Hi-gain preamp comparisons

[tubenit]

I thought this is an interesting look at 4 triode hi-gain stage preamps.  Note CH preamp is reportedly for EL84's.

Guys, note any observations here?   

The observations might give some insight into designing high gain preamps.

with respect, Tubenit

[tubenit]

Observations:

1) tendency to lower value cathode caps (vs. Fender) OR no cathode caps

2) tendency towards a mix of higher value plate resistors and lower value plate
    resistors.  (Note other D-style amps use 100k in first two gain stages)

3) at least one interstage coupling cap with a fairly low value .0047 to .0015

4) 68k to 100k slope resistors in the tone stacks

5) smoothing caps (or a "cut cap") to drop higher frequencies to ground

6) mostly 250k, 100k or even lower pot values (or resistor to ground values)
    used in interstage attentuation  (note 4.7k value to ground on CH schem)

7) use of 100k to 470k resistors after interstage coupling caps to drop gain
    and remove some higher frequencies

Summarized:  I'd say boost then drop gain values (with resistors and pots) using multiple gain stages  to increase harmonics and use something to smooth highs to mid highs to remove hash (such as smoothing caps, cut caps or lower value interstage coupling caps).

Other thoughts ..........................  ??

With respect, Tubenit

[SILVERGUN]

As always,,,THANK YOU for your insightfulness
I had been doing some tweaking without keeping track of my schematic very closely,,,so last night I sat and figured it out, and this is where I am.. (I'm not going to update that thread until I post a sound clip  )
At one point I was reading Merlin's book and comparing D styles with some other famous preamps,,,and this is what I came up with, as a result of all of that blending......I would like to believe that this is the best sounding point that I have reached, after spending hours re-wiring, and swapping components......I was following my ear and not really writing it out...
Now that I have built the breadboard, I hope to put this circuit on it and run them side by side, and hopefully be able to complete this thing....

The only BIG thing that stands out to me about the preamps you posted, other than what you mentioned, is the obvious missing cathode resistor on the Tad Cat V1 (typo?)
Your examples did get me thinking about possibly increasing plate resistor values to increase current, and then maybe using an unused triode for a cathode follower to tone stack insertion point before my PI...
Then again,,,after looking closer at the Dumblish example,,,maybe I want more current through the first stage?(I'm gonna try that tonight).....the tone stack is my stumbling block

I currently have no tone stack, and I don't hate it,,,it just seems like I should add at least a bass cut control.....
The one thing I was considering was adding an active EQ by using a D'lator loop with a filter "inside" of it ....any thoughts?
Maybe a bass cut, as a stand-alone traditional tone stack control, and then a sweepable "active" mid boost/cut in the D'lator driven loop??

Your thoughts are always invaluable 

The biggest thing that stands out to me now,,after much tweaking,,,is the effect of increasing gain in a stage that is "accented" by a bypass cap,,,and how that can change the sweetspot of the freq. response substantially......take a look at my 2nd stage, and you'll see a 1K with a 2.2U
Until recently, that 1K was a 4.7K,,,until I got "inspired" and dropped it 
Boy, did the "boost point" of that 2.2U cap ever take over that preamp----BAM.....now, I will probably back it off a little ,,,but I like to make big - obvious jumps, to really hear a huge difference.....
Your smoothing cap suggestions are the only things that I haven't over-experimented with,,,and I'm still hoping to use them as the icing on the cake 

Nice thread idea,,,,,,at least for me

[tubenit]

Silvergun,

Man, I appreciate seeing your updated schematic. Nice! Glad you're taking your time on this to make it what you want it to truly be. Bravo!

Your amp schematic really needs to be on this list for comparisons.

I think the tone stack or filter within an FX loop could be an interesting idea and I have thought about that idea several times but never put it into a schematic.
I'd love to see that done.

Are you reallllllllly going to do a soundclip?   

With respect, Tubenit

[SILVERGUN]

I went out and tried raising the plate resistor on that paralled V1 to 220K (because I'm a nut)...and I really expected that tube to start melting
It increased sustain (which was already endless) and made the note jump to harmonic quicker (obviously---more gain)

I'm gonna go back out and experiment with bypass cap values in that spot
Does the fact that V1 is paralled increase it's resilience.......meaning, can I beat it up a little harder, because it is paralled, does it increase it's ability to handle more current than a singular triode?
Or should I expect it to start meltdown shortly?  

I'll take the cath. resistor up to 1.5K and see if that's a good middle ground

Are you reallllllllly going to do a soundclip?   

