Sunday, September 7, 2008

12b4a amp, version 2

Looking to see if I could eke out any extra performance out of my 12b4a amp, version 1 , I tried the following
1. Increased cathode bypass resistor on the 12b4a output tubes to 250 uF . No audible change, so no changes there
2. Add film bypass capacitors to the power supply and cathode bypass electrolytic capacitors. Smoother sound, less grain. Definate yes.
3. Add a 0.5 ohm dropper resistor in one leg of the filament secondary to drop the filament voltage from 7V down to 6.3V. Ideally, you would want to use two 0.25 ohm in each leg of the supply, but come on, 0.5 ohm is miniscule compared to the power transformer's DCR. Worked like a charm.
4. Build a IXYS IXCP10M45 CCS and replace the film resistors on the 12AT7. I find that with the CCS
  • Gain is increased
  • Transparency is increased
  • Drive and also treble resolution are increased
  • The warm "euphonic" sound effect is reduced. To be expected, but I kinda miss it.
  • The miniscule amount of hum (only heard with ear 1 cm from driver) I had before is pretty much gone (increased PSRR, I guess).
I varied the current from 2mA to 4.5 mA but backed ff to 3.5mA . At this point, plate voltage is abotu 150V. I don't have a lot of B+ to play around with so I'm going to be conservative. At first glance, the higher current seems to give the amp a bit more drive and a "heavier" sound, whereas the lower current results in a "lighter" more airy sound. I'll verify this upon further listening.

Here's the new schematic:

Update: September 20, 2008 .
I occasionally noticed a little amount of "zzzzz" sound . It turns out there is a little blocking distortion action going on. "Kenpeter" suggested a couple of "fixes".I've tried the first circuit ( The debuging saga is documented at

Here's a link on blocking distortion

1 comment:

Anonymous said...

Hallo Everybody,

I am Ben from Holland and I took the whole thing to the next level and made a world class amplifier out of it. If anybody is interested, please e-mail me:

(My amplifier has constant current source on the 12B4A and a totally different driver stage. Works excellent.