preamp build

[ernie_jr]

I am thinking of building a tube pre-amp to use with my Sunn concert amp. I have a small PT that give 150 volts. using 150 x 1.4 I get 210 dc volts. what plate and cathode resistor should I use to keep the plate voltage at around 200?
Thanks,
ernie

[terminalgs]

With a 12AX7, I don't think that can happen. with that kind of B+, what you might get from a 12AX7 gain stage:  with a bigger 270k plate resistor (Rp) --and assuming a small cathode resistor (Rk) 1.5K-2.7k-- you'd probably see 60V @plate;  with Rp=100k, you'd probably see 120V @plate.  If you are driving a tone-stack, I don't think you'd want to go lower than a Rp=100k (output impedance  of the stage goes down as the Rp goes down).   Even if you do try to go lower, like Rp=47k, you might see 150V?  any lower and you are entering current-driver territory, and not gain producing voltage-drivers.  and these voltage guesses are optimistic..


things to consider:


is this for bass or guitar?  you want clean or overdrive and distortion?


if you are planning to plug into the 'power amp in' on a Sunn amp,  you'll need a lot of clean gain,  so you'll need a lot more B+.  you also need to see what the input impedance (Z) of that input is, and make sure your output Z is appropriate.


If you are plugging into the instrument input and all you really want is some boost, then you can probably do something with that PT. a simple 12AX7 gain stage with a volume knob work work.   You could add a single one knob treble roll off control if you want.   The Vox Top Boost stage is fun and uses the other 1/2 triode as a coupled CF.

[2deaf]

You could probably put a voltage doubler on the secondary.  That would allow plenty of voltage drop in the filter section and still have a high E_bb.

Even with a B+ of 200V or so, you can make all kinds of tube preamps.

The line in on a Sunn Concert Bass looks like it wants 0.5V, so headroom shouldn't be any problem.  It also looks like it has a minimum input impedance of 47K.  Even so, you would still want a very low output impedance from your preamp if you are using cables of any significant length.

[vibrolax]

I second the motion for a full wave voltage doubler.

Here is a link to a dual channel Fender blackface-like bass preamp I built with a voltage doubler: http://www.frontiernet.net/~jff/af2b/Dual_Channel_Blackface-style_Preamp_for_Bass_Guitar.html
This was pretty much a clone of the Alembic F-2B.

With your TX, raw B+ will be 2.8 x 150 VAC = 420 VDC.  Dropping the voltage 25% in 2 or 3 RC filtering stages will give about 315 VDC, very typical for 12AX7 gain stages.  If you really need a low impedance output, a cathode follower, or a MOSFET source follower might be in order.  But you may be ok without a CF.

Jon

[HotBluePlates]

I am thinking of building a tube pre-amp to use with my Sunn concert amp. ... what plate and cathode resistor should I use to keep the plate voltage at around 200?

Why does the plate voltage need to be 200v? Did you design the full preamp already, with consideration for input signal from the pickup, needed drive voltage to your Sunn amp, and amplification needed given any losses from volume/tone controls and/or other effects?

Assuming you did all the above, and the only way to use the tube stages you want with the desired control capabilities is to have a plate voltage greater than 200v, and therefore a supply voltage around 300v+, then why are you stuck with using the 150vac transformer?


I don't know that I'm reading the Sunn Concert schematic correctly, but it looks to indicate 0.25-0.5v peak at the power amp input is plenty to drive this amp. Granted, this should be from a low source impedance.

You won't be driving output tubes, so is a big supply voltage and a big output voltage necessary?

[terminalgs]

You won't be driving output tubes, so is a big supply voltage and a big output voltage necessary?

you are right, I was thinking of a 'power amp in' on a tube amp.

[Colas LeGrippa]

In order to keep it as simple as possible and to avoid parasitic oscillation, hook up the cathodes direct to grnd.
After when you 'll get the beast working, add unbypassed resistors ( 820, 1k5, 2k7 first stage and 10k other ). This will permit you to shape the tone progressively. Don't bypass all the resitors with caps to begin with. Do it as you'd build a house. So you'll pinpoint more easily what you like and what you don't. Don't mess around with dc heaters. You can build a noisefree preamp with ac heaters and good lead dress. Post pics when it's done !


Colas LeGrippa

[ernie_jr]

I have an ampeg B25 and i like the sound of its preamp over my Sunn 200s preamp. ( have sent both into the power amp in of the Sunn concert) I was thinking of building a stand alone ampeg pre-amp and it calsl for about 200 volts on the plate of the 12ax7.
Thanks,
Ernie

[sluckey]

It's more like 230 and 245 on the plates of the preamp 12AX7. But more important, the B+ at node E which supplies that preamp is 345V. That's the part that will be difficult to get from your 150VAC transformer. A FWB will only give you 212. You may need to look at some voltage multipliers like the tripler on this page...

     http://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/voltage-multipliers/

[ernie_jr]

Maybe easier to get a hammond 269GX P/T  (225 x 1.4 give 315 DC volts)
Ernie

[terminalgs]

Triode transformers/classictone has a dynaco preamp transformer 330-0-330 @ 15ma and 12V @ 800ma for under $50 and it's compact.


if you are using 12AX7s or 12SL7s the filament supply of 12v is perfect. if you are using 6SL7s,  you could rectify to DC, and use something like an L7806 to regulate it to 6VDC.

[ernie_jr]

that look perfect
thanks,
ernie

[sluckey]

Here's my favorite. It will give you exactly what you need. It will fit under a 2.5" deep chassis. I used this on my revibe project.

     http://www.tedweber.com/wrvbpt

[ernie_jr]

Thanks Sluckey
Ernie