WEM, Watkins, Rush - Pepbox, Rush Pep Box  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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Post by analogguru »

Electric Warrior wrote:That's how they did it. Vintage units are different. The grey wedge ones don't have a trimmer.
Thats BS, have a closer look: there is a slider-trimmer inside.

The 100 Ohm resistor is between emitter of the first transistor and ground for temperature stabilization.

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Post by Electric Warrior »

Image so that's what that thing is...

So in the later units the trim pot replaces the 56k resistor and in the earlier ones it serves a different purpose?

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Post by phibes »

The trimmer in the JMI does look like a feedback resistor from Q1B to Q2E. Would also make sense with the text fuzz gain written next to it. Probably 100K I'd presume. The trimmer in the later red box units controls the resistance to Q1's collector. I just glanced at the gray wedge photos on the database and it looks like there's five resistors and a trimmer compared to the later four resistors and a trimmer. My guess is there was a feedback resistor in the wedges. To my failing eyes and guess it looks like 100R, 1K5, 10K, 100K and 1M and the trimmer is probably on Q1's Collector like in the later models. With the small photo though those are just guesses.

AG's right on the 100R placement.
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Post by alnico »

I agree.

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Post by phibes »

To keep discussion rollin, here's how I see the studio 2. I'm not sure on the value of the cap running between collectors but it has to be higher than 5pf. My guess is 47pf-100pf. The 5 might of designated 50.
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Post by Electric Warrior »

phibes wrote:To keep discussion rollin, here's how I see the studio 2. I'm not sure on the value of the cap running between collectors but it has to be higher than 5pf. My guess is 47pf-100pf. The 5 might of designated 50.
Or 5 is not the value, but the tolerance. I'd expect the value to be in pf or nf. But pf*10? :shock: :scratch:

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Post by phibes »

Could be the tolerance. It could really be anything with old caps.

Where you getting 10pf from?
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Post by Electric Warrior »

not 10pf. Value in pf multiplied by 10. I don't think that's likely. :wink:

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Post by phibes »

Tolerance makes sense. Anything from 50-100pf would work. If you want to knock off more edge you could go higher.
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Post by mictester »

phibes wrote:Tolerance makes sense. Anything from 50-100pf would work. If you want to knock off more edge you could go higher.
Correct, but 5% tolerance caps were not common (and very expensive) in the 1960s. The WEM box I've had here was fitted with a 180 pF polystyrene capacitor (which measured 210 pF out of circuit), and another one from input to ground (presumably to keep RF out of the circuit). The feedback / bias resistor was 68k.
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Post by phibes »

Which WEM Box did you have?
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Post by Electric Warrior »

Turns out there was a germanium version as well:
http://cgi.ebay.de/Vintage-Rare-1960-We ... 3a6073de23

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Post by RnFR »

nice. can someone save the pics from the auction and upload them? not sure how to do that with ebay's new funky photo software.
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Post by jrod »

:thumbsup
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5.png
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2.png
1.png

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Post by Solidhex »

That looks like a Fz-1 without the input buffer, You got the 470k and the 1k5 on the first transistor. The Fz-1's attack control when turned all the way up is 50K in parallel with 22k in series with 2k2 (17.47K) hence the 16k resistor to ground on the base. The second transistor has different biasing on the base but still has the 10k and grounded emitter. Finally you have the .0033uf output cap and 56K in parallel with the output volume control. I would guess the fuzz pot to be somewhere between 50K-100K. Doesn't run on 3 volts apparently though. The Fz-1 has 20uf caps between the stages. I see a "25" on one and they're both electro caps so unless the "25" is the voltage there's a chance both caps are 25uf. I would guess the output pot is a 500k. Its dominated by the 56k anyways. Nice work guys!

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Post by jrod »

That pot on Q2's base is really odd. Kind of reminds me of the Selmer Buzz Tone.

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Post by Electric Warrior »

Solidhex wrote: I see a "25" on one and they're both electro caps so unless the "25" is the voltage there's a chance both caps are 25uf.

--Brad
Sure looks like the Voltage to me...
We can't be sure about their value. They're 1µF in the FZ-1A

I wonder if the silicon ones are actually based on the same topology. The parts layout is the same :hmmm:

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Post by Solidhex »

Electric Warrior wrote:
Solidhex wrote: I see a "25" on one and they're both electro caps so unless the "25" is the voltage there's a chance both caps are 25uf.

--Brad
Sure looks like the Voltage to me...
We can't be sure about their value. They're 1µF in the FZ-1A

I wonder if the silicon ones are actually based on the same topology. The parts layout is the same :hmmm:
The Fz-1a uses 1uf capacitors but this pedal is closer to the Fz-1, the earlier 3 volt version which has 20uf caps in those positions. Of course we can't know for sure but if we are stuck with making assumptions its a possibility. 25uf is crazy big but it wouldn't be the first time. The Selmer Buzztone has some 50uf caps in the signal path!

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Post by RnFR »

phibes wrote:To keep discussion rollin, here's how I see the studio 2. I'm not sure on the value of the cap running between collectors but it has to be higher than 5pf. My guess is 47pf-100pf. The 5 might of designated 50.
YAFF!
jrod wrote:That pot on Q2's base is really odd. Kind of reminds me of the Selmer Buzz Tone.
kind of, but the buzz tone just has some series resistance. it looks like this thing actually affects the bias.
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Post by Electric Warrior »

Solidhex wrote: The Fz-1a uses 1uf capacitors but this pedal is closer to the Fz-1, the earlier 3 volt version which has 20uf caps in those positions. Of course we can't know for sure but if we are stuck with making assumptions its a possibility. 25uf is crazy big but it wouldn't be the first time. The Selmer Buzztone has some 50uf caps in the signal path!

--Brad
And of course the Fz-1A wasn't available in 1965. :mrgreen:
The silicon version of the Pepbox used 12.5µF electrolytics - another possibility.

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