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General Discussions => Drawing Board => Topic started by: eskimo on February 14, 2010, 05:38:20 PM

Title: Tubes circuits and lesser fidelity
Post by: eskimo on February 14, 2010, 05:38:20 PM
I'm looking for color, and lots of it, no subtleness required. I hate buzz words as much as the next guy, but I'll give you an analogy and see if it sticks. I'm always looking the polaroid/super8 equivalent in audio, if that makes any sense. Not reality. Something prettier (to me), in an ugly way. In the visual format I'd say that's about oversaturated colors and grain. In audio I suppose the variable is THD?

Of course a pre doesn't do all this, but I've got the other parts down, so the pre is the last piece in the puzzle.

I've searched this forum in and out, and in tubes, it seems to boil down to:

- No negative feedback
- Adjusting plate voltage
- Playing around with bias

I've fooled around with this circuit (altec pre), replaced R2 with a 25K pot(that's what I had) and I can't say much happens when adjusted. Sure at zero and a bit up, it's all faint, but once you have a proper signal, much doesn't change.

I had the plates down from 135V to like 70V, and still no big difference.

(http://eskimo.creotia.com/438pre.jpg)
To my extremely limited knowledge, this circuit doesn't provide any negative feedback. No?

What does a man have to do?! ;D

Perhaps I'm barking up the wrong tree and should just insert a friggin distortion pedal, but I'm not sure I'm looking for distortion in that sense, but perhaps rather texture? (buzz word again...)

Any thoughts?
Title: Re: Tubes circuits and lesser fidelity
Post by: EmRR on February 14, 2010, 05:57:01 PM
The output is a cathode follower, which is 100% NFB.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 14, 2010, 06:07:54 PM
Nothing wrong with your current design, but it's surprisingly linear. as you have now found out the hard way.

What does a man have to do?!

More stages. Think guitar amp, but gentle. You'll probably want a tube that has a very nice break up curve, say a 6SN7 (or a similar variant), and no more shrill 12AX7's or variants. Lots of choice here, I've had my eye on 2c22 for example.

Then you'll find out you created a unit that puts out quite a bit of gain, a very usable preamp in fact, and you'll want an output attenuator. I would suggest something before the output transformer, because distorted output transformer rarely sounds good. The distortion there is usually a kind of ring modulated "death clip" you'll want to avoid at all costs. We want just the rich tube sound, and none of the side effects. Sure we're wasting gain, and adding noise with the pre-output trafo attenuation, but what the heck, that's what the specs of the project demand.

Here's an example "stage" that will definitely work (you don't need the input trafo of course). NYD drafted that a couple of years back when me and a couple of others were asking the same questions you are. I would say it's "not enough" for the type of rich cream distortion you're looking for, but stick it after your "altec" and you're bound to have a classic in your hands.

Projects like this are my absolute favourites to DIY.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 14, 2010, 06:09:08 PM
The output is a cathode follower, which is 100% NFB.
Snap, I'm laughin really hard right now.
Well, then I know were to start! ;)
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 14, 2010, 06:16:42 PM
Excellent post, thanks!
I recognize that schem from my searching, think I saved it. I've been pretty set on trying the collins 6Q-1, since doug praised it so highly in the same search.

Whatever I choose will be a front end of a 436 vari mu, as a separate build, or in the same box with a utc a-18 as interstage, haven't decided. While trying the 436 out(as a 438 at the time) I think I experienced the output trafo distortion. Had a attentuator after the trafo. Yeah, not nice at all.

Nothing wrong with your current design, but it's surprisingly linear. as you have now found out the hard way.

What does a man have to do?!

More stages. Think guitar amp, but gentle. You'll probably want a tube that has a very nice break up curve, say a 6SN7 (or a similar variant), and no more shrill 12AX7's or variants. Lots of choice here, I've had my eye on 2c22 for example.

Then you'll find out you created a unit that puts out quite a bit of gain, a very usable preamp in fact, and you'll want an output attenuator. I would suggest something before the output transformer, because distorted output transformer rarely sounds good. The distortion there is usually a kind of ring modulated "death clip" you'll want to avoid at all costs. We want just the rich tube sound, and none of the side effects. Sure we're wasting gain, and adding noise with the pre-output trafo attenuation, but what the heck, that's what the specs of the project demand.

Here's an example "stage" that will definitely work (you don't need the input trafo of course). NYD drafted that a couple of years back when me and a couple of others were asking the same questions you are. I would say it's "not enough" for the type of rich cream distortion you're looking for, but stick it after your "altec" and you're bound to have a classic in your hands.

