5879 triode voltages

[luthierwnc]

Hi guys,

I'm putting the finishing touches on a build using cathode-biased EL34 along the lines of a Plexi similar to a number of projects that have floated here over the last couple years.  Mine has a 5879 in the one hole with a switch (borrowed from here too) that changes from pentode to triode.  The rail has been set up to give me about 125VDC on the plate and 65VDC on the screen in pentode mode.  In triode I get 65VDC on the plate.  The triode volts seems low.  Triode has some good sounds but there isn't much improvement in the headroom given the loss in volume.  I've scoured the web for actual operating parameters for that tube in triode mode and either found wide divergences on the hi-fi side or nothing on the guitar side.  I'm not married to the switch in its current configuration.  If it needs a different rail arrangement to get it healthy, I'm all ears.

Is anybody running this tube in triode and where was your sweet spot?

Thanks, Skip

[PRR]

Post schematic or all values _including_ B+ voltage to the stage.

[HotBluePlates]

Also post your desired result with the switch, as well as what you want from each mode. Never assume we have a clue about what you want the amp to do, even if it should be "obvious".

Clueless people like me need some help... 

[luthierwnc]

Here is the circuit as it stands right now.  Pentode and triode voltages are shown with a P or T respectively.  I'll probably up the 82k PI resistor to 100k and reduce the size of the coupler into the PI.  The plate resistor started at 56k (as per Jeff's drawing) but I kept bumping it up until I got the pentode plates between 120 and 130VDC.

HotBluePlates: what I'm fishing for is a recommended plate voltage for the 5879 in triode mode.  The web is pretty thin on using this tube that way -- maybe for good reasons.  The switching circuit is convenient but I'm not married to it.  If I need to redo the rail (or even punch in another dedicated tube), that's fine.  I suspect, but can't prove, that 65VDC is too low for any headroom.  There are some usable noises in the triode mode but since the pentode handles the dirt side pretty well, I'd like better cleans.  The cathode resistor isn't carved in stone either.

Thanks for looking, sh

[tubeswell]

What is the HT voltage at "E"? And what is the bias voltage?

[HotBluePlates]

And what is the bias voltage?

That is, the voltage across your cathode resistor.

[luthierwnc]

I've updated the schematic with fresh voltages.

I also did a Google image scan for 5879 triode circuits and hit a little paydirt.  One came in a 60 plate volts but two or three more were in the 107 range.  K resistors were higher than mine at 1k2 to 1k5.  Mostly no bypass caps.  Plate resistors were small -- in the 33k range -- coming off a rail about the same voltage as mine.  I started at 56k but my pentode plates were 140ish and harsh.  Old Gibsons were usually 100k/750k -- on not a lot of B+ -- so I think I'm with the less-is-more crowd as far as the pentode side goes.  I do have a HD 3P2T toggle so I could add a parallel plate resistor mod on the third pole before I start hacking the board.

So, it might already be about as good as it gets.  I just want to check the options that the switch design (nicked here at EL34) doesn't give the triode mode short shrift.  If I were building this only to use the 5879 in triode, how would I do it to optimize the headroom?  The 5F6-A CF stack is going to add some raunch so clean (within reason) from the first stage is a plus.

FWIW, the amp is whisper quiet in triode -- even close to dimed.  The pentode has a little hiss but for a high gain channel I've got no gripes.  The 5879 is in a stock Hoffman 9-pin socket but that is suspended by a homemade shock mount that looks like a power-tube bear trap but splayed out using aluminum roof flashing and pop-rivets.  Two points of contact with the chassis 90 degrees off axis from the socket connection.  Not elegant but effective.  Good pricing too.

Cheers, sh 

[PRR]

Rg2 is too large.

Rk is too small.

Rg2 should not be switched-out in triode mode.

Plate voltage is not very critical in Pentode mode.

http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5879.pdf

[kagliostro]

Mine has a 5879 in the one hole with a switch (borrowed from here too) that changes from pentode to triode.

---

I'm not married to the switch in its current configuration.

You can give a try to a morph pentode/triode circuit

see Tubeswell schematic on this tread

http://www.el34world.com/Forum/index.php?topic=14393.0

this gives a larger control than a simple switch

K

[luthierwnc]

Thanks gents.  All values I have on hand.  I'll slap them in over the next couple of days and report back.  sh

[luthierwnc]

Values and volts changed on the amp and the schematic above.  Pretty snarly beast.  I still need to take some hair off -- probably with a 12AT7 in the PI socket.  It is now at the stage where I need to try different guitars and cabs.  The EVM Thieles are a bit splatty (not officially a word but you get the idea).

Thanks again, sh

[Backwoods Joe]

Very interesting build....please keep us posted.

[luthierwnc]

Amp's done.  I put a 1.5uf bypass cap across the cathode on V2a and a 12AT7 in the PI.  I also put in two 250pf caps; one is a snubber from the plate to the cathode on V2a and the other goes across the 82k PI resistor -- as per Jeff's suggestions.

It is a keeper.  The pentode/triode and the preamp bypass give it a good choice of noises although dead clean isn't one of them.  I knew that going in so no surprises.  It is also quite loud.  The El34's are pulling 68ma each.  As you can see from the schematic, I'm not going as low as the traditional Matchless K resistors which should help the tubes last a bit longer.  Probably wouldn't bother with the mid-boost if I was doing it from scratch.

The only problem I've had since is that the PEM nuts I had in the chassis worked loose so I need to get some floating nut plates and bolt them in to get the chassis in the cab (shown in the attachment).  

Thanks again for looking and for your comments.  I'm glad to field questions if you have something similar in mind.  sh

[jojokeo]

Nice to see and hear the info as I'm putting together a similar build w/ EL34s. Will run about 100 more volts to the 34s and in fixed bias but how do you like the master vol control's response here? Thinking of doing that and power scaling it to hit the sweet spot easier for smaller clubs making it more versitile overall.
Here's a preamp using 5879 I've been running w/ pentode/triode but honestly I don't hit the triode mode very often with it (but it is cleaner). I've been running pentodes differently since I built this amp a couple years ago but this works very well.

[luthierwnc]

I've never done a scaling amp.  For playing at lower volumes I'm not above just turning down and stepping on a stompbox.  For practicing at home I really don't care what it sounds like.  Most of my MV experience has been with Dumble clones (I'm almost done with #7) where it is just the volume control on the previous gain stage or Vox-type amps with post PI.  Gotta have a MV on this and it makes a difference.

I'd think a bigger drawback would be needing pure cleans.  It is all very usable but not the rig you would take to a hootnanny.  With the taken recommendation of changing the plate and screen values, the triode version doesn't lose quite so much volume against the pentode.  There is a little switch pop but not nastily so.  Definitely put in the PAB, though.  Too much in pentode but a bonus for the triode.  Very cheap and very cool.  Even better as a relay-activated stomper but on the panel is much better than not at all.  Mine is a push/pull mid pot since you are adding resistance to the tail of the tone stack right there.  A little trimmer as a variable resistor will let you dial in the right amount of honk.

I'm not sure the pentode shock-mount was necessary but that was also a simple mod.  I send the chassis DXF file to the laser shop with a 1" hole and mounting holes on either side.  Then I cut a piece of aluminum roof flashing in the shape of an outsized power-tube bear trap clip and bent it so it only touched the chassis at two points.  The tube socket is riveted to the piece on the raised part and bolted to the chassis in the lower part.  Attached is a 3D shot that looks better than the job I did cutting it out with a pair of shop shears.

Good luck with your rig, sh

[PRR]

> There is a little switch pop

Try a large resistor on the switch, so the screen-cap does not go flat when working triode.