Valve amp tone question

[OldGit]

I have a Peavey Valveking Royal 8.
It's a 5 watt class A valve amp. Mine's modded for harmonica (Valves and speaker)

I'd like to filter more treble out of the tone.
Is it possible to change a capacitor as I would on a P bass?

I have a schematic (attached) but have not opened the amp up to see what is there.

Any thoughts?

Thanks

[attachment=48156:Valvekin...matic_1_.pdf]

[escholl]

C6 is currently 10nF, change it to something larger. Should make the tone control darker. 22nF, 47nF, 100nF -- I don't know how dark you want it. I would try 22nF, and see how that goes.

You could also try making C12 smaller, or getting rid of it altogether.

When the tone is in the low position, C6 essentially bleeds the HF to ground. When it's in the high position, C12 enables the higher frequencies to bypass the gain reduction performed by the "Gain" control, effectively giving them full gain into the next stage regardless of where "Gain" is set. When it's somewhere in between Low and High, it does a combination of these things. Simple but effective, really.

Of course, I could be totally wrong! In which case someone will hopefully correct me.

[OldGit]

[quote name='escholl' post='817209' date='Apr 24 2010, 02:41 PM']C6 is currently 10nF, change it to something larger. Should make the tone control darker. 22nF, 47nF, 100nF -- I don't know how dark you want it. I would try 22nF, and see how that goes.

You could also try making C12 smaller, or getting rid of it altogether.

When the tone is in the low position, C6 essentially bleeds the HF to ground. When it's in the high position, C12 enables the higher frequencies to bypass the gain reduction performed by the "Gain" control, effectively giving them full gain into the next stage regardless of where "Gain" is set. When it's somewhere in between Low and High, it does a combination of these things. Simple but effective, really.

Of course, I could be totally wrong! In which case someone will hopefully correct me. [/quote]


Thanks.
I'll get a few caps and have a go.

[escholl]

[quote name='OldGit' post='817221' date='Apr 24 2010, 03:04 PM']Thanks.
I'll get a few caps and have a go.[/quote]

Ok, let me know how it goes, if that doesn't work for some reason we can try something else. I'm sure you know this, but I have to say it anyway: remember not to touch C5, C7, and C8 right after turning it off! I usually put electrical tape over the contacts of them if I'm going to be having the amp on and off while working.

[OldGit]

[quote name='escholl' post='817427' date='Apr 24 2010, 07:02 PM']Ok, let me know how it goes, if that doesn't work for some reason we can try something else. I'm sure you know this, but I have to say it anyway: remember not to touch C5, C7, and C8 right after turning it off! I usually put electrical tape over the contacts of them if I'm going to be having the amp on and off while working.[/quote]


Thanks.
I don't normally mess with stuff that plugs into the mains, or even has batteries P bass electrics tend not to bite back
I'll be careful.
Cheers

[Ancient Mariner]

I'd go along with escholl, but also recommend changing C2 for something much larger - 1000uF or there abouts. Having a big cathode cap on the power valve can strengthen and smooth the bottom end. Some guys I know have used up to 2000uF, but above that things start to get a little muffled.

If you're in the mood to experiment then change C1 to about 2uF or 4uF and C9 to 22nF or 47nF. This will reduce the bass generated in the first gain stage, but will also allow a wider frequency range through to the next stage. I would suggest trying the larger of both values to begin with, and if the tone gets harsh and ragged when you wind up the volume then the amp is suffering blocking distortion. In that case change C9 to the smaller value. You could also try bridging R17 with a 100K resistor.

Have fun.

[OldGit]

[quote name='Ancient Mariner' post='819438' date='Apr 26 2010, 02:38 PM']I'd go along with escholl, but also recommend changing C2 for something much larger - 1000uF or there abouts. Having a big cathode cap on the power valve can strengthen and smooth the bottom end. Some guys I know have used up to 2000uF, but above that things start to get a little muffled.

