Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cboysen on July 19, 2018, 10:58:46 am
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So I built myself a cool sounding ab763! I promise to make pictures of the whole project some day soon.
Unfortunately I find myself with a pot hole too many.. and now I don't know what to put there..
I could be a simple guy, and just add a dwell knob, but what's the fun in that?
Then I had this weird thought of splitting the signal. My idea is to use a 1Mohm Linear potentiometer to rotate between two pathways - One pathway going through the reverb circuit, the driver (V2), the tank and recovery (V3B) stage, while the other pathway bypasses the whole reverb section, and goes straight to V3A.
The idea is to get a more direct signal when the pot is counter clockwise, slowly blending in the reverb stage turning the pot clockwise, until the amp is "stock" when turned fully clockwise.
Looking at the schematics, the signal would be able to run "backwards" through the potentiometer, which could cause some sort of feedback loop, but I wonder if this is just theoretical or a real issue?
What do you guys think - Would it work?
Kind regards Christian
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Yes it would, it's a mixer setup. I am not sure the optimal pot for this use, but you may also still want a few m ixer resistors inline to keep a minimum of balance between the two paths so you keep things under control. This is a very typical type of circuit used for what's a 'balance' pot in stereo amplifiers, but you're using it to manage wet and dry signals between the reverb and the original channel.
~Phil
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Sometimes you just have to try and see or hear. If I was just trying to fill a pot hole I'd replace R9 with a 100K (or 250K) pot wired as a variable resistor. Call it RAW. I'm pretty sure you would like that.
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What do '1 2 3' on the schematic represent -terminals of the 1M pot?
The 1st thing I'd look to add is a midrange control :think1:
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What do '1 2 3' on the schematic represent -terminals of the 1M pot?
The 1st thing I'd look to add is a midrange control :think1:
Yes, it was supposed to represent the pot pin numbers, Sorry for that.
Yes it would, it's a mixer setup. I am not sure the optimal pot for this use, but you may also still want a few m ixer resistors inline to keep a minimum of balance between the two paths so you keep things under control. This is a very typical type of circuit used for what's a 'balance' pot in stereo amplifiers, but you're using it to manage wet and dry signals between the reverb and the original channel.
~Phil
Cool! I'll try it out!
Sometimes you just have to try and see or hear. If I was just trying to fill a pot hole I'd replace R9 with a 100K (or 250K) pot wired as a variable resistor. Call it RAW. I'm pretty sure you would like that.
Actually, the amp turned out more than great. I am so grateful for all the support and design plans! I hope to give back, as much as I can!
I actually already have a 25kOhm Mid pot, that is more than enough. and a 3 way switch, between normal, bypassed tonestack, and semi-bypassed (100kOhm resistor). And a 3-way NFB loop. The 3-way NFB is especially handy for that tweed clarity, and the bypassed tone stack sends you right to dirty tweed territory.
So the front panel looks - Input, vol, treb, mid, bass, "??" , Reverb, Master, On/off
This LP + Tweed Ab763 amp is my favourite grab and go combo, and it takes pedals so well!
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Well done!
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Yes, nice work :icon_biggrin:
I don't think that the pot mix control would work well without further mods, as the pot track would be in parallel with the 3M3 resistor, greatly reducing the total effective resistance.
How about a reverb dwell control (eg replace the 1M reverb driver grid leak with a pot)?