and you helped with the 250k. OTOH --I could be wrong ;-). It wasn't the transistor swap, though --nothing wrong with the gain of a 5088 in that circuit.

-Eric