Switchable resistance from gain / volume pot to ground for lead boost?

[walkman]

Hi all,

Wondering if the following will work or is not worth doing.

I want to make a passive boost for lead / solo lines for a simple el84 champ style amp. The amp already has switchable cathode caps for both halves of the 12ax7 preamp tube. These make some difference to tone and volume however not really enough for a lead / solo level boost to compensate for the volume of strum chords vs lead playing.

The idea would be to place a switch and /or switching jack for a foot switch followed by a pot for boost adjustment inline with the current gain / volume pot ground connection.

As the current volume / gain pot is 500k another 500k  switched inline to ground should be ok. It doesn’t matter if the overdriven tone changes as I’m not looking for a clean boost. Being more overdrive than the rhythm tone is preferable.

I can see that there may be a couple of issues.

Firstly a switching jack may fail then cause the tube to have no path to ground. As there is no other path to ground at this point prior to the first 500k vol/gain pot this could lead to runaway and destruction of the valve. An additional 1M to ground would lower the overall level out before any boost and is not what I’m looking for.

Secondly there may be some popping or other noise, (there should not be any DC level changing by switching ) though there could be some other reason I can think of for it to happen.

Thirdly noise may be introduced through the use of a switching jack and foot switch and the path to ground travelling through the foot switch lead and back to ground in the ground in the amp chassis. again I would think that as the noise could be introduced in the path to ground it is likely not to affect the signal , though again I may be overlooking something.

Has anyone one rigged up something like this?

Hoping to keep it passive and simple. There is no need for the switch and /or switching jack to work in tandem / influence the state of each other ( eg. if the there is a switch to be mounted on the control panel and switched to boost it can override the foot switch position or visa versa). If it is simpler to only have a foot switch or a control switch but not both without the use of relays then the preference is for a foot switch.Happy to set a fixed boost level with a resistor if pot introduces some complication, though do not see why that should be the case.

Belated happy new year to all.
Walkman

[Bieworm]

my advice would be that you try using the guitar volume pot for the boost. Crank the amp and roll of the volume on the guitar. If there is too much loss of highs then you can add a treble cap on the volume pot ...

[chocopower]

Hi.


I had done it and works "kind off".
I put a pot wired like a variable resistor in the bottom of the standard vol pot and use a pedal to short to ground. At mediun-low levels in the standard volumen work quite well. With volume pot over 6-7, dont do a lot.


My suggestion, don use a pot. Just wire a 1M resistor and switch it in and out with the pedal.
You even dont need a relay.


Mesa Boogie did it in some amps. Can't remenber the model now.
If i find it ill post it.

[PRR]

> could lead to runaway and destruction of the valve.

A 12AX7 fed from 300V is not going to melt-down. Especially with a 50+K resistor in series.

I think running volume-pot audio across the floor to a stomp switch is begging for hum/buzz, but maybe I'm missing the picture.

[chocopower]

In my amp, low gain, with switch open (boost on), some noise is added,  but a lot less than a standard booster or overdrive pedal.


I think, correct me if I'm wrong, that Dr Z amps do something similar, but adding resistance to the bottom of tonestacks. In fact he make it variable, with a pot in the remote pedal itself.

[jjasilli]

I think, correct me if I'm wrong, that Dr Z amps do something similar, but adding resistance to the bottom of tonestacks. In fact he make it variable, with a pot in the remote pedal itself.

This is basically a RAW control.  Works well inside the amp.  Be aware that added R under the tonestack tends to increase mids to "boost" volume.  Nothing wrong with that.


As PRR says, placing the control outside the amp may lead to noise issues.  This can vary from venue to venue.  One solution is to use a relay operated by a footswitch.  That way only relay power, not signal, leaves the protection of the chassis.

[walkman]

Thanks for all the replies...

Seems that the simplest thing then is the ‘raw control’ /‘raw switch’ approach without a foot switch. I will see how this works for the amp.

[chrisd]

Following - I've got an amp with a switchable tone stack that does a similar thing, and I'd like to put it on a foot switch so that I can (when I'm feeling lazy at least), gig without a pedalboard :)

Could you improve the noise situation by using a shielded stereo TRS cable to the footswitch, combined with some thoughtful grounding?

I _think_ you could take the Tip from the bottom of your 1M resistor, wire the Ring to signal ground, then use the Sleeve for chassis ground. On the footswitch end, connect the sleeve to the enclosure, and the tip+ring to the switch (or even another pot).

That way the whole lot is shielded. As a bonus, if you forget your stereo cable, it'll still work with a mono guitar lead - it just might be a bit noisier (the connection from sleeve to ring on the amp-side jack socket would cause another path from signal ground to chassis ground, so you might end up with a small ground loop).

[tubeswell]

Following - I've got an amp with a switchable tone stack that does a similar thing, and I'd like to put it on a foot switch so that I can (when I'm feeling lazy at least), gig without a pedalboard :)

Could you improve the noise situation by using a shielded stereo TRS cable to the footswitch, combined with some thoughtful grounding?

I _think_ you could take the Tip from the bottom of your 1M resistor, wire the Ring to signal ground, then use the Sleeve for chassis ground. On the footswitch end, connect the sleeve to the enclosure, and the tip+ring to the switch (or even another pot).

That way the whole lot is shielded. As a bonus, if you forget your stereo cable, it'll still work with a mono guitar lead - it just might be a bit noisier (the connection from sleeve to ring on the amp-side jack socket would cause another path from signal ground to chassis ground, so you might end up with a small ground loop).

Any part of the signal circuit on long wires AND outside the chassis, will be prone to noise, and shielding might reduce noise somewhat, or not.


If you want something footswitchable, relays are a better option noisewise

[chocopower]

One question..


Isn't the way Fender switch on/off the reverb in the ab763 series, the same thing we are talking here?


I know a relay is a more clean and hum-free method,  but i have used in low gain amps without major issues.
Going further,  i used the same method for preamp cathode bypass cap switching in a Guyatone ga1050 (fender twin rever clone, but 2x6L6). Works well without any hum.

[pdf64]

Yes, but note that the cable etc for the footswitch needs to be good, otherwise it picks up hum when reverb is enabled.

There are a few Fender 75s in my area, I guess a local dealer got a good deal on a batch of them. They’ve got a pull switchable mid boost, tone stack lift kinda thing.

https://el34world.com/charts/Schematics/files/Fender/Fender_75.pdf

A useful mod I’ve done on a few of them is to alter the function of the reverb footswitch, changing it to be in parallel with the mid boost. Provided the front panel mid pull boost is out, the footswitch can be used for Santana type tones when the ‘lead’ mode is selected.

I’ve further mod’ed a couple of them including my own, changing the lead mode to be more of a Mesa Mk / Dumble kinda thing, ie more gain. And the mid boost footswitch works fine, no appreciable increase in hum.