Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: baene123 on October 08, 2009, 01:17:15 PM

Title: First Hybrid
Post by: baene123 on October 08, 2009, 01:17:15 PM
hi, so this is my first time into this forum and it's my first message.... So, I'd like to built on my own a new guitar amplifier based on two stages:
1) preamp, I have two tubes by electro-harmonics like 12BH7a, than i want to use them to make a preamp, but only a clean channel, because these tubes have a "little" gain, than a distortion channel is too  difficult to do.
2) amplifier, I'd like to use two LM3886 in parallel to get 100W on 4 Ohms, but I don't know if LM3886 is a good chip for audio riproducing and a good audio quality... so, i'm not sure.... anyone have this experience by usuing a LM3886 chip? is it good? so, i'd like to use LM3886 because its cheaper than CMOS or BJT guitar amplifier

thank for all
and sorry for my bad english :-)
Title: Re: First Hybrid
Post by: armstrom on October 08, 2009, 01:40:34 PM
The first thing to keep in mind is that you will have to greatly attenuate the tube preamp. Depending on how many gain stages you use and what kind of other losses are introduced (tone stacks, for example) you will end up with very high signal voltages coming out of your preamp stage. Most SS power amps are set up to handle 1V RMS or less input signal and will clip horribly at any signal above this level. As for the suitability of the LM3886 I think you're on the right track. These chips are known for being very linear and transparent. They're very low noise when built with a good circuit layout.
-Matt
Title: Re: First Hybrid
Post by: baene123 on October 08, 2009, 04:36:22 PM
Quote from: armstrom on October 08, 2009, 01:40:34 PM
The first thing to keep in mind is that you will have to greatly attenuate the tube preamp. Depending on how many gain stages you use and what kind of other losses are introduced (tone stacks, for example) you will end up with very high signal voltages coming out of your preamp stage. Most SS power amps are set up to handle 1V RMS or less input signal and will clip horribly at any signal above this level. As for the suitability of the LM3886 I think you're on the right track. These chips are known for being very linear and transparent. They're very low noise when built with a good circuit layout.
-Matt
so, only 1 V or less input signal for LM3886.... what is the value (volt) of the signal that's coming out of my preamp make by tubes?
so, is it better if i make a preamp with jfet or opamp ? but, i want tubes...
Title: Re: First Hybrid
Post by: Brymus on October 08, 2009, 09:06:23 PM
I am gonna take a stab at this since I want to do the same thing.
Please guys correct me if I am wrong.
Mensur mentioned in his hybrid build that you need a Voltage follower to reduce the impedence,I asked if a cathode follower would work he said yes.
But I worry too with the plate voltage in my preamp being around 300V at the last stage,
about coupling that to a SS chip amp.
The Alembic F2B seems to have the output taken directly from the plate of the last triode.
And at Alembic they said they run them into McIntosh amps so IDK.
I now think a voltage follower using a mosfet with the gain set inverse and porportional
to the accumilated gain of the pre-amp is what is needed to make one work.
Meaning if the total gain of the preamp is 100 you would have the follower attenuate the signal by dividing by 100 so 300v divided by 100 would be 3volts out? HMMM still too much...
But you get the idea I just dont know how to do the math to show you.
Just set the gain of the voltage follower so it only has 1-2volts P-P at its output.
I guess this could be done with a tube too,or just get a mosfet rated higher than your max B+ voltage.
SO RG ,JM ,Teemuk, Armstrom and crew am I close?
Or should I delete this post?
Title: Re: First Hybrid
Post by: phatt on October 09, 2009, 06:38:46 AM
Hi folks,,, if you are trying to get your head around tubes into SS,, then download the Messa V Twin circuit and just build it less all the cab sim add ons,,and that will make your life a heck of a lot easier than trying to work out the maths.

Note; as long as the plate cap decouples the HV tube then the signal voltage is the only concern.
Generally the biggest signal AC voltage is only 100VAC max so as long as the *First* cap is 400volt rated then the rest can be 100 volt caps.  This is obviously how the Vtwin circuit gets around it all.

