Fun with '2461nico' and a 5654 Pentode

Hi

While diy-ing with a donor ldc mic, I've  been noodling about with a tube preamp circuit to use with a nice new capsule.

I really am liking as well as being a bit intrigued by this capsule amp  :-

It's a simple  circuit - a GE 5654 pentode in triode mode, low-ish gain.
Capacitor feeding a Cinemag 2461nico output transformer.

Balanced feed into a Motu interface bal line inputs.

OK - so I am powering with some standard settings :

-> B+ 125v, capsule pol +63v both zener regulated
-> 6.3Vdc heater chip regulated

I have it biased for :

-> plate 91V thru a 4K7 plate resistor to 2u2 output cap
-> cathode 1.5Vdc thru a 180R resistor unbypassed
-> 1Gohm grid bias resistor

That's around 8.2mA thru the tube.
Gain is low at x8.9 and rp is around 4.2K

And the output traffo is x6.4 stepdown.
Output of the traffo is feeding (prolly) a 10K bal line.

OK ---------------

First up, all working as expected. Bias spot on. Low noise - a beauty!
With my RTA sweep test, it showed the flatest, widest response I've ever saw.
Like 10Hz to >40KHz FLAT flat. Not really dependant too much on driving levels.
Coolio.

SO then, as is my wont, I connected my guitar to the input (capsule not yet in play).

WOWee. A really great hifi sound, but with everything else too.
A certain low-mid emphasis. Instant winner sound.

My test here is a pretty hifi Strat - dimarzio virtual vintage set.
Super quiet and great dynamics. Quite strong output levels.

1Vpp easy. 2Vpp fairly regularly. Peaks maybe 3Vpp and beyond occasionally.

-----

Well I think I heard the best guitar pre circuit ever.

After much noodling about, admiring this wonderful circuit with it's quiet, (lowish gain remember) but soulful and deep sound.

So I connected it back to the RTA for some spectra.

Well - as I said, flat, flat response. Nice reproduction subjectively.

Spectra shows quite High 2nd harmonic 
Sets in pretty much from the lowest levels and is there pretty much all the way up to some big input swings.

When pushed to the top of my RTA  range (around+9dBu) the 3rd harmonic distortion begins to show up.

-----

SO there it is! Beautiful sound, very hifi and dynamic to music input to it.

BUT (seems) to be quite a bit more 2nd harmonic distortion than I expected.

Now it's with the cheap capsule connected some mic tests.

Of course, its always cool to have links to the data sheets ...

Cinemag 2461nico
http://cinemag.biz/mic_output/PDF/CM-2461NiCo.pdf   

GE 5654 small pentode  (raytheon datasheet - last page for triode curves)
http://www.mif.pg.gda.pl/homepages/frank/sheets/138/5/5654.pdf

PSU is a re-jiggered apex donor psu - fixed capsule voltage.

And with the apex capsule, it sounds as expected - low sensitivity and quiet (low-ish gain).

And fairly thin.  ie emphasised highs

As expected with a k47 china capsule into a wideband low gain amp.

-----

So - next steps are to get a nice capsule and then focus on the 'eq' ing with tweeks.

AND I'm setting up to test this great traffo some more in 'NY Daves' One Bottle Preamp.

(thanks NYDave)

OK - so now to increase the plate load some, and rebias.

This time I'm doing : 

B+125Vdc, heater +6.3Vdc referenced to ground, capsule voltage +62Vdc

and

16K plate resistor (quite a bit higher than it was)  and +62V plate (quite a bit lower than it was)
-1.0V bias, 4mA cathode current (a lot less than before)  thru 250R resistor unbypassed
1Gohm grid bias resistor
giving rp 6K or so

Interesting to see how the distortion comes up.
Prolly lower me thinks. Despite the +1.5dB I should gain.

Will test flat (and how flat is the cinemag!) and then do some picocap eq'ing of that capsule.

It is so thin sounding with the ge nos tube, 2u2 coupling  cinemag flat response.