Hi end "Lift" for EF86/12AU7 preamp circuit?

[JW]

I built 2 of these preamps into one of these Akai amplifier blocks years ago. I really like it's sound especially with the impedance balanced output. Only complaint is I wish there was a little more "air" or "lift" available in the highs.

The guy's name is Jesse and here's link to his blog: https://xfmr.blogspot.com/2012/06/brute-force-of-jekyll-hyde.html
It's a no negative feedback circuit with an EF86 in front of a 12au7.

Is there an obvious modification for a little more high end that I can maybe put on a switch or a pot?

Schematic attached.

 

[RuudNL]

Did you measure the frequency response?
I don't see any reason why you would be losing high-end.
I suppose you will have to add a sort of 'EQ' circuit if you want to boost the high frequencies.
(Maybe have a look at guitar amplifiers, sometimes they have a 'bright' switch that adds a simple R-C network to the signal path.)

 

[Khron]

Adding a capacitor (or a capacitor and a resistor in series with it) in parallel with the 680 ohm resistor in the cathode of the lower triode (coming off pin 3) should give you a top end boost.

 

[JW]

Great! Thanks. Any value you'd recommend experimenting with first?

 

[abbey road d enfer]

Only complaint is I wish there was a little more "air" or "lift" available in the highs.

is the input xfmr actually a 1:15? The ones I know of have a rather low HF cut-off.

 

[emrr]

I'm certain Tablebeast posted that here years ago.  There should be a thread for it. 

You could also lose the cathode cap on the first stage and use a small film cap there.  Like 0.0047 +/- a lot, check by ear or measure. 

The other EQ method, look at RCA BA-2, right before the volume control, paralleled cap and resistor. 

Both methods will drop gain amount up to 6dB. 

 

[JW]

Yes to 1:15 I used a Jensen JE115K. Thanks for that Doug. Incidentally was planning on a BA2 for the next build.

 

[JW]

Still learning here, but research suggests cathode bypass caps effect tone, level, and freq. response. Okay!, good to know.

So, Doug, is replacing that 50uF cap going to lose me some level?

I've never even swept this preamp, but I will soon. I can't see what happens up past 20KHz though, as I'm limited by the converters.

Here's one that has a 220uF cathode cap https://www.tube-town.net/cms/?Info/How_To_Use_a_EF86
Is this because in this schematic it is assumed that 1 tube is doing all the work, and a higher value cathode cap gets you extra gain? So, having the second 12au7 stage (in the tablebeast preamp) will allow me a smaller film cap here?

Also, in the RCA schematic, you're referring to R11 and C8 which are connected to the first stage's anode? http://www.coutant.org/ba2c/index.html

So, I'd experiment with adding this network between the two stages, prior to the volume pot?

 

[emrr]

Right r11/c8.  R11 and the volume pot form a voltage divider, c8 bypasses r11 making more treble. 

Look at RDH4 about cathode bypass caps. 

 

[abbey road d enfer]

Yes to 1:15 I used a Jensen JE115K.

Well, JE115K is specified as 150:15k, so it's actually a 1:10.