Radial - Bigshot PB1 Class A Power Booster  [schematic]

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Radial - Bigshot PB1 Class A Power Booster

Postby Ripdivot » 23 Oct 2010, 18:03

This is a buffer / power booster that really sounds good. There is a mistake on the PCB with the labeling of the gain and drag pots. They are labeled backwards but I have labeled them properly on the photo showing the component values. Any questions just ask. Here is the write up from their web site:

"The Radial BigShot PB1 is a combination power booster and buffer designed to provide exceptional sonic performance for pedalboard users in a compact and simple package. It provides 15dB of boost, accessed using a guitar pick via a rear-mounted recessed control that is protected from foot stomp damage.
Unlike class-B preamps found in most pedals that color the tone and give buffers a bad name, the BigShot PB1 features 100% discreet class-A circuit topology for the utmost in sound quality. Class-A circuits are preferred due to reduced zero cross distortion, improved phase response and the resulting smooth Bessel curve of evenly cascading harmonics. This is augmented with Drag™ control, a variable load correction circuit that lets you dial in the perfect impedance to match the original tone and feel of your guitar and amp combination.

With the PB1, noise is reduced while performance is augmented. Long cable runs are finally possible. And when you hit the boost, you get smooth clean control like you never thought possible. The BigShot PB1 is likely the finest guitar power booster ever made."
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Re: Radial Bigshot PB1 Class A Power Booster

Postby analogguru » 24 Oct 2010, 00:38

Thanks for the guts.

Radial wrote:The BigShot PB1 is likely the finest guitar power booster ever made.

Sales lies.... I highly doubt that when they use a 78L08 with 9VDC supply voltage (to less for the droput voltage).

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Re: Radial Bigshot PB1 Class A Power Booster

Postby okgb » 24 Oct 2010, 03:06

presume the schematic will shed light on this ,m
is it great
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Re: Radial Bigshot PB1 Class A Power Booster

Postby Ripdivot » 24 Oct 2010, 04:07

I'm thinking the 78L08 might be for over voltage protection. Radial has some pedals that use 12 and 15 volt adaptors although if it were designed properly those voltages shouldn't be a problem anyway. Anyway it is a really nice sounding booster.
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Re: Radial Bigshot PB1 Class A Power Booster

Postby Ripdivot » 24 Oct 2010, 22:23

I started tracing this today. Don't have too much time to spend on it but hopefully I will have a preliminary schematic in a few days. So far there is some weirdness going on...
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Re: Radial Bigshot PB1 Class A Power Booster  [schematic]

Postby Ripdivot » 25 Oct 2010, 05:16

Here is a hand drawn schematic. It came together quicker than I thought it would. I have checked it over many times so I hope it is correct. I will cad it up at a later date. If someone wants to re-draw it go ahead. This is a strange circuit to me or maybe it is just the way I drew it. It is definately different than the usual buffer/booster pedals I have seen.
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Re: Radial Bigshot PB1 Class A Power Booster

Postby Ripdivot » 27 Oct 2010, 04:19

OK, I don't get it. How is the jfet being biased. I see no DC path to the jfet other than through Q5 (2n5087). Are Q4 and Q5 some kind of weird darlington arrangement? Also the booster section is hard to figure out. I checked my trace over again and couldn't find any mistakes but that doesn't mean there aren't any. I'm wondering if AG or Baga or someone could explain this circuit?
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Re: Radial Bigshot PB1 Class A Power Booster

Postby RnFR » 27 Oct 2010, 05:25

did you check the datasheet for the fet? it looks as if it would make more sense if the drain and source were flipped. then you would have something similar to a complementary feedback amplifier. it seems to me that the drag pot controls the bias. the second stage looks similar to some push/pull output stages I've seen. not exactly sure there, though.
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Re: Radial Bigshot PB1 Class A Power Booster

Postby Ripdivot » 27 Oct 2010, 05:38

ya I thought the same thing with the PN4393 source and drain. the data sheet says it is DSG looking at it from the front (flat side) with the pins facing down. so if that is the case i think i have it correct. as soon as i get time i will check the whole circuit over again. i have already done so a half dozen times already and found a few small errors but haven't been able to find any more.
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Re: Radial Bigshot PB1 Class A Power Booster

Postby RnFR » 27 Oct 2010, 05:43

looking at this some more I'm starting to really think that is the case. different manufacturers have been known to put different pinouts for the same device. I think the bs170 suffers from this. if it was flipped it would look a hell of a lot like a comp f/b pair.
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Re: Radial - Bigshot PB1 Class A Power Booster

Postby clintrubber » 16 Apr 2018, 13:20

Interesting they use dual pots while the schematic doesn't require them....

possibly duals for mechanical reasons ?

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Re: Radial - Bigshot PB1 Class A Power Booster

Postby bmxguitarsbmx » 18 Apr 2018, 07:59

Q2,Q3,Q1 are a discrete op-amp.
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