eskimo

Tubes circuits and lesser fidelity
« 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.


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?


EmRR

Re: Tubes circuits and lesser fidelity
« Reply #1 on: February 14, 2010, 05:57:01 PM »
The output is a cathode follower, which is 100% NFB.
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Kingston

Re: Tubes circuits and lesser fidelity
« Reply #2 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.
« Last Edit: February 14, 2010, 06:09:30 PM by Kingston »

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #3 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! ;)
« Last Edit: February 14, 2010, 06:27:16 PM by eskimo »

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #4 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.

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #5 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?

Kingston

Re: Tubes circuits and lesser fidelity
« Reply #6 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).
« Last Edit: February 14, 2010, 07:17:03 PM by Kingston »

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #7 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? 

Kingston

Re: Tubes circuits and lesser fidelity
« Reply #8 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:
« Last Edit: February 14, 2010, 07:17:28 PM by Kingston »

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #9 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!


PRR

Re: Tubes circuits and lesser fidelity
« Reply #10 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.

Re: Tubes circuits and lesser fidelity
« Reply #11 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.


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.

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #12 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.

Kingston

Re: Tubes circuits and lesser fidelity
« Reply #13 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.

Re: Tubes circuits and lesser fidelity
« Reply #14 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.

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #15 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.

Re: Tubes circuits and lesser fidelity
« Reply #16 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.

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #17 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.

eskimo

Re: Tubes circuits and lesser fidelity
« Reply #18 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?

Re: Tubes circuits and lesser fidelity
« Reply #19 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.


 

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