Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on December 04, 2022, 05:08:29 pm
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What changes would l made to this schematic to change to a Plexi 6v6 presence, 5kl pot ?
If l eliminated the 4k7, R26 by taking out the resistor, adding a 5kl presence pot replacing (R25) and connect a .1 uf to wiper and ground? Would taking that 4k7 out completely cause any issues. This is a PCB bd so a little tricky changing traces.
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Dude! Show us the entire power amp!
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I suggest to replace the 7k5 with a 10k lin pot wired as a variable resistor.
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I suggest to replace the 7k5 with a 10k lin pot wired as a variable resistor.
This is the same as BillM suggests, this is a Blues Junior. I was wondering if the knob would react different if wired as l mentioned like Sluckey posted above.
Here’s more of the schematic
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Yes, there was a recent thread on the amp garage where plot sims of the 2 arrangements were posted.
The variable resistor provides varying degrees of boost at a fixed turnover frequency.
Whereas the vintage type pot sluckey posted provides varying degrees of boost with a turnover frequency that reduces as more boost is applied.
Turned up full, they’re both the same, obviously.
https://ampgarage.com/forum/viewtopic.php?p=446018#p446018
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Yes, there was a recent thread on the amp garage where plot sims of the 2 arrangements were posted.
Thanks for the link, just been reading about the BillM presence mod, some like it some say it doesn't do much. I like the vintage Plexi presence in my 6V6 Plexi, works well, never all on or off.. The Blues Junior III (cream bd), being PCB I get one or the other, not going to take that board out again but lifting a cap and resistor can be done with out too much difficulty to add the Plexi presence, I would assume the feeding R from the OT, 8 ohm tap being 75 ohms might need to be lowered to say 33K, along with a 5KL pot and .1 uf ...?
Appreciate the advice as always.
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FYI, counterintuitively, too much nfb can result in pfb.
BJs often crop up with stability issues, so in the first instance at least, leave that stuff alone.
One change at a time is the preferred MO.
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FYI, counterintuitively, too much nfb can result in pfb.
BJs often crop up with stability issues, so in the first instance at least, leave that stuff alone.
One change at a time is the preferred MO.
I totally hear you, a friends amp. There are a few mods that seem worthwhile but just replacing that 4K5 R with a variable R pot has mixed results. Playing the amp stock, cream board 2003, it's certainly not a Fender sounding Deluxe type of amp to me. Besides a few coupling and tone caps (bridge the mid pot mod) and HT filters too, I feel a Presence set up like the Plexi would benefit the amp a lot. Don't like drilling a hole in the Face Plate but seems easy enough for me.
If it were my amp, I'd do Doug's new PTP board here or just get an amp that doesn't need all these changes.