Odd presence control

[capn_starsky]

Hello all! I'm trying to get my head wrapped around a circuit on an amp that I have right now. It's a 50 watt dual EL34 powered junker that I got for a complete steal and have been doing a little modification work to it. Looking at the schematic, I'm finding a "presence" control that I've actually not seen before. It is not a negative feedback circuit with a pot like I'm used to, but rather, just after the bias supply, there is a 1M pot and .001uf cap in series with each other connecting the two screen supplies together. Just wondering how this functions as well as if there'd be any cons to removing it. Thank you!

[SILVERGUN]

Hello all! I'm trying to get my head wrapped around a circuit on an amp that I have right now. It's a 50 watt dual EL34 powered junker that I got for a complete steal and have been doing a little modification work to it. Looking at the schematic, I'm finding a "presence" control that I've actually not seen before. It is not a negative feedback circuit with a pot like I'm used to, but rather, just after the bias supply, there is a 1M pot and .001uf cap in series with each other connecting the two screen supplies together. Just wondering how this functions as well as if there'd be any cons to removing it. Thank you!

Maybe you mean "connecting the two grid supplies together"?
If so, that is just a simple variable treble cut control which can be easily removed if you like the result.

https://robrobinette.com/Voicing_an_Amp.htm#Vox_Cut_Control

[sluckey]

Show us the schematic please.

[capn_starsky]

Yeah, I couldn't quite think of how to describe it and I didn't want to just take a picture of the schematic and upload a grainy image...that's almost exactly it. Pretty much the only thing different are the component values. Thank you tons for the speedy reply! Another little question or theory to bounce around. This sounds also like I could in theory remove the capacitor or otherwise bypass it and have a PPIMV control a'la VOX, does it not? Phase cancelling both inputs as the pot is turned and thereby turning down the volume? It's on page 2...don't judge me on the amp, like I said, I got it for a 100 buck mod platform!

[capn_starsky]

New to posting on this forum, just in case it doesn't show up, it's a crate VFX5112. Upon further research, it indeed looks like a cut control labeled as a presence control. I believe the .001 uf cap controls the frequency that is sent out of phase to the other tube via the 1 meg pot to the other power tube grid to be canceled out.

[sluckey]

If you short out the cap the pot will become a "Crossline" master volume. Some amps have both...

     http://sluckeyamps.com/lightning/matchless_lightning.pdf

Many people including me do not like the crossline mv.

[capn_starsky]

Again, thanks everyone and thank you sluckey! I've owned numerous amps but I'm new to building new and modifying existing amps and this is the first time I've actually come across this circuit before! I'm definitely going to try shorting that cap because I'd been planning on tinkering with master volumes.

[PRR]

> the first time I've actually come across this

Back in the 1940s, that was just the Tone Control. Cap and pot across a suitable point. In 1 Watt radios it was simply a 0.05u cap and a pot across the 2K OT winding.

[pdf64]

...Many people including me do not like the crossline mv.

My (limited) experience with type 3 master vols is that they sound good with a regular 12AX7 type LTP eg 5F6a, but horrid with a regular 12AT7 type eg TR AB763.

[sluckey]

I never liked it in my Matchless Lightning with 12AX7 LTP. But then I never bonded with that amp for other reasons too.

http://sluckeyamps.com/lightning/matchless_lightning.pdf