PS, I've built the Vtwin circuit into a SS Amp so I know it works just fine.
Cheers, Phil.
Title: Re: First Hybrid
Post by: Brymus on October 09, 2009, 02:41:57 PM
How about a schematic Phatt?
Title: Re: First Hybrid
Post by: Brymus on October 09, 2009, 08:11:03 PM
Here is a better way of expressing what I wanted to say before.
If your guitar has a signal of 1V and your pre-amp amplifies it by a factor of 10
Then your last stage- a voltage follower needs to have a gain of .1 to bring the signal back down to 1volt to input your chip amp  :tu:
Title: Re: First Hybrid
Post by: awdman on October 09, 2009, 10:46:17 PM
is this the schematic? http://schematicheaven.com/boogieamps/boogie_vtwin.pdf (http://schematicheaven.com/boogieamps/boogie_vtwin.pdf)
Title: Re: First Hybrid
Post by: phatt on October 10, 2009, 05:27:40 AM
Quote from: awdman on October 09, 2009, 10:46:17 PM
is this the schematic? http://schematicheaven.com/boogieamps/boogie_vtwin.pdf (http://schematicheaven.com/boogieamps/boogie_vtwin.pdf)

Hi awdman,
          Yes! that is the schematic I based my ideas on.
It is a most intelligent way to intergrate Valves with opamps.

Here is a rough circuit drawn from memory of how I built mine,,, bare in mind this was many years ago but might help you to come to a better understanding of how to go about such things.

Note the mistake on Vtwin drawing; *R24-270k* around U1b is *Wrong* 27k will make it much more civilised.
I did not use all the addon bits just the clean section thru the tone and onto the pwramp output.
I've moved on from this stuff as I've found that it is better to throw money and time at the business end of an amp,, ie, A Tube Poweramp is more useful as that is the one thing SS can't do to well.

Having said that, a Valve preamp circuit like this will certainly improve a SSAmp.

I can't stress enough that any *One off* ciruit like this you really need to spend a lot of time breadboarding it all *RD FIRST*,,,Then build,, Otherwise there is a good chance you are just adding to the land fill.
Have fun, Phil.
Title: Re: First Hybrid
Post by: phatt on October 10, 2009, 05:34:46 AM
Quote from: Brymus on October 09, 2009, 08:11:03 PM
Here is a better way of expressing what I wanted to say before.
If your guitar has a signal of 1V and your pre-amp amplifies it by a factor of 10
Then your last stage- a voltage follower needs to have a gain of .1 to bring the signal back down to 1volt to input your chip amp  :tu:

Brymus,
       Here is a simple way ,, just hang a 10k pot off the plate *After the HV Cap of course* and run that straight into a SS Amp, Not perfect but keeps the voltage contained a fair bit at the expense of some freq loss. Still the Vtwin idea is better.

Yes I've built this simple circuit and it is very contained,, I've drawn it with the *Drive* switch.  If you build it into a pedal the Switch can be Hard wired to a stomp sw,, no relay needed.

The Vtwin circuit makes intelligent use of the massive voltage loss when using passive high imp tone stacks. example; a 10 volt swing looking into the tone stack results in only about 3volts swing out of TStack.  Bingo Now the AC signal is smaller and can be handled easily by an opamp. Still you would want to use the highest possible supply for the opamps to avoid any clipping.

Extra Vtwin notes;
C7-22pF (across R3-3m3) is likely to cause some harshness,, try deleting it.
Remember into a Valve power stage this would not be a problem as valve circuits tend to wipe off a lot of exagerated treble response but SS does not develop a lot of natural treble loss. If anything some tend to make it quite harsh as they don't have a transfomer to magically wipe off the HF hash.

