Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

Return to Tube DIY Asylum

Message Sort: Post Order or Asylum Reverse Threaded

6AF4A Useful?

97.105.99.138
Posted on May 17, 2017 at 13:19:38
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 Anyone using this tube? It has a reasonably low Rp, so I was thinking it might make a good CF. Then again, maybe it won't do anything other than oscillate. Looks like it could be temperamental...

 

Hide full thread outline!
    ...
RE: 6AF4A Useful?, posted on May 17, 2017 at 13:36:29
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 With a gm of 6.5 mA./V., the 6AF4A should easily get you an O/P impedance under 200 ohms. That looks pretty darned good to me. Given the type's UFH capability, put a 4.7 Kohm CC stopper on all grid connections, as you would do for other "touchy" types, like the 5842.


Eli D.

 

RE: 6AF4A Useful?, posted on May 17, 2017 at 16:24:45
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 I have not had any issues with much higher triodes singing when stoppered a bit. Type 6C45 is the worst, but left me OK with 1k on each ov the grid pins. 511 has been fine for 5687/6H6's too...:) I'd not worry much.
cheers,
Douglas

ps...if I didn't have an adequate supply, I'd be slightly worried about mentioning such a fine tube positively around here...LOL


Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Plus they are cheap , posted on May 17, 2017 at 16:39:59
Johno
Audiophile

Posts: 388
Joined: June 9, 2002
 Trending $1-$2 per tube on auction sites

Johno

 

RE: 6AF4A Useful?, posted on May 17, 2017 at 18:03:03
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Doug,

The 5842 and 6C45П (6s45p) come annoying close to oscillating, while sitting in the cardboard box. FWIW, I say take no prisoners. A 10 Kohm CC stopper on each grid connection, a 100 Ω CF stopper on the plate connection, and ferrite beads on the heater wires is (IMO) not overkill.

The "in box oscillators" were designed for grounded grid RF duty. Setting them up common cathode requires stringent precautions. I've seen documentation with RF chokes in the heater wiring, when set up grounded grid.



Eli D.

 

To tell the truth, I'm sorry I asked..., posted on May 17, 2017 at 20:31:53
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 Eli, I looked into these more after getting home from work. I missed the dual grid connections earlier, and like you say, that means they're designed for UHF grounded grid service. They want to oscillate, and they need the shortest possible connections from the grid pins to RF ground. And the 5842 you mentioned - four grid connections and transconductance of 25,000-30,000 umhos! Holy Moly! Sorry, this is just too much to deal with. Any piece of audio gear built with these tubes needs to be vetted with a microwave spectrum analyzer. Even then, the circuit might age into instability in a year. That's exactly the kind of thing I don't want to deal with in hi-fi equipment that should be routinely and unconditionally stable. Now I know why the 6AF4 is so cheap.

Anyway, I've gone back to published tube curves in search of other types for use as choke-loaded CFs. I'll post more in the thread below. Thanks for your comments on these little triodes.

 

RE: To tell the truth, I'm sorry I asked..., posted on May 18, 2017 at 05:56:56
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 it did not help me confusing 6AH4 with the 6AF4 either.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: 6AF4A Useful?, posted on May 18, 2017 at 09:09:47
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002
 I just increased the 1500 ohms CC grid stopper on my 6gk5 to a 10k ohm CC.

The 6gk5 has a in circuit Miller of about 45pf.

1500 ohms gave a HF -3db point of over 2 meg Hz.

10k ohms gives a HF -3db point of 350kHz.

Should I do more?

Thanks

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: 6AF4A Useful?, posted on May 18, 2017 at 11:59:36
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 I don't know. As gm increases, I increase the stopper value. My seat of the pants value for the low gm 'X7 triode is 100 ohms. OTOH, the high gm 5842 has 4 grid connections and I advocate 4X 10 Kohm parts arranged in a "cone" on that tube.

A tiny ferrite bead cemented around, but not touching, the grid's socket lug(s) is another anti-parasitic oscillation method. I like that idea on the "upper" triode of a cascode, as that grid should be at AC ground.


Eli D.

 

Page processed in 0.024 seconds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]