Tubes circuits and lesser fidelity

[emrr]

To be entirely contrary, lots of feedback is the bane of listenable distortion and saturation.   I have a f'in shitload of tube preamps, and all the high feedback units have crappy obvious distortion, with no real useful distortion knee for signals with long envelopes.  You can't even push drums in very far before it sounds pretty cheesy.  

The units with no feedback are not obviously more hi-fi.  If anything, they are slower and rounder sounding.  You can push them pretty far into distortion and it keeps sounding smooth at the same time.  

I don't hear much that I call nice distortion coming out of anything done at Abbey Road.   When I hear what I consider to be preamp overload, it sounds pretty crappy, and reminds me of the various high feedback tube preamps that I call "clean".   I don't hear anything coming out of Abbey Road that sounds like a Little Richard or Chuck Berry record, and IMO that stuff has excellent distortion characteristics, the sort that can be mistaken for fatness rather than distortion, and even when obvious sound so controlled as to seem on purpose (by modern standards), rather than by accident.  

As to the cathode follower effect as found in guitar amps, I find it a total one trick pony that works great on guitars, in certain instances.    I definitely don't want everything sounding like that.   It seems like the sort of distortion that is most easily mimicked by a billion plug-ins, and is almost a waste to bother building.  OTOH, if you just absolutely love that sound, and really want everything to sound that way, then go for it.   It's not for me, and is not what I think of in regards to saturation, it's way past it.   It's like the difference between soft knee low ratio compression and hard knee limiting.  

Grid stoppers are a waste of resistors, unless you find a problem that needs fixing.  There's hardly a decent preamp circuit out there that requires them.   I would suggest that they are a band-aid for poor design or poor layout.  

 

[Schmilsson]

I simply read something about grid stoppers being a way to drive stages without changing to the load on the previous stage, and thus not crapping out as much on peaks. But I'm pretty much clueless.

 

[Schmilsson]

PS. Why did you have to post that direct coupled C/F link now that I've very nearly finished designing a guitar amp-like tube preamp/overdrive for tracking duties.  :'( I had no idea I could do that kind of asymmetric but controlled distortion.

haha, totes!

 

[DaveP]

emmr is right,

You won't get distortion from Abbey Road pre-Amps, what I was meaning was that the feedback in them never did the sound any harm, as it was Tape compression/distortion that did the business.  I recommended the C/F option because you sounded like you wanted something really noticeable rather than subtle.

Best to go the no feedback route then, but beware of too many stages as you could turn the amp into an oscillator even without deliberate feedback.  There will be feedback through the power supply unless its regulated and there may even be enough capacitance at ultra sonic frequencies between the input and output from the wiring layout, especially with low gain low Z tubes.  Best to use a scope to spot that.
best

 

[lassoharp]

I'm finally getting around to building this thing. Gonna be a pretty hifi unit after all, but I hope driving it as hard as I want will get me a long way.

Doing the nyd 6sn7 circuit, copying the first stage and slapping it on up front. So three 6SN7 for a 2-channel pre. Probably gonna add grid stoppers to the original design. Any hints on the values on those?

Also, I wanna pad the signal before hitting the output iron, can I use a simple pot? I don't mind if it's a crude solution.

Eskimo,

In hopes of saving you some possible grief - have you built up a test version of the 3 stage 6SN7 amp you sketched out earlier?  Doing 3 stable stages with no feedback is likely going to be difficult - esp when you are wanting to run it at higher gain.  Grid stoppers may tame some problems but I wouldn't use them unless there's a problem to be fixed.

By saying you wish to pad the output before hitting the iron I assume you mean putting a 10K pot in as the plate load resistor of V3. The SN7 may be happy part of the way but at a certain point you're going to start getting (unintended for a pad) distortion as the load gets really low.  It may make for a fun experiment but I would suggest just using the gain control in front of V3 to control this level.

But again I must say that building a test circuit up and trying these things out is the best answer you'll likely get.  You can build and audition 5 different circuits in a days time once you get going.  There's a certain popular vintage tube pre I had intended building - 5 minutes of listening to it on the bench told me otherwise and saved me a lot of later disappointment.

 

[Schmilsson]

Dave, the common cathode thingy was news to me, so big thanks for that, gonna check it out!

lasso, a test circuit would be very wise indeed, can't agree more. my favourite expression when it comes to actually trying out classics and hyped stuff is usually "meh", so there you go! :lol:

 

[lassoharp]

I can name some fifties rock records where I know the vocals have been tube distorted but even there I'm waiting to be proven wrong.