This is the one thing that has become obvious to me when swapping identical preamp circuits from Tube to SS Amplifiers, The treble can become extreme.
Again just my tonal taste so some might want extreme treble,, which is why I say R&D First.
Phil.
Title: Re: First Hybrid
Post by: awdman on October 10, 2009, 09:06:57 AM
Phatt,
Do you have any schematics of a Chip pre-amp going to power tubes, and is there a way to bypass the output transformer? It seems that the Transformers are why Tube amps cost so much to build.
Title: Re: First Hybrid
Post by: phatt on October 11, 2009, 09:57:34 AM
Quote from: awdman on October 10, 2009, 09:06:57 AM
Phatt,
Do you have any schematics of a Chip pre-amp going to power tubes, and is there a way to bypass the output transformer? It seems that the Transformers are why Tube amps cost so much to build.

Hi awdman,

Re bypassing OTr's,,, yeah like you wish!
Yes give me $10 million for the R&D an I'll get back to you in about 10 years :)
Some People claim to be able to bypass the OT or at least emmulate it but even with the best minds and tecks workin overtime trying to do such things, well let's just say from a hobby perspective a Valve output section (using an OT) might work out quite a lot cheaper. (If you catch my drift?)

I have no doubt that some things come close but most will want an arm an a leg for their efforts. A bit of effort using simple equipment (both valve and SS) can be setup to give you most of what you will ever need.
To save repeating it all,,
My setup is found on this page: http://www.ssguitar.com/index.php?topic=1017.msg8118;topicseen#new  (http://www.ssguitar.com/index.php?topic=1017.msg8118;topicseen#new)

The preamp is the Passive input tone box found here; http://www.ssguitar.com/index.php?topic=1136.0 (http://www.ssguitar.com/index.php?topic=1136.0)

Yes I've messed with electronic circuits for over 20 years,, played guitar on an off for close on 40 years, built heaps of pedals, Amps (both Tube and SS) and all sorts of other audio stuff.

I have arrived at a very simple way to understand the vastly complex,,
EQ> dist/gain> EQ> dist/gain> EQ> speaker.
Get the right EQ happening in the right places in the signal chain of events and even with very basic equipment you may be quite suprised at how good it can sound.
Even with an all SS affiar just EQ before a reasonable dist pedal and again after it will reap great rewards.

Behind the viel of all the amazing mojo Tone heaven teckno gear you guys drool over lies a really simple concept that I believe all guitar players should know.

""Great tone is arrived at via the culmination of one tonal response curve which is then imposed onto the next element with a different response shape,,, add them all together in the right combination and *Magic stuff* will happen.""

The simplicity of it all is *LOST* because there are just so many places in any given circuit that can alter the tone that you can quite literally go on forever making small or large changes to the resulting sound. Without a solid understanding of *Which part to tweak?* You are just guessing.
If they made every internal paremeter adjustable then your front panel would have 100 knobs on the front, Not very user friendly ay? :(

You have to do a lot of reading between the lines when it comes to tone tricks as each tone guru has his own idea of which internal bit to tweak ,,, more often than not it won't be what you want.
You need to work out what is going to give you enough of a tone change to be of *True Value*

The Pickup is an obvious but mostly over rated major tone tweak;
The aftermaket Pup industry is rampent with wild claims,, Yet the simple truth is that you would get more value from better amplification. If you think that better pups will magically transform your tone you need help,,, it is an *Electric* guitar for gods sakes,, so the AMPlification is the *Active Tone Altering Factor*.
(yes the right pups can help but it's only a marginal improvement)

You want tweed sound,, then a dip at 1kHz will help
You want british tone,,, then all mid boost  (from 100Hz up to 3kHz)
You want big Fender clean then,, Boost at 100Hz with a big dip at around 400Hz and then boost all the treble from there up. 

*Obviously* That's a simplified rule of thumb but it might help you get your head around what you are trying to achieve. Each Amp will have it's own natural tonal Q factor which will of course make it sound better or worse.

