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First Hybrid

Started by baene123, October 08, 2009, 01:17:15 PM

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baene123

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 :-)

armstrom

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

baene123

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...

Brymus

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?

phatt

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.

Brymus

How about a schematic Phatt?

Brymus

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:

awdman


phatt

Quote from: awdman on October 09, 2009, 10:46:17 PM
is this the schematic? 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.

phatt

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.

awdman

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.

phatt

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

The preamp is the Passive input tone box found here; 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.

J M Fahey

*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.

armstrom

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

Minion

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 ....