Tube preamp - output

Started by Unlikekurt, May 18, 2015, 09:01:57 PM

Previous topic - Next topic

Unlikekurt

Hey guys.  I'm working on a standalone tube preamp similar to the blackface / alembic preamp schematics.
There are some differences with respect to plate and cathode resistors and cathode capacitors.  Also i'm using a banxandall tonestack.
All that said, the preamp works fine and i enjoy the way it sounds.
The issue is that I really want to include a master volume due to the output level of this thing.
I measured the output impedance to be just about 42K. 
I've also hooked up a 4.1:1 matching transformer which brings the impedance down to about 2.5K.
Now the signal coming off of the transformer secondary can still get pretty hot.

Ideally, I wanted to maintain the normal output jack and then include a jack off of the transformer for a lower impedance.
Therefore I think it would make the most sense to utilize a MV before the tranformer primary.

I've tried a variety of things.  Such as replacing the 1M to ground after the plate coupling cap with a log potentiometer.  I've tried a variety of values here and some different arrangements.  That said, I've found that the preamp SOUNDS best with the 1M in place.  I've also tried a 220K resistor in series from the coupling capacitor to the output jack, while keeping the 1M in place to ground.  This obviously creates a divider and dumps some signal off.
Again, even in this arrangement, the preamp seems to sound best with the full 1M.

I have also tried putting a trimmer between the transformer secondary and its respective output jack.  This does trim back the level while pretty much preserving the tone.  But, of course, it does nothing with respect to the higher impedance output jack.

Can anyone suggest any ideas or avenues to look down?

Thanks

I'm posting a link to the Alembic preamp at Geofex as extremely close to my schematic (which i have no image of to post).

http://www.geofex.com/FX_images/alembpre.gif

PRR

A 250K pot seems the best compromise.

Or 200K+50K (180K+47K) resistor divider.

Leave the 1Meg there also if it sounds better.

This plan was only intended to feed a short wire across a small chassis. If you must feed external stuff, and transformers, you may be forced to add a buffer (and a very high-level buffer at that).
  • SUPPORTER

Unlikekurt

Thanks for the response PRR.
Are you suggesting to put the 200K resistor in series between the 1M to ground and the 50k resistor / or pot?

PRR

  • SUPPORTER

Unlikekurt

Thank PRR.  That's what Id thought.
Tried it out.  Found that when feeding the power amps of some amps, it was still too hot.  Even if I took the signal after the transformer.  Also found that I wasn't ever getting past the first 1/3 or the pot's taper.

So I tried this:

http://i.imgur.com/NOuLmX7.jpg

I omitted the 1M.  Raised the 180K to 200K and replaced the 47K with a 10K log pot.  Seems to give me better range for feeding most things.  Could end up wanting to do something with a rotary switch perhaps.

Stock / Attenuated / Lower Impedance via the transformer.