Help with Peavey Tube Sweetener

[imloggedin]

Hey guys. I have a Peavey Tube Sweetener and I would like to use it on my 2bus. The problem is that it has a horrible low pass built in with a tone control. I would like to bypass the tone control completely. I have attached a schematic of the input section of the first channel. If i put a jumper between the red arrows that i highlighted, do you think that will be enough to test bypass the tone "CONTOUR" circuit? Maybe they have a lowpass somewhere else in the circuit also? I don't see it, but I'm not good at EQ's. Any help is appreciated! I really want to use this thing.

 

[Tubefreak]

A jumper alone might not be enough. There are several optional steps, maybe some more enlightened persons can help us.

1. jumper as proposed
2. remove R3 (to prevent the first connection
3a. disconnect ground from CONTOUR pot
3b. if not enough, remove R4, R7, C4 and C5.

But if we eliminate the tone circuit, it might sound funky. Maybe a simple RC setup is needed to 'tone' it properly and/or get the gain staging right.

Please send in the troops.

 

[imloggedin]

This is probably a silly question, but will cutting the ground to the pot harm those electrolytic caps at all? (C3,C4,C5)?

 

[JohnRoberts]

Do you want to make the tone controls go away for ever, or just temporarily bypass? Perhaps lifting a ground connection might work as a temporary bypass, there is also an overall LPF pole in there you need to defeat. Easier to permanently remove the reactive ciruitry, but that begs the question does a simple tube path actually sound that magical alone?

I was not a fan of these me-too "touched by a tube" type products and resisted making one for yearts, but after I left product management Hartley got another different product manager to do one. 

JR

PS: The Peavey tube products I liked were the VMP preamp and VCL limiter.. They actually do something useful. [/rant]
PPS: Sorry did I mention I don't like these kind of products?

 

[imloggedin]

John,

Thanks for your response! I am sure you are well informed of this product. Can you please point out the LPF pole? I am not good with filter designs. Bypassing temporarily or permanently would be fine. I just need to understand what can and cant be removed. Any help is appreciated.

 

[imloggedin]

...also if possible could you comment on why there is such a hard LPF in the design? Its very obvious and is something like -6 db down at around 20k at the flattest.

 

[Kingston]

Without the filter network there are two almost equally set up 12AX7 stages at very high gain into a cathode follower output stage.

No "sweet" to be found here, very little tubes either. The first two stages represent the closest thing tubes can do to emulate a very high gain BJT transistor and the last stage is pure feedback. All stages will scream harshly and in agony when clipped, and below clipping point there is only very clean sound. It's one hell of a distortion.

 

[JohnRoberts]

John,

Thanks for your response! I am sure you are well informed of this product. Can you please point out the LPF pole? I am not good with filter designs. Bypassing temporarily or permanently would be fine. I just need to understand what can and cant be removed. Any help is appreciated.

This is first time I saw that schematic, I dragged my feet on ever doing one.

The additional LPF pole is formed by C31 by the volume pot and series resistance... Your jumper would make the series resistance 0 Ohm .  ;D oops.

You didn't answer whether you wanted to permanently disable the EQ or temporarily bypass it. Lifting the grounds from VR2 might accomplish the temporary bypass.  more permanent is remove the sundry caps.

please do not take my comments as a suggestion for how to make this "better"... No doubt an engineer and product manager more enthusiastic than me about the product concept voiced that EQ contour because they though it sounded good...or what they expected the customers for such products to like. I pretty much kept my distance.

JR

PS Coincidentally around that time I was in management dealing with trying to sell some slow moving inventory. I had a bunch of 4 input tube mixers designed for high end guitar rigs... Despite being a real tube path, I couldn't give them away. 

 

[Coldsnow]

Probably a dumb question from someone that knows very little but, with the tone control at midway or any point, is the eq and low pass filter neutral?
Also, I have a couple of these and always think they sound better in parallel.  Maybe that is why?

 

[imloggedin]

The additional LPF pole is formed by C31 by the volume pot and series resistance... Your jumper would make the series resistance 0 Ohm .  ;D oops.

You didn't answer whether you wanted to permanently disable the EQ or temporarily bypass it. Lifting the grounds from VR2 might accomplish the temporary bypass.  more permanent is remove the sundry caps.

please do not take my comments as a suggestion for how to make this "better"... No doubt an engineer and product manager more enthusiastic than me about the product concept voiced that EQ contour because they though it sounded good...or what they expected the customers for such products to like. I pretty much kept my distance.

