fizzy overdrive vs crunchy overdrive

[Derelict]

what can you do to an amp that has fizzy distortion that you want to make more crunchy? am i correct in that the plate resistors act as a low pass? if so, could i increase plate resistor values, cutting of more of the highs? or does it have to do more with the anode or cathode and which of those contributes to more gain?

i am wanting to improve my humble little backup amp, a peavey valveking 112. the clean channel is ok, but the drive could use a bit more crunch.

[El Caballo]

Cross-line post-phase-inverter treble cut, like a Vox?

[rockon1]

Sometimes a simple pre amp tube change will get you there. Perhaps a lower gain 5751 in the V2 position (or even lower gain). Stating the obvious but keep the gain down and volume up! Also make sure the speaker is thoroughly broken in too. Bob

[Derelict]

Sometimes a simple pre amp tube change will get you there. Perhaps a lower gain 5751 in the V2 position (or even lower gain). Stating the obvious but keep the gain down and volume up! Also make sure the speaker is thoroughly broken in too. Bob

ok

ECC83 vs ECC82

with the gain on the ECC83 on about 3, its equal to the gain of the ECC82 around 8, but the 82 sounds less fizzy. with a little research, the 82 bias is warmer in the same slot. so, i could decrease the cathode resistor and get the same effect w/the 83: a warmer bias.

interesting.. :munch

[dk_ace]

I had that problem with a marshall dsl401. I ordered a mullard 4024 and a GE 5751 from kcanostubes (bluestrat on TGP), and it fixed the problem completely. I settled on the mullard in V2 and the GE in V1 for right now. I also clipped out the bright cap. Start with a tube swap (I don't think you could beat that mullard for the price). The tube should get you close. If you are still hearing a little fizz (like I did), look for a cap that may be adding unnecessary brightness (fizz).

D

[Madsman]

I think a lot of that has to do with the cathode resistor or cap on one of the gain stages. Wait, I took notes! I'll look it up when I get back home, and get back to ya. I remember that the resistors and caps on each preamp gain stage were working to effect the gain as well as which frequencies were allowed to pass. One of the mods we did in the Egnater class really demonstrated that fizz thing.

[Derelict]

I think a lot of that has to do with the cathode resistor or cap on one of the gain stages. Wait, I took notes! I'll look it up when I get back home, and get back to ya. I remember that the resistors and caps on each preamp gain stage were working to effect the gain as well as which frequencies were allowed to pass. One of the mods we did in the Egnater class really demonstrated that fizz thing.

please let me know; any information is greatly appreciated.

i think, basically, the cathode resistor is responsible for a lot of this, once again, due to creating a warmer bias. the gain control is right at V1B in the schematic.

[El Caballo]

i think, basically, the cathode resistor is responsible for a lot of this, once again, due to creating a warmer bias.

And also more gain -- unless Ck is so large that the gain is effectively the gain of the tube, and Rk is purely for biasing purposes. What are the values of Rp, Rk, and Ck in the stage you're looking at?

This is the case with the Bassman/JTM45 inputs, and the Normal channel on all 4-input Marshalls: a Ck of 250/330uF covers the entire audio range, so Ck just biases V1.

It's on my list to get some 10K pots and try some experiments with this, but I haven't done it yet.

[Derelict]

And also more gain -- unless Ck is so large that the gain is effectively the gain of the tube, and Rk is purely for biasing purposes. What are the values of Rp, Rk, and Ck in the stage you're looking at?

This is the case with the Bassman/JTM45 inputs, and the Normal channel on all 4-input Marshalls: a Ck of 250/330uF covers the entire audio range, so Ck just biases V1.

It's on my list to get some 10K pots and try some experiments with this, but I haven't done it yet.

thinking of R123, changing to 800 ohms or so.

http://i74.photobucket.com/albums/i247/doggage5050/overdrive.jpg


also have a question about the input stage. would it be overkill to
decrease R110 to 10K, remove C108 and replace with a jumper, bump R114 up to 1MEG, maybe lower R120 to around 800 ohm?

http://i74.photobucket.com/albums/i247/doggage5050/input.jpg

[El Caballo]

My experience is that too much gain in a single stage = fizz. I'd be careful about trying to get more gain out of these stages and reducing fizz at the same time. But there's no substitute for soldering and listening.

800 Ohms on R123 may push it too far towards the hot side, but there's no way to tell without trying it. If you really want to push the gain hard, put a big fat Ck bypass cap on the 820 Ohms, like 15 uF or greater. A smaller Ck will give you the Plexi-style midrange boost...try things in the 0.25uF-1uF range.

I'm not confident enough in my analysis of that input stage to offer my advice. However, if you're talking about that many changes, I advise breadboarding up what you want and bypassing the original entirely instead of hacking up the PCB too much. I'd hit R120 first and see what happens.

