Solo boost / switchable FX loop

[tmac]

Hi!

I'm thinking about making a solo boost function on my Express 5:25, and and FX loop switch. So I would like some opinions on my thoughts about this. Obviously the solo boost should be remotely switchable (I will use a 12V relay), but I'll settle for a simple on/off switch on the FX loop for now. I think the solo boost will be a switchable global master volume pot.

First, some relevant info:

1) The "channel master volumes" are pre FX loop.

2) According to the manual, V3A is the last gain stage for both channels. Therefore, the output from V3A is attenuated, sent to the FX loop. The return signal is amplified in V3B according to the manual.

3) According to another thread here, there is a 47kOhm resistor between V3B grid and ground, and it was suggested as a good place to put a global master. But I cannot use this, since I will be bypassing V3B entirely when the FX loop is deactivated. I want the solo boost to work independently of the state of the FX loop.

* Notes on FX loop switch:

To turn the FX loop off, I should simply take the output from the plate coupling cap on V3A, and send it to wherever the plate coupling cap output of V3B goes. I'll worry about volume differences here some other time...

* Notes on solo boost:

Since I am also building an FX loop switch, I cannot use the V3B grid to ground resistor to do this. That would only be effective when the loop is not deactivated. So instead, I'll have to insert a proper switchable master volume at the point to which V3B plate cap output is connected. This will work independently of the state of the FX loop, since in both cases, the signal is connected to this volume control. I'll probably just use the same resistance volume pot as the individual master volumes, be it 500kOhm or 1MOhm.

I have some success with mechanical 12V relays in a homemade amp. I will use the already existing 12VDC in the Express to drive the relay(s). I'll also have to put in an external jack to control the relay, of course. I figure I'll have the global master enabled when the relay is not powered, since the solo boost will typically be used only once in a while. Most of the time the solo boost will be off (i.e. global master enabled), and the relay not powered.

Any opinions/improvements?

Torquil

 

[tmac]

Wow, I don't like PCB! Not only does it make it more difficult to modify, but it's also difficult to understand the circuit topology...

Per chance, does anyone have a schematic of the preamp?

Torquil

 

[tmac]

Ok, I think I know where the "bypass return" signal should return to the circuit, before the phase splitter. It's at the rectangular plate coupling cap of V3B, called C31 and it is next to the two 10uF vertical capacitors near V3. The bypassed signal should return to the circuit here, on the "front of amp" side of this cap, using a switch/relay.

But I'm not yet sure exactly where to extract the signal before the FX loop. For some reason the signal path is raised to 5V immediately after the master volumes. It is lowered again to 0V just before sending it to the FX loop. Any insights on this? I think the signal runs through the contour circuitry at this point.

I'm pretty sure the contour is after the master volumes, but before FX send. Has anyone confirmed this from listening to the amp. E.g. those that have used FX send into another amp/recording unit?

I don't want to extract the signal before the contour, because they would then be ineffective when the loop is bypassed. It might be easier to build a "FX loop and contour" bypass switch

EDIT: I'll just try to bypass both the contour and FX loop, since I think I already know how to do that and it seems to be simpler. Maybe bypassing the contour will be an improvement, compared with going through the contour circuitry, even when the contour switch is off.

Torquil

 

[Don]

I'm pretty sure the contour is after the master volumes, but before FX send.

This is correct.

 

[tmac]

I have bypassed the contour and loop. It works fine apart from a lack of volume. So there is some attenuation going on somewhere. I think the master volume pots are only 100kOhm and that may contribute, and/or it is in front of the phase splitter. I'm hoping it is mostly the last one, so that I only need to "tap in" closer to the phase splitter to fix it. We'll see.

Now I don't remember: Is the stock Express 5:25 able to produce distortion purely in the phase splitter and/or power tubes on the clean channel? I.e. turn the gain know low enough to get a clean sound when the master is low. Then turn the master up to get distortion, without touching the gain knob?

EDIT: Oh, I forgot: Thanks, Don!

Torquil

 

[Don]

The channel 1 Master pot is 250k linear taper and the channel 2 Master is 250k audio taper.

 

[tmac]

Conclusions:

I now know how to make a switchable solo bost. For both channels, the audio signal goes through the 475kOhm resistor close to the master volumes. So for me this is a good position to put a solo boost "global master" volume. Of course, one could have two independent master volumes, one for each channel, by just connecting them next to each individual master volume, but I don't know if this is necessary.

Description of the mod I tried: Bbypass the contour circuit and the FX loop. I did this by extracting the signal after the last gain stage V3A, and plugging it back in where the FX loop return V3B output drives the reverb and phase inverter.

Problems with this mod:

1) The signal strength driving the phase inverter becomes too weak. I tried to fix this by skipping/lowering some voltage divisor resistors, but I still haven't managed to fix the problem entirely. I did reduce the amount of attenuation caused by the voltage dividers, and it helped, but still I wasn't satisfactory.

2) The reverb becomes very weak! Even when the reverb is on maximum there is almost no reverb to be heard. On the clean channel I heard a faint reverb, but on the burn channel I heard nothing...

3) It would be complicated to make this mod switchable, since several resistors different places will have to be skipped/reduced, so several relays would be needed.

4) I "had" to skip the contour along with the FX loop, which was not what I intended. My original idea was to just skip the FX loop. This reduces some important versatility.

So in conclusion, I won't keep this mod, but I will use what I learned to make a solo boost switch instead. When I do, I will describe it here in this thread, with pictures.

EDIT: Actually, since I won't be bypassing the FX loop, I can even put the solo boost somewhere on the FX loop return circuitry, not necessarily near the pre FX loop master volumes. It could be like a remotely switchable "FX return level" knob. A phase inverter master volume is yet another possibility.

Best regards
Torquil

 

[tmac]

Let me wrap this thread up. I already have a simple footswitchable passive volume control unit that I can put in the FX loop. So I'll use that instead of modding the amp.

Instead I would like to modify the solo boost function in my AX84 amp. Compared to the Express, that amp is a pleasure to work with/modify, since it's non-PCB and therefore components can be moved around. Of course, if the Express were non-PCB it would be mighty expensive! Its circuitry is quite impressive...

Torquil