Projects like this are my absolute favourites to DIY.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 14, 2010, 06:42:12 PM
Since I have that pre allready wired up, is there an easy way to simply make it a plate follower instead to try it out with no feedback?
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 14, 2010, 06:50:03 PM
[edit]

whoops! hope you didn't read what this post originally said.

Anyway,

The easiest solution is to just copy the first stage (or the first stage of that NYD schematic).
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 14, 2010, 07:06:26 PM
As in: C4 to plate instead of cathode, and add an electrolyte (don't think I have one at home) to the cathode? 
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 14, 2010, 07:11:20 PM
the cathode bypass cap is not completely necessary, but that's the part that blocks negative feedback. And dammit, I screwed up with that image, plate resistor is very much needed. http://www.aikenamps.com/CommonCathode.htm

update:
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 14, 2010, 07:24:43 PM
In other words, the bypass cap is essential, given the topic. ;)
Thanks for the link, got some reading to do!
Title: Re: Tubes circuits and lesser fidelity
Post by: PRR on February 14, 2010, 09:54:53 PM
THD color is a lot about the ratio of signal voltage to supply voltage.

Run your Altec on a 9V battery. With typical studio levels, it will give not-clean to grossly bent.

> lots of it, no subtleness

Also put graphic EQs front and back, steep complementary curves.
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on February 14, 2010, 11:53:45 PM
I'm looking for color, and lots of it, no subtleness required. I hate buzz words as much as the next guy, but I'll give you an analogy and see if it sticks. I'm always looking the polaroid/super8 equivalent in audio, if that makes any sense. Not reality. Something prettier (to me), in an ugly way. In the visual format I'd say that's about oversaturated colors and grain. In audio I suppose the variable is THD?

Of course a pre doesn't do all this, but I've got the other parts down, so the pre is the last piece in the puzzle.

I've searched this forum in and out, and in tubes, it seems to boil down to:

- No negative feedback
- Adjusting plate voltage
- Playing around with bias

I've fooled around with this circuit (altec pre), replaced R2 with a 25K pot(that's what I had) and I can't say much happens when adjusted. Sure at zero and a bit up, it's all faint, but once you have a proper signal, much doesn't change.

I had the plates down from 135V to like 70V, and still no big difference.

(http://eskimo.creotia.com/438pre.jpg)
To my extremely limited knowledge, this circuit doesn't provide any negative feedback. No?

What does a man have to do?! ;D

Perhaps I'm barking up the wrong tree and should just insert a friggin distortion pedal, but I'm not sure I'm looking for distortion in that sense, but perhaps rather texture? (buzz word again...)

Any thoughts?


Some more things to add to your "boil down to" list:

 - triode vs pentode
 - small tubes vs big tubes
 - different varieties of iron ( some telecommunications pieces can snuff out the "reality" pretty good) 
 - worn out funky, murky sounding coupling caps.
 - old out of spec carbon comp resistors top to bottom
 - trying a plate to voice coil transformer from an old radio/hi-fi set as a crude plate choke
 - PP vs SE
 - using compensation networks to deliberately bump up different sections of the band.

These will probably be somewhat subtle but do effect the fidelity and distortion profile. You may be in a mode of expecting something dramatic.  Subtle can be strong over time.

Do you have some particular sound in mind? - real recorded examples make it much more tangible and clear than any adjectives.

I think Kingston's suggestion of more stages is an excellent way to increase the density of the sound for sure.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 15, 2010, 01:00:41 AM
Care to elaborate on the small vs big tubes?
I think I'm gonna stay with the iron I have. Peerless K308D on mic going to either an utc a-18 as interstage, or an utc H-23 as output.
And yup, definately going SE.

As for what I'm aiming for..hmm, I suppose it's mainly 60/70s sounds. The wrecking crew stuff, as well as their detroit counterparts.
Adore the kinks' grittyness: http://www.youtube.com/watch?v=zL9tyzE83nc
Creamy smoothness of S&G: http://www.youtube.com/watch?v=-IwYQ1Vqf_4

This is were I'm at the moment, going 4-track cassette really got me inches away from were I wanna be:
http://eskimo.creotia.com/hmm.mp3 (this really was a comparison of the 4-track pres and the 438)
http://eskimo.creotia.com/244.mp3

I do agree that multiple stages might be key. Slap a pot in between and then just tune for that sweet spot.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 15, 2010, 03:53:57 AM
Care to elaborate on the small vs big tubes?

