Greetings, and Apologies.
I have the feeling that I have posted this in the wrong place, but darned if I know where else to post it. Please, pardon my ignorance.
I recently bought a VHT Special 6 Ultra guitar amp(made for modding). The stock amp is really over the top as it comes, but I have some mods to make(reverb, add a choke and shielded cables to input, etc.). With knobs and switches galore, the only disappointment is the Texture control. VHT assured me that it was a subtle but effective treble shaper. My ears are not the best, but I hear zero change in tone from the use of this switch, which I am informed is a Boucherot Circuit, used as, or in, a Zobel network They said it was most effective when one played around with the other adjustments in conjunction with it😕, and I've adjusted the Hell out of everything and...Nadda ! I looked up both Boucherot cell and Zobel network, finding very little info on either that appeared to apply to my situation. It seems that most of what I found out concerned impedance matching, and nothing implied it's use as a treble adjustment circuit. Perhaps it's just beyond me.
However, if anybody wishes to tackle this, I would be very appreciative. The schematic can be found at vhtamp.com. Select Special 6 ultra, and choose manual, then download the manual to find the schematic on page 15. I suggest zooming to 200%, to better see the values and fine print.
I see one cap(C50) seems to be a 2.2uf/100V cap, and the other is a 3.3uf/100V cap. I am wondering if increasing the value of the two caps might give more noticeable results, without harming anything. VHT, who has been good to answer all of my questions so far, are silent about this one. I actually sent the first amp I received back, because I found no effect by using this switch, thinking that I had a bad switch. It's hard for me to believe that 2 in a row had a problem. I had my brother, who plays bass and has excellent hearing(so far) listen to it, adjust it, etc., and he thought it worthless.
I would appreciate any opinions, and would especially like to hear any thoughts to my intent to change the value of the 2 caps in question.
Thanks so much for ANY replies. I figure if d.y.i audio can't cover this one, I'm probably out of luck !
tonequester.
I have the feeling that I have posted this in the wrong place, but darned if I know where else to post it. Please, pardon my ignorance.
I recently bought a VHT Special 6 Ultra guitar amp(made for modding). The stock amp is really over the top as it comes, but I have some mods to make(reverb, add a choke and shielded cables to input, etc.). With knobs and switches galore, the only disappointment is the Texture control. VHT assured me that it was a subtle but effective treble shaper. My ears are not the best, but I hear zero change in tone from the use of this switch, which I am informed is a Boucherot Circuit, used as, or in, a Zobel network They said it was most effective when one played around with the other adjustments in conjunction with it😕, and I've adjusted the Hell out of everything and...Nadda ! I looked up both Boucherot cell and Zobel network, finding very little info on either that appeared to apply to my situation. It seems that most of what I found out concerned impedance matching, and nothing implied it's use as a treble adjustment circuit. Perhaps it's just beyond me.
However, if anybody wishes to tackle this, I would be very appreciative. The schematic can be found at vhtamp.com. Select Special 6 ultra, and choose manual, then download the manual to find the schematic on page 15. I suggest zooming to 200%, to better see the values and fine print.
I see one cap(C50) seems to be a 2.2uf/100V cap, and the other is a 3.3uf/100V cap. I am wondering if increasing the value of the two caps might give more noticeable results, without harming anything. VHT, who has been good to answer all of my questions so far, are silent about this one. I actually sent the first amp I received back, because I found no effect by using this switch, thinking that I had a bad switch. It's hard for me to believe that 2 in a row had a problem. I had my brother, who plays bass and has excellent hearing(so far) listen to it, adjust it, etc., and he thought it worthless.
I would appreciate any opinions, and would especially like to hear any thoughts to my intent to change the value of the 2 caps in question.
Thanks so much for ANY replies. I figure if d.y.i audio can't cover this one, I'm probably out of luck !
tonequester.
I notice from the curves given in the manual that it only claims to give about 4dB of cut, at and above about 5kHz - which is not very much. I think it might have a more
noticeable effect when the output stage itself is overdriven (i.e. playing with all volume, power and gain controls on full). In this situation the output tube and transformer will be adding their own extra high frequencies, due to distortion.
