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The Knight KN724 Vacuum Tube Amplifier

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     I believe this is the only documentation on this amplifier anywhere on the web.  I had to reverse engineer the amp to get a schematic.  Exhaustive web searches by me have yielded no evidence at all that this amp even exists.  There is a 724A that is not too hard to find that uses a slightly different tube complement.  Basic topology may be the same, I don't know.  I found my 724 in my neighbor's trash when I lived in Elgin, IL.  It needed new tubes and one broken bias resistor fixed.  Today, it sounds very, very good. Be warned:  If you're a Knight fan and a purist, turn away now!  This amp has been so heavily modified that even to identify it now by it's stock name is pretty much meaningless.  If you'd like to know how I turned this sow's ear into a rayon purse (it's not quite silk), read on!
 
     These are schematic drawings of the Knight KN724 in both its stock form, and as modified by me.  The last four large pics are schematics of the amp power supply (PSU), phono, driver, and output stages as modified by me. Tone and balance controls have been removed, and the screwy volume control from the stock arrangement has been replaced with a simpler, higher quality, lower value Noble pot.  This made the volume taper much more gradual and usable over the entire range.  Stock, more than the top 1/3 of the range was distorted--totally unusable.  Also, the second wiper in the pot altered the frequency response curve as volume was adjusted.  I have no idea why this was done--best sound was at the point where resistance to ground at the wiper was at a minimum, hence the circuit you see now.  Tone is uniform throughout the dymanic range now, and audio just begins to distort at full volume. I also altered parts values in several areas to tweak attenuation levels and frequency response.  Until I did the PSU upgrade, removal of tone controls was far and away the biggest single improvement.  Replacement of all coupling caps with new parts was also significant, and pretty much mandatory if you're tweaking a vintage tube amp.  That goes for the electrolytics too, and quality parts are worth the extra $$.  The output couplers I'm using now are Multicap RT's.  PSU caps are important too, read on.
     A standby mode for the tubes was been added, but is now partly removed since doing the PSU mod.  Many parts have been upgraded to higher quality components, though that has little or no impact on the schematic.  More info on these details will be forthcoming. 
     Size reduction of the (one remaining) hand-drawn schematic for viewing here has rendered resolution of the pics rather poor, at least from my computer.  To see in good, clean resolution, right click and use "Save As" to download the jpegs.  This also applies to the thumbnail at the bottom of the page which combines the driver and output stages in a single drawing. 
    The power supply upgrade started some time ago is nearly complete and a schematic is now here (10-2-04).  This was probably the last major upgrade that could be done to this amp, and it's a doozy!  I'm now enjoying true high-end sound with tons of detail, finesse, and even decent imaging--all from an amp that was rescued from the trash.  The price paid for this is in the massive size of the PSU.  The chokes and huge ASC capacitors meant the PSU would have to be in a seperate chassis--one that is much larger than the amp itself.  Totally worth it, and a nice side benefit was that removing the power transformer from the amp chassis eliminated the 60 Hz hum that had always been present at low levels in the Aux/Tuner mode and at just-enough-to-be-annoying levels in Phono mode.  I have yet to finalize standby mode arrangements with the new PSU--I may favor instead a slow power-on arrangement that will be more transparent.  When everything's done and pretty, I'll post some photos of the rig. Stay tuned...

Stock Power Supply
kn724psu.jpg
This supply circuit was adeqate, but not the super performer the amp sports now.

Phono Stage and Audio Inputs
kn724phono.jpg
This schematic shows one channel only.

Driver Stage
kn724driver.jpg
Again, only one channel shown.

Output Stage--Stock and Modded are the same.
kn724outputmod.jpg
One channel only shown.

The redesigned PSU is now in a seperate chassis.
kn724psumod.jpg
Bigger than the amp itself, the improved PSU yielded MAJOR sonic improvements.

Modified Phono Stage (Mag/Xtal switch removed)
kn724phonomod.jpg
Not much different here. The cart type switch is removed and the RIAA filter has been rebuilt.

Modded driver stage--no tone or balance controls!
kn724drivermod.jpg
The convoluted volume control is replaced with a Noble pot. Better taper and more uniform tone.

Output stage--no modifications have been made.
kn724outputmod.jpg
The ouput stage is unchanged. It's shown again here for your convenience only.

Right click & "save as"
kn724driver-op.jpg
Combined schematic of driver & output stages together.

Questions or comments? You can send me e-mail at:

wb9k@acd.net