Narrowing down would be the common example of 12AX7 variants vs. 6SN7 variants. A difference there to be heard, as the shape of THD in a "roughly equal" setting is quite obvious. 6SN7's will give you mainly lots of round 2nd harmonic - a distortion that might not be so easy to hear - while the 12AX7's clip in grittier ways. There are some very good and powerful "small" tubes, say 5687 or 12BH7, so generalisations should be avoided.

As for what I'm aiming for..hmm, I suppose it's mainly 60/70s sounds. The wrecking crew stuff, as well as their detroit counterparts.
Adore the kinks' grittyness: http://www.youtube.com/watch?v=zL9tyzE83nc
Creamy smoothness of S&G: http://www.youtube.com/watch?v=-IwYQ1Vqf_4

That kinks example is simply early multi-track sound. Only tape distortion there. I suspect the rest of their signal path was more pure than most of today's better digital set ups. Tape was a really bad bottleneck in the early sixties.

S&G vocals certainly shows the type of cream on the vocals, but even that is tainted by tape.
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on February 15, 2010, 11:33:14 AM
Care to elaborate on the small vs big tubes?
I think I'm gonna stay with the iron I have. Peerless K308D on mic going to either an utc a-18 as interstage, or an utc H-23 as output.
And yup, definately going SE.

As for what I'm aiming for..hmm, I suppose it's mainly 60/70s sounds. The wrecking crew stuff, as well as their detroit counterparts.
Adore the kinks' grittyness: http://www.youtube.com/watch?v=zL9tyzE83nc
Creamy smoothness of S&G: http://www.youtube.com/watch?v=-IwYQ1Vqf_4

This is were I'm at the moment, going 4-track cassette really got me inches away from were I wanna be:
http://eskimo.creotia.com/hmm.mp3 (this really was a comparison of the 4-track pres and the 438)
http://eskimo.creotia.com/244.mp3

I do agree that multiple stages might be key. Slap a pot in between and then just tune for that sweet spot.


You got that from 4-track cassette?  Its hard to completely tell on my small computer speakers but it sounds as big as the Kinks stuff.  I'd say you've got that sound nailed - particularly the drums.  Very nice.  I worked on 4-track cassette for years and could never get anything decent from the built-in pres.

I don't know for sure but the Kinks track was dated 1968 so it may have had SS on the front end.

Wrecking crew did a lot of work at United Western Recorders - Bill Putnams place.  I assume that would have been a UA console - small tubes.

Big tubes - what Kingston said plus 6V6s, 1622s etc like you might see on program amps and limiters.

This is one of my favs on the smooth and silky sound: (from 1953)

http://www.youtube.com/watch?v=cUrUtJi2bIw


Did you mean H-22 instead of 23?  The 22 is speced for SE use - the 23 is more like A-26 in that regard.

If you decide to do the 'interstage' arrangement - try pulling out some 1:1 types for different textures. You'll still need the output transformer on the first circuit -  _ _ _:600, then into the 1:1 at the grid of the second amp ( attenuator in between). Should work nice for that sound.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 15, 2010, 01:21:02 PM
Tascam 244, yup. The pres are weird, sort of phasey sounding, but they're workable. Dynacomp on the drums there, fun stuff. :)

I bought a H-23 to use as the output on the 436. Ended up with a pair, so that's why. Not ideal perhaps.
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on February 15, 2010, 01:36:23 PM
Tascam 244, yup. The pres are weird, sort of phasey sounding, but they're workable. Dynacomp on the drums there, fun stuff. :)

I bought a H-23 to use as the output on the 436. Ended up with a pair, so that's why. Not ideal perhaps.


244?  Same one I had!  Yeah, I thought the pres were noisy.  The EQ is helpful though.  Workhorse 4-track.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 15, 2010, 02:06:58 PM
I'm toying with the idea of doubling NYD's 6SN7 circuit. Using 2 tubes, 4 stages. Slap a pot in between each stage for gain staging. Perhaps use a three deck so they can be controlled with one knob, if one can find the right values.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 15, 2010, 03:35:05 PM
I think I understand most of this circuit, but what does R1, C1 and C7/C6 do?
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on February 15, 2010, 03:42:46 PM
I think I understand most of this circuit, but what does R1, C1 and C7/C6 do?


C7, C6 are power supply decoupling caps.

The 100K and cap are a part of a network to tame any ringing from the transformer.  The cap value would be different depending on the particular iron you use.

