GPM 45 Resistors

Get support and show off your MetroAmp JTM 45 kit builds.

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Silmaril
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GPM 45 Resistors

Post by Silmaril » Tue Sep 04, 2018 2:59 am

I found this Picture , as I understood the one who post it, it is a GPM45. I see nearly Allen Bradley in all positions except for 3 around the bias trimmer. I wonder what kind of resistors these are. Seems like 1 Watt Metal Film or Metal oxide?
And has it a purpose to not use Allen Bradley in these positions, like to reduce noise?

danman
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Re: GPM 45 Resistors

Post by danman » Tue Sep 04, 2018 7:45 pm

There are some 1 watt and 2 watt AB resistors used as the B+ droppers. These need to be at least 1 watt in this position. I also see some 1/2 or 1 watt carbon film (I believe) in the bias circuit. They are providing bias voltage and have no effect on the signal path.

RockinRocket
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Re: GPM 45 Resistors

Post by RockinRocket » Tue Sep 04, 2018 8:34 pm

Your right, some sort of metal film or oxided. Probably selected because they blend in visually but used for reliability I would assume why George used these.

I do believe the 56k CC in this position would contribute to some tone as a good amount of voltage on is present.

Silmaril
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Re: GPM 45 Resistors

Post by Silmaril » Fri Sep 07, 2018 5:58 pm

In the normal JTM 45 Kit from Metro there were optional Allen B. CCs suggested for resitors on the signal path. Compared to the Metro GPM45 on the Picture, did the other CCs that arent on the signalpath have an influence on the tone or were they used for the vintage look only?

Sorry if my english is bad.

RockinRocket
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Re: GPM 45 Resistors

Post by RockinRocket » Sat Sep 08, 2018 11:26 pm

George uses AB CC resistors for what ever reason but they are not historically correct for a 45.
MArshall may have used all CC on the first (10? prototypes) but later mover to Piher resistors. You can still find these but are not cheap. Wazzgrover has them on ebay.

A few CC made it in to Marshall after the prototypes because of the length of the lead on the cathodes ect

If you have never built a amp Id just use cheap stuff because you will learn a crazy amounts of what not to do on your first build.

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