koss A-1220 headphone amps

[DummyLoad]

a good friend bequeathed unto me a pair of vintage koss a-1220 headphone amps. one is a basket case and the the other is complete - well, more or less  anyway as both are missing covers. one is stamped apr. 25th, 1962, and both have transformers with the same year date codes.

the basket case amp seems restorable. i have not fired-up the complete unit yet. i reverse engineered them as i cannot find much information on this particular model of koss; perhaps they were an OEM sourced product.

link to some pics.

http://s2.beta.photobucket.com/user/pmitchel/library/Koss%20A-1220

schematic(s) attached.

respectfully,

--pete

[PRR]

Oh, my. That's old.

Koss was struggling to make a market for StereoPhones. Not like today when the earbuds never come out.

Later Shure had a transistor phone-amp of very similar frontal aspect but less depth.

Interesting that it is unity gain.

That 2% resistor.... their builder musta had a box leftover from some other contract. "Kinweusethese?" "Whynot."

Ah... and not a stereo pot!

[DummyLoad]

Oh, my. That's old.

i was just over a year old when they were built. oh, my. i'm old too...   

they have that letsmakeitchaeapasshit quality about them. one of the output tubes bumps the power supply cap - whoops!

1/2wave PS - 110V B+ - at least they tapped plates after first RC network. 

no load Z rating marked - i assume 32-500R would be ok?

thanks,

--pete

 

[PRR]

I would really like to know the OT ratio.

I don't think the load impedance can hurt the amp.

Going from the 180V+250V suggestions on GE 6005 data, 7.5K would be a target zone for "best" power at 110V B+. But that gives maybe 0.7 Watts output, which is far more than headphones need, even with severe impedance mismatch.

[DummyLoad]

I would really like to know the OT ratio.

since it's more or less already dismantled, i'll measure the pair in the basket case.

--pete

[DummyLoad]

ok pulled the B+ wire off the term that feeds the top of both OTs primary.

both measure 62.5V applied to primary w/ 1V measured at the secondary.

so 62.5x62.5 = ~3900 x 8R = 31.2K -or- 3900 x 6R = 23.4K and likely what they are?

--pete

[PRR]

Strange loading.

If we take the estimated 7K5 "optimum" load for that tube and voltage, it wants a 2-ohm load.

Something rings a bell that the first Koss were 4-ohm speakers in plastic cups, and it's got two jacks, so that's not crazy.

Later Koss were 100 ohms, a stiff under-loading. However they probably weren't using 100 ohms when this amp was made? And with possible 0.7W in happy-load, it sure could be way off the optimum and still be plenty loud. Looks like 10mW in 100 ohms which is respectable (and a lot better than tapping 4 or 100 ohms off a tube "Line Out").

[PRR]

And oh yeah.... you should restore these. While the build quality may be lowest-bid, the circuit (without shorts or leaks) will be fine.

Might take advantage of 4-cent rectifiers and use a FWB instead of the half-wave. But there's plenty of hum-filter already with C1a 270r C1b and a screen/driver dropper.

If, in the end, it's short on gain (some modern outputs are not as hot as some old gear), the 1.5K could be changed to 3K ground to cathode, 3K cathode to "X".

[DummyLoad]

Strange loading.

If we take the estimated 7K5 "optimum" load for that tube and voltage, it wants a 2-ohm load.

Something rings a bell that the first Koss were 4-ohm speakers in plastic cups, and it's got two jacks, so that's not crazy.

Later Koss were 100 ohms, a stiff under-loading. However they probably weren't using 100 ohms when this amp was made? And with possible 0.7W in happy-load, it sure could be way off the optimum and still be plenty loud. Looks like 10mW in 100 ohms which is respectable (and a lot better than tapping 4 or 100 ohms off a tube "Line Out").

thanks for the info - i figured most of the early hi-fi headphones resembled small speakers attached to a headband.

i am planning restoration of both. new covers will be the biggest hurdle; have access to a brake and the owner is pretty handy with it.

agreed, loading is odd and to me seems quite high for 6AQ5. i measured 3 times just to be sure. FWB is doable, but the multi-lytic in both is shot. i guess too much heat from the power tube resting on it - caps are bloated but both fire up on a limiter; first task is to relocate the replacement lytics and install a polarized 2 conductor power cord.

again, much thanks for the help & information.

--pete