Hello all,
I am new here, this is my first post.
As for Intros, I live in the Detroit area. Like to tinker with tube audio as a hobby.  Not an expert by any means.

I own two KOSS A-1220 headphone amps. Each share the same model number, yet have completely different internals: One uses just a single 12AU7, while the other has a 12AT7 and a pair of 6AQ5A's.  I have searched the forums and found one or two threads about this amp, but nothing that addresses my problem....

I have recapped each of them and they sound 'ok'. The problem I am having is that there is little to no bass present (in each unit).  And there is a slight hum in the background.  The hum does not raise when the volume knob, and is still present when the preamp tube is removed.

HOWEVER, my first goal is to find a way to adjust the EQ circuit and get more low end.  *please note that neither of these units have tone controls.
From what I have read online, you can sometimes adjust filter caps and resistors to get different tone, correct?

Here is a link with a scan of the schematic (for the single tube version).  Does anyone have any suggestions on what I can do to get a better sound?

Many thanks!

 
http://i9.photobucket.com/albums/a60/darmen/Koss1schematic_zpscf357a61.jpeg

Hm no replies?
 
What headphones are you using?  A lot of really old headphone circuits from back then were designed for use with high impedance load cans.  The fact that you are getting an audible hum in the background, "can be" an indication that the headphone impedance is too low for optimal use with that circuit
 
Electrolytic caps and DC filter caps would be the first thing I'd go for, because those can degrade over time.  Looks like you've already got a handle on that, although you should consider increasing mfd capacitance.  There could be some improvements there to be made.  I don't know enough though to steer you towards the right exact values... let someone else reply with that detail.
 
Have you metered the voltage coming out of the power transformer circuit?  I don't know if step down transformers "degrade" with time, but I guess anything s possible.  My Earmax amp OEM power supply was spec'd at 19V / .750A, I bumped that up to a military surplus 20V / 2.5A.  It didn't severely alter the overall tone of the circuit, but it opened it up the sound more.  Sounds became more effortless and relaxed.  The amp doesn't "gasp for air" as much as it used to.  It doesn't strain/struggle as much, and it definitely got louder with lower impedance headphones.  I think the tubes get a "little" warmer to the touch now, they are just barely too hot to comfortably touch.  When I tube roll, I now have to use a glove to comfortably pull them.
 
Do output transformers degrade with time?  Are the OEM ones "right" for the impedance of the headphones you are using?  You should look into this as well.
 
I dont think resistor swaps will gain you much, again in terms of the overall tonal balance of the circuit.  Unless they are intentionally starving the tube of plate voltage.  A lot of guitar preamp manufacturers will do that to increase / extend tube life and keep temperatures down.  You should meter them out just to make sure they are correct value.
 
Side note... is the circuit earth grounded, hard to tell from the schematic?  thats kind of scary if its not.  If they have spec'd C1 at 600V (!!), thats some spicy mustard there.  I would look into that just as a safety precaution / mod.  I just finished a simple mod to my darkvoice 337 and it has some big can ~500V caps that were right next to some chassis screws.  My heart raced... but I HAD to do it.  I put sheets of layered electrical tape around everything and the screwdriver.  Managed to get through the mod without giving myself a heart attack or arc-welding any of my tools!!
 
Good luck with the restoration!!  Do you have pics of the inside?  pictures always seem to trigger ideas in DIY communities.

You should request a moderator to move this to the DIY sub forum... it will ge the proper attention there.
 
EDIT... I dug up this old thread...
 
http://www.head-fi.org/t/46891/need-help-w-koss-1220-tube-headamp
 
good luck!!

Thanks for the info!
 
On the single tube amp (the one with schematic you are seeing) I replaced all the caps with new electrolyics that had a higher value. This was in part to not being able to find ones that would properly within the chassis. The 40/30/20 group is now rated at 80/40/37.  
With the three tube version, I was able to find much closer replacements.
 
Anyway, each amp has the near exact hum in the background.  So I am now thinking this is something that may be stemming from them using a 2 prong cord.  The 600v cap is a ceramic disc that is only rated at .01uf, so nothing to be afraid of.
 
But my main concern is trying to get the EQ issue solved.  Not trying to invest any more money if all I can solve is the hum issue.
I'll see if getting higher impedance headphones will sound any different.  I'm currently using a pair of Grado 225i (with 32imp).

Alternately you could wire up an impedance adapter.  resistors on a wire in line with the Grados.  I do this all the time and it effectively kills any hiss or hum with higher gain amps.  250 ohms works fine with my Grados.  I think it was a kevin gilmore article I was reading that resistor values over 3-4x the headphone impedance will not color the sound.  I have found this to be the case as well.

Good suggestion, I'll look into it, thanks!

i recently acquired a pair of of a-1220s. both of mine are have the single 12AX7 and a pair of 6AQ5. one had been partially dismantled and missing the level pots. the other was complete and unmolested, however, both are missing the chassis cover. 
 
thread link below to topic i started on another forum regarding these amps.  
 
http://www.el34world.com/Forum/index.php?topic=15163.0
 
links to some pics:
 
http://s2.beta.photobucket.com/user/pmitchel/library/Koss%20A-1220?fromLegacy=true&postlogin=true
 
and i did reverse engineer them. both are the same circuit & there's a schematic in the photo album as well. 
 
respectfully, 
 
--ISO