Mu-follower gain
The upper valve in a mu-follower bootstraps the lower valve's anode load resistor, making it (roughly) mu times larger.
Similarly, looking down into the anode, an unbypassed cathode resistor in the lower valve is mutiplied by a factor of (mu + 1).
In any practical circuit, the bootstrapped Rl is so much larger than ra + rk (mu + 1) that the gain does not change when the cathode bypass capacitor is removed.
The upper valve in a mu-follower bootstraps the lower valve's anode load resistor, making it (roughly) mu times larger.
Similarly, looking down into the anode, an unbypassed cathode resistor in the lower valve is mutiplied by a factor of (mu + 1).
In any practical circuit, the bootstrapped Rl is so much larger than ra + rk (mu + 1) that the gain does not change when the cathode bypass capacitor is removed.
Frank,
Yup, whisky in hand, running RH linux 7.3 (8.0 was too slow), fully networked, SMB, VM's, it all works!
Okagesama'de
BTW, I bought some Cahors yesterday, haven't tried it yet (£3.49).
EC8010,
The mu valve was an EL86 when I checked this, now it's an EL84. As I said, I'll check and report back....
Cheers,
Yup, whisky in hand, running RH linux 7.3 (8.0 was too slow), fully networked, SMB, VM's, it all works!
Okagesama'de
BTW, I bought some Cahors yesterday, haven't tried it yet (£3.49).
EC8010,
The mu valve was an EL86 when I checked this, now it's an EL84. As I said, I'll check and report back....
Cheers,
Req.
Hi,
Ah,I thought already that was what you hinted at.
Don't know about E-bay price for tubes..I always buy my valves where I can see them and have absolute trade in garantee.
Moreover,professional series penthodes aren't necessarily less noisy than the standard run of the mill stuff.
Low noise and penthode is a bit of a contradiction in terms won't you say?
I must say in all fairness that noise considerations were a major factor for me to stay away from mu-followers in the first place.
Since you point to this factor,I think I was correct in doing so...at least for highly sensitive stages anyway.
Cheers,
Hi,
Ah,I thought already that was what you hinted at.
Don't know about E-bay price for tubes..I always buy my valves where I can see them and have absolute trade in garantee.
Moreover,professional series penthodes aren't necessarily less noisy than the standard run of the mill stuff.
Low noise and penthode is a bit of a contradiction in terms won't you say?
I must say in all fairness that noise considerations were a major factor for me to stay away from mu-followers in the first place.
Since you point to this factor,I think I was correct in doing so...at least for highly sensitive stages anyway.
Cheers,
Re: EL822
I wanted to make a pentode sink to sit underneath a 6BX7 differential pair that was DC coupled form a 6SN7.
I realised that for a good sink one required:
A pentode that could handle the power
A pentode with high anode resistance
A pentode with high gm
A pentode with lowish output capacitance
And a pentode with nice flat curves.
I studied the Vade-Mecum and came up with EL822. EL84 would have done but its anode resistance and gm are a bit lower (ra = 40k)
EL822's anode resistance is around 100k and Pa is 12W. Gm is 15 mA/V. I have about eight of them; I suspect that they will be long-lasting and being Mullards, test as they should.
I have heard of its being used in control mechanisms in textile machinery!
It appears to have had no US equivalent, but the Russian 6P15P is very similar in specification and has very nice-looking curves.
7N7
fdegrove said:Hi and good morning to you too,
The EL822 seems a good candidate indeed.
I'm not sure whether it was widely used outside the British Common Wealth,maybe 7N7 can tell us more.
I must say I've never seen any in Belgium.
Cheers,
I wanted to make a pentode sink to sit underneath a 6BX7 differential pair that was DC coupled form a 6SN7.
I realised that for a good sink one required:
A pentode that could handle the power
A pentode with high anode resistance
A pentode with high gm
A pentode with lowish output capacitance
And a pentode with nice flat curves.
I studied the Vade-Mecum and came up with EL822. EL84 would have done but its anode resistance and gm are a bit lower (ra = 40k)
EL822's anode resistance is around 100k and Pa is 12W. Gm is 15 mA/V. I have about eight of them; I suspect that they will be long-lasting and being Mullards, test as they should.
I have heard of its being used in control mechanisms in textile machinery!
It appears to have had no US equivalent, but the Russian 6P15P is very similar in specification and has very nice-looking curves.
7N7
Re: EQUIVALENTS.
Well the EL821/6CH6, look OK until you see the curves; they are not nearly as nice as those of EL822/6P15P.
In reality I expect that the 6CH6 wouldn't make a bad sink, but having seen the curves has put me off
7N7
fdegrove said:Hi,
The closest equivalents to the Mullard EL82X series are the the US made 6CH6 and 6CL6s.
The reason I proposed the EL86 to John instead of the EL84 is based around the same reason you have picked the EL822.
Cheers and thnx,
Well the EL821/6CH6, look OK until you see the curves; they are not nearly as nice as those of EL822/6P15P.
In reality I expect that the 6CH6 wouldn't make a bad sink, but having seen the curves has put me off
7N7
6CH6
For a simple base re-wire these 6CH6 would probably fit most EL84 applications quite well , maybe the biasing is similar also . The main stumbling block is the fact that no-one has really heard of the things . Can you imagine the bunch over at the tubes asylum arguing over which shaped getter on their 6CH6 was the best ? They'll use up the world's supply of new old stock EL84 then go on to lousy current production . More than likely in 4-5 years time most of the 6CH6 will be disposed of without ever finding usage in audio .
316a
For a simple base re-wire these 6CH6 would probably fit most EL84 applications quite well , maybe the biasing is similar also . The main stumbling block is the fact that no-one has really heard of the things . Can you imagine the bunch over at the tubes asylum arguing over which shaped getter on their 6CH6 was the best ? They'll use up the world's supply of new old stock EL84 then go on to lousy current production . More than likely in 4-5 years time most of the 6CH6 will be disposed of without ever finding usage in audio .
316a
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