Yeah,,,I know I have to  ....besides promising you,,,I made a deal with jojo----no new builds until I put up a clip......something about bad mojo.....
SO if jojo is warning me about mojo,,,I'll take it seriously

I'm not sure where to start looking for a mid-boost/cut circuit for the loop,,,,but I think that sure would be cool

[PRR]

> 1) tendency to lower value cathode caps (vs. Fender) OR no cathode caps
> 3) at least one interstage coupling cap with a fairly low value .0047 to .0015

Guitar is not gut-less. If you amplify EVERYthing the same, the bass overwhelms the sound; when over-driven, bass overtones splatter all over the rest of the chord/passage. So gain-to-fuzz usually "must" drop some bass along the way.

> 5) smoothing caps (or a "cut cap") to drop higher frequencies to ground

Heavy overdrive makes harmonics too-strong and too-high. Very unlike any acoustic source (natural or musical), not pleasant. Anyway the hihat/cymbals own that turf. So we usually "must" trim the extreme highs.

[SILVERGUN]

Guitar is not gut-less. If you amplify EVERYthing the same, the bass overwhelms the sound; when over-driven, bass overtones splatter all over the rest of the chord/passage. So gain-to-fuzz usually "must" drop some bass along the way.

During tweaking I tend to go back to the guitar and play some lower,artificial harmonic type riffs---a-la Billy Gibbons on steroids,,,,,and when those harmonics really punch (which I really like), any chords that I play usually do get muddied up........so this is a delicate balance, that I dont handle so delicately....
I built a little cap decade box, and spend a lot of time between stages trying to "get it right".....unfortunately for me, what may sound right by myself, sitting 3 feet from a 2x12 cabinet, and fully focused on sounding like a wall of LesPaul,, will probably be a problem for a soundman

Fortunately, I have you guys here to talk some sense.....
Last night I went out and tried the decade box on the .02uf coupling cap coming off the 3rd stage, and wound up preferring it raised to .068uf,,,because it increased the bass harmonics, and filled in a space that I find important.....BUT, I may have been too focused on single notes, and not really playing through all of the chordal options....
This is the toughest part for me----where to stop? 
Increase a cap here, lower a resitor value there,,,,drop some bass here,,,try to get it back there......and voila, it sounds better,,,or does it?

I did go back and forth with the plate resistor value on the first stage and wound up taking it back to the posted schematic values,,,,basically because, to me, the introduction of more current there, brought in some high hash that I wasn't willing to deal with (even though there was a noticeable increase in punch), it seemed to bring in some new high freq. oscillation
The reason for the lowered component values in that position, as far as what I read,,,is to lower the noise floor in that 1st (paralleled) stage.....maybe that is what I heard?

I hope this isn't too much of a hi-jack, T 

[Tone Junkie]

Silvergun if we look at what you did there by raising the 1st stage to 220k in parallel ,breaking it down to what a single side would see it would be like taking it to 440k which adds a lot of hash and resister noise thats real noticable.
 I have seen some high gain amps that go to 330k but in trying them thay sound real noisy to me compared to the extra gain you pick up, and I usually end up dropping them down to 220k.
Then I almost always have to put smoothing caps in to get rid of the extra high end hash it adds.
I have 2 cap boxes just for smoothing caps and probably should make 2 more to be in a perfect situation .
I almost have enough resister and cap boxes to do a  whole pre amp without ever soldering anything for caps and resisters . Im waiting for somemore make before break switches so I can make 2 more and I will be in the perfect spot it sure helps to dial in the perfect sound.
Thanks Bill
PS. Im going to cut apart a few pages I have and hopefully add to the collection here I have a bunch of high gain schematics 

[SILVERGUN]

I almost have enough resister and cap boxes to do a  whole pre amp without ever soldering anything for caps and resisters . Im waiting for somemore make before break switches so I can make 2 more and I will be in the perfect spot it sure helps to dial in the perfect sound.
Thanks Bill
PS. Im going to cut apart a few pages I have and hopefully add to the collection here I have a bunch of high gain schematics 

Thank you Bill,
I'd love to see the amp laid out on your bench consisting of just decade type boxes 
I'm looking forward to see the collection you have,,,,to see if I have overlooked any, and just compare more

I'm hard at work meshing's T's D'lator schematic with a 3-band parametric EQ schematic I found today.....I think I have to see this through to know if i'm onto something  .....or not

[Tone Junkie]

Parametric EQ sounds interesting Keep us posted.
 I want to draw some schematics of my boxes so i can get some feedback from the guys here on things Ive missed or hadnt thought of, I will post those on a differant topic of course.
There is such a wealth of knowledge to draw from here .
 I printed out some schematics of high gain amps i have i will try and get them posted so we can look and see if they follow the patterns of the others
 Bill
                                                         

[SILVERGUN]

Parametric EQ sounds interesting Keep us posted.

