Quote:
Originally Posted by Alex Pogossov
"patrick-turner" wrote in message
...
Me again folks. I wrote a new page about using a differential input pair
of triodes for a PP amp. See my page at
http://www.turneraudio.com.au/basic-tube-5.html
Brick bats, buckets of poop, poisoned arrows, drone delivered ordinance
all tolerated, but returned to sender with gusto.
Patrick Turner.
Alex:
I skimmed this page.
Noticed an error in the MJE340 current source circuit in the first
schematic. (Correct in the others.)
Noticed a typo "oft3en" in the first line of the section "DRIVER STAGE
DIFFERENTIAL AMP".
Was pleased that the term "differential stage" is widely used, and that
arcane "long tail pair" is only occasionally -- sort of being phased out.
Did not understand the purpose of long and distracting
calculations/speculations about mu and gain... Was it all to point out that
the actual mu of 6CG7 is 22, not 20 as quoted in the datasheets?
There were no explanation why the tubes with mu around 20 are preferred.
Obviously, 12AX7 is not a good candidate because it can not work with a
decent current, but 12AT7, 6BQ7, ECC85 seem even better than 6CG7 because of
higher mu, larger Gm and about the same current.
Again, I did not read the whole article attentively. Perhaps there are some
other minor errors... |
In the two clones of Norman Crowhurst's Twin Coupled Amps I built for publication in AudioXpress back about 10 years ago (AUG 2004). I found the 6BQ7, 6BK7, 6BZ7 family to be much better in terms of THD & IM than one might expect. The amps were both driven nominally by two stage differential amplifiers using 6SL7 & 6SN7. The OP section in this example is PP 6LU8s. The triode section is connected as a CF into the pentodes.
I built simple adaptors for the 9-pin miniatures & did comparative measurements at similar levels with these adaptors in both circuits. Each has a one K, ½ W resistor in series with the grid leads to calm down any tendency to oscillate at RF. The 6SN7 replacement also has 2.2 K, ½ W resistors in series with the cathodes to get its gain down somewhat more like what it replaces. Some of the results with the 6BQ7 family are attached.
Cheers to all. Lots of manual labor here on the acres!