Alembic F2B build 6N1P

[Spencerleehorton]

Hi,

Have started to build a Alembic F2b preamp and noticed I had a few Russian 6N1P tubes available, I checked the datasheets comparing to to 12au7 and 12ax7 and seem to be same pinout and both are dual triode.
Just need to buy a 1U case and mains transformer and it should all come together.
Will post some pics of the boards later.

 

[rock soderstrom]

Interesting project! I don't want to be a smart arse because the pinout of the 6N1P is not really identical to the 12AX7/12AT7/12AU7 family. The Russian tube is 6.3V only (like the E88CC) and has a shield at pin9 between the two triode systems. The Alembic F2b calls for a 12AX7, the 6N1P is pretty different but there is the Russian 6N2P. This one is 6.3V only as well but otherwise technically very close to the 12AX7.

But I guess with smal changes you can build up this preamp with 6N1P too.

 

[Spencerleehorton]

Oh right? Thought I’d probably read the datasheet wrong!
So it could work I’ve just got to change a few bits?

 

[Spencerleehorton]

So only 6.3v for the heaters rather than 12.6v

Will the other voltages be ok?

 

[rock soderstrom]

Thought I’d probably read the datasheet wrong!
So it could work I’ve just got to change a few bits?

Yeah, check the datasheet. The 6N1P has lower gain, lower plate resistance, needs more heater current, has better drive capability and so on. It is basically a total different tube as the 12AX7/ECC83.



The question is, what do you want to use the preamp for?

If you want to operate this preamp before a power amplifier, use the original circuit with an ECC83 or the Russian equivalent.

If you want to use it as an instrument amp in front of a mixer or audio interface, you'll have to make some changes anyway from my point of view, check the forum, there are already quite a few threads about it.


Then you can also use the 6N1P and make it fit...

 

[Matador]

6N1P has a mu of 33 versus 100 for a 12AX7. It's more like a 12AY7 (6072). It's also significantly lower plate impedance, which may not be optimal with the 100K plate resistors.

F2B is basically a Fender AB763 front end with some component changes. A 12AX7 with a 100K plate resistor and 1.5K cathode resistor is roughly overall gain of about 65, however stuffing a 6N1P in there yields an overall gain of about 30 (roughly half). So (overall) gain will be significantly lower. If you plan on using it as a bass preamp and are expecting it to run in the territory of breakup/distortion you might be disappointed.

On the flip side, the lower gain tube can take a significantly hotter input signal before distortion exceeds 1%, so if that's what you are looking for, it might just fit the bill.

 

[Spencerleehorton]

Ah ok, again im Learning a lot from my choices and lack of knowledge in these areas!!
I’ll Stick with the trusted 12ax7 but the second tube is a 12au7 so
As with the noble preamp I used, I will have a bit of a play with the tubes and the resistors to get it to where I’m happy.

 

[Spencerleehorton]

quite a specific transformer for this is needed, 260v @ 0.09A sec with 2 x 6.3v @ 2A
i can get a 2 x 220v @ 58mA per winding with 2 x 6.3v @ 2A, so could put the 220v in parallel but would only get 308vdc after diode bridge.
I have asked the same company whether they can wind the correct one but not heard back yet.
the antek one comes from USA and its shipping is twice the amount of the txf and it will incure another cost on the customs so isnt worth getting.

 

[Rob Flinn]

quite a specific transformer for this is needed, 260v @ 0.09A sec with 2 x 6.3v @ 2A
i can get a 2 x 220v @ 58mA per winding with 2 x 6.3v @ 2A, so could put the 220v in parallel but would only get 308vdc after diode bridge.
I have asked the same company whether they can wind the correct one but not heard back yet.
the antek one comes from USA and its shipping is twice the amount of the txf and it will incure another cost on the customs so isnt worth getting.

Look at R core transformers on ebay or aliexpress. I think I have seen one in the past with a 260v secondary, but they can also do custom ones pretty cheaply & they work well. I used them in my RS124 builds.

Here for the custom ones with free shipping !!!

https://www.ebay.co.uk/itm/22576086...11fKFoj6pfyvS6LW7YdU4KDA==|tkp:BlBMUI7Ixr2lZA

 

[Rob Flinn]

This one is pretty close

https://www.aliexpress.com/item/329...oHWXamRQ&utparam-url=scene:search|query_from:

 

[Tubetec]

A 50W transformer with 4 A @6.3v seems like a very large amount of power for a double triode which consumes either 300 or 600mA heater current in total depending on if you use the 6N1P or 12AX7 .

I often find myself using seperate HT and LT transformers on small tube projects these days , I can find a suitable LT transformer for free from electronic recycling , then I use a standard bathroom shaver transformer for HT which costs around 11 euros from the electrical wholesale , it can be configured for three different secondary voltages , 120/165/240 AC . Its rated for around 80mA @ 240v , more than twice that @ 120v .

Im not sure what schematic your looking at , but thats the one I found .

Id go with the 12ax7 but this circuit will only drive a hi-z load like a DI box .

 

[Spencerleehorton]

I have this toroidal, its pretty chunky so i would have thought the 6.3v and the 10v were at least 2A each and the 220v about 100mA.

would like to use this if i can?

is it possible?

 

[Accelerator]

@Spencerleehorton​

For the Half Wave Capacitor Input Load Rectification that you see in the Alembic Tube Pre-Amp. F2B Schematic, your transformer will be a little bit too high for the ECC83 – 12AX7 tube as you will have:

V (Peak) D.C. = 1.41 X Sec. V A.C. = V (Peak) D.C. = 1.41 X 220 Volts A.C. = V (Peak) D.C. = 310,2 Volts D.C.

So, for this 310,2 D.C. Volts as Vb voltage and in this circuit with 100KΩ Plate Resistor(s), I will preferred the 12AY7 – 6072 tube with 1716,1290… Ω Cathode Resistor(s), aka nearest value 1K7 Cathode Resistor(s)…

 

[Matador]

I wouldn't sweat so much the exact voltages: because the current draw overall is so low, you can pretty easily adjust B+ by just tweaking the 1.5K series droppers that generate B+. The tubes will operate fine overall over a pretty large voltage range.

I would just use that 220V torroid in a full wave configuration, yielding around 300V after the diodes, then adjust the 1.5K's up until you get 250V at B+ under operating conditions.

 

[Spencerleehorton]

what is the current draw for B+ as it was said earlier that the heaters were 600mA.
would each plate current be 1.2mA @ 250v?

so each channel would be 2.4mA im only making one channel first of all.

 

[Spencerleehorton]

so if this is about right and the current draw is somewhere between 2.4mA and 5mA then replacing the 1.5k for 2 x 12k 2W should drop the required 60v to get me 250v ish.
i'll have a go at getting the PSU together tomorrow.

 

[Spencerleehorton]

I did wonder what the 250 R pot and the 68 ohm, diode and led was all about on the 6.3v?

 

[Matador]

I would just build it with what you have, use the default 1.5K series R, then measure all of the voltages and then you'll be in a place to tune it. The preamp will work with anything between 150 and 350V to get you up and running.

 

[Tubetec]

250 ohm pot is a humdinger across the heater supply , 68 ohms and diode drops the voltage across the led a bit .

 

[Spencerleehorton]

I don’t think that’s a led as to drop to 2v @ 10mA it would have to be 470 ohm, I think it’s a 100mA 6v lamp?
So I will go with the 470R, 1n4007 diode then a led?