These extra high frequencies are coming too late in the chain to be affected by any of the other tone shaping and would only be tamed by the Zobel network.
noticeable effect when the output stage itself is overdriven (i.e. playing with all volume, power and gain controls on full). In this situation the output tube and transformer will be adding their own extra high frequencies, due to distortion.
These extra high frequencies are coming too late in the chain to be affected by any of the other tone shaping and would only be tamed by the Zobel network.
A Boucherot cell or Zobel network is a CR circuit added to dampen resonance or control HF impedance, often at the output of a feedback amplifier. There seems to be a modern habit for some to refer to any CR circuit as a Zobel - this is confusing and should be avoided.
An identical network used as part of a tone control may be called a shelf network, as it causes a shelf in frequency response.
An identical network used to help loop stabilisation is known as a lead-lag network, as it causes both a lead and lag (at different frequencies) in phase.
An identical network across a switch or relay contact is known as a snubber.
So the name of the circuit depends on the context and purpose for which it is used. Google results will lean heavily towards the purpose for which you searched, even if you meant to find out about a different purpose.
An identical network used as part of a tone control may be called a shelf network, as it causes a shelf in frequency response.
An identical network used to help loop stabilisation is known as a lead-lag network, as it causes both a lead and lag (at different frequencies) in phase.
An identical network across a switch or relay contact is known as a snubber.
So the name of the circuit depends on the context and purpose for which it is used. Google results will lean heavily towards the purpose for which you searched, even if you meant to find out about a different purpose.
I think this will get more responses in the appropriate forum which is Instruments & Amps so I will move it there.And since its is a guitar amp I would expect that it has a "mid-range" -oriented speaker setup that starts to roll down the frequency response in a steep slope at around 3 - 5 kHz.
Most likely as the harmonics from clipping distortion will add more content to the higher end of the frequency spectrum.
As usual with guitar amps, most effects really start to show up only when you enter the distortion territory. ...and 4 dB really isn't that much.
Zobel network feed back welcome.
Greetings guys.
Thanks to Malcolm Irving for your reply. I still don't get this. I can hear a 4db difference, my ears are not THAT bad. Believe me, I have driven this amp as hard as it can go, and there is still ZERO noticeable difference using the effects 2 settings. No difference from bypass, period. Thanks to DF 96 for the search tips concerning this subject. Thank you kevinkr for moving my post to the proper section of the forum. Apologies for missposting it. Thanks to teemuk for the advise, However, as I said before, I have definitely overdriven the output section. I had the amp "smokin" with great overdriven tone, even getting controllable feedback. No complaints there. I just don't like switches and effects that don't do squat. My brother has played bass professionally for 30 years, yet has excellent hearing, the guy can figure out every nuance of a song.
He adjusted the amp himself, and jacked with it for a half hour. His call ? "What am I supposed to hear" ? "All I hear is the click of the switch". Thanks gentlemen for your comments. I still don't know if changing values of C will produce anything audible ! tonequester.
Greetings guys.
Thanks to Malcolm Irving for your reply. I still don't get this. I can hear a 4db difference, my ears are not THAT bad. Believe me, I have driven this amp as hard as it can go, and there is still ZERO noticeable difference using the effects 2 settings. No difference from bypass, period. Thanks to DF 96 for the search tips concerning this subject. Thank you kevinkr for moving my post to the proper section of the forum. Apologies for missposting it. Thanks to teemuk for the advise, However, as I said before, I have definitely overdriven the output section. I had the amp "smokin" with great overdriven tone, even getting controllable feedback. No complaints there. I just don't like switches and effects that don't do squat. My brother has played bass professionally for 30 years, yet has excellent hearing, the guy can figure out every nuance of a song.
He adjusted the amp himself, and jacked with it for a half hour. His call ? "What am I supposed to hear" ? "All I hear is the click of the switch". Thanks gentlemen for your comments. I still don't know if changing values of C will produce anything audible ! tonequester.
Hi,
Increasing the value of C would make the same amount of treble cut (say 4dB) start happening at a lower frequency. E.g. doubling the C would start the cut from 2.5kHz
instead of 5kHz. Before you buy any cap.s you could try putting the two existing ones in parallel (to get a combined 5.5uF).
I think any effect (if there is one!) would be more noticeable in 'pentode mode', as
the output stage is then more like a 'current source' and the voltage produced at the speaker is more sensitive to the change in total load impedance when the 'Zobel' becomes effective.