I've learnt enough to say hey don't get caught up in every new gizmo as a lot of it is just sales hype. Sure you can add a lot of teck stuff to my simple explanation of tone but most guitar players start to get lost in tecknical mumbo jumbo and in the end just rely on *Name Dropping* because it sounds so kool to say;
I've got a triple recto with a matched quad of cryogenicly stabilised 6550's in my rig.
Which of course means nothing to those that know better,, it's just a trick way to sell more tubes ,,AT Inflated prices.

My advice to your original Q; Build yourself a very simple tube amp (a champ if you want). If I've done a good job of explaining all this then it should be obvious now that all the rest can be done with relitively cheap external workarounds.
Have fun, Phil.
Title: Re: First Hybrid
Post by: J M Fahey on October 18, 2009, 08:51:56 PM
*If* you have an Alembic preamp, or any tubed type "pulled" from a Classic Fender, you´ll have to do the following, to keep the good original sound; not screeching trebly, chip killing level, noisy , etc.
Let´s do some math:
To begin with, a tube fed with 300 Vdc can swing some 40 volts less, say around 260 Vpp , or around 80 Vrms.
To safely lower it to chipamp levels, you´ll need a *resistive* attenuator (flat response) , and not a capacitive one (bass killer).
I suggest the following: take your signal from the triode plate through a .047x400 cap, as originally specified, and connect it to the hot pin of a 10K pot, *through a 470K resistor*.
That way, the triode will be happy seeing a load higher than it´s original 100K plate resistor, overloading gracefully as intended, and you´ll get somewhat more than 1V rms at the chipamp input.
You will need no cathode/voltage follower, and the output impedance will be acceptably low: 10K (the pot´s value)
I think it´s clear enough, but if not, I´ll post a drawing, now I´m on a borrowed computer away from home.
Title: Re: First Hybrid
Post by: armstrom on October 18, 2009, 09:22:54 PM
Another good option to reduce the voltage coming out of your tube preamp is to run it through a tube power amp and output transformer :) All joking aside, I believe that is the best way to achieve true "tube tone" with SS cost/weight per watt. While the transformers used in tube amps are where a great deal of the cost goes, the cost of those transformers is directly related to how powerful you want the tube amp to be. If you're bulding a tube amp with 5W or less of output then you can get away with much cheaper transformers. You can pick up nice 5W SE output transformers for less than $30. The output transformer also does a great job of getting the output voltage down to a more manageable range. Think about it, 5W of power across an 8ohm load is only ~6.3V RMS . Much easier and safer to attenuate down to the 1V RMS range than say 60V RMS. Now if you use the 4ohm tap on the transformer then you will only have to contend with half the voltage, so ~3.15V RMS. Now at that voltage range you should be able to just add a 10-20K volume pot in front of the power amp and be done with it. That will give you tons of clean headroom (run the tube amp at a low volume to get a clean sound and crank up the SS power amp volume to make it loud) while still being able to achieve low volume overdriven sounds (crank the tube amp volume and lower the SS power amp volume). Of course, the trade-off is that you could in theory crank both volumes and still clip your SS amp, but that's the price you pay for lots of clean headroom AND the ability to push into overdriven sounds. To solve this you can use something like a VGA (Variable Gain Amplifier) stage on the input of the SS amp to guarantee that you will never exceed its maximum input voltage, or you can add some simple diode clipping right before the amp. It's not going to sound like tube amp clipping, but it will surely sound better than clipping the SS amp and also afford you some protection.
-Matt
Title: Re: First Hybrid
Post by: Minion on November 02, 2009, 08:07:32 PM
You can also make a low voltage Tube preamp , I have the plates in one of my hybrids running off of 40v , that way you can use the Chip amp PSU to power the plates and just get a 12v Adapter to run the heaters and you don"t have a 200V signal going into your chip amp .... I just have a buffer after the tube stage then going into a 15 band EQ and into a TDA7293 , sounds great ....
Title: Re: First Hybrid
Post by: joecool85 on November 03, 2009, 02:50:00 PM
Quote from: Brymus on October 09, 2009, 08:11:03 PM
Here is a better way of expressing what I wanted to say before.
If your guitar has a signal of 1V and your pre-amp amplifies it by a factor of 10
Then your last stage- a voltage follower needs to have a gain of .1 to bring the signal back down to 1volt to input your chip amp  :tu:

