3 channel with integrate fx loop. Build from scratch

[Tiopepe]

Hi to all! After a longo time reading and finding right informatico, it's time to share what I learned.
I just want to share my latest build, my amp number 14 th.
This is really crazy (for me) because I didn't follow an existent schematic. My goal was to build an amp with 3 selectable individual channel, that goes from sparkly clean to screaming lead, to a bluesy/rock crunch. In addition I wanted a switchable built-in tube driven FX loop (a dumbleator). Finally I chose 2 EL34 for power amp section, slightly reduced in power using 36W OT and PT with 300-300V secondary.
So, as you can see in the block diagram, the first 12AX7 half is in common with all channels.
The CLEAN channel follows “5F6” topology, with only one channel, balanced to sound in between to “Bright” and “Normal".
The CRUNCH channel adds basically another gain stage, the cold clipper one, in the similar manner of “JCM”.
The signal goes to a cathode follower that drives a tone stack; this stage is in common between those 2 channels and finally there are 2 master volume, one for each channel that give an almost total independence of that different voices of this amp section.
The last channel is basically the SLO lead with some changes to sound less extreme. After the first half of 12AX7 in common, there is a second stage, a cold clipper, another gain stage before the cathode follower and the tone stack/master volume.
From the preamp output the signal goes to a mechanical DPDT switch to be sent to the tube drived FX loop that allo some regulations in Dumbleator style, or directly to the phase inverter.
The frequencies shapes of global negative feedback are regulated via “Presence” and “Depth” controls.
I wanted to build this in totally point-to-point wiring, military turret style. Altough I know that it would have been an “high gain” amp, I trusted in a properly grounding, properly wiring, expecially for filaments which if connected right, would not have required a DC power supply.
So you can see, I used one piece wire for each heater, straight and not twisted, one next to the other.
I used shielded cable, expecially in FX loop, that is extremely sensible circuit.
I installed 3 gorund point:
- preamp section in upper right side: here they come all the grounds of respective cathode resistors/RC, connected under the board from the one with the respective highest plate voltage to the lowest one (from B+3 to B+6). Furthermore grounds from pots, input jack and NC/NO relay switch.
- Power amp section in lower left side: here they come grounds from filter caps, in the same manner of preamp, highest to lowest (from B+1 to B+6), CT power transformer HV winding, output transformer secondary, bias circuit and grounding from elevated heater circuit.
- Near to power socket, the main earth.
The amp is very quite, despite the high gain, expecially in lead channel.
Now the real challenge was the switching system. I needed a mutually exclusive switch in order to select one of 3 different path of preamp section, one-at-a-time. I found the answer in a discrete CMOS CD4043 that is a set-reset Flip-Flop switching circuit. This is pretty simple, I located the little stripboard in vertical on the right side of chassis. In short, when we apply a tension to circuit with a momentary button, the CD4043 let flows the tension from one output (i.e Q1) and stops it from others (Q2 and Q3) and so on. I used the output voltage to activate a base of a PNP transistor and I used this to drive the relay as switch for the floating ground that will reach relay coil itself only when the button is pressed. The IC can handle up 4 output, and of course I used only 3 of this, Q1, Q2 and Q3.
Now, if you can see the block diagram, I have indicated 3 relay that shows their “Normally Closed” status.
So if we started from RL3, when we pressing the “LEAD” momentary button (that can flows the tension through Q3 of CD4043, activating the base of transistor that act as switch and connect the floating ground to RL3 coil), the relay is activated in “Normally Open” status: in the lead channel the grid of Cold Clipper is lifted from ground (RL3A) and the wiper of Lead Master Volume is connected to the mechanical DPDT switch that goes to FX loop or directly to phase inverter; the master volume wipers of clean and crunch channel are disconnected.
When we pressing the “CRUNCH” momentary button, it flows tension to CD4043 circuit, activating Q2 and deactivating Q1 and Q3. So RL3 can return in “Normally Closed” status and RL1-RL2 can enter in “Normally Open status”. The pin1 of crunch gain pot is so disconnected from the ground; the cathode follower is connected to crunch cold clipper (and disconnected from clean stage 2) and the wiper of crunch master volume is connected to mechanical DPDT switch via “Normally Closed” status of RL3B.
But what happens if we press the “CLEAN” button? The tension is applied to CD4043 circuit and Q1 is activated, Q2 and Q3 are deactivated. To switch independently and not sequentially from one channel to another I needed another relay that function as SPST switch and not correlated to the circuit preamp.
When activated, in “Normally Open” status, it disconnect the floating ground that goes from transistor activated by Q2 of CD4043, so RL1 and RL2 can returns in “Normally Closed” position, therefore, the Clean Channel.
I probably said things already known, and I am not a technician, just a doctor with a passion for electronics and lutherie.
I hope that my experience can be of help to those like me who want to build an amplifier with a "complete" sound