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tube voltage amp in ss amp

Started by Wuchi, June 09, 2009, 06:18:12 PM

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This is my first post in this forum.

I've built few tube amps from kits and I have some understanding about how they work. Now I'm trying to learn something about ss amps and I'm building one with my son.

The idea of the amp we're building is like this:

two different preamps driving a stereo amp(lm3886). One is driving the left channel and other the right channel. The idea was that you could use one or two amps the same time using an a/b/y box. I have a 2 x 12" combo that I'll use with two different speakers, alnico and ceramic. And the two preamps would drive naturally separate speakers.


Here is the question: If I use for example stompboxes from runoffgroove as preamps, do I need a voltage amp before the poweramp? If I do, what if I use a tube circuit here? Will it contribute tube(ish) sound to overall result?

Hopefully you will  understand my writing :)



J M Fahey

Hi WuChi. : you can use both halves of a 12AU7 , each triode working as a very simple preamp for one LM3886, and allowing it to be fully driven by a 9V powered stompbox.
Why 12AU7?: because it :
A)has the least gain (around 10x) among common double triodes, and
b)is comfortable working with as little as 100V dc. supply (even 60V DC in a pinch) while
c)12AT7 provides around 40/50x and 12AX7 almost 100x which is way too much.
Besides they´re happy with around 250V DC, preferably not less than 160Vdc, deadly voltages.


Ok. This was something I was looking for.

Well what kind of power transformer I should have? The lm3886 wants 18v+/18v-, the stompbox preamps want 9v and the 12au7 preamp wants that 100v and 6,3v for heaters.

Any tips how should I do it?



for the different voltages build a voltage multiplier, I would use 12AX7 tubes . ANY TUBE will operate correctly down to 90V if biased correctly.

                                                 Rock On

J M Fahey

Hi Wuchi/Jari
I find your +/-18V supply quite low for your LM3886.
Yes, it will work, but will provide no more than 18W into 8 ohms. +/-30V (or thereabouts) will give you around 45 Watts per channel, good for stage use.
Where do you live? Or, more precisely, what´s the power line voltage there? There´s a simple/cheap trich using small power transformers that can give you both those 6,3 volts (or 12,6) *and* your +B tube power.
J M Fahey


Actually there are a lot of designs out there that use the 12ax7 with voltages as low as 12v , or use 12v AC and use a Voltage doubler to get about 32v for the Plate , that 12v ac can also be made into a Half wave voltage doubler to give +/-15v DC for solid state stuff like opamps .....

So you can concieveably Build the Tube pre , solid state/Stompbox curcuits and Power amp with a Transformer at 18v ,0v ,18v at 200Va and a 12v tap at 15Va ......



Hello Wuchi, In reference to "will it deliver tubish sound?"
Having done same or similar to what you propose,, the short answer is you're wasting your time.
The real secret to tubes is in the Power Stages where the signal is huge and passes through a transformer. (more to do with *Pentodes driving Transformers* than a Valve in general)  In my humble Opinion preamp distortion (however you derive it) is always a little harsh and somewhat lacking in richness and depth.
Heck even the preamp stages of an old Marshall stay quite *undistorted* ,,
it's the power section that delivers most of the magic.

All that can be achieved with triode preamps is a bigger signal along with a bit of half wave clipping which will in most cases deliver some improvement but not enough to make it a *Winner*.
Mixing tubes with SS is always a tricky one but If you just can't help yourself and you just have to build a Valve stage then you will be hard pressed to find a better circuit than the *Messa VTwin pedal*. (you will need to find a 200VHT PSU though) Running valves way below there standard DC OP is just wasting a valve.
I only needed to build the clean section of the Vtwin as I found the crunch ch a waste of time and my SS units did a better job of preamp crunch anyway.

You mention Fet circuits and then ask if they would benifit from a valve stage.
Understand the in-escapable truth about Audio circuitry; (Valve or SS) The bigger the supply voltage then the bigger the signal that can pass through.

So a Fet working from 30VDC has no chance of competing with a triode working from a 200VDC supply. From my experience even if you use a valve with at least 90VDC HT you will have no need for a Fet.
We could get all complicated and mention *Current gain verses Voltage gain* but the dynamic way that a High Voltage, High Impedance triode type circuits work is sonically different. That does not mean SS is crap,, just means the whole design/ game plan has to be approached in a different manner. Now mix the two together and wow that can get you in a lot of strife.

To generalse it; Transistors are Low Voltage, Low impedance, High Current devices,, Whereas Valves are High impedance, High Voltage, Low current devices. That helped me get my head around the reason they are so different.

Just shoving a triode into a SS circuit does not costitute a Valve tone,,, sadly a lot of crap on the market tries to sell that school of thinking. Even more sad that so many believe it.

Re Vtwin schematic,
R24 270k around u1b is Wrong,, try a more civilised 27k instead.
You may not need that part but if you are driving the Powerchip direct from the Vtwin then you may need it.
You may wish to get ideas from my *Passive tone circuit* listed under *schematics and layouts* on this site.
It's just the tone section of the Vtwin without the Valves in front. (of course I opted for the Hiwatt tone design)
Don't forget *C2* 50nF 400volt,, as it decouples the HT from the lower voltage rated tone components.
You can use any OD circuit in front of it as it's still fairly clean but you may wish to add the second crunch Ch instead.
But Again I stress this still won't sound totally convincing through a SS powerchip.
I would want to hunt down a really good Distortion unit to add in front.

Re Stereo guitar, A lot of messing about for little gain. In my view it would have to be at the very least Twice as good as mono,,, sadly it's not.

Re Speakers,
More focus on circuit design is going to reap far greater rewards than swapping speakers around all the time. Even the implimentation of simple things such as a graphic EQ before the power stage is a lot easier to work with than swapping speakers.

I strongly suggest you bread board anything you conjur up and test it first before commiting to building it as there is nothing worse than building something only to find it ends up in the parts bin. BTDT :(

I have spent over 20 years trying to come up with circuits that really work for guitar sound/tone ,,sadly most of them have been complete disasters.  I have built a lot of DUD circuits that come under the catorgory of;
"seemed like a grand idea at the time".
So be warned it is not as simple as just soldering a few resistors to a board and putting it in a pretty box with a shinny coat of paint.
Hope it helps,, Phil.