Triode Trick

Only insofar as triode connection is an alternative to UL operation. Its an alternative to a ~100 ohm resistor.

yes

the reports are that this solution sounds better than a resistor.

An alternative to UL connection is not what is intended here. All he is saying is, if you want to connect a pentode/tetrode as a triode, it's better to do it his way. The arguments make sense to me, but how much better it will sound I wouldn't know.

That's also known as "boosted triode".

You'll also see that used on some Lafayette and Hallicrafter tubed CB radios on the RF final.

I have tried a similar aproach. But I do it with a programable tl431
shunt regulator at around 25V , with a Elna 450uf cap across it.
The power is a little bit smaller, but the sound is bit better to me, more relaxed or open.

Regards

Cosme

Hi,

I've incorporated this 'trick' in my el84 se and must say that it makes an excellent improvement. My grievances before the mod were a somewhat slow, romantic but also muddled sound lacking attack; after the mod the amp sounds much more detailed and tighter. I wouldn't consider building another triode-strapped amp without using the trick. BTW the schematic contains a few small errors, ask me if you're really interested than I'll take a peek into my amp and see how I did it. fwiw Eddie Vaughn made a few suggestions for optimising the circuit (he uses something similar in his carina), namely using 5,6V zeners IIRC (same number would do the job) + one 1000µF electrolytic instead of the two 470µ and resistors. He also explains the benefits on his website (think he calls it black box).

Simon

TRIODE TRICK

Hi,

Simon,

What are the few small errors in the schematic ?

Bien à vous .

Robert

10 ohms resistor should be connected to the plate; all the rest is as Bill Perkins prescribed in the original joelist thread. Less complicated than I remembered

Good luck

Simon

This is the schematic

This "trick" sounds interesting - but a bit confusing

There is this article in EDN about Get more power with a boosted triode , but they it are running g2 at a higher voltage than the plate

SveinB

Hi,

to me, that doesn´t make much sense from a technical viewpoint:

What is done there, is lowering Vg2 while maintaining the same AC swing at g2 as being present at the anode.

That means, theoretical possible total AC swing of g2 before catastrophic distortion havoc is lowered by 2 times the DC voltage drop from anode to screen grid introduced by this circuit.

In turn that means when using that circuit you limit usuable (not severely distorted) anode swing by the same factor, since AC swing still is the same.

This _may_ work for lowish gm pseudo-triodes to some extend and within limits, because the g2 DC voltage drop introduced will not intefer with the possible AC anode swing at its lowest possible value along the load line, but one would have to check each case in detail to make sure it actually doesn´t.

Anyway, this is not a good "general" recipe to use, since for correct triode-operation of pentodes you want the screen grid to be able to swing the full swing the anode may swing - in perfect synchronicity.

Besides that, it escapes me how a "g2 swing limited pseudo triode" can sound better than a "non g2 swing limited pseudo triode" - there is not the least traceable hint in this article why it should.

Actually, shifting Vg2 a little bit lower DC-wise while maintaining AC swing at anode and g2 only will shift the plate family of curves a little bit "horizontally" towards higher Va values; you can obtain pretty much the same effect by simply increasing the bias voltage slightly towards more negative values.

Regards,

Tom Schlangen

Besides that, it escapes me how a "g2 swing limited pseudo triode" can sound better than a "non g2 swing limited pseudo triode" - there is not the least traceable hint in this article why it should.

Click to expand...

If the title is well chosen the aim of the circuit is to get higher output power, I didn't find any reference to sounds quality in the article. Also this looks like a very different animal than the g2 conditions as per Bill Perkins.

Simon

I experimented with the "boosted triode" idea that was proposed in the EDN article. They used a 6L6 type tube in their article, so that is what I tried. The result, a dead 6L6GC. Their boost idea applies a positive (G2 higher than the plate voltage) offset to the screen grid. This does allow the plate saturation voltage to drop down to about 20 volts allowing slightly higher power on a lower plate voltage. What the article did not mention (and the authors may not have noticed) is the screen grid dissipation will be exceeded at idle and it will shortly melt. Mine lasted about half an hour. Do you want to run any 6L6 type tube with 400 volts on the plate and 500 volts on the screen, not for long. Their simulator (Microcap) will not melt but a 6L6 will! Been there tried that.

The Triode Trick pointed out in this thread is highly useful for operating sweep tubes in triode SE. Many sweep tubes have a low G2 max voltage which limits their usefulness in triode mode.

tubelab.com said:

I experimented with the "boosted triode" idea that was proposed in the EDN article. They used a 6L6 type tube in their article, so that is what I tried. The result, a dead 6L6GC. Their boost idea applies a positive (G2 higher than the plate voltage) offset to the screen grid. This does allow the plate saturation voltage to drop down to about 20 volts allowing slightly higher power on a lower plate voltage. What the article did not mention (and the authors may not have noticed) is the screen grid dissipation will be exceeded at idle and it will shortly melt. Mine lasted about half an hour. Do you want to run any 6L6 type tube with 400 volts on the plate and 500 volts on the screen, not for long. Their simulator (Microcap) will not melt but a 6L6 will! Been there tried that.

The Triode Trick pointed out in this thread is highly useful for operating sweep tubes in triode SE. Many sweep tubes have a low G2 max voltage which limits their usefulness in triode mode.

Click to expand...

I saw that article a while ago, and that was my thought... using the grid OPPOSITE it's intended purpose. i will eventually try it on a rainy day with a with a more rugged-screened tube just for kicks and giggles. I did consider trying it on small signal pentodes, but never got around to it.