Bass tube preamp (Alembic F2-B + full EQ + XLR + tuner)

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Hi everybody,
who can help me?
I would to build a tube bass preamp (from Alembic F2B) with these specifications:

Led Clip
Lo Mid & Hi Mid (or Freq pot and Mid)
Led Power On
Mute switch
XLR D.I. output
Pre/post EQ switch
Tuner out (active also in mute mode)

All components must be in pcb (resistors, capacitors, tube, pots, input jack, switches, leds,….no XLR jack, tuner jack and output jack)!!!!
Please, who wants to help me in this project? There’s nothing similar already exsisting….and for me it’s a good project
Contact me!
Dear davebass: you are trying to mix two absolutely incompatible projects, for no good reasons.
An F2b is what it is, a killer sounding very simple Tube preamp, in fact a Fender preamp, with time tested passive controls, period.

While there are also very good but absolutely different SS preamps, with the features you mention.

1) you do not need "gain" since tubes have huge headroom, it's easy to clip a +/-15V fed SS preamp (maximum output swing ~30Vpp), that's why it's often needed a gain control, active/passive inputs, etc. while the ~300V fed F2b triodes can swing almost 300Vpp :eek:

Besides, gain in an SS stage is defined by its NFB, and can be set to any value between, say, 1 and 100, while a tube stage does not use NFB and has fixed gain, usually around 50X .
You may or may not use cathode caps which allow for a fixed gain shift, maybe split load resistors if you want to lower gain or signal level, or add some passive attenuator between stages, a VERY different way to do things.

2) Bass and Treble?: it already has TMB controls, straight from Fender, they work very well and are what they are.

3) hi mid/low mid? ... those frequencies are close to each other, to achieve any separation you need tuned circuits, in Tube amps needing actual wound inductors.
Possible but ... it will stop being an F2b

4) mid with freq pot? ... definitely an active circuit ... easy with (SS) Op Amps
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, sort of impractical with Tube Op Amps:
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Not physically impossible, mind you, and somebody could do it to beat a Guinness Record or something, but not practical either .

and on and on and on ......

Yes, you may add a Led light (in fact I think original F2b has one) , a (passive) mute switch (just short output to ground) , you may pull some signal for a Tuner out and if you do this before the Mute, it won't be affected.

As of an XLR out, it needs either addition of a cathode follower to drive a small line transformer or (ugh!!!!!) , you'd need to add a couple SS Op Amps.

In a nutshell: just looking at rack bass preamp front panels, it might be cool to imagine to mix functionalities, but they are so different inside, use such different ways to achieve things, that the mix becomes, at least, very impractical.

Yes, but it will not be an F2b

Regarding gain, this can be added, but is more suited to getting overdrive tones. Low gain guitar and such.
You can add some tricks to the tone stack to add some flexibility.

For the rest, see jmfaheys reply :D

Transformer balanced outputs are not hard to add (you need an extra tube, and you can use the other triode section in that tube to get more gain if needed).

The psu needs attention, and if you haven't built tube circuits before, do a lot of studying first.
Hi Guys

Everything the OP wants to do is perfectly easy to do with tubes. You just have to accept the fact that it cannot be made extremely small.

Almost every tube bass preamp is a standard fender preamp with two gain stages that have an EQ and level control between them. The values are standard as well.

Add another tube to buffer the tuner output and to buffer the main output. Either or both can have an isolation transformer added to eliminate ground loops. So, two tubes overall and nothing new to design - all dead standard stuff.

It is most often the case that when someone tells you that you cannot do something, that in fact you can.

Have fun
Dear Struth, when you answer to a post, at least *try* to answer what's being asked, or at least somewhat related.

Your "Everything the OP wants to do is perfectly easy to do with tubes." doesn't stand the chance of a snowflake in Hell.

