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preamp with tone controller

aj84

Member
2016-01-24 8:51 pm
Preamp plans for solid state power amp.

Have googled a while but i do not find a schematic with the tubes i already have.
Closest thing is this:
tone_control.bmp

(found on this side: Audio Asylum Thread Printer )

This is the tubes i have: 6N2P-ER, 6N3P-EV and 6N1P-EV.
I was thinking just to swap 12AX7 with 6N2P-ER but someone told me that tube has to little current to preform well with a cathode follower so i was thinking mabye 3P or 1P tubes for this. The 3P is made for HF but i have seen many well working diy preamps with this on the web.

But i do not find a tone controller schematic with this combo. 2P tube has 10mA cathode current an 3P has 18mA. So i was thinking mabye just modify this schematic with that factor?

IF that is in some way the right way to think the schematic will look like this:

572335d1582630865-bass-diskant-justering-pa-diy-preamp-gastor.bmp


Im sure many if not all changes is wrong but im here to learn :)
 

aj84

Member
2016-01-24 8:51 pm
Thanks but that schematic is for 6N6 or ECC88, when i cross reference ECC88 to russian versions i get 6N23P. But i do not have these tubes and i want to use those i already have. Can anyone look at my modifyed schematic and let me know what you think before i build this
 
True.
Schematic shows one tube type (they must write *someting*) but in general similar tubes can be used.
Sometimes straight plug-in, sometimes with some bias tweaking.

That said , tubes are *struggling* there (every stage *needs* a CF just to provide reasonable current and impedance, plus los of NFB, and individual stage gain is not that high) , when Transistors can do that with much less trouble and Op Amps SHINE in that application.

The "Tubes are better" Mantra does not apply everywhere, definitely not here.
 
You need to calculate the plate dissipation of your tubes. 12AX7 really struggle with the load of the tone control section of a PAS3 preamp. 6n3P had better have higher than 12AX7 plate wattage limit with all the current load you've hung on it in post 1. My PAS2 with 12AX7 tone control+output cannot drive more than a 6' RCA cable, which is only about 200 pf. Else highs fade away.
Until proved otherwise, I think you calculate plate wattage as plate voltage x plate current. You have to use the mesh analysis of impedance on the plate to calculate the plate current. Kirchoff's 3 laws. For example the 3.3 uf C8 in post 8 doesn't actually draw as much current as if output was grounded, C8 is series some input resistance of the load (power amp) and possibly parallel cable capacitance which cuts final impedance to something reasonable.
The advantage of tube preamp IMHO, is they will take a huge lightning hit without damage. The PAS2 took a strike that vaporized the turn-off pop capacitor & carbon tracked the power switch to "on". With no further damage. Disadvantage, it draws >100 W, takes a B+ capacitor and rectifier tube every 10 years, won't drive a 12' RCA cable or a 100k input impedance power amp. My ST120 power amp has 250k input impedance to be compatible with PAS3. Sound, equivalent to my RA-88a op amp disco mixer after a lot of improvements. Note most mixers as original won't be that good. The PV8 sounds good on line level inputs, but lacks a RIAA MM phono input.
 
Last edited:

waltube

Member
2005-02-19 1:46 pm
roma
I can't understand why someone is looking to have a complicated life.
In the diagram on post 4 you can use the 6N1P with little changes and it qorksa fine because is tested on lab, not on a virtual world of simulation!
In addition the use of 500k pot, double, is little bit complicate respect a 100k because is easy that ( in the center position) the value of one section is little bit different form the other.
With 100k double pot, a standard value, you can find a good parts and the error will be lower.
Walter
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
...calculate the plate dissipation of your tubes....

It is practically impossible to melt a 12AX7 with just 250V supply.

Assume NO added resistance (8k is practically none) on top of the ~~60k resistance of a saturated 12AX7. 250V^2/60k is 1.1 Watts.

If you have "significant" added resistance, even less. If you do NOT have "significant" added resistance, it won't be a happy amplifier.

Fender and Mac worked 12AX7 in 400V systems and had to check. Fender had another 82k-100k in plate and some dozen k in cathode, he was fine. Mac, I forget.

I'm not EVEN going to talk about what Russian tube would work unless the datasheet (preferably in English) is at my fingertips in the thread.

The 3-band EQ may be pushing your luck with any likely triode. The 2-band has errors enough due to limited gain.
 

aj84

Member
2016-01-24 8:51 pm
i belive i read somewhere that my power amp has about 14K ohm input, and it will be mounted in the same enclosure so no long rca cables is needed.

waltube: i want to use at least one 6n2p-er because of its sound signature

Here are the datasheets (english on the right side)

6N1P-EV : http://www.sovtube.com/pdf/6N1PEV.pdf

6N2P-EV (did not find datasheet for ER version) : https://hfc-fs.s3-eu-west-1.amazonaws.com/s3fs-public/6n2p_russian_datasheet.pdf

6N3P (did not find datasheet for EV version) : http://www.r-type.org/pdfs/6n3p.pdf

EV and ER are more rugged versions, i believe it can be compared with the difference between a 6SL7GT and a 6SL7WGT
 

aj84

Member
2016-01-24 8:51 pm
Hi

Look here
Tube tone control baxandall

post nr. 15

Walter

I might build this point to point in stereo but i have some questions

Can you explain where "-Vbias" should be connected?

What value should "test point" be?

Are there any more changes needed if pots are changed to 500K?

You say there are smal changes to use this with 6N1P but im thinking if there is possible to use 1/2 6N2P-ER where V1A is drawed and 1/2 6N1P-EV where V1B is. And if so, are more changes needed?

I really want to use this 6N2P-ER tube because of its sound character, even thou it makes life hard :)