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Help with modifying cold cathode on guitar amp

Hi all,

As a pre-amble, I should say that I'm 'electrically compentent' however I do not have experience with valves, until a day ago I hadn't even looked at the pin out of any valve.

I have a Bugera 333XL guitar amplifier, which uses 4 of 6L6 or EL34 tubes in the power output section, and 4 12AX7's in various positions around the circuit.

In a lot of places online there's discussions about 'cold clipping' where the cathode is 'cold biased' with a higher value resistor, i.e. around 9.5 to 40kOhms instead of ~1.5k. I've even read about it being reduced as low as 500ohms.

As far as I understand, there's no reason not to have this on a potentiometer, so that potentially to adjust from .5 to 40k, though I'll probably do something a little fancier with a pot and a few series mounted resistors with on-off toggles.

I've found the schematic, and I think I know which valve/pin I need, and hence which resistor I should remove. So here's my question:

Please can someone confirm the correct resistor to target to change distortion on lead/crunch channels?
Bonus points for advising if I should remove the bypass cap, if there is one...
 

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It is an IMPORTANT part of Marshall sound.

Tons of newbies think they "discovered the wheel/fire/gunpowder" and replace it by a "normal" value resistor to get a little more gain and pat their own shoulders for cleverness.

Do you think Marshall are fools?
Why has this seemingly "wasted gain" stage kept alive and copied by others?
Think again.

A pot there is a terrible idea, it will scratch horribly because it is passing DC current through, a no no for pots.
 
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Thanks JMFahey, but if you read the post you'd see it isn't a marshall. If you look at the schematic you'd see it doesn't use the same values as a marshall, you'd know I'm not trying to be like marshall, but I could as one of the options. I don't think I've discovered anything, but it would be nice to find out how this part ofd the circuit contributes to the sound. the motivation is to learn more than to improve.

Scratching isn't an issue, this would be a set and forget adjustment at the back of the amp not front.
 
"It has a Marshall inside" 😉
It's in its DNA 🙂
Modded values? ... sure, why not.
But cold cathode circuits always work the same way, minor differences notwhistanding.
Basically:
* VERY Cold biasing means plate is WAY closer to +V rail than to ground; so it can swing UP, say, 10-15V tops while it can swing DOWN 200-250V.
So it will clip giving you a VERY assymmetrical waveform , TONS of even harmonics, lows modulating highs in a very characteristic, very Marshally way.
You will not find this in, say, a Fender amp or any other.
Losing all this character for a few dB extra gain is a very poor exchange.
* A symmetrically biased stage will swing symmetrically (duh).
So it can slam next grid with, say, +100/-100 V peaks
That huge positive one will he rectified by next grid and cause all kinds of wild bias shifts, including ugly blocking distortion.
Specially in a heavily overdriven amp.
Now if you limit positive to 10-15V peak, situation becomes very controllable.

So cold cathode biasing is not an error waiting to be corrected but an important part of Marshall/Heavy metal sound.
 
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JMFahey is exactly right, IMHO. Tube guitar amps are designed to give a particular sound. If want a super high gain sound then the whole amplifier topology may need to be redesigned to get that sound right. IOW, amps are a big part of the sound of electric guitars, often having more to do with 'tone' than the guitar itself. Fingers come first of course when it comes to tone, but choice of amp and speaker may be next most important thing that matters most for a particular style of music (and to fit into the mix of a particular band's sound, which is in itself a complex subject). Trying to re-optimize a guitar amp from one type of sound to another usually involves a number of variables. Changing just one thing without fully understanding its original purpose in the overall design usually will only make things worse.

If you want to understand more about guitar amp circuit topology, the Valve Wizard book on guitar and bass preamps is a pretty good introduction.
https://www.amazon.com/Designing-Tube-Preamps-Guitar-Bass/dp/0956154522
 
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change distortion on lead/crunch channels?
Previous discussion considered and respected, it appears that V7A drives both the lead and crunch signal paths, as indicated by their respective Volume controls. It has the "~1.5K" resistor, R109, you mentioned in your OP, connected to the cathode.

You could certainly rig a 3 position switch to choose your 500, the stock 1.5K and your 9.5 - 40k modification. You may want to turn down both Volumes when changing positions on that switch...

There's a cap C67 which is apparently a 50V part. You'll have to check if, when the higher Ohms cathode resistor is selected, you begin to approach that 50V rating. If so, you may want to replace it with a higher voltage 22uF part. If you simply remove it, you'll reduce the gain of that stage and - with two circuit changes at one go - might get yourself lost as to what did what.

Technically, the combination of 22uf/1.5K has a time constant, which should be preserved as you change to 500 and, say, 25K. Otherwise another character of the circuit moves around when the resistor is changed alone. Perhaps it's most important for the 500 value; not so much for greater than 1.5K. You could click to 500 and have two things changing, again, getting lost as to why you're hearing whatever it is that changed in the sound. Perhaps replacing it with a 68uF part to accommodate the 500 Ohm value will be sufficient, or, better than just leaving its value at 22, as - is.

Hope this helps,
 
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Fingers come first of course when it comes to tone...
Exactly. Listen to the 1st Zep album from 1969. Who would have thought that guitar wizzard Jimmy Page, one of the greatest and most influential guitarists in rock biz, played all the tracks on his psychedelized Telecaster, perhaps the only axe he owned those days? He just was able to get all the sounds he wanted just out of his fingers.

Best regards!
 
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Changing R96 is probably your best best, you can stick a 50k pot in there. It will rustle a lot though since there will be DC current in it. The bypass capacitor is up to you; put it on a switch? I'd prefer to leave V7A alone because it is driving the tone controls, and changing it's bias will change its output resistance.
 
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Cold Clipping, and Cold Biased . . .
Before today, I never heard those terms used on Tubes / Valves threads

Perhaps this Guitar Amp thread should be on Instruments and amps.
Or, are you trying to convert a Guitar Amplifier into a Hi Fi amplifier?

Many of us in Tubes / Valves threads attempt to reduce distortion, and make amplifier's frequency flat across the 20Hz to 20kHz frequency range.
That does not "sound" like a guitar amplifier to me.
 
Kay Pirinha,

My friend played the Roast of Jimmy Page and group . . . Highway to Heaven, sung by Heart and with a large assembly and a whole chorus, etc.
I hope you have seen that video.
I will miss my friend, he passed away Dec 9th, just a few days ago. Ahh.