Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: GlideOn on February 07, 2025, 08:26:45 am
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Have a Single Ended with parallel EL34B power section based on AX84 SEL amp. Marshall JMP style currently. I recently put in a V3 tube for a faux PI section that gains presence and NFB which worked better than I had hoped, but now I'm looking to utilize the other unused side of the V3 triode. I love distortion I guess. I decided to emulate the Soldano Avenger circuit with the extra first gain stage, extremely cold 39k clipper and cathode follower.
But I'm wanting to utilize more than one voice. As you may know, the Soldano Avenger does not have a dedicated Clean channel, it copies the Marshall JMP and makes a Low input, skipping a gain stage. Not a fan of the volume drop, but I rather like the tone with a clean boost so I'm happy to utilize this.
We have the age-old issue of High Input and Low input when the low skips a gain stage (JMP, Soldano HotRod, Avenger, etc) and they are not only out of phase, but only one can be used at a time using the stock configuration.
I plan on using a few tricks to mitigate this:
Made up some crude schematics to illustrate - forgive me this isn't my strong suit - but the basic premise is this.
- Use JHS Mini AB switcher, split the signals into stereo instrument cable.
- Switchcraft 14B input jack to accommodate two switched signals into amp.
- Separate gain pot for Low input.
So far so good, but the puzzling part is where the signals are potentially summed.
First the "High," which goes to the first gain triode via small grid stopper (68k, 33k etc). Generates gain, goes out via the anode coupling cap into gain pot input, goes out from gain pot to grid stopper into second gain triode. Shorts to ground when unplugged via 1M resistor so the V1 grid is happy. Pretty standard stuff.
But the "Low" input goes directly to its' own gain pot; specifically in this instance a shared stereo pot I special ordered from TAD Germany - 1M stacked pot with push/pull - from gain pot goes into same second triode the High Input is connected to. So it is skipping a gain stage, out of phase with the High signal.
The normal second triode's input grid stopper in this Soldano design I have is a 470k.
To sum two signals while mitigating "channel bleed," one would use two grid stoppers to form a "mixer," no?
Except a mixer is usually a voltage divider where the other end of the signal that is currently not being used gets sent to ground. Not necessarily looking to do this though.
I wrote up 3 different styles.
One sums the signals into a single shared 470k. Least confident in this one.
Second uses dual 470k to form a mixer/voltage divider.
Thurs uses dual 270k to form a mixer/voltage divider.
If mixers are the way to sum these two signals, what are the side effects of doing so?
Can I compensate for this later in the circuit?
Or do I have a solution present and/or close to one?
Again, drawing schematics isn't something I'm experienced with, I rely mostly on my DIY Creator layout to convey my thoughts which I've also included. Tried to scale it 1:1 as much as possible.
LMK your thoughts.
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Bump!
Any thoughts?
I think it's most likely going to need independent mixers per signal, or add a summing/mixer resistor from the Low gain pot's lead.
Should then the following voltage divider stage be changed to compensate?