Wait a minute...1v into a chipamp?  I ran my guitar straight into my LM3886 and didn't get full volume, my pickups put out 2+ volts.  Then I hooked it up to my LM386 and ran that into it and got full volume, not sure how many volts it was putting out, but had to have been 5-6v.
Title: Re: First Hybrid
Post by: 5thumbs on November 03, 2009, 04:20:41 PM
Quote from: joecool85 on November 03, 2009, 02:50:00 PM
Wait a minute...1v into a chipamp?  I ran my guitar straight into my LM3886 and didn't get full volume, my pickups put out 2+ volts.  Then I hooked it up to my LM386 and ran that into it and got full volume, not sure how many volts it was putting out, but had to have been 5-6v.

I'm working on projects that are based upon the Tripath TA2020 and the LM1875 chip amps.  The TA2020 has a max input voltage of 6V.  (Yes, that's an absolute max rating...the recommended RMS input voltage wasn't specified on the datasheet, so it could be 1V or perhaps higher...unknown to me at this time.)  So you can feed 6V to a TA2020 chip amp and not fry it, but that doesn't mean it will not distort like crazy when fed that level.  (I don't know what voltage levels I can feed my TA2020 chips for clean operation, but I'll get the voltages "in the ballpark" in the design phase, then tweak to final values during the "prototype tweak" phase.)

The LM1875 is max rated for input voltage from -Vee to Vcc, which in my application is 48V (-24V -Vee to +24V Vcc.)  I own a couple of commercial hybrid amplifiers which use the LM1875.  The last valve section feeds +80V into in an inverting 4558 which effectively halves the signal before presenting it to the LM1875 chip amp.  So in this implementation, they have played exactly "by the book" and are halving the voltage to keep it within the spec'd input voltage range.  (In this particular amp, they use a single supply of +40V on the LM1875, but they are still within spec for that chip amp.)

So all of that is more or less reinforcement for what others have said about making sure your preamp output is within spec for the input voltage rating of your power amp.  After reading this thread and thinking it over, I'm going to add an inverting op amp with variable gain reduction just before the power amp outputs.  It's a trivial task to do at the design stage that will make tuning the first prototype that much easier.

Thanks to all on this thread for giving me some good food for thought as I work through my own design issues.  :)
Title: Re: First Hybrid
Post by: phatt on November 11, 2009, 07:52:37 AM
Quote from: J M Fahey on October 18, 2009, 08:51:56 PM
*If* you have an Alembic preamp, or any tubed type "pulled" from a Classic Fender, you´ll have to do the following, to keep the good original sound; not screeching trebly, chip killing level, noisy , etc.
Let´s do some math:
To begin with, a tube fed with 300 Vdc can swing some 40 volts less, say around 260 Vpp , or around 80 Vrms.
To safely lower it to chipamp levels, you´ll need a *resistive* attenuator (flat response) , and not a capacitive one (bass killer).
I suggest the following: take your signal from the triode plate through a .047x400 cap, as originally specified, and connect it to the hot pin of a 10K pot, *through a 470K resistor*.
That way, the triode will be happy seeing a load higher than it´s original 100K plate resistor, overloading gracefully as intended, and you´ll get somewhat more than 1V rms at the chipamp input.
You will need no cathode/voltage follower, and the output impedance will be acceptably low: 10K (the pot´s value)
I think it´s clear enough, but if not, I´ll post a drawing, now I´m on a borrowed computer away from home.

