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

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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby drewl » 27 Feb 2010, 03:17

Check this one out, looks a little different layout wise.


http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... ink:top:en
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 27 Feb 2010, 12:21

Compare to the pictures I linked in the very first post on this thread :wink:
It has a 56k resistor instead of the trim pot. That's why I concluded that the trimp pot should be 100k.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 01 Mar 2010, 02:23

Electric Warrior wrote:nice! I presume the cap between the transistors was put there to prevent oscillation.

There's a 5 printed onto one of them in the pictures. Could that possibly mean it's a 5pF cap? Seems rather small to me..


I just noticed the 90's Tone Bender reissue has a 10pf cap in the very same spot, so 5pf probably is probably correct.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby drewl » 02 Mar 2010, 21:51

So why do you think they used silicon NPN's in this when all the contemporaries were using PNP germaniums?
Ahead of the curve? he knew they were more stable?
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 02 Mar 2010, 22:29

Well, somebody had to be first.. Maybe they just didn't want to cheap out on transistors or top out their competitors. It was probably common knowledge that silicons don't have the temperature issues that germaniums have.

If my schematic is correct, they were way ahead of the curve with this one. The cap across the transistor and the trimmer are just like today's "boutique" mods.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby yeeshkul » 02 Jun 2010, 20:38

The 5p cap is a standard way to get rid of fizzing trebles in gain stages in general. I believe it was a well known trick at that time.
Thanks for the schematics, it looks interesting.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 03 Jun 2010, 15:12

just keep in mind that it's still unconfirmed :wink:
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby mictester » 03 Jun 2010, 17:33

Electric Warrior wrote:just keep in mind that it's still unconfirmed :wink:


Is the 1M correct in the collector of the first transistor? Seems a bit high to me.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 03 Jun 2010, 21:04

That's the problem: I don't know. Would swapping the 1M and 1k5 resistors make more sense?
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby phibes » 03 Jun 2010, 21:21

My guess would be that the 1M resistor goes from Collector to Base on Q1. The 12.5uF on lug 2 of the Pep pot should also probably go to ground instead of meeting with the 100R resistor.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 03 Jun 2010, 22:44

I agree about the elctrolytic - that would be just like in a Vox Tone Bender.
Not so sure about the 1M resistor, though. Do you think there's resistor from Q1's collector to 9V? or a direct connection?
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 08 Jun 2010, 01:42

There's pictures of the JMI clone: http://www.jmiamplification.com/EFFECTS/stud2.html

The 1k5 does connect with the battery, indeed. I'm gonna rethink everything tomorrow.. probably not a single resistor value right :lol:
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby phibes » 08 Jun 2010, 07:12

I drew something up the other day but never finished and it looks like I wasn't too far off so I'll attach it. Where the 100K trimmer is I meant to right in the 56k on my schematic (not sure why I was thinking 68K). The only thing that I can see so far that looks different is where I placed the 1M resistor. Looks like it just goes from input to ground in the JMI shots. The trimmer looks like it goes from from Q1's collector to base. You can hack this apart if you want and put the right values in. Looks like this one can go to bed.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 08 Jun 2010, 07:45

That looks about right. I agree about the 1M resistor. I'm not sure what to do with the 100Ω. I looks like it might connect to Q1's emitter and ground.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 08 Jun 2010, 12:42

:scratch: it would work with the later circuit board, but not in an obvious way. What the hell were they thinking? Tr1 is Q2?

circuit3.jpg
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 08 Jun 2010, 17:17

here's the schematic:

WEM Rush Pep Box.png
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby alnico » 08 Jun 2010, 17:25

From the JMI pics, it appears that Q1's collector resistor is 56K and the trimmer is adjusting the DC feedback from Q2's emitter into the base of Q1?
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 08 Jun 2010, 17:51

That's how they did it. Vintage units are different. The grey wedge ones don't have a trimmer. In the later, larger boxes it replaces the 56k resistor.
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby alnico » 08 Jun 2010, 17:56

No feedback resistor then? :scratch:
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Re: WEM, Watkins, Rush - Pepbox, Rush Pep Box

Postby Electric Warrior » 08 Jun 2010, 17:58

We can't be sure. The 100Ω resistor could be a feedback resistor. What do you think?
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