Schematic Understanders, i need you!

timmysoft

Morning!
 I love the sound of my Peavey Prowler, but i'm finding the drive channel a little dark and bass heavy for my tastes, especially as i play in Drop C with heavy strings on humbuckers.

 I'm really no expert when it comes to amp schematics, give me a wiring diagram for a subsea oil and gas termination and i'm golden but amps are just full of unicorns to me.

 So what i'd like to do is add a global presence pot or maybe just a bright switch for the drive channel, but maybe theres some components relating to the drive channel i can change out to up the brightness and cut the bass a little?



The Prowler was the follow up to the Bravo, the circuit was then taken and modified for the 1st generation of Valveking amps.

ICBM

Can you post the link to where you got the schematic? The resolution isn't good enough to read it in that screen shot. I can't find it on Peavey.com… (I may be looking in the wrong place.)

timmysoft

https://www.peavey.com/forum/viewtopic.php?t=38787

I got it from here! 

ICBM

Aha - yes, I can read that.

There are a couple of caps which will take top-end off the high-gain channel - C117 next to the Ultra Gain pot (but mostly when the gain is up a lot), and C105 and C149 after V2B. The value of R173 will also affect this.

Cutting one or more of those caps will brighten the amp up, but be careful - they may also be there to suppress instability. Make sure you can reconnect them if odd things happen…

Cutting bass is a little trickier, but replacing C100 with a smaller value is where I'd probably start. That won't have any bearing on stability either.

timmysoft

Cheers @ICBM ;

what do you mean about instability? 

ICBM

timmysoft said:

what do you mean about instability? 

When the circuit feeds back on itself electrically - usually at high frequencies - this is a significant problem with a lot of high-gain amps, and often it's necessary to add caps to stop it. The trick is to do it without killing the top-end response.

It can sometimes happen in the audio range in which case you'll hear it as squealing or whistling, but it more often happens at higher frequencies - which you can't hear, but what you'll notice is that the amp sounds low on power, oddly muffled or 'soggy', because it's still trying to produce a lot of power even though you can't hear it, so the proper signal gets swamped. In the very worst case it can cause too much power to be developed in components which don't normally need to handle it, and so then cook themselves… so be careful.

timmysoft

Cheers for the info, I think I'll try new valves first, then it's definitely time for mods!

timmysoft

I think i've fixed my issue! 
 The stock speaker was the fairly dark but uniform Blue Marvel, i replaced instantly with a Creamback Neo, so today i switched it out again for a V30, although the amp is heavier now, it also sounds more like i want it to, i think once i get the horrible Harmas out of it then it'll be a real tone monster. The great thing is, i have two of these amps!

ecc83

Just a word about instability? As ICBM says, an amp can be delivering 20 watts ultrasonically and not give any obvious sign of distress. For those that wish to dabble with tone shaping components and don't have a 'scope, you can make a wide band meter with almost any moving coil meter movement , 1 mA FSD is about perfect but a speaker out will drive almost anything.

Build a bridge rectifier with OA91 Germanium diodes (think Maplin still carry them) and feed from a series limiting resistor, part of which can be adjustable for calibration. The latter can be done from a 6.3V 50Hz heater supply and using a digital meter for best accuracy. N.B. Digital meters are bog useless for detecting ultrasonics, most are 3dB down by 1(yes ONE!) KHz and then the readings go off a cliff.

Such a kludged meter will read way into the RF spectrum.

Dave.