John,

I would rather temporarily bypass (or put it on a switch), but if removing it permanently is easier than I want to do that. My main goal is to get rid of the low pass filter. To answer Coldsnow: the low pass filter is always active. I have tested the frequency response, its almost impossible to make flat. John: Will removing C31 alone get rid of that LPF? I realize you are not recommending this as a "better" mod, but I would rather have the option of no EQ.

 

[JohnRoberts]

Yes removing C32 will remove the LPF, perhaps consider putting a much smaller cap in it's place if there is a stability issue.

JR

 

[imloggedin]

John, I removed C31 and there is still a low pass, or high shelf (negative) on the top end. It must be part of the tone circuit. Can you recommend a way to install a bypass switch? I guess I will try removing the ground next.

 

[imloggedin]

Removed the ground going to VR2 and signal is like 12db higher now. When I bleed some of the signal off to ground from R3 (through a 2k resistor), the signal decreases... but then it has a mild low and high shelf. wtf. What is C1 doing? Does that have to be removed too? Don't even know what to try now.

 

[JohnRoberts]

John, I removed C31 and there is still a low pass, or high shelf (negative) on the top end. It must be part of the tone circuit. Can you recommend a way to install a bypass switch? I guess I will try removing the ground next.

To make a bypass switch break the circuit where top of volume pot connects to C31. Connect wiper of SPST switch to the pot. Normal position connects pot to original path (C31). Bypass position connects a new added resistor in series from C2(?) to bypass switch. This resistor will scrub off some signal gain so bypass isn't too loud. You will need to determine a final value but i would guess something like 50-100k ohm.

Please don't tell anybody I'm involved with a tube sweetener...

JR

 

[imloggedin]

Its our dirty little secret John. I will try with a resistor in series later tonight. Hopefully theres no LPF still  :-\

 

[imloggedin]

John a have a few more questions:

1. What is the purpose of C1, R3, R4? Does it makeup another low pass filter?
2. Per your suggestions, why should I use a resistor in series with the signal to attenuate it, instead of bleeding it off to ground?

Thanks

 

[JohnRoberts]

John a have a few more questions:

1. What is the purpose of C1, R3, R4? Does it makeup another low pass filter?

C1, R3, R4, and C3, R10, VR2 form a bandpass filter.

The way to evaluate EQ or filters is to look at reactive components (like caps or inductors). In this case the capacitors look like open circuits at very low frequency ans short circuits at very high frequency. The details or where the filter poles are depend on relative resistor and capacitor values.  Looking at that circuit it is flat at very low frequency with loss, then C3 starts conduction causing more loss with increasing frequency, until C1 starts conducting, shunting across the whole network eliminating the loss and flat above the frequency. So a low cut shelf that dips some more before it restores the cut and flattens out. 

2. Per your suggestions, why should I use a resistor in series with the signal to attenuate it, instead of bleeding it off to ground?

Thanks

Typical tube EQ are lossy.. instead of active boost/cut they apply cut only EQ with overall make up gain. It is my speculation that if you remove the lossy EQ, you will still have the make-up gain with nothing to make up for... Of course you could just turn the volume down more.

JR

PS: I am not a tube guy either....

 

[imloggedin]

Typical tube EQ are lossy.. instead of active boost/cut they apply cut only EQ with overall make up gain. It is my speculation that if you remove the lossy EQ, you will still have the make-up gain with nothing to make up for... Of course you could just turn the volume down more.

I realize the point of the attenuation. I was asking why use a series resistor to do it, instead of using a resistor to ground to do it?

 

[JohnRoberts]

Typical tube EQ are lossy.. instead of active boost/cut they apply cut only EQ with overall make up gain. It is my speculation that if you remove the lossy EQ, you will still have the make-up gain with nothing to make up for... Of course you could just turn the volume down more.

I realize the point of the attenuation. I was asking why use a series resistor to do it, instead of using a resistor to ground to do it?

Six of one, half dozen another... Adding a series R is more of a general purpose solution that works with more kinds of circuits. Adding an R to ground looks like it is parallel with R1 (plate load?) but could impact the HPF pole formed by C2 and this R... if too low... Again guessing.  R to ground shouldn't hurt anything.

I repeat I am not a tube guy... never designed one tube product... no desire to start.

JR

 

[imloggedin]

Thanks again for all the info. Big help.