Here's the link to the schematic I'm looking at:
http://www.sabrotone.com/?p=578

Nice work, and I'm very grateful to the originator for posting, and sharing....THANK YOU HARALD!
He also has a link to a very cool looking hi-gain design here:
http://www.sabrotone.com/wp-content/uploads/2013/04/Schematic2.png

I just have to wait for the op-amps and pots to get here,,,I don't have any vero-board, so i'm doing my layout on these little radio shack DIP socket boards that come connected in the package of 2
It's actually working out nice just leaving the 2 boards connected, for the majority of the circuit,,,and then using a piece of turret board for a pot mounting "rail" with some of the components right at the pots....at least it's working out in my head 
 

[Tone Junkie]

Sorry it took me so long, here are a couple good ones

[Tone Junkie]

Cant forget my favorite at the moment.
Bill

[nandrewjackson]

great thread, I love the hi-gain stuff.

I've got a demo video uploading to youtube right now of my new preamp project.  I'll post it here as soon as it's online.

it's such a crap-shoot when you start raunching the signal.  I know there is lots and lots of math involved in getting a valve amplifier to perform properly with low THD, but when you are purposely wanting it to scream at you, things get funky....

 

[Ed_Chambley]

Cant forget my favorite at the moment.
Bill

Very Cool.  My type of amp. Could you share the PT specs and power tube plate voltage?  I am currently working on a design of my own which is so similar to this it is uncanny.  I plan to use 4, KT120 with a half power switch, but am unsure of the total load needed.  To be on the safe side I am using 780vac, 300ma, but the PT can be wired 800vac @ 200ma.  I am wanting to hit a loaded plate voltage on the power tube plates of 550vdc give or take.  UL design is the difference.

[nandrewjackson]

homemade preamp demo

[tubenit]

Hey, that was a pretty cool demo!   Thanks for sharing it.

Looks like a very versatile approach.

With respect, Tubenit

[phsyconoodler]

I built a 5 watt high gain amp recently and experimented with a few approaches.I did the marshall cascaded preamp but tried both a cathode follower and simply a gain stage in place of that,and used the coupling elements like a trainwreck.The Trainwreck approach turned out to have too much uncontrollable feedback and the marshall only was also in the same boat.
  I have only a gain,volume and VVR on the amp. When I added a cold-biased gain stage in place of the marshall style cathode follower it not only added the perfect amount of gain,it also added total control over the feedback.
  The little amp rocks like you wouldn't believe.I was closely following Silvergunn's threads to see if that's what I wanted but ended up slightly different.
  It responds so well to different output pickups. A strat is lovely in the extreme and a humbucker drives the little amp into gain-heaven! I would assume that the lack of a tone stack is a major factor here and would definitely re-think the gain with one. The addition of a tone control was not good in this case as it sucked the life out of it.
  Careful manipulation of the PI coupling cap and cathode caps made for the perfect transition between single coils and humbuckers. I have absolutely no desire to use a tonestack with this amp.However,if I do another i will use a rotary tone control to switch out a coupling cap.As is it's near perfect.


  Drop tuning is incredible! I have the high and low input that bypasses one gain stage.Very marshall there.

[SILVERGUN]

The Trainwreck approach turned out to have too much uncontrollable feedback

There's no such thing as too much uncontrollable feedback 

Try putting a Rangemaster in front of that Turbo-wreck and hold a note,,,,,,fast forward 3 days later, and it's still singing 

I wonder what would happen first:
A) I would start bleeding from my ears and eyes
B) 12AX7 meltdown
C) The 9v battery would run out
D) All of the above

 

homemade preamp demo

Loved that demo,,,,what power amp is it coming out of?
You should sell that design to Egnater as "the real" TWEAKER 

[Willabe]

I wonder what would happen first:
A) I would start bleeding from my ears and eyes
B) 12AX7 meltdown
C) The 9v battery would run out
D) All of the above

E) None of the above.

Speakers voice coil will over heat and burn up.



              Brad        

[John]

C) The 9v battery would run out

 

[SILVERGUN]

Speakers voice coil will over heat and burn up.

Nuh-Uh 

[Willabe]

I'm not joking.

Speakers do not like having a high frequency feeding back because of the very small movement of the speaker cone and with that the voice coil. 

Speaker have been known to burn up on stage while the guitar was feeding back for longer periods of time. Some have even caught on fire.


                 Brad     

[John]

Some have even caught on fire.

Now that's a concert!! 

Seriously dude, just stop with the techinal info. We are obviously all focused on burning stuff up. 

[Willabe]

Seriously dude, just stop with the techinal info. We are obviously all focused on burning stuff up.  

 


http://youtu.be/8mQwZLr_qRI



              Brad       

[nandrewjackson]

thanks.

