General question about tube preamps and gain

Following up on the reply from mganga,
I have a Decware CSP3 tube pre amp.  In addition to the standard volume attenuator there are adjustable pots for gain on both the input tube and on the output tubes.  My understanding of these pots is for "gain riding".   Adjusting the gain affects the performance of the tube (s) and changes the character of the sound so you can may it darker and syrupy or not.  This allows for fine tuning of the pre amp to find the sweet spot of the tubes.  The pots change the performance of the tubes a lot. This is a nice little tube pre amp.  It does benefit from changing out the stock tubes, particularly the rectifier tube.  

I used to have a Decware amp (TORII mk3) and a Decware preamp (CSP2+) both of which were wonderful.  Decware amps have gain attenuators and I found that I enjoyed music most with the amp gain turned to about 25% turn and the preamp to 75% turn.  It made the music more holographic, dimensional, real.  Both the preamp and the amp were tube designs but the preamp probably had a different distortion profile than the amp and it was this distortion profile that I like, so I turned up the preamp and turned down the amp.  Perhaps more 2nd harmonic distortion?  At least that’s what I always thought.  It was nice to be able to adjust the sound profile somewhat in this manner and it was cheaper than buying more tubes, which I of course did anyway🙂

More distortion?   Really?
Seems to me that the knock on tube amps to begin with.  Okay, it's an amplifier.  The farther you are from clipping, the better.  My c-j amps and preamps sometimes have bit more gain than I prefer.  Years ago I took an Act2 pre back to c-j and had them attenuate the gain.  That lasted a couple of months until I had it returned to normal.  Careful what you wish for... you might get distortion you don't want.

gochurchgo, FWIW I think your experience re systems sounding fuller at higher volume is not unusual, but I believe it has much more to do with the speakers than one's electronics. Most noticeable  in the bass  but also in the soundstaging which really opens up at higher levels.

Thanks. I don’t know where my acquaintance was headed with his question. I feel like my new preamp sounds fuller as I turn it up but I thought that with the passive I had before so..

anyway, interesting things to think about here.

A properly designed tube line stage for home audio should not have issues vs 'volume level' In fact tubes inherently have more linear operating range signal-voltage-wise than solid state typically. +/- 50V swings are not unachievable. Driving the tube stage 'harder' for wetter (more harmonics) is what musicians do, not necessarily what we want in high end audio.

But if that is what you really want, look up "vacuum tube filament starvation" via a search. This may be what you're really looking for. It could also be an adjustable thing.

My bad, Class B isn’t common. Let’s not throw the baby out with the bath water. 

mkgus"it also depends on the implementation: Class A, AB, B, etc."

I am not aware of any reputable manufacturer of components intended for use in Music Reproduction Systems that utilize, employ, or leverage Class B amplification for they're products so you're suggestion is nonsensical.

My gut tells me yes. Driving a tube harder should result in an increase in even and odd order harmonic distortion which is a big part of “tube sound,” however it also depends on the implementation: Class A, AB, B, etc. 

One other element to keep in mind is majority of the volume controls are nonlinear (logarithmic) and the degree of resolution increases as you turn up the volume. This is another reason for proper matching of the gain between the preamp and the amp to allow for the operation in the mid-upper region of the volume control range.

@mkgus  so if one were to do this, Would driving the tubes bring out more of what that particular tube imparts on the sound?

yes theoretically?

gochurchgo, what you are asking makes sense. Putting a resistor downstream of the tube preamp isn’t all that different than putting a passive preamp downstream of the tube preamp. The only difference is that you get a “resistor” with volume control. If you did that, you could drive your tube preamp to full output and then attenuate the volume with the passive preamp. I’ve done this before. What I noticed was that the overall tone changed with tube preamp output level so with this setup, I could push the tube at a constant level and then control the volume with the passive preamp.

@f917025  I was under the impression though the pin outs are the same you cannot swap 12AT7 for 12AU/X7 tubes.

No?

 jj 12at7 :as the first gain stage or preamp tube  the nature of the 12at7 type tube is is a bit bright with a 60-70% gain factor.the nature of jj tubes is even with nice mids and not bright. a bit colourless and dull compared to nos greatest hits off course but not harsh like the chinese or russian tubes usually. ide use a 12ax7(100%gain factor) nos early 70's mullard 12ax7 i63(great low mids ) with low gain ratings or 5751(80% gain factor). that will raise the preamp gain which will result in earlier breakup or clipping and more overall output so if the breakup started at volume 5 now it will be from 3 or 4 so and you will be on lower volume with more of the tube’s gain in the sound and less of the circuit’s gain .the other option is opposite -to lower the gain : more clean range later breakup less overall output and then you can dial more volume  which is also nice and makes more sense in audio gear and angry guitar amps  i highly recommend mullard 70’s or 60’s 12au7 this type has more lows and about 30% gain factor so the added bass sort of makes up for the loss in preamp gain .within the 12at7 type you can also choose a higher or lower gain tubes and get less dramatic results . you don't need much knowledge or consulting the amp maker for swapping preamp tubes .

Perhaps relevant...I installed an attenuator potentiometer in my Akitika GT102 amp in order to lower its gain to match my Dyna ST70 (w/VTA driver PCB upgrade) to enable hybrid bi-amping (SS on woof, VT on Mid-tweets).  The pot was supplied by the designer of the amp to allow a single source input to be run w/o preamp.
See: https://akitika.com/documents/VC102AssemblyManualRev2p00.pdf

I'm new to tubes.  I recently purchased a used Herron VTSP 3a ro2 pre amp, which has a High and Low gain option.  It's paired with a NAD M22 v2 power amp, which has High, Mid, and Low settings.  I futzed with the settings for several days and finally settled on High for the Herron and Low for the NAD.  This combination makes my Tannoy DC8 Ti speakers really sing.  