Malcolm
Increasing the value of C would make the same amount of treble cut (say 4dB) start happening at a lower frequency. E.g. doubling the C would start the cut from 2.5kHz
instead of 5kHz. Before you buy any cap.s you could try putting the two existing ones in parallel (to get a combined 5.5uF).
I think any effect (if there is one!) would be more noticeable in 'pentode mode', as
the output stage is then more like a 'current source' and the voltage produced at the speaker is more sensitive to the change in total load impedance when the 'Zobel' becomes effective.
Malcolm
Hey Malcolm.
I understand what you are saying. I think that I'll try what you advised. Believe me, I got this thing overdriving, in pentode mode(triode mode sounds muffled). I was even getting some pretty cool controllable feed back. Yet, the Texture switch had NO TEXTURE !
It's funny how sometimes the numbers just don't jive with the "real world experience". As I said before, I can hear a 4db change in volume. However, it isn't really much of a change. I have a Villex Mid range boost on my guitar, and it is VERY noticeable with a 5-6db boost. You could hear it across the yard. I tried headphones, attempting to detect the Texture effect, with no luck at all. Hell, I sent the first amp back because I thought the switch was defective. Now, I know that the first amp was just fine(except that the effect is only heard by dogs and maybe a cat or two). I'm going to change those cap values like you advised. I'll let you know how it turns out. Thanks again for all the help.
Best Wishes Malcolm ! tonequester..........still questing !
I understand what you are saying. I think that I'll try what you advised. Believe me, I got this thing overdriving, in pentode mode(triode mode sounds muffled). I was even getting some pretty cool controllable feed back. Yet, the Texture switch had NO TEXTURE !
It's funny how sometimes the numbers just don't jive with the "real world experience". As I said before, I can hear a 4db change in volume. However, it isn't really much of a change. I have a Villex Mid range boost on my guitar, and it is VERY noticeable with a 5-6db boost. You could hear it across the yard. I tried headphones, attempting to detect the Texture effect, with no luck at all. Hell, I sent the first amp back because I thought the switch was defective. Now, I know that the first amp was just fine(except that the effect is only heard by dogs and maybe a cat or two). I'm going to change those cap values like you advised. I'll let you know how it turns out. Thanks again for all the help.
Best Wishes Malcolm ! tonequester..........still questing !
Hi,
As teemuk said, guitar speakers don't respond much at these high frequencies anyway. Another thing you could try (just to see if any difference can be detected) would be to put a tweeter in series with the Zobel path, if you have one sitting around. (Not recommended for normal playing and watch out for blowing the tweeter, even at 6 watts, but it least it would show if anything is happening in that circuit.) The cap in the Zobel would prevent bass frequencies going through the tweeter.
Malcolm
As teemuk said, guitar speakers don't respond much at these high frequencies anyway. Another thing you could try (just to see if any difference can be detected) would be to put a tweeter in series with the Zobel path, if you have one sitting around. (Not recommended for normal playing and watch out for blowing the tweeter, even at 6 watts, but it least it would show if anything is happening in that circuit.) The cap in the Zobel would prevent bass frequencies going through the tweeter.
Malcolm
More on Zobel network.
Greetings Malcolm.
Thanks for the "tweeter tip". I do have one......I never throw away anything electronic. Sometimes it pays off ! Hey, VHT finally got back to me about several questions. They say that I can increase capacitance 50-100% to increase the "effect". However, they told me to PARALLEL the existing caps to increase C.
I thought caps added up like resistors in series. I'll have to look that one up. My memory isn't what it once was. Man, I appreciate all of your help in understanding this texture switch. They say that it helps with "fizz" encountered when overdriving the output. They also told me to turn down the Depth control. I hate it when they tell you to turn a control down as a fix for a problem. If it's a fact, then they should have designed the Depth control better. If I were an engineer(fat chance), I'd minimize the "damage" one could do with the stock controls ! Thanks again Malcolm !.................tonequester.🙂
Greetings Malcolm.
Thanks for the "tweeter tip". I do have one......I never throw away anything electronic. Sometimes it pays off ! Hey, VHT finally got back to me about several questions. They say that I can increase capacitance 50-100% to increase the "effect". However, they told me to PARALLEL the existing caps to increase C.