The 100K by itself would be for loading the secondary so that the mic sees a proper impedance.

I'm not sure but that Peerless you mentioned may be designed to work unloaded in the grid circuit.  Check the online Peerless catalog and see if they mention it.  I know the K-241D is specified this way.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 15, 2010, 04:55:50 PM
I think I understand most of this circuit, but what does R1, C1 and C7/C6 do?

R1+C1 are the zobel network for the input transformer. Transformers can ring in very high frequencies depending on loading conditions and transformer type. Sometimes audibly, sometimes only visible on oscilloscope and maybe negligible in real world use. Basically, square wave response (transients) might be skewed and you might want to tune that away. Most transformer spec sheets will tell you the optimal zobel, and sometimes only a resistor is needed. If you need to tune it yourself (difficult!) there are some very good threads around here.

And sometimes (most times?) zobel network isn't needed.

C7 and C6 are B+ smoothing caps, sometimes called "stabs", reducing ripple. These values are not critical at all, and anything between say, 10-200uF works. In fact they aren't needed for the amp to work. Depending on how "stiff" (regulated or capacitive) your B+ power is, they might not actually help at all.

[edit]

aww heck lassoharp answered these questions already. oh well the more the merrier.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kit on February 15, 2010, 05:18:40 PM
Also put graphic EQs front and back, steep complementary curves.

+1.

Pre/post emphasis EQ is very, very important when designing nonlinear distortion circuits.
Generally speaking, boost high mids going in, and cut high mids going out. 20hz-20khz distorting circuits usually sounds like a 70┬┤fuzz face pedal. (Tubes sounding marginaly better than bipolar parts)
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 15, 2010, 08:34:24 PM
C7 and C6 are B+ smoothing caps, sometimes called "stabs", reducing ripple. These values are not critical at all, and anything between say, 10-200uF works. In fact they aren't needed for the amp to work. Depending on how "stiff" (regulated or capacitive) your B+ power is, they might not actually help at all.

Ah, of course! I'm so used seeing them in the psu section of the schem I got confused.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 16, 2010, 07:02:16 PM
Here's what I sort of had in mind. The values are off since I'm pretty much clueless.
Two NYD stages. The two gain pots goes to a stereo pot for 1knob control. Output attentuator before the interstage trafo. Thoughts?
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on February 16, 2010, 08:43:21 PM
4 stages can be a plateful in terms of stability, distortion, phase shift etc. Without feedback its even tougher.

First thing I see is the A-18 most likely won't give you a low enough output impedance - you really want something like a 10 - 20Kish:600 here.

Second, I sense some overload problems at the grid of the 3rd tube.  Thing to remember is that 3rd = 1st tube in terms of it being designed to give max voltage gain to small signals(mic level).  By the time it gets out of the 2nd tube its a much larger signal (line level).  You may wind up having to keep the first stage volume control super low to prevent hard clipping at normal levels.

To do 4 stages without trouble would probably take a redesign. As is you'd be looking at some 90 or more dbs of gain.
 
For reference, here's one of only two 4 stage tube designs I know of: (note large amount of feeback)

http://www.groupdiy.com/index.php?topic=35357.0




 
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 16, 2010, 09:02:52 PM
If I wasn't clear, I wouldn't use the a-18 as an output but as an interstage between the NYD and the 436.
As for the 3rd stage, I left those values out, figuring the would rather be the numbers of stage 2 and 4, rather than stage 1. And yeah, 4 stages is a lot, i just figured it would make maximum use of the two double triodes.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 17, 2010, 04:39:40 AM
Maybe you'd like something a tried and tested, instead of somewhat experimentally slapping four stages together like above?

Here's what I would suggest, these are all from NYD's plate of goods, but that's just because his designs are easy to understand and make sense from a builders point of view. Well actually it turns out the "two bottle" first stage is similar to some Gates mixer input stages.

1. Build the 6SN7 "one-bottle"
2. attach the below "second bottle" for additional 23dB of gain to drive your gentle 6SN7(EF86 works fine if you can't find 5879). Or just use the Altec "drive" you already have.
3. Use the pre-output-transformer gain attenuator to keep gain below transformer saturation. this is quite important because the above configuration puts out stupid amounts of gain.
4. experiment with lower plate voltages especially on the 6SN7 stages.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 17, 2010, 11:54:49 AM
That doesn't look bad at all! Would sure be nice to try out.

