Triode connecting a pentode?

[Original Burnedfingers]

Just wondering how to triode connect a pentode for line stage usage.
For a EF86 would you connect pins 6,8, and one together?

For a 6AU6 would you connect pins 2,5, and 6 together?

Thanks

[Giaime]

Hello!

Always connect G3 to cathode: however in small signal pentodes it won't make big difference.

See also here:
http://geek.scorpiorising.ca/GeeK_Zo...p?topic=2345.0

[alexmoose]

Sometimes people (such as myself) prefer to connect G3 to the plate through a small resistor generally around 100 ohms

[Tubes4e4]

Hi Alex,

Originally posted by alexmoose
Sometimes people (such as myself) prefer to connect G3 to the plate through a small resistor generally around 100 ohms

What is the reason of that preference, and why using a resistor to do so?

Tom

[alexmoose]

in Pentodes, especially the EL34, having a small potential difference between the plate and G3 either in pentode or Triode mode will make the tube behave better, using a resistor is the most basic way to do this, however there are other ways that sound a little better

[Geek]

Hi Tom,

Originally posted by Tubes4e4
Hi Alex,



What is the reason of that preference, and why using a resistor to do so?

Tom

Tames the possible parasitics

[Tubes4e4]

Hi Alex,

in Pentodes, especially the EL34, having a small potential difference between the plate and G3 either in pentode or Triode mode will make the tube behave better

Have you ever measured what you are talking about?

I just tried it with EL34. The current from g3 to plate connected through a 100 ohms resistor is in the lowish two-digit microAmpere range, so the voltage drop across a 100 ohms resistor producing the

small potential difference

you claim is about a single milliVolt compared to direct g3-to-plate connection.

Even a 1000-fold larger 100k resistor would drop only about a single volt or two.

You still claim that it

will make the tube behave better

?

Welcome to Voodoo-Land

Tom

[ray_moth]

Hey Tom,

Don't you think it would be a more appropriate approach just to ask Alex to explain why he believes having a small potential difference between the plate and G3, either in pentode or triode mode, will make the tube behave better?

[Tubes4e4]

Hi ray_moth,

okay, the chant ("Tonfall" - I hope this is the right translation) of my response may be have been not appropiate, I admit. Sorry about that.

Certainly I am interested to learn on what Alex´ claims are based, if not on measurements.

Tom

[tubelab.com]

This thread is going down a road once travelled. There was a similar thread a while back that was dealing with this subject and the EL-34. It was obvious then that there are some STRONG opinions on both sides. They are not always backed up by facts. In that thread I ran curves on a single sample of an EL-34. I came to the conclusion that the tube was more linear with G3 connected to G2, at least at low plate voltages. The results were disputed by some, and I had planned to repeat them at a later date, but never did.


EL34 Triode Mode - G3 where?


I have EL-34 triode mode SE amplifier currently running (with JJ EL-34's) on my bench where I can easilly make the switch. I tried the G3 connection both ways. I can tell you that there is an audible difference, but it is not profound. I can't even tell you which I like best. I haven't ran any measurements yet. I am running about 420 volts across the tube, which may mask the effects seen in the curves.

The original question referred to the 6AU6. It is stated in the RCA tube manual to connect the plate, G2, G3, and the internal shield ALL together when triode connecting this tube. Many other tubes state that G3, should be connected to the cathode. So there is even some difference of opinion within RCA.

It would seem that there are two opinions on what to do with G3, and neither is invalid. So if you have the opportunity, try it both ways, and form your own opinion.