EF91 Line Stage questions

JDeV · 14th August 2005, 02:35 PM

I recently got hold of 5 new Mullard EF91 valves and want to know:

1) Is it a good valve to use for Hi-Fi quality projects?
2) Are there any good line stage circuits,using this valve, that I may copy?
3) Can anybody design a nice line stage or buffer, using this valve - if it is worth to do?

Thanx for all assistance

kopite · 14th August 2005, 07:01 PM

Probably not as this valve is a pentode and usually used in IF stages.
The EF94 (6AU6) is a similar valve and can be wired as a very good triode.
You may want to have a look at this.

Morgan L · 14th August 2005, 10:26 PM

EF91 has a very high mu (70) in triode connection, which isn't desirable unless you want to use feedback (unnecessary in a line stage). I haven't seen any triode-ct curves, but it seems likely it would make a fair (1st) voltage amplification stage, as in a power amp. The plate resistance will be fairly low trioded (ca 10k), which is nice. But for line stage VA, no. If you're going to include a cathode follower on the output of the line stage, however, the EF91 ought to be about as good as anything in this function. Output impedance ca 150 ohm, and the high mu won't hurt linearity either (due to the 100 % degenerative FB of a CF). So don't throw them away! They also make good constant-current sources (ref. M. Jones' Valve Amplifiers). Not sure which op point might suit line stage CF use, but guessing 120-150 Vak and 5-8 mA or so (trioded, that is).

Morgan

JDeV · 15th August 2005, 02:40 PM

I found this article but can not see the relevant schematics. Can somebody maybe re-draw them?

http://members.lycos.co.uk/griffonco...line_stage.htm

Thanx to all.

Morgan L · 16th August 2005, 01:59 AM

The schematic seems to be missing from the page. But there is an e-mail address on the index page. Maybe you could write the author and inform about the problem (or just ask for the schematic)?

griffon 'at' ntlworld 'dot' com

Do you need just a cathode-follower buffer (unity gain)?
I don't have access to a server to upload to, but if you write me, I
could probably draw something that might work with an EF91.

morgamp1 'at' yahoo 'dot' se

Morgan

hacknet · 16th August 2005, 04:47 AM

i`ve seen tons of mullard ef91s at a local surplus store too... i had a look at the curves and they don`t look too linear on top of that i have doubts about the microphonics. am i wrong? i`m think about them in a phonostage. arn`t these 70mu valves built for stages like these?

ray_moth · 16th August 2005, 06:54 AM

The EF91 was intended for RF and was used in the thousands 50+ years ago, including in some early computers. It is not a partricularly good tube for audio. Used as a pentode, it will be noisy. It also suffers from microphonics. However, as already stated, it can probably do duty as a CCS or CF.

14th August 2005, 02:35 PM	#1
JDeV diyAudio Member Join Date: Oct 2002 Location: Cape Town	EF91 Line Stage questions I recently got hold of 5 new Mullard EF91 valves and want to know: 1) Is it a good valve to use for Hi-Fi quality projects? 2) Are there any good line stage circuits,using this valve, that I may copy? 3) Can anybody design a nice line stage or buffer, using this valve - if it is worth to do? Thanx for all assistance __________________ "Be carefull who's advice you buy, but be patient with those who supply it." From Baz Luhrmann's - Everybody’s Free (To Wear Sunscreen)

15th August 2005, 02:40 PM	#4
JDeV diyAudio Member Join Date: Oct 2002 Location: Cape Town	I found this article but can not see the relevant schematics. Can somebody maybe re-draw them? http://members.lycos.co.uk/griffonco...line_stage.htm Thanx to all. __________________ "Be carefull who's advice you buy, but be patient with those who supply it." From Baz Luhrmann's - Everybody’s Free (To Wear Sunscreen)

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14th August 2005, 07:01 PM	#2
kopite diyAudio Member Join Date: May 2005 Location: NZ	Probably not as this valve is a pentode and usually used in IF stages. The EF94 (6AU6) is a similar valve and can be wired as a very good triode. You may want to have a look at this.

14th August 2005, 10:26 PM	#3
Morgan L diyAudio Member Join Date: Jan 2004 Location: Umea, Sweden	EF91 has a very high mu (70) in triode connection, which isn't desirable unless you want to use feedback (unnecessary in a line stage). I haven't seen any triode-ct curves, but it seems likely it would make a fair (1st) voltage amplification stage, as in a power amp. The plate resistance will be fairly low trioded (ca 10k), which is nice. But for line stage VA, no. If you're going to include a cathode follower on the output of the line stage, however, the EF91 ought to be about as good as anything in this function. Output impedance ca 150 ohm, and the high mu won't hurt linearity either (due to the 100 % degenerative FB of a CF). So don't throw them away! They also make good constant-current sources (ref. M. Jones' Valve Amplifiers). Not sure which op point might suit line stage CF use, but guessing 120-150 Vak and 5-8 mA or so (trioded, that is). Morgan

16th August 2005, 01:59 AM	#5
Morgan L diyAudio Member Join Date: Jan 2004 Location: Umea, Sweden	The schematic seems to be missing from the page. But there is an e-mail address on the index page. Maybe you could write the author and inform about the problem (or just ask for the schematic)? griffon 'at' ntlworld 'dot' com Do you need just a cathode-follower buffer (unity gain)? I don't have access to a server to upload to, but if you write me, I could probably draw something that might work with an EF91. morgamp1 'at' yahoo 'dot' se Morgan

16th August 2005, 04:47 AM	#6
hacknet diyAudio Member Join Date: Nov 2002 Location: sg	i`ve seen tons of mullard ef91s at a local surplus store too... i had a look at the curves and they don`t look too linear on top of that i have doubts about the microphonics. am i wrong? i`m think about them in a phonostage. arn`t these 70mu valves built for stages like these?

16th August 2005, 06:54 AM	#7
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	The EF91 was intended for RF and was used in the thousands 50+ years ago, including in some early computers. It is not a partricularly good tube for audio. Used as a pentode, it will be noisy. It also suffers from microphonics. However, as already stated, it can probably do duty as a CCS or CF.

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