Sorry JMF,, I missed this one,,,, :-[
Once again you nailed it ,,,Yes of course that idea would be the better way to go.
Good on you,,, :tu:  your a wealth of info.  Phil.
Title: Re: First Hybrid
Post by: Brymus on November 12, 2009, 08:13:49 PM
I just about finished putting a 4 gain stage Marshall type tube pre amp(a modified Dockery TMB in cascade mode with selectable boost and an NFB circuit) into a small SE amp (Valve Jr).
I added an FX loop right after the MV breaking directly before the input to the power tube
an EL84.
I can drive anything with it !!!!!!!!!!!
Seriously with the MV down I can drive the input of any of my SS gear.
And it sounds sweet.
And it works super going into another tube amp as well.
Still needs a boost in front to get the gain I am after  8| But wow with some drop tuning
it is the $h1T.
I see no reason why a Hybrid tube pre/SS PA shouldn't be as simple.
FWIW the Dockery pre uses a cathode follower into the PA and it made the tonestack sound better to me.So the last stage is a CF followed by the TMB tonestack and then into the MV into the FX loop into the EL84(PA)

Yeah Joe I found the same thing with most chip amps they need more than 1V input.
I thought the question was in regards to limiting the input to 1 V for a certain chip.
Title: Re: First Hybrid
Post by: J M Fahey on November 12, 2009, 09:37:43 PM
 :P :P :P(Drooling) :P :P :P
Hey Brymus, post something else. Pictures, schematics, MP3s, whatever.
What you write is enticing. :tu:
Congratulations.
Juan Manuel Fahey
Title: Re: First Hybrid
Post by: Brymus on December 30, 2009, 04:09:17 PM
Hey JM sorry its been awhile,been busy.
Here is a schematic,This is the original I think I changed a couple of small things since in the FX loop and clipping section nothing major.
No MP3s yet,
(http://i419.photobucket.com/albums/pp276/Bry928/High%20Gain%20EVJ/schematic002.jpg)
The "guts"
(http://i419.photobucket.com/albums/pp276/Bry928/High%20Gain%20EVJ/pic001.jpg)
More pics
http://s419.photobucket.com/albums/pp276/Bry928/High%20Gain%20EVJ/
I should probably start another thread for this. Later when i have more time.
Title: Re: First Hybrid
Post by: phatt on January 02, 2010, 08:13:49 AM
Nice work Brymus ,,
but my concern is the NF path ,,, efx loop in between?? Head scratch?? Like what's gonna happen if the returning signal is in phase??

Surely that would then be a positive FB situation?
Hey if it works then no matter,, an I maybe missing something. ;)

Phil.
Title: Re: First Hybrid
Post by: J M Fahey on January 02, 2010, 10:38:47 PM
Hi brymus. Two things:
1) I *loved* thios preamp, i'm sure it's a killer.
2) agreeing with Phatt, you can't bridge so far-away points in the amp with a NFB loop; even less if there are "modifiers" included in iot: Volume, tone stack and, horror of horrors !! an effects loop.
Just forget it, and, besides, the amp sounds better without it.
3) URGENT URGENT ¡¡¡¡¡NOW!!!! solder a 1M resistor across the effects return jack or even better, from EL84's grid pin to ground !!!!!!!!!!!!
The SECOND your output tube loses ground reference (such as a faulty jack or an output DC  isolated effect) your poor power tube will go to Tube Heaven, to play harp and fly from cloud to cloud.
4) If you make Phatt and I scratch our heads too much, we´ll start to get bald early in our youth !! (now that's a good excuse, hehe)
5) Liked the layout too.
Title: Re: First Hybrid
Post by: Brymus on January 03, 2010, 08:48:38 PM
Ah yes it did run away w/o the ground reference,caused me quite a headache .
That has been fixed.
The NFB works great when the MV is at full volume.
Tames it just enough,but if there is a better way I am all for trying.
The FX loop was for trying the pre-amp with other amps more than adding FX but works both ways,still not very user friendly unless you know how its wired.
This should really be a two channel amp instead of having those switches to take the pre from clean to over the top but I was working with in the confines of the EVJ chassis and PT.
Thanx for the feedback any revisions are more than welcome.