If you're in the mood to experiment then change C1 to about 2uF or 4uF and C9 to 22nF or 47nF. This will reduce the bass generated in the first gain stage, but will also allow a wider frequency range through to the next stage. I would suggest trying the larger of both values to begin with, and if the tone gets harsh and ragged when you wind up the volume then the amp is suffering blocking distortion. In that case change C9 to the smaller value. You could also try bridging R17 with a 100K resistor.

Have fun.[/quote]


Well it's supposed to sound like an over-driven raggedy Chicago blues harp sound. Is that what you mean?
Does Ok at present but not quite enough bass as my mic is not quite dark enough.

[Ancient Mariner]

Blocking distortion is just unpleasant, rather than 'nasty'.

I'd try the 1000uF cap on the EL84 and bleed more treble off first.

[OldGit]

[quote name='Ancient Mariner' post='820059' date='Apr 26 2010, 10:25 PM']Blocking distortion is just unpleasant, rather than 'nasty'.

I'd try the 1000uF cap on the EL84 and bleed more treble off first.[/quote]

Cool.

[OldGit]

So, got all excited, got a space and time to try this op and went to Maplins to the tech guys in the box at the back and asked for the stuff. They asked what type of capacitor and resistor I need ...

er... seems there are many types.. Any idea which I need?

Metallised Ceramic plate
Resin dipped
Decoupling ceramic disc
Polypropylene Film and Foil
Audio grade polypropylene
Class Y RFI
Metallized Paper RFI class X2
Electrolytic (of many sub types)
Double Layer
Variable miniature film dielectric trimmers





Thanks

[escholl]

[quote name='OldGit' post='823167' date='Apr 29 2010, 04:56 PM']So, got all excited, got a space and time to try this op and went to Maplins to the tech guys in the box at the back and asked for the stuff. They asked what type of capacitor and resistor I need ...

er... seems there are many types.. Any idea which I need?

Metallised Ceramic plate
Resin dipped
Decoupling ceramic disc
Polypropylene Film and Foil
Audio grade polypropylene
Class Y RFI
Metallized Paper RFI class X2
Electrolytic (of many sub types)
Double Layer
Variable miniature film dielectric trimmers





Thanks[/quote]

For resistors, their standard .6W metal film ones Maplin sell will work fine (the blue ones), assuming they fit as the schematic mentions SMT for some resistors (surface mount).

For capacitors C6, C12, C9 (remember, C9 is rated at 400V, the others at 50V) and similar, anything related to either the polypropylene or polyester film (including multilayer and metallised/metal foil versions) will be fine. Don't bother with the "audio grade". Assuming they have the values and voltage ratings you need, their following ranges should work:

[url="http://maplin.co.uk/Module.aspx?ModuleNo=98156"]http://maplin.co.uk/Module.aspx?ModuleNo=98156[/url]
[url="http://maplin.co.uk/Module.aspx?ModuleNo=361"]http://maplin.co.uk/Module.aspx?ModuleNo=361[/url]
[url="http://maplin.co.uk/Module.aspx?ModuleNo=12031"]http://maplin.co.uk/Module.aspx?ModuleNo=12031[/url]

For the electrolytics, which are C1, C2 and similar, pretty much any of them will work fine as long as they are the value you need and are equal to or above the voltage ratings given (which in both cases here is 50 volts). You don't need non-polarised ones, and you don't need high temperature ones (the red ones they sell).

You might want to check though if the caps you are replacing are axial lead (one wire comes out of each end) or radial lead (wires both come out the same end of the cap). Odds are they are radial. Getting the lead package wrong is not the end of the world, it's just inconvenient.

Good luck!

[OldGit]

Ta can you explain the nF uF pF and the relationship?
Ta

[BigRedX]

In descending order:

µF = microfarad = millionth of a Farad

nF = nanofarad = billionth of a Farad

pF = picofarad = trillionth of a Farad

[OldGit]

[quote name='BigRedX' post='823472' date='Apr 29 2010, 10:09 PM']In descending order:

µF = microfarad = millionth of a Farad

nF = nanofarad = billionth of a Farad

pF = picofarad = trillionth of a Farad[/quote]

Thanks BRX

[Dood]

If there was ever a valve amp schematic you could call 'cute' that would be it! It's right up there with Fabian's Hack-a-day Business Card Valve Amp - a project i STILL haven't gotten around to building!