[Rich M]

If it is too fizzy, you might try lowering the plate resistors (R103, R104) to 100K. Then you might need to tweek the cathode resistors (R120, R123) a bit to reestablish the bias points. This will lower the gain a bit, similar to putting 5751's in.

[Trowerfan]

I just fixed a problem with "fizzyness". My Allen Encore had a 12AX7 in the PI so I put in a Jan Phillips 12AT7 in place and when using an OD the Fizz is now gone. The amp does sound great with the 12AX7 if an OD isn't used.

[El Caballo]

If it is too fizzy, you might try lowering the plate resistors (R103, R104) to 100K.

An excellent point, especially if you want to use a lower Rk.

Can you post the rest of the schematic? A lot of "fizz" comes from overdriving the phase inverter, because it's after all the tone controls. But that depends on how they do the master volume. Is it fizzy even with the master set low? Does turning down the presence reduce fizz?

[Derelict]

An excellent point, especially if you want to use a lower Rk.

Can you post the rest of the schematic? A lot of "fizz" comes from overdriving the phase inverter, because it's after all the tone controls. But that depends on how they do the master volume. Is it fizzy even with the master set low? Does turning down the presence reduce fizz?

there is no presence control, and yes, there's some fizz down low. also, the overdrive is mushy and there's a loss of high frequencies; it really bugs me when i use my guitar volume to clean it up; there are no highs and it sounds muddy. i want crunch and punch; think jcm800.... (i know i know), as well as more tightness and better highs (not fizz) in the overdrive department.

so far, (input) i changed R110 to 10K, removed C108 & replaced w/a jumper, and upped R114 to 1 meg. the clean channel sounds great now. i may, in the future, replace C107 and C110 w/ silver micas (of the same or close to the same values), but for now am very satisfied w/ the clean channel, as well as the bright switch, and want to leave it alone.

for V1B, i reduced R123 to 820 ohm. that took some of the fizziness out, but not all of it. i think i should have gone around 500ohm.

There's not quite enough punch and crunch; the attack is just a bit too soft, even if its turned up loud. most of the drive comes from V1B, with a little coming in form V2 (the boost is also there).

here's what i am thinking so far:

for crunch (removing mushiness):
V1A - lowering C114 from 22u to 2u
V2B - lowering C112 from 22u to 2u

for a bit more high frequencies:
V2B plates - removing R107 (680k) & replacing w/a jumper, and reducing R111 to 400K. then, if any changes are needed in terms of tone, i make them to the overdrive channel's tone stack cap values.

i am also curious as to the effect of lowering C109 and R115 (V1B) would have on the drive tone.

for the schematic, i had to take screenshots ( i don't see anywhere to upload files). the second screenshot overlaps the first one as i couldn't get all the first one in there at one time. the last is V3, the phase inverter.

http://i74.photobucket.com/albums/i247/doggage5050/1.jpg


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http://i74.photobucket.com/albums/i247/doggage5050/2.jpg



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http://i74.photobucket.com/albums/i247/doggage5050/3.jpg

[El Caballo]

so far, (input) i changed R110 to 10K, removed C108 & replaced w/a jumper, and upped R114 to 1 meg. the clean channel sounds great now.

You could ditch R110 entirely. I'd recommend a grid resistor in either case to filter out AM radio...that's what the 68K in Marshalls and Fenders does. Since it's after the 1M to ground it doesn't attenuate much.

There's not quite enough punch and crunch; the attack is just a bit too soft, even if its turned up loud. most of the drive comes from V1B, with a little coming in form V2 (the boost is also there).

here's what i am thinking so far:

for crunch (removing mushiness):
V1A - lowering C114 from 22u to 2u
V2B - lowering C112 from 22u to 2u

I would go to 0.1u or even 0.68u on V2B. 2u isn't going to make much difference (3dB around 90 Hz). On V1A it's more a matter of taste. If you have a place to mount another switch, it's a great place to mount a couple different Rk/Ck combos.

Not sure about the big hairball around V2A.

[Derelict]

You could ditch R110 entirely. I'd recommend a grid resistor in either case to filter out AM radio...that's what the 68K in Marshalls and Fenders does. Since it's after the 1M to ground it doesn't attenuate much.



I would go to 0.1u or even 0.68u on V2B. 2u isn't going to make much difference (3dB around 90 Hz). On V1A it's more a matter of taste. If you have a place to mount another switch, it's a great place to mount a couple different Rk/Ck combos.

Not sure about the big hairball around V2A.

lol.... that V2A is a hairball... :roll


i am really digging the clean channel now. very articulate and clear.

excellent suggestion on V2B. My math skills are still very newb-ish on this stuff. this is great, though, as i some doors are really starting to open up.

thanks el caballo :JAM

[El Caballo]

How did this all work out for you?