So "nothing new to design" huh?
1) you do NOT explain how to get Gain (control, NOT a passive volume)
2) you do NOT explain how to get Led Clip
3) you do NOT explain how to get Lo Mid & Hi Mid
4) you do NOT explain how to get Freq pot and Mid
you also leave out a couple minor points, but will set those aside, I'd LOVE to see your
nothing new to design - all dead standard stuff.

But ok, I'm open minded, please explain how will you solve those problems.

Just don't tell me "the answer is in TUT *** , yours for only $***" ;)
If you suggest a working solution, please post it here :)

As of:
It is most often the case that when someone tells you that you cannot do something, that in fact you can.
... would you please care to post just *one* full tube preamp (hidden Op Amps or DSP inside count as cheating ;) so don't :nownow:) which does what the OP is asking for?

Maybe there's some reason he wrote:
There’s nothing similar already exsisting….
Hi mGuys

JMFahey: Are you unable to read? Do you feel threatened that someone wants to do something you wouldn't do? or that you don't know how to do?

Read post-5 again. Everything is addressed quite simply.

It is definitely a good thing to have a bass preamp with a gain control considering the high output of modern active basses and even some passives. Having true variable gain for the input stage is done in two different ways, both of which are standard tube methodology from the fifties and earlier.

One variable gain approach is to have two cascaded stages within a feedback loop. The loop gain is made variable from a low number (not equal to one, more like 3-5) up to whatever the feedback resistor values dictate (within tube capability). I've used this circuit since 1990 for my Spectrum bass amps but it hails from the '50s.

The second way to have variable stage gain is to set the stage up like a concertina. The cathode output has a pot to ground that varies the gain. This circuit is even older and I use it in my current All-Tube Bass preamplifier, Studio power amp, and KC25 EL-84 power amp.

So, either one triode or two for variable gain. Then add another stage or two for more character. Like I said above, the standard Fender preamp is what most bass amps use: triode stage, tone stack, volume, triode stage. This actually works extremely well for bass which is why everyone else does it that way.

Three-way EQ in a bass or guitar amp? Who has ever heard of that? Look at any vintage Fender schemo. You can have 3-way EQ using the venerable tone stack or usinga Baxandall EQ. If you want to go "modern" you can make the bax active, as the original 1947 article shows, or follow updated circuits that do not need tapped pots. Again, all standard stuff for anyone who can or does read.

A tuner output. However could you do this with tubes? A cathode follower is the usual way, with its grid signal tapped from the input jack. Want to make it switchable or mute the main path for tuning? However can you switch things? Fortunately Michael Faraday invented relays back in Victorian times - or was it Elizabethan? Anyway, it was a really long time ago and fortunately modern relays are tiny and consume little power. Set up the control for this to default to playing, then the relay is only powered for muting and tuning.

Cathode followers are also used for the DI output and main output. The main output should have its own level control. The DI should tap after all the preamp and EQ NOT right from the input. TUT2 explains why in detail, but the reason is obvious. Why would you want the raw instrument tone to go to the house PA rather than the tone from your preamp which you actually hear on stage?

A tube-driven clip LED. You can do it but it is unnecessary. What is the poijt of indicating clipping visually? You can hear it when it clips. The only reasonable point for a clip LED on a tube amp is at the DI or house output where signals beyond a specific level will overload the input of subsequent gear. In this case, a tube stage with variable gain can be used, or a simple rectifier in series with a zener and resistor and the LED will work. here is where using a single transistor would be effective as a BJT has a lot more gain than a common tube does and will drive the LED current easier.

All the circuits above are available online and in most guitar amp books published, and/or in manufacturer's schematics. Nothing new at all.

DaveBass: DO NOT be put off by people like JMF. Your project is easily do-able but you should build it in stages. Start with the standard Fender circuit with tone stack and simply add a level control after the second stage. This will give you a feel for how tubes sound and what just two stages (one 12A_7 bottle) can do.

Have fun
Oh, I can read very well ;)
Just in case, can you self quote and show all of us where you address this? :

Now you are suggesting a way of, "sort of" adding a little more (why didn't you on post #5 is beyond me).