When you say "altec drive" are you referring to the whole circuit or simply the first triode stage?
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 17, 2010, 12:23:11 PM
So, basically this?
I noticed the mod stage is identical to the 6SN7's first stage.
Title: Re: Tubes circuits and lesser fidelity
Post by: craptical on February 17, 2010, 05:34:21 PM
I noticed the mod stage is identical to the 6SN7's first stage.

Except the original NYD one-bottle is made up around a 12AV7 tube. See:

http://www.groupdiy.com/index.php?topic=6711.0 (http://www.groupdiy.com/index.php?topic=6711.0)
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 17, 2010, 06:23:37 PM
Yeah, I know, just saying. :)
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 18, 2010, 04:36:09 AM
When you say "altec drive" are you referring to the whole circuit or simply the first triode stage?

I thought you had already built the altec preamp. could as well drive the 6SN7 stages with that.

So, basically this?
I noticed the mod stage is identical to the 6SN7's first stage.

Notice the first stage tube is a pentode wired as triode. The original draft by NYD shows the correct wiring, your new version doesn't.

Also remember to design the output attenuator.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on February 18, 2010, 12:32:13 PM
Oh, right, forgot to connect 9-7 to plate.
As for the output att. would a simple pot work? I'm not too picky.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on February 18, 2010, 12:52:59 PM
I don't know if a simple pot is a good idea. Might work, maybe others could tell how exactly.

I would instead do something like a 4-6 step(10dB steps) L-pad, matching it for the 10k impedance of the output transformer primary.

http://www.uneeda-audio.com/pads/

[edit]

here's a better calculator where you can just stick your impedance and decibel values. you can pretty much ignore watts in this circuit. 1/4w resistors will work.

http://www.bcae1.com/lpad.htm

PS. you could simplify the stepped pad by setting the series resistor to something like 8k, and use a 10k log pot for the parallel one.
Title: Re: Tubes circuits and lesser fidelity
Post by: craptical on February 18, 2010, 06:43:45 PM
Yeah, I know, just saying. :)

Figured so, just making sure you hadn't missed another great project by NYD!  :)
Title: Re: Tubes circuits and lesser fidelity
Post by: funkydiplomat on March 10, 2010, 10:44:51 PM
grid leak bias on input.  lower bandwidth transformers.  lower operating voltage coupling capacitors.  open-loop pentodes.  really high value plate resistors.  there's plenty of stuff left which most people try not to do.  take a look at some of the older and cheaper tube tape machine mic inputs for some vibey ideas.  i think the ampex 601 runs an open-loop pentode and cascaded triode stages, for instance. 

you could try using blocking/high-value grid stopper resistors on grids of cascaded gain stages.  this way you can saturate the second one without changing the load the first tube's output sees as drastically.  This avoids a total meltdown on peaks but still sounds 'driven'.  The aiken amps site has a Norman Crowhurst (sp?) article posted about this though it's geared toward power amp output stages.

I agree, what you are after is not gross clipping.  The guitar amp analogy is right on.  More stages each contributing to the subtle distortion.  I think it's really hard to achieve this in one box.  I have better luck doing a little during tracking... bounce a few tracks around on tape... eq to make up for the crappy playback response of the record head... and so on.

None of these particularly apply to your specific circuit.  But maybe it'll give you some things to research/think about.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 11, 2010, 03:37:55 PM
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.
Title: Re: Tubes circuits and lesser fidelity
Post by: DaveP on October 11, 2010, 05:45:14 PM
Eskimo,

You described it well, its like the difference between video and film only in sound.
But before you ditch your original Altec Pre with the cathode Follower read this article:-

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

You can use that C/F to good advantage by arranging to operate near cut-off a' la Vox AC30/ Marshall amps, this does work.

You will need to use a chart for the 12AY7 (Franks Site) and adjust the operating point of the C/F to give you positive drive.

I agree with you that many of the changes do nothing, I found the same, but remember, the mic pre's used in Abbey Road decks (V72 and REDD47) all used plenty of feedback and they got the sound you want.  HiFi Geeks don't use feedback because they say its a purer sound, if that's true, then that's going in the wrong direction for you.  If you dump the feedback all you'll notice is the extra noise.  If you want colour then, use the C/F biased hot and it will give you some nice compression rather like tape does.

Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 11, 2010, 06:20:46 PM
Thanks, that's an interesting read for sure!
The 12ay7 is long gone in my mind, and I've been set on a pair of nyd's for ages.

Interesting bit about getting some sort of compression and second harmonics out of it.