Dissecting the various distortions on the old records is always interesting.  The Little Richard records are a good study.  My vinyl versions of 'Tutti Frutti' and 'Slippin & Slidin' have a good deal more distortion on them than most all of the versions readily hearable on youtube - some from vinyl, some apparently from digi-remasters into mp3. 

If you're looking for a good example of recorded distortion from the tube era it's hard to top this one. Total overload and more lo fi than the one (probably first) version of 'Tutti Frutti'.


http://www.youtube.com/watch?v=NlHO7OEzHQk

 

[emrr]

Cool track.

You will never get a high feedback ANYTHING to make that sort of distortion.    The peaks on that are going as square as a non-NFB amp will do, and a high NFB amp will go way past that point immediately, and then you gotta do.......take 2.......

 

[lassoharp]

Here's a priceless interview with Cosimo Matassa.  Great anecdotes and some relevant info on distortion in the vinyl cutting process.  Too bad the limiters aren't revealed . . . . but - I tell you what:  ;D 

http://www.toddcollinsmusic.com/index2.php?option=content&do_pdf=1&id=84 

 

[emrr]

He talked about having a Fairchild at the 2004(?) TapeOp conference. 

 

[Kingston]

If you're looking for a good example of recorded distortion from the tube era it's hard to top this one. Total overload and more lo fi than the one (probably first) version of 'Tutti Frutti'.

http://www.youtube.com/watch?v=NlHO7OEzHQk

No wonder white suburbia was absolutely terrified! That one tops most modern psychobilly tracks!

Here's a priceless interview with Cosimo Matassa.  Great anecdotes and some relevant info on distortion in the vinyl cutting process.  Too bad the limiters aren't revealed . . . . but - I tell you what:   ;D

what could it be... ;D

Funny they had loudness war even back then, resorting to nasty tricks like clipping the cutter. At least they knew what they were doing. Or did they. Anyway, great article.

 

[Schmilsson]

Could someone explain the open loop pentode to me? How does it work, benefits and so on.

 

[abbey road d enfer]

A pentode has more gain than a similar triode. As an example, the EF86 in pentode mode can achieve gain of ca. 200x. In triode mode, max gain is ca. 70x.
This is obtained at the cost of distortion, which is not only higher numerically, but also has a different harmonic content that is judged harsher. The output impedance is higher too, which may be a disadvantage in certain topologies.
Generally, a pentode would be the 1st stage of a multi-stage circuit, with some NFB around. The large gain of the 1st stage makes the noise of the second stage almost irrelevant.
The EF86 (without NFB) is supposed to be responsible for the "jangle" of the famous Vox AC15.

 

[Schmilsson]

odd order harmonics?

 

[emrr]

See my RCA thread for a specific pentode case, and what happens when you run it without NFB. 

 

[Schmilsson]

I'm definately gonna try that cathode follower mentioned earlier out.

http://www.freewebs.com/valvewizard1/dccf.htm

He's pretty set on the 12ax7, are there others that could work? How much gain does that circuit produce?

 

[emrr]

I put an input transformer and a pot in front of that circuit in the very first preamp I built years ago.    I found tube QC for current production was all over the place, and one side would draw far too much current compared to the other, no matter what I put in it.  This affected sound and performance drastically.  I wasn't sorry to see that one go away.  YMMV.

 

[Kingston]

Did you experiment with the load/bias resistor of the cathode follower, say 220-30k? I get the idea from the article it makes the asymmetric "shape" at least partially controllable. The shapes in those oscilloscope images, especially the 100k (and perhaps 220k) setting should not sound completely awful. In fact I've done the same digitally many times and there's definitely a use for it. But I would at least boost some bass before this, otherwise it's going to sound raw.

It's odd how few - if any - guitar amp-like circuits there are for "line level shaping" usage. That to me sounds like something to develop further.

 

[Schmilsson]

Would be fun to see what you're up to kingston.

 

[emrr]

Did you experiment with the load/bias resistor of the cathode follower, say 220-30k?

If you're responding to my comment, my point is that I found any change in tube changed the characteristics, and you'd have to change parameters to suit the specific tube.  Change to another 12AX7, and change the parts around it.   

I'm sure it's fine for some things, but it didn't work when trying to make a stereo pair of preamps that acted like one another.