What do you DO with ROG effects?!?

[frank]

Yes it would be more complex. But if you take a push-pull-ot stage with NO negative feedback, it would be a less high first step at trying to do so.  I would have to work with a simpler idea and then think about the presence thing (later=procrastination, I know). No?   Am I doing my simulation for NOTHING (but knowledge)?  Mark, I will never do that solder WAX MASK now !!!  My concentration is on three things at the same time.

[JDoyle]

I'm looking at the schematic for the Carvin Vintage 33 at the moment, which has a different sort of front end. the schematic shows a 2+2 back-to-back pair of 1N4745 diodes used in some sort of clipping manner.  I see these are 16V zeners, which would give the diodes a clipping threshold of +/-32v.  Assuming one wanted to make a FET-for-triode version of such a fron end, what sort of dioes would be suitable replacements?  Clearly you can't leave the 1N4745s in there.

The schematic can be found here: http://www.bnv-gz.de/~ooehmann/schematics/carvin/Vintage_33.pdf (Vintage 50 has pretty much the same front end).

I don't think the 16V Zener voltage comes into play because for the opposite polarity a Zener diode acts (and IS) a 'standard' diode. If the diodes were drawn as actual Zeners AND they were opposed to each other, THEN the zener action would come into play, but as diagrammed, they are just regular, 0.6V diodes...

So in my opinion, someone at Carver either didn't think this through and ended up with a happy accident that sold, or they had a lot of 1N4745s hanging around and used them as regular diodes...

Regards,

Jay Doyle

[puretube]

The curiosity about the transformer partly stems from the absence of anything that one can use as a source of "presence" the way it is used in true amplifier circuits, where harmonic components from the transformer output that are not present in the input to the transformer are fed back in varying amounts as negative feedback.  Now, whether you could get a dinky little PC-mount interstage transformer to behave like a power output transformer....that's a different kettle of fish, and well beyond the perimeter of my expertise.

Hi Time for a new generation of Distorters... 

[frank]

Hi Time for a new generation of Distorters... 

$%?&*!  I scraped my wax mask as I was thinking of that O.T. simulation. Now both are becoming a comedy.   ...  

[mac]

I only breadboarded the 18 and the prof tweed at about 30v and they sounded very good as pedals. Also experimented using the fx loop to bypass the pre amp.

mac

[trendyironicname]

Mark, I think you are a genius.  From day one back in 2000, if I see something with your name on it, I read the whole thing.  A lot of people use the, "ah, we came up with it at the same time" excuse when it comes to stealing ideas, see elisha gray..  um but it really is weird how i was sitting looking at this tiny transformer I got from a grab bag I ordered day before yesterday and thought, "i wonder what tone transformers would add in the output of a pedal."   I would love to say that great minds think alike but mine is no good. But you sir, your mom should be proud.  Must've read to you as a child.

[arawn]

I use rog designs as effects pedals to make up for what my amp lacks in character or balls depending on my mood

[DougH]

IMO, output transformer distortion is way overrated. Case in point, I'm working on an amp right now. Last night I subbed the smallish output transformer for a bigger one with a higher wattage rating. The sound instantly improved- better bass response, more headroom, a more interesting sounding midrange- etc. The smaller transformer was adding some distortion, but compared to the bigger one it made the overall tone of the amp sound like mush. These are both run-of-the-mill Hammond transformers, nothing special or expensive.

There's a reason that amp builders prefer bigger iron in an output transformer. It's not because they want more distortion- it's usually because they want less. They usually want more headroom and better bass response among other things. IMO, amps are kind of like pedals in that distortion is easy- headroom and EQ are the hard parts. There was nothing 'special' about the OT saturation I was hearing. It was just more distortion from another source and it smeared the frequency response of the amp, not in a good way.

IMO, the "effect of an overdriven output transformer" is a red herring. Yes, there may be all kinds of complex interactions but at the end of the day what does it really add to the mix of everything else going on in an amp? Not much - I-M-O. It may make for an interesting academic discussion to some, but practically speaking I don't think it's ever going to come to much in a pedal circuit.

[frank]

For me it's an "academic" issue.  Trying to understand how the transformer can affect tone is just interesting for my home experiments and understanding.  I am not trying to produce a "magic" effects that would make every guitar player happy (plus I don't have the ability to do so).  Like I said before, if you want the real sound, just buy a small tube amp or make one.  Plus, you have the peace of mind of playing a REAL musical instrument that is embedded in a tradition.  Scaling down a tube amp is a "trip" that is left for curiosity.  Playing with small low voltage components is just a way to learn and do a hobby, such as plastic miniature models.  The ROG are fun interesting effects, that sound good and have "flaws".  I don't know who is working with OT in FET designs and I don't "care" who had the idea first (the word care is not pejorative here).  Who will claim the idea, I will leave him the "TITLE", NO PROBLEMS.  I have mechanical patents to take care of and this kind of activity is taking a LOT of energy.

What I understand:   For the transformer, bigger is not necessarily affecting tone (that much) as long as you are not saturating the core (or under powers it).  What I mean is that the transformer is designed to "handle" a particular electronic design (and power).  The size of the iron core is a function of the magnitude of magnetic flux that will be generated by the primary winding and the current that goes through it.  If you have same windings and vary the size of the core you will have saturation sooner or later and you may have different leakage inductace (negligible).  So probably when you swapped transformers in your amp, it was more of an impedance matching issue.  For scaling down,  the problem is that FETs and tubes ARE NOT THE SAME THING AT ALL.  The output impedance (and the power) of a pair of tube triode design and a J201 design is why you have the two windings of the OT different.  Therefore the "filter" action of the OT will be different.  So again, what I understand, is that the advantages of putting the transformer in a FET design is for hysteresis in the core, core saturation and also, as Mark said, you also have the feedback loop that is also a parameter. 