I've never experienced what you describe in any of the tube equipment I ever used.

Perhaps the volume control needs a bath in contact cleaner.

If cut the signal strength anywhere, between preamp and amp, and I personally would want remote control of that and easy removal of it.

Loudness Compensation. Old tube preamps and receivers had Fletcher Munson Loudness Compensation circuits, whereby frequencies were progressively adjusted for equalization at lower volumes.

Have your friend look for this feature and enable/disable it, perhaps it is involved in the differences heard.

https://en.wikipedia.org/wiki/Equal-loudness_contour

Don't forget the lowly 12AY7. Even less gain, but ohh soo smoooth. My incremental increase in volume per notch is smaller. For Rock 'N Roll I now need to go to 2 o'clock on the volume. With the 12AT7 I had to stop at 1 o'clock.

While this question isn’t about my preamp
specifically, I’ll play devil’s advocate and use it for reference.

Lets say the preamp comes stock with 12AT7 JJ’s.

To answer your question, I am not sure that I am driving the tubes harder, just that with the attenuation I am getting more into the sweet spot of the volume controls. Both the pres I have seem to have benifited greatly with attenuation employed, please note both these pres have dual stepped volume controls, with that said as you add volume each step seems to be smaller allowing for closer adjustment. Hopefully someone will jump in with a better explanation than I am able to provide. Both these pres have been used with the same amps with the same results, VTL MB125s, Pass XA30.5, Canary CA306 and Quick Silver M120s all needed the attenuation.

It couldn't hurt to try your idea. Does the power amp have adjustable sensitivity?  Lowering that would also be an option. 

The gain of the amplifier has a direct correlation to the output of the preamp, if an amplifier especially a solid state amp ,Has too much gain it can impact also a higher noise floor 
I have seen this a lot with direct heated Triode preampsIt depends on direct or indirect tube circuit , there are many factors
tubes also , you didnot mention what types of tubes 
for example a 12au7 ,and a lower gain 5963 variant may be warmer sounding 
but have 15% less gain , you just have to turn the volume up a bit 
more to get the same spl levels , any time you add a another Attenuator or something in the signal path less sonic purity ultimately.

@tooblue  do you find that driving the tubes harder adds to the sound?

the 3B was a preamp I was looking at on the used market after glowing reviews and good words on forums about it.

I have used attenuators (10db) between my Audible Illusions 3B and amps to allow greater range of the volume controls, my Lamm LL2.1 has a 15 db attenuator switch on the front panel that I use as well for the same reason. Enjoy the music.

Aric Audio has preamps with adjustable gain. I run his Unlimited. I don't know if this is what your looking for.

So it sounds like my hypothesis was wrong. Which is fine. And I appreciate the feedback as always. 
So semi-on topic question then is what’s the point of adding attenuators between amp and preamp?  Just curious.

Call the manufacturer of the preamp and ask if you can replace with a lower gain tube.

Changing a higher gain tube for a lower gain tube won’t do much, you need to change the feedback loop components to reduce the gain, a job for a good tube tech.
One of my tube pre’s has a switch 1 x or 3 x gain https://ibb.co/ygM6WHt

Cheers George

@snapsc you are probably right.

@gochurchgo


It seems like you are asking if a home audio preamp, might behave somewhat  like some guitar amps...where you can turn up the gain to increase distortion...some call it crunch??  I'm sure that there are people on this site ....like @atmasphere that can give you the answer you are looking for.

I suspect that the distortion component present as you turn up the volume increases minimally until you hit maximum gain...which 10db of attenuation is unlikely to allow the volume to be turned up to that level.  Therefore, I'm thinking that the best way to add more tube coloration...as has been suggested, is to try different tubes until your acquaintance finds the one that best suits their preferences.

This has nothing to do with the preamp I just bought. There was a discussion I had with an acquaintance which is why I ask. It has nothing to do with me personally.

i was asked the question and I said that it seemed to me that by dropping the out put 10db the added volume to make it up might add more of the tube coloration.

You didn't mention what signal tubes you have in the preamp but if you have 12AX7 tubes you could switch to 12AT7 or even 12AU7 tubes which have lower gains, respectively. 

@gochurchgo, you need to just leave that Quicksilver alone and let it break in till you get a couple hundred hours + on it. The builder is very conscious of the tubes he uses and the matching of them. Enjoy the music.

Call the manufacturer of the preamp and ask if you can replace with a lower gain tube. I do this on my tube phono preamp with no problem!

Now installed, you notice that the more you turn the preamp volume up, the more the tubes impart that sound on the signal. But you can’t play it loud.
So could you, theoretically, put attenuators (lets say -10db) between said preamp and the power amp to lower the output signal which you’d then turn the volume up and drive the tubes a little harder to impart more of the tube’s sound at lower levels?

You will increase the load the tube preamp sees, as then you'll have two parallel loads to ground, and it "may" not be a very good impedance match.
Better off getting the gain reduced in the preamp by a competent tube tech. 

Cheers George

It makes sense. You are saying you have gain related syrup.

But please, no, just switch your preamp. :)

If you are otherwise enamored of your preamp, consider new coupling caps instead?

It might help to mention the model of preamp and tube types it uses. 

In my mind it makes sense. But then I don’t have any actual knowledge so maybe I’m way off.

I don't see why not. Realistically a lot of preamps and power amps have more gain then they will need. So the preamp many times is running at the bottom of its dynamic range.