6SN7=6CG7?=?6FQ7
Ian Bell wrote:
Patrick Turner wrote:
Ian Bell wrote:
The 6SN7 is said to be a good tube for audio and the 6CG7 is apparently
its B9A direct equivalent. Looking for sources of the 6CG7 on the
internet I found more examples of its supposed equivalent the 6FQ7 than
of the 6CG7 itself. Looking at the data sheets the two seem extremely
similar and the 6FQ7 even has slightly less plate to grid capacitance.
So, is the 6FQ7 a worthy alternative to the 6CG7? Are there any real
differences between the two? Presumably there must be some difference
else why have a different number.
Cheers
Ian
The 3 tubes have virtually the same gm, µ, and Ra wherever you find
them.
Many varieties exist, with Siemans NOS probably the best. The CG7/FQ7
have slightly lower Pda ratings than the 6SN7.
I found the Siemans 6CG7 to have a slightly higher µ.
Anode and grid size varies with varieties, but as long as you keep the
relative distances inside the tube about the same the
gm, Ra and µ stay about constant.
Patrick Turner.
I simulated a 6sn7 in mu follower circuit and it gave very good
distortion figures - best I've seen yet - this was at 8mA Ia. So I
ordered some from Colomor valves in the UK so I can try them out. They
are the 6FQ7 version made by Matsu****a - didn't know they made tubes.
Does Matsu****a have a good or bad reputation for its tubes??
Japanese. I doubt the japs made worse sounding tubes than anyone else.
Its not going to cost you much to find out for yourself.
The secret to the µ-follower is that you have a large enough R between
top cathode and bottom anode.
If the Ia = 8mA, and that would be only good if output voltage swing is
smallish, then the Ea can be say 100V for top and bottom tube
and if the B+ = 300V, then you can have 100V across the R between the
two tubes, so R = 12k.
If the top tube internal gain is 18, then the bottom tube has an anode
load = 18 x 12k, plus any biasing R.
Its still only 216k effectively, or about 25 x Ra. But you should get
less than 0.1% THD at 10Vrms output,
which means less than 0.01% at 1V output, which isn't much, eh.
Patrick Turner.
Cheers
Ian
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