Clipping diodes

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jestaudio
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Clipping diodes

Post by jestaudio »

Love or hate them, I have a really nice Carvin V33 but want a all tube signal path, can anyone see any real issues with removing the diodes from the signal path, I,ve attached a schematic
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Cliff Schecht
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Re: Clipping diodes

Post by Cliff Schecht »

If you remove D1-D4, you will need to replace it with something around a 1M resistor. Otherwise the grid has no reference to ground and it can't use it's absolutely minuscule (but still important!) grid current to develop a bias. If you leave it floating you will end up with flabby/farty sound if you get any at all, many times when you leave inputs floating they will work their way towards one of the power supply rails and you get no sound at all.
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jestaudio
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Re: Clipping diodes

Post by jestaudio »

Cheers Cliff, correct me if I am wrong but having a diode clipping circuit in a amp reduces the output of the circuit ? so in theory I should have a little more output on tap, I have notice that the clean channel output has more "volume" than the Drive channel,
matt h
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Re: Clipping diodes

Post by matt h »

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cxx
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Re: Clipping diodes

Post by cxx »

I tried this in a JTM-60 a while back. I added a switch to put them in or out of the circuit. It didn't turn out like I wanted. A lot more volume and a lot more bass. Without the diodes you'll have 3, capacitor bypassed, gain stages and the tone stack.You could try switching out the diodes for a capacitor, kind of like the tuning circuit on the express/liverpool. Possibly removing one of the capacitor bypasses. Or changing one of the bypasses on an early stage to limit low end like to a marshally .68 . The unfettered low end has a tendency to dominate unless controlled. But then again, some people like really bassy overdrive. These changes would be relatively easy to try. Changing the order of the stages wouldn't be worth it. Might as well start fresh.
jestaudio
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Re: Clipping diodes

Post by jestaudio »

cxx wrote:I tried this in a JTM-60 a while back. I added a switch to put them in or out of the circuit. It didn't turn out like I wanted. A lot more volume and a lot more bass. Without the diodes you'll have 3, capacitor bypassed, gain stages and the tone stack.You could try switching out the diodes for a capacitor, kind of like the tuning circuit on the express/liverpool. Possibly removing one of the capacitor bypasses. Or changing one of the bypasses on an early stage to limit low end like to a marshally .68 . The unfettered low end has a tendency to dominate unless controlled. But then again, some people like really bassy overdrive. These changes would be relatively easy to try. Changing the order of the stages wouldn't be worth it. Might as well start fresh.
not intending to go mad on it, I,ve found some details on the VT50 which is the same topography as the Vintage 33, there is a fair amount of loss in the tone stack that can be changed plus the amp is bass heavy anyway , I,ve heard a V33 with the tone stack and diode modded, sounds night and day to mine
cxx
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Re: Clipping diodes

Post by cxx »

jestaudio wrote:
cxx wrote:I tried this in a JTM-60 a while back. I added a switch to put them in or out of the circuit. It didn't turn out like I wanted. A lot more volume and a lot more bass. Without the diodes you'll have 3, capacitor bypassed, gain stages and the tone stack.You could try switching out the diodes for a capacitor, kind of like the tuning circuit on the express/liverpool. Possibly removing one of the capacitor bypasses. Or changing one of the bypasses on an early stage to limit low end like to a marshally .68 . The unfettered low end has a tendency to dominate unless controlled. But then again, some people like really bassy overdrive. These changes would be relatively easy to try. Changing the order of the stages wouldn't be worth it. Might as well start fresh.
not intending to go mad on it, I,ve found some details on the VT50 which is the same topography as the Vintage 33, there is a fair amount of loss in the tone stack that can be changed plus the amp is bass heavy anyway , I,ve heard a V33 with the tone stack and diode modded, sounds night and day to mine
If you know of one that you like the sound of that's great, start there.

For a given volume, the bass frequencies have much higher amplitudes, and are clipped sooner. That's why you have to filter some out before clipping and your tone stack is at the very end of the preamp.

Check out the excellent sticky with scope plots of gain stages at the top of this forum. I think that's where it is anyway.
Cliff Schecht
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Re: Clipping diodes

Post by Cliff Schecht »

If the amp gets too loud after removing those diodes, you could always add a trimpot instead of a resistor that you can tweak until the volume between both channels is equal.
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jestaudio
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Re: Clipping diodes

Post by jestaudio »

Cliff Schecht wrote:If the amp gets too loud after removing those diodes, you could always add a trimpot instead of a resistor that you can tweak until the volume between both channels is equal.
I don't think that will be a issue lol :D , the clean channel on these is mega and pretty close the stated output, the dirt channel seems to be attenuated by the diodes and as such I generally struggle for good balance without micing up, I,m a sound engineers worst nightmare, I use feedback within my technique and tend to turn up enough to get decent amounts of it lol
gingertube
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Re: Clipping diodes

Post by gingertube »

I don't think those diodes are doing anything anyway.
The 1N4745 is a 16V zener diode.

With them wired 2 in series and back to back like this the maximum signal voltage will be 32V peak to peak.

On the positive going cycle the grid signal will clamp due to grid rectification and that will have the effect of charging the coupling cap such that the postive signal peak is at about 0V.

On the negative going signal peak the tube will be cut off long before the zener conducts.

EDIT: Hang on that is not right - with the series connected diodes wired back to back like this it is the forward voltage of the diodes which will determine the clamp/clip not the zener voltage (zener voltage is a reverse "breakdown" voltage), that is, signal is limited to say 1.4V peak to peak, so why the hell use zeners at all and not just ordinary diodes?

Any insights?

Cheers,
Ian
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Reeltarded
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Re: Clipping diodes

Post by Reeltarded »

just lift them by 10k at ground..

I like diodes.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
eniam rognab
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Re: Clipping diodes

Post by eniam rognab »

here is a link to the Hasserl mod page on the clipping diodes in a VT series amp

Hasserl has lotsa mods, ayuh, wicked pissah

http://members.dslextreme.com/users/rlh ... odeMod.pdf

looks like he uses a 470k instead of the diodes
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roberto
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Re: Clipping diodes

Post by roberto »

That configuration gives no more than 2,4 V on each side.

Is the same concept of jcm900, mesa v-twin and so on.
A diode clipping plus a tubification of the sound.
jestaudio
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Re: Clipping diodes

Post by jestaudio »

well I went ahead a did it, used the mods detailed by Hasserl and its a change for the better, not as much drive but far more pleasing, I also changed the tone stack a little which is a definite improvement, just need to figure out if its possible to do a effects loop.
Thanks for all those that chimed in
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