Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: prongster on September 15, 2021, 01:39:40 pm
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Hello there, this is my first post here:
In adding a mid pot to an AB763 DR clone, is there any effective difference between replacing the 6800K resistor with a pot, and wiring the pot in as it is AB763 amps that come with a mid pot? Seems like a much easier wiring job to just replace the resistor with a pot than to run the blue wire from the mid cap to the pot, then into the bass pot as it's done in the SR and Twin.
Thanks
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I think you are asking the same question in two different ways? Replacing the 6.8k resistor with a 10K pot is the correct way to do it, which is exactly what Fender did for the big amps. If you’re asking about layout, I’d follow the the super or twin layout to see how it’s wired.
Another option is keep the 6.8K in circuit and do a 25k mid pot in series. At minimum setting it’s stock. Up to 25K you get more mids and gain. It’s often called a raw pot.
https://robrobinette.com/images/Guitar/Marshall/RR2104_Master_Volume_Micro/RR2104_Master_Volume_6V6_No_Bias_Tap_Layout.pdf
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Well specifically, I'm asking if there's any circuit difference between these two layout scenarios:
1. running a wire from where the blue wire connects to the bass pot over to the middle lug of the newly mounted mid pot and back to the ground bus from left most lug as seen from the back (this is just replacing the resistor with the pot).
2. running the blue wire from the mid cap over to the new mid pot, connecting the middle lug of the mid pot to the left lug of the bass pot, and grounding the left lug of the mid pot (ie like it's done in the big amps).
It seems to me that these are effectively the same electrically, but #1 uses 1 less wire, which is more desirable.
My question is: is there no real functional difference between these two? IE, do you lose anything by using the simpler wiring?
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1 and 2 are electrically and functionally identical.
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Awesome thanks.
I want to use RG174 for the connections to the pot - which ends are the best to attach the shield please?
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It seems that you are way overthinking this.
You don't need any RG-174. Just remove the 6.8K resistor and connect one short wire between the Bass pot and Mid pot. Then ground one lug on the Mid pot.
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What Steve says..I have done this several times. Works just fine.
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I assume from the RG174 he’s putting the pot somewhere else. Like a ground switch hole or something. I did this on my 1164 Princeton. Grounded the shielding at the preamp ground. Worked great. No added noise.
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I assume from the RG174 he’s putting the pot somewhere else. Like a ground switch hole or something. I did this on my 1164 Princeton. Grounded the shielding at the preamp ground. Worked great. No added noise.
Makes sense. Maybe he could tell us exactly what he's doing if he needs a precise answer. :wink:
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Yes, it's going in the ground switch hole for the time being, so the wire will be run across the chassis. I thought that was implied but obviously not - my bad.
Thanks so much for the replies - much appreciated!
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In that case, I would use shielded cable.