Pentode preamp, help!

[ppaappoo]

Hi, anyone knows how to make a preamp with an EF41 pentode?
Here is the datasheet.

http://www.r-type.org/pdfs/ef41.pdf

bie!

[PRR]

Distortion will be bad (which may be "good"):

B+ = 150V-300V
Rp = 25K (27K) 1 Watt
Rg2 = 75K 1/2 Watt
Rk = 330

For about 250V supply:
current demand ~~ 6.5mA
cathode voltage ~~ 2V
plate voltage ~~ 100V to 170V (not critical)
gain ~~ 40
max output ~~ 40V peak
max input ~~ 1V peak
cathode cap for -3dB at 20Hz ~~ 45uFd (try 10uFd 6V for guitar)
G2 bypass cap for -3dB at 20Hz ~~ 3uFd (try 0.5uFd 400V for guitar)

[ppaappoo]

Distortion will be bad (which may be "good"):

B+ = 150V-300V
Rp = 25K (27K) 1 Watt
Rg2 = 75K 1/2 Watt
Rk = 330

For about 250V supply:
current demand ~~ 6.5mA
cathode voltage ~~ 2V
plate voltage ~~ 100V to 170V (not critical)
gain ~~ 40
max output ~~ 40V peak
max input ~~ 1V peak
cathode cap for -3dB at 20Hz ~~ 45uFd (try 10uFd 6V for guitar)
G2 bypass cap for -3dB at 20Hz ~~ 3uFd (try 0.5uFd 400V for guitar)

Wow, how you get that values?  
I'll use a 150VDC  power supply, keep that values?

[ppaappoo]

Somethink like this? again using a 150VDC power supply.

[ppaappoo]

so?

[panterafanatic]

You may want to parallel the cathode bypass cap with a 100nF or so, electrolytic caps have low frequency distortion, but since it's not a coupling cap and we aren't doing hifi it's not critical. The output cap should be bigger too, 100nF for that should that do, you want it big incase the next thing in the signal chain doesn't have such a high input impedance like tubes/FETs. The cap to filter the screen voltage can be made smaller if you'd like as well, 100nF would do that well, but the value you've got now will work just fine too.

How'd you calculate that 100k grid stopper?

[ppaappoo]

How'd you calculate that 100k grid stopper?

No, just read in somewhere that grid stopper resistor must be larger and i saw it in vox ac4' scheme.

[merlinb]

so?

It will work. Values for valves are not critical. You can play with the anode resistors, cathode resistor and screen resistor to your heart's content, to get a sound you like.

[ppaappoo]

Gracias por las réplicas, estuve pensando en usar una segunda valvula como etapa amplificadora.

El circuito seria el siguiente, solo necesito ayuda para calcular los valores de los componentes faltantes.

[PRR]

""Thanks for the replies, I was thinking about using a second valve as amplifier stage. The circuit would be the next, I just need help to calculate the values ​​of the missing components.""
================================================

Please review this post: http://www.diystompboxes.com/smfforum/index.php?topic=88481.msg747070#msg747070

[PRR]

Operating conditions for the power-amp stage can be copied right off the datasheet, page 4.

http://www.mif.pg.gda.pl/homepages/frank/sheets/046/e/EL41.pdf

[amptramp]

Paul is correct about the distortion that can be expected because this is a remote-cutoff tube (also called variable-mu in some places) whose main purpose is to amplify a signal with a gain in accordance with the negative bias applied to it - the more negative it goes, the less gain there is.  This is not an issue when you are handling millivolt inputs with several volts of bias which this tube does in IF amplifiers in radios, its main application.  It is more of an issue when your singnal is more than a tenth of the bias, because the curvature of the transfer function (that is, the ratio of plate current to grid voltage) begins to be noticeable in this range.  There are some compressors such as the Fairchild 670 that used 6386 remote cutoff dual triodes for gain control and the Hammond Solovox uses 6SK7 remote cutoff pentodes for volume control, so it can be done.

Remote cutoff is achieved by varying the helical winding pitch of the grid wires so that at high bias only the loosely-would section allows current through whereas at low bias, all of the cathode current comes through and full gain is available.

BTW I was once asked by Davey Loprinzi a.k.a. Davey Nipples, bassist for Everclear then Storm Large and the Balls to locate a 6386 for his Fairchild 670.  Note that these compresors go for incredibly high prices now - about $30,000 - and the only 6386 that was listed on the net was $65.  By the time he tried to buy it, it was gone.

But what would be bad for high fidelity use may be good for a stompbox - distortion is often welcome and you will get that smooth entry into distortion as the input gets louder that made the fuzz face a popular favourite (although the pentode should have less distortion).

[PRR]

> this is a remote-cutoff tube

Datasheets all say it is an Output Pentode. Should be fairly linear.

Is not common. Is not cheap. Has no apparent advantage over more common and inexpensive tubes. I suspect very-very few builders will ever try to build an amp around EL41, which is why I will not spend much time on this proposed project.

____________________________________________________________

> this is a remote-cutoff tube .... in radios, its main application

If it WERE a remote-cutoff tube---

It's not just that radio signals may be weak. Indeed the main need for gain-control is when signals are strong. The point I had never appreciated: when recovering an Amplitude Modulated signal, huge amounts of even-order distortion do NO harm at all. You can even cut the wave in half, and all the information is still contained; and this is how AM is usually demodulated, with a diode. Between IF stages, huge 2nd harmonic in one tube is flywheeled-out in the resonant circuit between stages.

> some compressors

Any "good" compressor/limiter will work two tubes in push-pull. Here 2nd harmonic must be kept low. Yes, you can find some SE gain-changers in old books. These were often aimed at 78-quality sources, and expansion was popular (to undo the severe compression needed to put symphonic sound through a 78 or an AM radio).

Also note that SuperControl (remote-cutoff) tubes are generaly specified for >40dB gain reduction. They really do go this low around strong local AM signals. OTOH in Audio we rarely ask-for, and really can't expect, 40dB gain reduction by decreasing tube current. Maximum output must happen at minimum current. But audio loads are lower impedance than AM IF loads. A hard-slammed remote tube can't make much audio. 20dB clean and 30dB ugly is good performance. (660 only does 12dB clean, AND uses a rather different tube made for aircraft radio, and more than a pair of them.)