I thought caps added up like resistors in series. I'll have to look that one up. My memory isn't what it once was. Man, I appreciate all of your help in understanding this texture switch. They say that it helps with "fizz" encountered when overdriving the output. They also told me to turn down the Depth control. I hate it when they tell you to turn a control down as a fix for a problem. If it's a fact, then they should have designed the Depth control better. If I were an engineer(fat chance), I'd minimize the "damage" one could do with the stock controls ! Thanks again Malcolm !.................tonequester.🙂
Hi,
They are right, two caps of 2.2uF and 3.3uF in parallel gives 5.5uF. Increasing the cap value will put extra current through the resistor in the 'Zobel' but I notice that it is a 10 watt resistor, which should be fine.
Thinking further about the tweeter idea, I am not sure what it would actually prove, except whether there is some 'fizz' coming through the Zobel path.
Malcolm
They are right, two caps of 2.2uF and 3.3uF in parallel gives 5.5uF. Increasing the cap value will put extra current through the resistor in the 'Zobel' but I notice that it is a 10 watt resistor, which should be fine.
Thinking further about the tweeter idea, I am not sure what it would actually prove, except whether there is some 'fizz' coming through the Zobel path.
Malcolm
Hearing the subtle effects of 4 dB isn't exactly the problem, it's hearing them under very specific constraints. You are not listening to a fixed frequency test tone, you are listening to an entire complex musical signal, of which the range where that subtle 4dB change could take place is very narrow and basically already attenuated highly by the speaker system.
As said, it might need some serious overdrive, "bright" speaker setup and ears trained enough to distinguish what you actually should be listening to in the first place. The highest frequency from an electric guitar, the pinch harmonics of the highest notes, are still practically limited to about 5 kHz (acoustic guitars actually extend to 10 kHz). Ovedrive will increase amplitude of even higher harmonics but a usual design tradition is attenuating them because, coupled to highly compressed and sustaining overdrive tone, they sound "fizzy", obnoxious and fatiguing. Hence choices of speakers that do not extent too far towards higher frequencies in bandwidth.
So overall, there may be a lot of sublety to the effect due to design and due to generic limitations of electrig guitars as instruments. And after all that it's still going to be just 4 dB.
So? As decibel scale is logarithmic 5-6 dB is actually much more than 4 dB. Also, and let's not forget this, human hearing is much more sensitive at mid-range frequencies than it is at low and high frequencies.
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Zobel network and other stuff !
Greetings Malcolm.
I hope that you will get this, as I'm not very forum informed ! I got a message from you on my email, but when I typed up a reply then and there, I got the message saying that I couldn't do this with the d.i.y.. forum. So, I came here, and I don't see your last message.
Anyway, I'll try now. VHT finally answered me about the Zobel network. They agree with what you and others who replied said. It's only a noticeable effect when the amp is full blast(although I don't even hear it then). They advised me to play with the caps values by 50-100%. They also answered questions that I had about excess noise(pre-amp) when using the high gain channel. Besides telling me to turn the Ultra control down !??? They suggest that I change R43 to 470-680 Ohms, and R42 and R49 to 22 kOhm, plus add a choke. I already had the choke and shielded cable(for the input jacks) on order. It seems that pretty much everything that you told me about the Zobel network was spot on ! I appreciate the help.....big time !
If you by chance get the time, I would appreciate hearing what you think about the resistors to be changed by VHT's recommendation. A second opinion is always good in my book. I'm hoping that the Ultra model is not too complex a design to get ahold of. I originally was going for the plain jane Special 6, but Bruce Hutcheon of Bitmo Mods advised me to go for the Ultra. For one thing, his reverb mod won't work on the cheaper amp. He actually tried to sell me an Ultra head that had been modded to the max for $549.95. The only reason I passed, was that I wanted a combo. It looks like I'll have at least that much in THIS amp before I'm done, what with the extra mods to correct that texture effect, and get rid of the noise(if possible).
Hey, I've taken up too much of your time. Once again, I truly appreciate the advise/explanations. You really know your "stuff".......tonequester.
Greetings Malcolm.
I hope that you will get this, as I'm not very forum informed ! I got a message from you on my email, but when I typed up a reply then and there, I got the message saying that I couldn't do this with the d.i.y.. forum. So, I came here, and I don't see your last message.