[OldGit]

[quote name='dood' post='823563' date='Apr 29 2010, 11:36 PM']If there was ever a valve amp schematic you could call 'cute' that would be it! It's right up there with Fabian's Hack-a-day Business Card Valve Amp - a project i STILL haven't gotten around to building![/quote]

Ha ha .. It's a cute little amp that punches beyond it's weight.
Not so good with a bass through it though

[umph]

there are a few mods i'd recomend to these kind've amps that i've been working on for my circuit. Replace both of the cathode bypass resistor / caps with a Super bright red Led + 0.1uf cap, Replace c3 and c9 with 47n. From there you could increase the size of r4, 13 and 17 to attenuate more highs but there already very large.

Also as has already been said replacing c2 with anything up to about 1000uf is always a good call or implanting an auto bias system!

[OldGit]

[quote name='umph' post='823983' date='Apr 30 2010, 01:02 PM']there are a few mods i'd recomend to these kind've amps that i've been working on for my circuit. Replace both of the cathode bypass resistor / caps with a Super bright red Led + 0.1uf cap, Replace c3 and c9 with 47n. From there you could increase the size of r4, 13 and 17 to attenuate more highs but there already very large.

Also as has already been said replacing c2 with anything up to about 1000uf is always a good call or implanting an auto bias system![/quote]


This is for blues harp rather than guitar.
Does that all still apply?

[Ancient Mariner]

[quote name='OldGit' post='823987' date='Apr 30 2010, 01:04 PM']This is for blues harp rather than guitar.
Does that all still apply?[/quote]

I've built an experimental preamp with a switch to change from conventional cap + resistor to LED bias, and the LEDs give a more aggressive tone with a bit more drive. Might be want you want to help cut through but you might also find it a bit hard-edged..

[escholl]

[quote name='OldGit' post='823987' date='Apr 30 2010, 01:04 PM']This is for blues harp rather than guitar.
Does that all still apply?[/quote]

Well, it's not going to hurt I suppose, but if you like the sound and just want is a darker tone, seems most logical to start with the two caps that are either side of the tone control C6 and C12. No sense overcomplicating things, and if you don't generally work with circuits like the one here, as some of us do, then keeping it simple will reduce the likelihood of anything going wrong and you then having to try to fix that too...

[Dood]

You guys make me wish I stayed studying electronics longer!!

[OldGit]

[quote name='dood' post='824131' date='Apr 30 2010, 03:02 PM']You guys make me wish I stayed studying electronics longer!![/quote]

Yeah me too
Took me 30 years to learn how to solder ....

[umph]

[quote name='Ancient Mariner' post='824100' date='Apr 30 2010, 02:28 PM']I've built an experimental preamp with a switch to change from conventional cap + resistor to LED bias, and the LEDs give a more aggressive tone with a bit more drive. Might be want you want to help cut through but you might also find it a bit hard-edged..[/quote]

LEDS give full gain at all frequencys, where as the conventional resitor + cap can prove to be a slow slow in the low and high end.

just had a thought it may be worth dropping the anode loads down to 47k it'll lower the gain but it'll make things sound more tubey and less shrill or harsh.

[OldGit]

[quote name='umph' post='824315' date='Apr 30 2010, 07:08 PM']LEDS give full gain at all frequencys, where as the conventional resitor + cap can prove to be a slow slow in the low and high end.

just had a thought it may be worth dropping the anode loads down to 47k it'll lower the gain but it'll make things sound more tubey and less shrill or harsh.[/quote]


Steady on ol' chap .. This is all getting way beyond my confort zone
For now I'll just swap over the main tone cap and see how that works