pentode pre with a bias

[Andy Le Blanc]

I've been noodling on the bench, with pentode pre amps. I had a a client build
with a EF86 and got envious to have one of my own.

So I tried a 6SJ7 and got an alright tone with a 6L6 50 power side, 6SN7 inverter.

The cathode is held at a fixed bias with voltage division from the B+.
It seems to really stiffen the tone, and fun to dress.

[Bear]

I've elsewhere seen a guy swear by 6SJ7 in a grid-leak bias config. May be worth a try.

[Andy Le Blanc]

I have tried that, some of these "combination bias" approaches are very usable.
It's more a matter of the kind and amount of degenerative feedback you want.
Its feed back effect with out global feed back.

[PPL]

The Attached Circuit I think You might find useful it takes the cathode regulation concept further and adds Ultra linear operation to the Picture. You trade gain for linearity however output swing is not limited but is in fact unproved. Note

the grid leak of the following stage after the coupling capacitor is returned to the cathode, This not only makes the Grid leak resistor of the next stage part of the feedback loop but allows the Grid of the next stage to look deeper onto the Preamp output, or tighter preamp to next stage coupling.

The Voltage divider is no longer from the B+ but from the Plate providing yet another feedback path ans well as substantial power supply rejection for low Hum.

The Input the B+ as well as the cathode of the next stage must be ground referenced. Forgive me not deleting the default 1k resistor values as you will want to Bias the Circuit your way.

[Andy Le Blanc]

That looks neat, my brain wants to see it as a inverter.

What V dc will be present at the output? Will it affect the G bias of the next stage?

[PPL]

Hi Andy, yes the DC Bias voltage of V1’s cathode will be present upon the Input grid of the next stage. Since this affects the DC Bias of V2, you will need to adjust the cathode resistor of V2 to maintain the same Grid to cathode voltage for V2 as was without the mod. AC Phase relationships are the same as for a Conventional Pentode stage.

Attached is a working Circuit I designed for Hi-Fi use. The input stage uses a cascode configuration as the gain stage Element rather than the Pentode Type. however I treat Both cascodes and Pentodes as the same with the Pentode requiring more screen Grid current that a cascode configuration don’t require so places less of a load upon the tube as well as providing less DC Voltage on the cathode Requiring accommodation of
the DC Bias voltage of the next stage. Pentodes also suffer from additional noise that is absent in the dual triode cascode configuration. The output stage uses a Power pentode and as can be seen, substantial current must be accommodated in the screen grid voltage divider as well as recalculation of the Plate AC Load resistance. Cathode feedback around the output transformer accommodates different load impedances with less of a penalty than standard Pentode or UL configuration.

The Voltages shown in the attached Schematic are for a working amps actual measured Voltages with Fresh but burned in Tubes…

[Andy Le Blanc]

Now that's using your noodle, I've seen all the elements in other applications,
but not together as a whole like that. Why cascode?, and not SRPP or a mu-follower?,
and why the fuse between the cathode bias resistor and opt sec.?
It looks like a fixed UL, there's a benefit there, I saw a build where a 6l6 was
dressed in that manner with a capacitive coupling driving a tube with specific
requirement. I'll dig around and see if I can find the ref.. The inner and outer
feed back loops must be from N. Crowhurst, what was your inspiration?

[Andy Le Blanc]

found it..... The guy was doing a build around 6B4G and needed a way to
to drive them with the least distortion. He settled on a 6l6 dressed as UL tetrode,
and chose a resistance coupled circuit so he could safely double the plate voltage.
he got less than 1% distortion of the stage with no feed back and no cathode bypass
holding the tubes at a static 23w plate dissipation.
He did how ever apply a global feedback to keep the total distortion less than 1%.
The only cited references are RCA RC-19 and RCA TT-5.

audioXpress, Aug. 2002

[Andy Le Blanc]

getting back to a pentode pre, the discussion leads me to propose taking something
like 6sj7 with a 8N base where you have access to the screens and try the
various alternative dress, as a screen-grid tube, and exploring a bit.

[PPL]

Thanks mike for the 6L6 Print. My love for cascodes comes from my previous Solid State background as did my Love for Multi-Loop feedback systems. Yes Mr. Crowhurst Inspired me a lot. http://www.scribd.com/doc/27573111/Crow ... uit-Design.

Also Walt Jung had my interest when i was into Op-Amp based headphone Amps. http://elvencraft.com/ppa/

using pentode you can set the Divider ratio differently to try out different UL ratios. This would alter the DC Current through the tube however adjustment of the pate load and or cathode resistor will be needed to keep the Bias current where you want it.

I have been thinking if trying a EF184 since it is a frame grid tube with Mesh Plate http://www.electronicstudio.net/images/ ... /ef184.jpg

[PPL]

Andy are you sure you can connect the Suppressor grid to B+ I always amused perhaps incorrectly that the Suppressor grid and Beam forming plate be connected to Ground and in some instances at a potential slightly above Ground like 12 volts?

I have a friend that Plays Acoustic and he is begging me to do a Guitar amp that has the clean articulated sustain that this Circuit produces. He keeps Bowing my amp to play through his EV stage monitors. Obviously what he needs is a dedicated Guitar amp Optimized for guitar use?

This Brings me to yet another opportunity to apply Multi-loop feedback techniques to this application. I'm Thinking a Platform like the JMT45 optimized for Acoustic guitar, But versatile enough for Electric. I amuse that the ever so popular EF86 tube will be available forever and so would be a good choice to use?

[Andy Le Blanc]

That notion with screen grids comes from a very old reff., and does pose
exactly the same question in my mind too. I figured with a rf pentode it
wouldn't to far of a stretch to try with a reasonable voltage. An UL try
with a pentode pre...

That ef184/6ej7 is a "if" tube, funny in the data theres a distinction between
supply v , 550, and plate v, 250.

sound like your buddy needs to set up a proper stage monitor, nothing is
gonna help him there.......

But as a pre amp/ DI , a clean boost and tone source could be very useful.
With multiple feedback loops you holding the response of the amp in a
stable peaking region of operation, even over only a few simple stages,
too little and its lifeless, too much and the instabilities are not very nice.

[PPL]

ya tubes are somewhat forgiving of abuse so can't hurt to test. I am thinking of replacing the 12AT7 in The 777 Amp to a EF-184 and see how that performs. This will give me a seat of the pants feel for the Clean tone difference between the 12AT7 and EF-184.

Apparently the EF 184 is starting to gain popularity at least with the Hi-Fi Crowd.

[PPL]

I installed one EF184 and left the remaining cascoded 12AT7 in the Other Chanel so I could compare the two.

[PPL]

An old yet Informative site is Jukka Tolonen Tube DAC pages has great Info on what we are talking about. http://koti.mbnet.fi/siliconf/JukkaTolo ... part1.html