No power amp. Running into Presonus Audiobox USB/PC interface, Presonus studioLive software.  No comps, FX, Eq, just in and recorded.  The audio is about a second off from the video because I had to delete the Camera's audio and try to line it up, I got pretty close.  Here's the build thread link

http://www.el34world.com/Forum/index.php?topic=15609.0

[Ed_Chambley]

Nice.  Looks like a lot of fun as well.  Thanks for sharing it.

[alerich]

it's such a crap-shoot when you start raunching the signal.  I know there is lots and lots of math involved in getting a valve amplifier to perform properly with low THD, but when you are purposely wanting it to scream at you, things get funky....

Sometimes the best things happen when you toss convention out the window. A Marshall 2204 preamp first stage has a 2K7/.068uf cathode R with a 100K plate R. This feeds a .022uf coupling cap into a 470K/470pf treble peaking circuit into a 1MA gain pot (with or without a bright cap) and onto the next stage. Pretty standard stuff and the tone that launched a thousand ships. With all the mods in my amp I have a 1K1/10uf cathode R and a 220K plate R. This feeds a .022 coupling cap into a 1M/1000pf treble peaking circuit and then into a 100KA gain pot (that I never dime even when I run straight in). The wiper of the gain pot goes directly to a passive series effects loop then onto the grid of the next stage. The subsequent stages are gained up a bit but the first stage is the kicker. Complete left turn from convention but it sounds great. In the end I'm running a string of Boss pedals unbuffered and inserted between the gain pot and the grid of V1B and it sounds great.

[SILVERGUN]

The wiper of the gain pot goes directly to a passive series effects loop then onto the grid of the next stage. The subsequent stages are gained up a bit but the first stage is the kicker. Complete left turn from convention but it sounds great. In the end I'm running a string of Boss pedals unbuffered and inserted between the gain pot and the grid of V1B and it sounds great.

Well,,,, that alone was worth the price of admission  ....thanks for sharing
Great tip,,,,,definitely something I will try sooner than later, as I try to insert built-in effects into the signal chain

[alerich]

Well,,,, that alone was worth the price of admission  ....thanks for sharing
Great tip,,,,,definitely something I will try sooner than later, as I try to insert built-in effects into the signal chain

It sounds so good and works so well in that Mig60 that I tried to implement it in the same circuit position in my JCM 800 Mini. It sounded like ass in the JCM. I guess it's a signal level component thing.

[Tone Junkie]

Ed ive been away a few days sorry. I will try and get those specs for you, I found most of the high gain stuff at slo clone forum.
Im sure some were in my maze of stuff i have the voltages at least and I can find the rest at the other from a guy whose forum name is  (The Haunted) over there .
Bill

[SILVERGUN]

I did get around to building that ParaEQ from the print I posted above....I just haven't built the D'lator loop yet, so I was limited to using it in front of the amp,,,and I was able to get some nice boost effects, without too much noise...
It just doesn't have the same effect that I think it will have once I put it further on in the signal path.....

I think that it would make a useful tool as a single band mid-boost cut, if you wanted to simplify things

The 3rd pic is of a bi-polar power supply that I built to power the op-amps,,,,as a prototype for the PS I would need to install in the amp if I want this stuff built in 

[frank57]

Is it just a preamp you're building?
because you're leaving out the other side of the equation:
the power tubes and power section.

Look at the English muffin By electro harmonix.
It's a mess. It's a preamp not a pedal with 300 volts coming out and nowhere to go.
It works better in the efx loop of a tube and you use it as a preamp for the amp.
Lousy higain 12ay7 and on the edge of crap tone with 12ax7.
When you go in the front of the amp it messes up the tone stacks and you get harshness.
If you mod it to a soldano style preamp it becomes way more versatile and can use both 12ax7 and 12ay7.

[SILVERGUN]

Is it just a preamp you're building?
because you're leaving out the other side of the equation:
the power tubes and power section.

No, actually,,,,,most of these guys know what I'm building, so I'm leaving out details.....sorry for that
It's a carry over from another thread for me : http://www.el34world.com/Forum/index.php?topic=14575.150

And I totally hi-jacked T's thread but he's such a nice guy that he doesn't ever yell at me 

The EQ I just built was designed to be a pedal,,,but I'm thinking of building it in as a "tone stack"........in all my free time 

T's original intention was to get US to see what makes these pre-amps similar, but each unique..........I'm shooting for unique 

[frank57]

Have you tried anything like the old Laney protube amps?
The one I tried the other day was a hell of a Marshall type sound.

[SILVERGUN]

Have you tried anything like the old Laney protube amps?
The one I tried the other day was a hell of a Marshall type sound.

No sir,,,,,,the sad part of my story is that I've only ever had Mesa....so now that my eyes have been opened, everything is interesting to me

I pulled a snipet from the AOR 50 preamp, that I will take a closer look at the tone circuit,,,but I figured it would make a nice addition to this thread