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Which pentode for input?

Hello everyone, a forum newbie here.

I've been designing an EL34 SIPP-amp, and since I've zero experience on the sound of small signal pentodes, plus I need ample gain and swing, I decided to try pentode input just for the fun of it. Now I'd like to hear your opinions on which one to use: I have loads of E83F, EF184, E180F and E810F laying around. Which one of these would be the best? I'm looking for Av round 50 and a swing of 140Vpp from a 300 V B+.

I already was given a strong opinion in favor of the E83F by a person much wiser than me, and have drawn a circuit for that, but I'm still open for suggestions.
 
If the E83F is anything like the EF83 then it is variable-mu so will give you lots of second-order distortion with anything other than a very small input.

A pentode is likely to give you too much gain. If you reduce the load resistor to reduce the gain then you may have insufficient swing.

The valves you have listed are quite different from each other, as they are intended for different purposes.
 
If the E83F is anything like the EF83 then it is variable-mu so will give you lots of second-order distortion with anything other than a very small input.

It isn't. Even though for example the National Valve Museum website quotes it as a special quality EF83, it is a sharp cutoff pentode (this from the datasheets).

A pentode is likely to give you too much gain. If you reduce the load resistor to reduce the gain then you may have insufficient swing.

The valves you have listed are quite different from each other, as they are intended for different purposes.
With a 15k plate load and a 200 ohm cathode resistor I came up with something like Av ~ 130 with a bypassed cathode. I was thinking of leaving it unbypassed, and it should give something around 50 for gain, but these are probably inaccurate and I have not yet made this circuit. Though for the E83F this operating point is anything but conservative...

My idea is that using a pentode the input capacitance would be kept lower than with triodes. I could easily get the gain from a 12AX7, but then I'd have more input capacitance after the volume pot. If I use a cathode follower after the pentode, I should have very fast rise times throughout the amplifier, which is a concept I want to maintain in this design. Just for the fun of it.
 

leadbelly

Disabled Account
2002-12-22 2:13 am
Calgary, Alberta
Doesn't look like a very sharp cutoff to me:
 

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That looks quite typical of a sharp cutoff. If it were remote, the lowest grid voltage would be 10 or 20V, not 5.

As for pentodes, I'm partial to 6U8, 6GH8, etc. There are some particularly excellent ones, including frame grids (ECF801?). Cheap, plentiful, and quite handy. Pentode gain + triode cathodyne makes a lovely 6V6 PP amp. Bit of a stretch for 6L6, but quite doable. Not quite enough gain for NFB, you'll need a preamp to get away with it.

Or use two, of whatever type has the low-mu triode, for gain + LTP + CCS. You can drive quad 6L6s, sweeps, etc. Plenty of gain for NFB.

BTW, don't be afraid of common cathode combos (6X8, etc.). They make quite excellent LTPs, DC coupled amps, etc. Break with tradition, dare to be different...

Tim
 
Wavebourn, I've considered the EF184 also. It would give more gain, which could of course be lowered by local feedback, as I'm also considering with the E83F.

euro21, those results look very good indeed. A nice thing is I have a handful of Siemens E810F in my closet!

I'm trying to get away with tubes I have. That's why I asked for preferences on the list I gave. I could find more prospective tube candidates from my "small" stash (about 2 cubic meters), but that might complicate things too much... I already did a quick'n'dirty E55L spud with tubes found from my stash, and it's currently commanding my open baffles, even though I originally made it as a one evening joke build. Joke was on me, it sounds quite good...
 

Gorgomat

Member
2009-08-07 8:34 pm
Hey,

I also did some experiments with EF184 valves, they are strong enough to drive a triode strapped PL36 valve ( 6,8W RMS Output @ 8ohm load ). I was just wondering if the russian equivalents are worth the money, in Germany EF184 are quite hard to find ( ebay offers horrible prices ). I might try to buy some russian 6j51p since I´m quite satisfied with my results.

Best regards,

George
 

vas

Member
2007-08-12 8:55 pm
Bulgaria
Note that it was tested as a triode. The results look pretty much like what I've seen, which is why I use that tube a lot.

:) I see.
I use D3a in triode mode too, but with choke load (Electraprint 100H/30mA) - I get Au=73 for driving 300B - maybe the best of results for one tube.
In add. you'll get low Ri too. I not use it in pentode mode, (too high Ri for driver) or with CCS, but only with choke load.