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Default 6SH7 vs. 6SJ7 for an audio high gain stage

6SH7 vs. 6SJ7 for an audio high gain stage (mic/phono)

I was messing about changing 6SJ7's for 6SH7's just for fun... and
because I had just got some nice looking NOS mil. spec. glass ones.
Then I found this...

One of my tube data books says that the 6SH7 is not good for high gain
audio use as it may induce 60 Hz hum. I decide to check this with a
Heathkit A-8A amplifier to hand. It has a 6SJ7 mic/phono preamp stage
in which I tried various 6SJ7 and 6SH7 substitutes while measuring the
hum on the speaker output. Of course, it's not a rigorous test as
other noise is in the output, too, and I did not optimize the preamp
circuit for the two dissimilar tubes.

The preamp circuit was as follows: Rk = 1K, Rp = 100K, Rg2 = 470K, B+
185VDC. g1 shorted to ground (for test.) Heater winding CT to
ground. 2nd AF tube after the volume control and inside the NFB loop
was a metal 6SJ7 for all tests. Hum was measured with a Heathkit VTVM
but you could use any multimeter on the AC range.

Output voltage was measure across a speaker load (not at AF tube
plate) with Bass control at maximum boost and Volume at maximum (but
NOT while changing tubes!) In all, just a Q&D relative test.

Results (I hope the table holds up! If not, I'll re-post more clearly
somehow...)

The 6SH7's
Envelope Manufacturer Hum voltage (mV rms), approx
National Union
Glass NOS, mil.spec. 300
" " 600
" " 230
" " 300
" " 450
" " 300
" " 380
" " 330
" " 400

Glass 6SH7 average 365 mV

Metal Not noted 70
" Not noted 70
" RCA 200
" GE 100

Metal 6SH7 average 110 mV

The 6SJ7's
Glass Sylvania 150
" Sylvania 100

Glass 6SJ7 average 125 mV

Metal RCA 190
" GE 30
" GE 100
" GE 30
" Marconi 30
" Marconi 40

Metal 6SJ7 average 70 mV

Both types metal average: 86 mV
Both types glass average: 321 mV

Tube tests
All the glass 6SH7 NOS mil.spec tubes tested at 80% (+/- 1%) on my
TC-2 emission (only) tester.
The four metal 6SH7 tubes all tested in the 71% to 74% range.
The two glass 6SJ7 tubes tested 88% and 91 %.
The four metal 6SJ7 tubes all tested in the 88% to 94% range

My conclusions
Results only hold for this circuit and amplifier which, anyway, could
use some further "humbugging".
Results only hold for my random inventory of these types
Metal tubes introduce a bit less hum than glass tubes in the same
circuit.
My mil.spec. NOS glass 6SH7's are decidedly NOT low hum but the glass
(civilian?) 6SJ7's are a somewhat better.
6SH7's do have a bit more hum than 6SJ7's in the same circuit but we
would need to test in circuits optimized to the same gain to be sure
Could calculate the theoretical gain for each tube type in this
circuit, then calculate the "hum per unit gain" for the two types
(that's the homework assignment!)
Select lowest hum, metal tubes for mic/phono preamps and use a 6SJ7
over a 6SH7 (as the book says!)
BTW, I've left a low hum metal 6SJ7 in the mic/phono stage but used a
glass 6SJ7 in the AF high level, not pre-amp, position of my A-8A...
just because it looks nice! The 6SH7's will be left for RF use...

Anyone want to add anything?

Cheers,
Roger

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