Anyway, I'll try now. VHT finally answered me about the Zobel network. They agree with what you and others who replied said. It's only a noticeable effect when the amp is full blast(although I don't even hear it then). They advised me to play with the caps values by 50-100%. They also answered questions that I had about excess noise(pre-amp) when using the high gain channel. Besides telling me to turn the Ultra control down !??? They suggest that I change R43 to 470-680 Ohms, and R42 and R49 to 22 kOhm, plus add a choke. I already had the choke and shielded cable(for the input jacks) on order. It seems that pretty much everything that you told me about the Zobel network was spot on ! I appreciate the help.....big time !
If you by chance get the time, I would appreciate hearing what you think about the resistors to be changed by VHT's recommendation. A second opinion is always good in my book. I'm hoping that the Ultra model is not too complex a design to get ahold of. I originally was going for the plain jane Special 6, but Bruce Hutcheon of Bitmo Mods advised me to go for the Ultra. For one thing, his reverb mod won't work on the cheaper amp. He actually tried to sell me an Ultra head that had been modded to the max for $549.95. The only reason I passed, was that I wanted a combo. It looks like I'll have at least that much in THIS amp before I'm done, what with the extra mods to correct that texture effect, and get rid of the noise(if possible).
Hey, I've taken up too much of your time. Once again, I truly appreciate the advise/explanations. You really know your "stuff".......tonequester.
Then there's always the chance that something related to the particular feature is broken. Has this possibility been investigated?
Hi,
Thanks for your kind remarks.
If you want to check if the Zobel path is working, you could measure the AC voltage across the Zobel cap when the amp is playing. Be careful, as usual, use clip-on probes, attach then when power is off and after you have checked that electrolytic caps are drained.
The resistor mods to R43, R42 and R49 would reduce the DC supply voltages, especially to early stages of the amp. This would reduce the noise level.
Another suggestion would be to reduce R1 (68k) down to 10k. (Keep it close to the valve socket.) This is the first grid stopper in the amp and is usually the main noise source. It is there mainly to supress radio-interference, but 10k will usually do the job and will produce less noise.
Malcolm
Thanks for your kind remarks.
If you want to check if the Zobel path is working, you could measure the AC voltage across the Zobel cap when the amp is playing. Be careful, as usual, use clip-on probes, attach then when power is off and after you have checked that electrolytic caps are drained.
The resistor mods to R43, R42 and R49 would reduce the DC supply voltages, especially to early stages of the amp. This would reduce the noise level.
Another suggestion would be to reduce R1 (68k) down to 10k. (Keep it close to the valve socket.) This is the first grid stopper in the amp and is usually the main noise source. It is there mainly to supress radio-interference, but 10k will usually do the job and will produce less noise.
Malcolm
The entire thing just seems bizarre? - a zobel network is there to help prevent instability in a transistor amp, not to offer tonal changes.
Any possible audio changes due to the network are going to be absolutely insignificant, and only at very high frequencies - I'd be amazed if anyone could hear any differences at all, and certainly not without a tweeter.
Any possible audio changes due to the network are going to be absolutely insignificant, and only at very high frequencies - I'd be amazed if anyone could hear any differences at all, and certainly not without a tweeter.
As was mentioned earlier, though it is a series RC circuit shunting 16-ohm secondary to ground calling it "Zobel" is a bit misleading in this case. In generic Zobel filter the capacitance is about hundred nanofarads, in the texture control it's few microfarads (2u2 in one position, 3u3 in another). Big difference to overall response in comparison to more generic Zobel.
Yes, it would be better to call it a 'high-cut shelving filter' rather than a Zobel. That would be a more realistic representation of its purpose here.
I've seen larger capacitors used in proper zobel networks, although personally I always use 0.1uF - and I've done so when repairing amps where the larger zobel capacitor has burnt-up (contacting the manufacturer proved that was the recommended action as well).
But neither of those two capacitors, in series with a 15 ohm resistor, is going to have any audible effect - and certainly not (as I said before) with no tweeter in use. For that matter, what sort of output levels at these high frequencies do you get from a guitar?, presumably next to nothing as they don't use tweeters with guitar amps.
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