Footswitch-able FET Boost

[martin manning]

Circuit diagram and layout for adding a FET boost relay to ODS.

[erwin_ve]

Thanks Martin, this works great for not messing up the input impedance of the FET circuit!

[martin manning]

Norburybrook tested and approved too!

[norburybrook]

Indeed, the whole FET thing worked perfectly first time using Martins 18v battery method to set up the voltages, then the wiring for the footswitch/relay too.

I must say I'd be tempted to wire up a convention 4th gen ODS like this too and have a triple footswitch. It's a great addition to the amp. Only having the FET as a separate input doesn't really make it that usable live IMHO.


M

[martin manning]

My ODS doesn’t have switchable boost but I can see where that would make it much more usable. I did squirrel away an extra relay board, and built the footswitch with 3-buttons... I might just go ahead and convert it ;^)

[Aaron]

I like the FET with OD if using single coil pickups. Not as a boost, but I find it thickens up the sound.

Thanks,
Aaron

[rogb]

It's a must-have addition especially with a 3 button fsw!
I do get a fair bit of hiss if the gain is too high, so I wonder if it is wired up right?
It's on a Jborders board.
Great for goosing up a clean single-coil sound.

[dbeasley]

I recently replaced the FET circuit in one of my builds with a straight up EP3 Preamp booster and made this footswitchable, and I have to say love this setup so much more than the FET input.

The EP booster tends to have a natural EQ push around the 2khz region, and I've voiced it to naturally trim a little lowend so that when the OD mode gets boosted it doesn't make it flabby.

FWIW I want to say that a properly built FET dumble setup is a pretty flat boost.

Sent from my Moto G (4) using Tapatalk

[martin manning]

It is, but you can easily trim the low-end gain of the D FET boost by reducing the source resistor bypass cap.

[alkuz1961]

Martin tell me please, if I want to use in this place some kind of booster, like the Katana or the EP, how to power it? Is it possible to use a resistor divider as in the FET diagram (possibly with a voltage regulators), or it is desirable to use an additional source?

[dbeasley]

Martin tell me please, if I want to use in this place some kind of booster, like the Katana or the EP, how to power it? Is it possible to use a resistor divider as in the FET diagram (possibly with a voltage regulators), or it is desirable to use an additional source?

You can still use the divider setup, just be very mindful that it doesn't radically exceed 9v-12v on amp startup. It helps to use a standby switch here because unloaded B+ could exceed save voltage levels at the divider.


This is all speculation though. I cannot say with experience that this is true.

Sent from my Moto G (4) using Tapatalk

[martin manning]

The impedance of the FET supply is very high due to the large voltage drop required from B+5. This is not a problem if the current draw is low: it's only ~1mA for the standard FET boost, and maybe twice that for the two others you mentioned. You will of course need to adjust the voltage divider to get the right voltage for the circuit in use. If you use the Katana circuit, it's switching regulator will maintain a steady input voltage. If you power up without tubes in, the supply voltage might be almost 2x nominal, and could damage something. Coming off standby before the filaments are warmed up might also deliver a voltage spike, as mentioned above. An easy way around that is a simple Zener shunt regulator, and that will work for whatever boost circuit is used. Just choose a diode with breakdown a bit over the normal operating voltage.

[alkuz1961]

Thanks for the answers. I was kind of thinking about possible problems. Most likely I'm using zener as a protection against accidental voltage.

[Matt J]

Thanks for posting this diagram up!

I've started work on my #183 build and plan on having a three-button pedal with FET being footswitchable, so this certainly helps! I'm using one of Taylor's boards from Amplified Nation which correspond to these layouts seen at multiple places here on the Amp Garage.

Has anyone drawn up the proper way to wire the DIN plug and footswitch internals with this type of design? I'm a little unclear on what needs to go from there and wanted to research it and ask around before I did anymore soldering!

- Matt J.

[martin manning]

Please don’t repost the files above. It will lead to confusion as to which is the correct and/or latest version.

The foot switch wiring is like this:

Using a 5-pin DIN connector and moving the current limiting resistors to the foot switch enclosure, you can run three relays, leaving one pin for +12VDC and one for ground.

Connect an LED and a resistor in series from the +12V pin to one side of each switch.

Connect the relay pins to the same poles as the LED’s on each switch.

Put a cap across each switch if you like.

Connect the other pole of all three switches to the ground pin.