EF 86 pentode

[mooreamps]

gary, you're going to have a tough time with the microphony, even with the relatively low gain of a clean channel.

the hiss aka noise is not much of an issue ime... but finding an sv83/el84 that doesn't rattle sure is!

I was assuming the microphonics would be mitigated by using the spring clip that holds the tube in. But, I guess I'll find out.

-g

[ecc83]

Not confusing Ken, just putting my personal view.

I would not presume to tell people what form and how much distortion they should have, I was just pointing out the charcteristics of the EF86.

Some say, for a pentode the EF makes a pretty good triode!

BTW even in the halcyon days of Mullard the EF86 was never THE most reliable of valves.

Dave.

[soundmasterg]

Pentodes that are usually used as a power tube tend to have low gain and high current capability, so they don't work well in a preamp, though they might work well as a phase splitter amplifier. I seem to remember reading that the EL84 has a gain of around 20, whereas the EF86 is upwards of 200. That makes a big difference when voltage gain is your main consideration.

Greg

[mooreamps]

Perhaps voltage gain is not my main consideration. Perhaps it's the second or third stage in the clean channel, and instead of using a 5.6 K plate resistor, have the plate load as the primary winding of a reverb driver with a static plate current of 10 mills.

-g

[ecc83]

I have no details to hand of the impedance or other spec' of a reverb driver coil but I would have thought putting DC through it was a bad idea?

Also if the EL84 shorts it will bugger it.

Dave.

[black_labb]

wouldnt the el84 use alot more current than nessesary? or is that dependant on the voltage coming into it?

[kg]

Pentodes that are usually used as a power tube tend to have low gain and high current capability, so they don't work well in a preamp, though they might work well as a phase splitter amplifier. I seem to remember reading that the EL84 has a gain of around 20, whereas the EF86 is upwards of 200. That makes a big difference when voltage gain is your main consideration.

Greg

hi greg,

mu = gm* rp

if we take typical values for gm and rp for an el84 we come up with values for mu around 400-450.

you may be confusing the mu-g2g1 value which is indeed around 20. this would represent the mu while in the tube is run in triode mode.

with a few key exceptions you would be very hard-pressed to come up with a multi-grid tube with a g1 mu of <20, and a triode with a mu >100.

hth
ken

[mooreamps]

wouldnt the el84 use alot more current than nessesary? or is that dependant on the voltage coming into it?

No, not if you set it up right. I'd still run the plate at full voltage, and set up a static power scaling circuit to regulate the plate current to about 10 mills. Of course, this would be constitude a starting point to work from. The driver transformer I'm looking at should be a good fit for this. I would also vary the value of the SGR to check for optimal perfromance.
I wonder if the lack of a SGR for the EF 86 would be a contributor toward it's reliablity problems. I've never spec'ed a SGR for this tube.

-g

[soundmasterg]

Thanks for the corrections kg. I read something in TUT2 awhile back about using power tubes like the EL84 as a gain stage. I'll have to go read it again to get it correct.

Greg

P.S. Good to see you back posting on the forums.

[mooreamps]

wouldnt the el84 use alot more current than nessesary? or is that dependant on the voltage coming into it?

No. The primary coil of the reverb transformer I selected is rated for that.

-g