I've never thought of no feedback as a purer sound, but only as a way of getting and keeping lots of harmonic content.
Don't know what's the best approach really.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on October 11, 2010, 06:25:19 PM
but remember, the mic pre's used in Abbey Road decks (V72 and REDD47) all used plenty of feedback and they got the sound you want.

This is one of those odd myths that won't go away. V72 and especially REDD47 are awfully clean preamps and even the transformers they used in those were top of the line flat and THD-free. No distortion to be found there. Subtle "mojo" at best. Almost without exception the distortion and overdrive we hear on Beatles is tape. Slow tape with bad quality media they were still trying to perfect. Hello distortion and noise!

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.

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.
Title: Re: Tubes circuits and lesser fidelity
Post by: EmRR on October 11, 2010, 06:28:03 PM
To be entirely contrary, lots of feedback is the bane of listenable distortion and saturation.   I have a f'in sh*tload 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.  
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 11, 2010, 06:36:29 PM
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.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 11, 2010, 06:38:35 PM
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!
Title: Re: Tubes circuits and lesser fidelity
Post by: DaveP on October 11, 2010, 06:59:01 PM
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
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on October 11, 2010, 07:43:22 PM
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.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 11, 2010, 08:13:18 PM
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:
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on October 11, 2010, 08:53:33 PM
Quote
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




 
Title: Re: Tubes circuits and lesser fidelity
Post by: EmRR on October 11, 2010, 09:02:43 PM
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.......
Title: Re: Tubes circuits and lesser fidelity
Post by: lassoharp on October 11, 2010, 09:55:04 PM
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 
Title: Re: Tubes circuits and lesser fidelity
Post by: EmRR on October 12, 2010, 01:13:17 AM
He talked about having a Fairchild at the 2004(?) TapeOp conference.   
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on October 12, 2010, 04:32:08 AM
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

 :o 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.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 12, 2010, 12:54:25 PM
Could someone explain the open loop pentode to me? How does it work, benefits and so on.
Title: Re: Tubes circuits and lesser fidelity
Post by: abbey road d enfer on October 12, 2010, 01:30:12 PM
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.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 12, 2010, 01:37:09 PM
odd order harmonics?
Title: Re: Tubes circuits and lesser fidelity
Post by: EmRR on October 12, 2010, 05:03:56 PM
See my RCA thread for a specific pentode case, and what happens when you run it without NFB. 
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 13, 2010, 02:55:51 PM
I'm definately gonna try that cathode follower mentioned earlier out.

(http://www.freewebs.com/valvewizard1/DCCF2.jpg)
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?
Title: Re: Tubes circuits and lesser fidelity
Post by: EmRR on October 13, 2010, 03:24:47 PM
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.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on October 13, 2010, 05:03:01 PM
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.
Title: Re: Tubes circuits and lesser fidelity
Post by: eskimo on October 13, 2010, 05:04:17 PM
Would be fun to see what you're up to kingston. :)
Title: Re: Tubes circuits and lesser fidelity
Post by: EmRR on October 14, 2010, 12:22:21 PM
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.
Title: Re: Tubes circuits and lesser fidelity
Post by: analag on October 15, 2010, 03:49:56 PM
The circuits I see won't give you the sound you're looking for. You have to think outside the box and not many will do that. Ever played with positive feedback coupled with low plate voltage with current held constant with sandstate? Let's see some creativity boys...c'mon. If you want ugly start with 12AX7 that tube is all for it.
Title: Re: Tubes circuits and lesser fidelity
Post by: Kingston on October 15, 2010, 04:24:42 PM
I'm all for positive feedback, even if non-feedback stages alone will give more than enough distortion. Same for low plate voltages, that one is a proven trick. Possibly ruined in reputation by Art, Behringer and the ilk...

But doesn't constant current (plate choke, or a FET?) - instead of a plate resistor for example - just take us directly to a blocking distortion fart out? Not exactly "soft and pleasing".

What else could constant current provide us, distortion wise?
Title: Re: Tubes circuits and lesser fidelity
Post by: analag on October 20, 2010, 07:20:06 AM
I'm all for positive feedback, even if non-feedback stages alone will give more than enough distortion. Same for low plate voltages, that one is a proven trick. Possibly ruined in reputation by Art, Behringer and the ilk...

But doesn't constant current (plate choke, or a FET?) - instead of a plate resistor for example - just take us directly to a blocking distortion fart out? Not exactly "soft and pleasing".

What else could constant current provide us, distortion wise?

Experimentation is needed.