Implementing a silent mode DI on a tube bass amp

[briane]

you need to build in - say a 200 watt load dump.

do a search on resistive attenuator - remove the adjustable resistor.

wire to a plug that switches this in when speaker jack pulled. done.

[matt h]

(deleted)

[Gaz]

You wouldn't need a 200W load either way. If you really cut the input to the PI when in silent mode, then the power amp could safely idle with a much smaller 'dummy load'.

[xk49w]

Is there any real advantage to a reactive dummy load?

[EtherealWidow]

Absolutely. The frequency response is much closer to that of an actual speaker, since an actual speaker is reactive itself and doesn't have a constant impedance.

[eddie25]

Is there any real advantage to a reactive dummy load?

Good luck with this, the inductors needed are seemingly impossible to find. There was a thread on TGP where someone made one, but I think they ended up winding their own. I for one researched the creation of one of these to make my own Palmer style DI box, but ended up buying a Palmer which was very disappointing anyways IMO, but I think it would be MUCH more useful for your purposes (bass).

[martin manning]

I guess I could use a tube rectifier for the power tubes and a simple SS for the preamp. It's the large power draw from the power tubes that causes the rectifier to sag anyway, so having SS rectification for the preamp wouldn't change the sound much (I'm assuming).

In the usual set-up the preamp voltage sags along with the power amp because it is downstream on the same rail.

[matt h]

(deleted)

[jjman]

On the original question, the answer is yes if done properly. Most guitar amps have dc on their PI input grid but there is no dc on the input-grid to the PI on the scheme you linked to. If you are using the existing "Ext Amp" output you could probably replace it's jack with a multi-switching one that will also ground the input of the PI. The switch needs to also break the signal's path to the PI which is stock setup does not do. The PI grid must be grounded when you disconnect the signal wire from it so the grid does not "float up" and also allow noise in.

Mouser has ¼” jacks with multiple switching contacts. For xlr a separate multi-pole switch may be needed but maybe they have multi-switching xlr jacks too.

If you were tapping your “send” from the output tubes/tranny, things would be different.

[teemuk]

I guess it falls down to design goals. How much power amp / loudspeaker tone you really want captured?

If answer is "none" (or reasonably close to that) you can safely just mute the power amp's input (with somekind of a switching scheme) and it will protect the amplifier, even if its running to open load. (Because there is no input signal no harmful voltage potentials are generated at the plates). I would personally recommend using a resistive dummy load in addition, for extra safety, but with power amp input muted the dummy load doesn't have to dissipate high power (e.g. 2W power resistor is perfectly safe). Overdesigning naturally does no harm but high power resistors are somewhat more expensive. (For a DIY project this really isn't a big issue and I might even take that extra step).

If power amp and speaker sound is important part of your tone then you probably want to run the power amp to somekind of high power dummy load, attenuate the output and feed that to DI. And yes, definitely use a reactive dummy load for more realistic "simulation" of loudspeaker-amp -interaction. Somekind of a speaker simulation might also be needed.

Alternatively you might try something that fakes the power amp circuit with "reasonably close" approximation. But with this one you'd once again have to tackle a scheme to mute the power amp section and possibly provide dummy load for extra step of safety. Power amp emulation circuits are really different story, though, and naturally add even more complexity to the project. I don't know if it's worth to discuss that idea any further (it can be pretty extensive topic) unless you have interest on it.

[10thTx]

What about a PPIMV with fixed biased (vs. cathode biased)?

You can keep the speaker attached and just dial the PPIMV down to eliminate volume to the speaker.

Example to consider ..........

AND there is some info & schematic for fixed biased here:
http://www.el34world.com/Forum/index.ph ... #msg144178

With respect, 10thtx