You are still missing the very important demands of:
3) you do NOT explain how to get Lo Mid & Hi Mid
4) you do NOT explain how to get Freq pot and Mid
Care to share how to do it *here* ?

Because you already did what I suspected and warned against, instead of helping others you are using this Forum to sell your stuff:
TUT2 explains why in detail
I warned:
Just don't tell me "the answer is in TUT *** , yours for only $*** ;) "
If you suggest a working solution, please post it here

As of:
Your project is easily do-able
Ok, if so please post a schematic .
Here, of course.
Meanwhile, I'll grab the popcorn and get a comfortable seat.
Hi Guys

As always, the most negative viewpoint is the loudest.

DaveBass, follow the advice of post-5 and at the bottom of post-7 and you will have a fine bass preamp. Lo-mid and hi-mid are simple equalisation controls to implement but I would first try a tone stack - it is more useful than you think.

If you want to go active with 4-way EQ, then build two inverting gain sections, with bass and hi-mid in the first and lo-mid and treble in the second. This separates the control centres enough to reduce their interaction. Doing this with tubes will not result in deep cut/boost or narrow bands, however, it will be very musical. The gain stage needs a lot of gain to throw away to be accurate, and a single triode or pentode won't cut it, but they work okay if you relax the amount of boost/cut required.

Have fun
As luck would have it I was just looking at this circuit - the Twin Reverb - today with a view to building it. Just one 12AX7 makes it really simple.

I was thinking of using a Lundahl (or similar) line out transformer to bring down the output impedence.

Can any of you guys suggest one, or at least what I should be looking for in terms of specifications?

The idea would probably be to drive a digital output stage.
Hi Guys

Andy, the simplest way to bring the output level of the two-stage preamp down to what is acceptable for the sound card is to use a voltage divider. Since the sound card has an extremely limited capability, the bottom resistor of the divider will be very low, maybe 1k. The dropping R might be 100k or something approaching that value. Using a pot here is simpler again and allows continuously variable output.

The output of the second stage can be as high as 80Vpk when driven hard, and operating at typical voltages found in an amp.

The divider above does not give you galvanic isolation like a transformer and is not an impedance transformer in itself. The output impedance of the 12AX7 plate is quite high and very high turns ratio would be needed to bring the impedance down to a line level.

If you do not mind adding a second tube, then you have the ability to add a pot after the second stage to control output level independent of sensitivity to the input, and the new tube can be wired as a cathode follower. In this case you can use a 1:1 transformer, which are all-around easier to make very high-performance and at low cost. They are also the most common types so will be the lowest priced ones.

If you are intent on having only one tube, then turn the second stage into a CF and add the iso-transformer.

There is no such thing as a digital power amp or digital output stage, rather there are class-D amps which are dreadful for audio. Unless the PA is far from the preamp, there is no need to use a transformer in between them, although it does help to separate the quiet analog ground from the noisy class-D ground. Despite class-D having been around since 1965 or so, it is still a very immature technology. Good enough for cell phones...

Interfacing to any solid-state PA from the second tube gain stage is simplest using a level control - a pot. Ideally you add a buffer after the pot if the input impedance of the amp is very low. In most cases, you could place a resistor in series from the wiper and this would allow the pot taper to stay more linear sounding while providing some attenuation that is likely needed.

Have fun
Looks like a cathode follower is the simplest solution, and the lightest in terms of weight. If there are going to be 2 tubes anyway, then the Fender Bassman 5F6A circuit might be a better idea, something like this:


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Hi Guys

Leave out the CF driving the EQ as it is a tone sucker. The EQ is much more dynamic and DOES something when plate driven. Use the spare triode for a second output.

Also, place a level control between the EQ and the output CF. This gives a lot more control over the signal and allows one to access mre of the tube touch response character without over-driving whatever follows.

Have fun
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