NFB - on/off on a foot switch

[angelodp]

Hi, any issues in using a switched jack and foot switch to have NFB on a foot switch? It would be all or nothing.

A

[pompeiisneaks]

I think the NFB on a switch will almost always pop due to DC being on the pot, but you can put large value resistors between the leads to give it somewhere to discharge...

Usually it's something you either want on or off for a specific tone and often isn't needed mid song anyway, so the pop isn't a big deal but then also footswitchable becomes less important too.

~Phil

[angelodp]

Ok, yeah, I was thinking it acts as a boost on this amp I am working on, and can be a route to lead tone.

I guess I better use isolation washers for the jack.


Thanks

[maxkracht]

I might be missing something, but you could put a cap in line to block the DC? Not sure if a long cable run in a NFB loop would add noise. I think about half the time I bother with shielding when doing a mod for someone, never noticed a difference, but no idea if long runs would add problems. I think in the valve wizard preamp book he references something to the effect of, "arbitrarily large" caps to block DC in any NFB tone shaping. 100uf to get out of the audible range? Please correct me if i'm misunderstanding.

[Stevem]

I would not want to run NFB up and back on what would need to be atleast 12 ft of wire!!

I would get a add on relay or opto kit to do the switching within the amp.

[pompeiisneaks]

I might be missing something, but you could put a cap in line to block the DC? Not sure if a long cable run in a NFB loop would add noise. I think about half the time I bother with shielding when doing a mod for someone, never noticed a difference, but no idea if long runs would add problems. I think in the valve wizard preamp book he references something to the effect of, "arbitrarily large" caps to block DC in any NFB tone shaping. 100uf to get out of the audible range? Please correct me if i'm misunderstanding.

There's nothing wrong with the reasoning there. I think the reason most people don't worry about it is presence for an amp is quite often 'set it and forget it' so nobody's worried about a scratching pot occasionally while setting presence on an amp.

Using the on/off switch, on the other hand does imply that your idea may be worth the effort to remove the DC, but yes it would need to be pretty large, I think 100 or 250 uF were used on bassmans cathode bypass caps to not impact the tone of the bass side of things but I think even around 20uF is good enough for guitar frequencies as I recall.

For a one of thing that's probably not a big deal at all, for mass produced amps, the cost was probably never worth it for somewhat of a 'one trick pony' setup.

~Phil

[fred.violleau]

Hey Angelodp,

That is part of my plans as well on my next TD build.
I will use a relay to switch between the settings of the NFB switch I added on the first build. It should allows the amp to go from clean to mean, without changing the settings.

I would also add another relay to have the cascading gain switch under my foot so adding a small transformer with a PS boards makes senses for these two relays

My 2 cents here!

Fred.

[roberto]

Hi Fred,

I would look for a small SMPS board, if I were you. Simple and cheap.

[angelodp]

Hi Fred, ok so down the relay road sounds plausible, works great on Dumble amps. Be great to see how you configure it all with power supply and relays.

Best A

[angelodp]

Roberto, SMPS can you describe what that is please.

A

[angelodp]

Like what we used on the D-lite's

[pompeiisneaks]

smps means switch mode power supply. I don't think you'd want one in a tube amp. they work in really high frequencies but might not be the best for tubes, I think they prefer linear power supplies no?

Could be mistaken.

~Phil

[Heikki]

smps means switch mode power supply. I don't think you'd want one in a tube amp. they work in really high frequencies but might not be the best for tubes, I think they prefer linear power supplies no?

Tubes don't care if you make DC from 50 Hz or 200 kHz. If I want DC supply for heaters I almost always use SMPS.

[roberto]

This is what bogner amps do for heaters:
https://www.rsdsound.co.uk/product/repl ... ner-shiva/

Close view of the photo:
https://www.rsdsound.co.uk/wp-content/u ... .08.59.png

PS
You can find the exact same SMPS for ten times less.

[pompeiisneaks]

smps means switch mode power supply. I don't think you'd want one in a tube amp. they work in really high frequencies but might not be the best for tubes, I think they prefer linear power supplies no?

Tubes don't care if you make DC from 50 Hz or 200 kHz. If I want DC supply for heaters I almost always use SMPS.

Oh gotcha, guess I was thinking B+ but again, I made it pretty clear I was asking a question about something I wasn't very sure of

So for DC heaters that totally makes sense.

I also don't think I've seen commercially available SMPS for 450VDC Also, though, I don't know enough about SMPS to know if using dropper resistors on a B+ line, if you're using them, were to cause it's feedback to try to raise the voltage?

You're making me curious if we could fashion some kind of SMPS for B+ now I know they are way less expensive, but just don't know why people haven't used them for B+? Just no voltage regulators for that high voltage I guess? Or am I even more out of my depth than I think