     

[Mark Hammer]

My intent in suggesting use of output transformers was to permit use of traditional presence controls that actually use the input-to-output differences in transformer signal to clean up the signal.  Not that the goal of any circuit with a gain and master control is to make the sound clean, but I figure if a control like that is good enough for a Bassman or Marshall, then it's good enough for the rest of us. as a way to make the overdriven signal as smooth as possible.

[frank]

My intent in suggesting use of

1- output transformers was to permit use of traditional presence controls
that actually use the input-to-output differences in transformer signal to clean up the signal.

2-  Not that the goal of any circuit with a gain and master control is to make the sound clean,

3- but I figure if a control like that is good enough for a Bassman or Marshall, then it's good enough for the rest of us.

4- as a way to make the overdriven signal as smooth as possible.

I  follow you more,

1- I have putted a presence control on my ROG Princeton, but there is no Master volume and no transformer.

2 - What I think you mean is that the ROG design is "incorrect" because the feedback loop is not over an real output stage and transformer, but only over the simple gain stages, and you would like to put it like the 100W Twin reverb, at the end of the master volume.
http://www.ampwares.com/ffg/schem/twin_reverb_sf_100_schem.jpg

3- I understand that the Master is going at the same point as the input of the feedback loop and I see what you mean by the complexity of the whole thing.  The bassman don't have a presence control. Are you referring to a "mod"?

4- OUF!

I have to study that.  I think you are referring to the input of the output stage.

I was continuing at thingking at core saturation.
http://www.diystompboxes.com/smfforum/index.php?topic=65915.0

[trendyironicname]

they really need the foot in mouth smiley up there.
I re read my post and it may have sounded stupid and completely backwards to the way I was intending it. 
It should have read,"mark, you're a genius. I'm stealing your idea."

That whole thing was my poor attempt to write down my amazement at realizing sometimes ideas just want to be had.  Like one day the lightbulb wanted to be invented and gave the idea of itself to as many people as it took to get it properly realized.   <----I'm really not this dumb.  I'm dealing with only a couple of hours sleep here lately.  Past two weeks have been absolutely heck.  I've had ONE(1) actual full night's rest out of the whole thing. That's when my grandpa gave me one of his super old guy, "hey you got arthritis? we'll make you sleep," "there's no way this should be legal, I think I'm in a coma." medicine so you can rest, medicine.  I don't know if you can die from insomnia but I know I have it right now.  And I just sat there and looked at that sentence for 5 minutes.  I don't know why I typed this.  G'night guys.
Robbie 

[frank]

Am I entering a private zone (I have insomnia too).
Maybee I should go bacK to my solder mask methods.

[Mark Hammer]

I shovelled a foot of snow yesterday and fell asleep earlier than you folks.  Glad to be called a genius, if only by accident, and if only once in a while.

The turn this thread is taking makes me wish I had an inductance meter for measuring all those little interstage transformers I cannibalized from circuits over the years.

[frank]

Yesterday, in Montreal the snow was a disaster.  Everybody is so tired of shoveling here.  The cars are stuck everywhere.  I wish I had an inductance meter and a snowblower too !!! 

[Mark Hammer]

I think total snowfall this winter is up to around 400cm+ in both Ottawa and Montreal, with roughly 50cm (that's a foot and a half for you non-metric types) falling in the last 36 hrs.  There is simply very little place left to put the damn stuff whether you have a snowblower or not.  We have a picnic table that seats about 8 in the backyard.  At the moment, you have to actually know there is such a table out there to be able to see any trace of it because the snow is about 4ft deep in the whole back yard.  Normally, our driveway can fit 2 cars side by side.  Right now we are collectively trying to make a passage so that the people who bring the newspaper and the mail can actually GET to the door on foot.  

Bruce %^&*burn can wish for a rocket launcher all he wants.  What I really wish I had was a flamethrower.

[ItZaLLgOOd]

If I was to use the Professor Tweed as a preamp for a chipamp could I do away with the filter at the end of the circuit and let the speaker do it's thing?? 

[Mark Hammer]

T'wer I, I'd lift the 2nd 2n2 (.0022uf) cap but leave the 1st one in.  You are correct in assuming that a speaker emulation is not likely required, but:
a) There probably needs to be a series resistance at the input to the master volume such that when you dime it there is a series resistance between the PT output and the input of the chipamp, so keeping the pair of 10k resistors as smart.
b) With the PT tone control set to max treble, there can be a risk of hiss, so a 10k-50k linear pot can go between the remaining 2n2 cap and ground to provide tweakable treble trim.

[ItZaLLgOOd]

Thanks for the help. 

So to answer the question of the original post,  I plan on using a Professor Tweed and an Eighteen as preamps for a small combo amp.

[frank]

My intent in suggesting use of output transformers was to permit use of traditional presence controls that actually use the input-to-output differences in transformer signal to clean up the signal.  Not that the goal of any circuit with a gain and master control is to make the sound clean, but I figure if a control like that is good enough for a Bassman or Marshall, then it's good enough for the rest of us. as a way to make the overdriven signal as smooth as possible.

Mark, I think you refer to Nyquist.  Stability is a nightmare... I forgot about poles but...
Harry Nyquist was not thingking of a Marshall stack, now he is too in the Mojo Zone.
Thanks again Mark, I am having flashbacks...