Triode connected Tetrodes

[DMOD]

Instead of adding to a Necro thread

http://amfone.net/Amforum/index.php?action=post;quote=159870;topic=22176.0;num_replies=20;sesc=3c51bc39e6573b185855c1a12651d24b

Here is a question: Instead of copying and or adapting old RCA, etc., circuits I am curious as to whether or not anyone has actually characterized or seen any published curves for the 813 or other tetrodes in triode circuits?

Here is an example:

In order to develop a feedback circuit to stabilize a Screen Grid transmitter I decided to characterize the second unit of the 6DE7/6EW7 series in order to determine what voltages on the grid resulted in what cathode voltages with a constant plate voltage.

Yes, this is a triode but has anyone connected say an 813 or a 6DQ5 as a triode and determined plate currents vs grid voltages? It would seem that by running the tests and determining their characteristics one could learn how it really functions and how to use this configuration to its max, especially by those who have HV supplies in the 1500V to 2500V range.   

I.e., has anyone run these tests or have seen any published average characteristic curves of tetrodes in triode configurations?

Phil - AC0OB  

[w9jsw]

Here is the correct link to prior article...

http://amfone.net/Amforum/index.php?topic=22176.0

John

[DMOD]

Here is the correct link to prior article...

http://amfone.net/Amforum/index.php?topic=22176.0

John

Thanks for the other link John but unless I missed it I did not see any actual tube curves for triode-connected Tetrodes.

Phil 

[w9jsw]

I was just fixing your link from above, not trying to answer the question. It tries to create a post, not just view the article. It gets this error when clicked:

"Session verification failed. Please try logging out and back in again, and then try again."

[WD5JKO]

I attach and link to hand drawn plate curves for a triode connected 813..


http://www.audiodesignguide.com/Claudio845/813triode1.gif

GM70 and 813 SE Amp designs:
http://www.audiodesignguide.com/Claudio845/813amp.html


Jim
Wd5JKO

[K1JJ]

Hi Phil,

Well, it's 12 years later since that thread and I am still running the 4X1 modulators as tetrodes, class AB1.  They have about 6-8 dB of audio negative feedback with the MOSFET GFZ SS audio driver.. This turned out to be the cleanest config based on tests and operation on the air.

I suppose the classic answer is to simply run the tubes the way they were designed to be by Eimac.

However, the 4X1s and 813s still performed very well as triode connected modulators. I run my current 813s X 813s triode connected and it is very clean. (driving grid and screens tied together) The difference is mostly the ability to run with little to no grid current (and little screen current) in tetrode mode, which is always a cleanliness advantage.

I also think that unless there is a SS driver or direct coupled tube audio driver, then the slight advantage of running the pristine tetrode mode is lost due to driver audio transformers, etc.
Then there is the modulation transformer, that if of poor quality, will swamp any slight advantages that AB1 tetrode modulators will provide in quality.

I look at it as a cumulative thing. Use the best systems and components as you can everywhere and it all comes together the best you can make it in the end.



Jim: Cool on the 813 triode connected hand drawn curves. What is your opinion of that?  Are they close to  straightness as the tetrode connected curves?



Here's my last comment back in 2009 about the subject:

"Tetrode config on the modulator 4X1's -  update:

Well, I took out the 680 pF RF bypass cap on the 4x1 modulator screen grids and replaced it with a 4 uF for audio bypassing.  The screens now have regulated 400V and the bias is about 90 diodes in the fil CT (-45V) for a stiff bias.  The screen spike problem is gone - fixed! I ran some tests with the audio sweep gen and also some voice checks. I think it's the best config so far!  It's also easy to drive with about 10 watts of audio or so.    The screens are drawing only about 15mA on peaks, while the Eimac specs say 95 mA is for full power - this tells me there's plenty of headroom left.  The grid is drawing no current at all. I'm idling the plates at about 300mA total and the meter barely moves under voice peaks.  This tells me we are running close to class A, but really about push-pull AB1. That's a great mode for cleanliness.  This is on medium power tap, so it will increase the modulator requirements once I bump it up to full strap.

I'm real pleased with the tetrode configuration so far. I think it's gonna stay that way."


T

[DMOD]

I attach and link to hand drawn plate curves for a triode connected 813..


http://www.audiodesignguide.com/Claudio845/813triode1.gif

GM70 and 813 SE Amp designs:
http://www.audiodesignguide.com/Claudio845/813amp.html


Jim
Wd5JKO

Thanks Jim, good info.

It appears from the curves that for any plate current > = 50 mA this results in a very linear plate current operating area.

From the curves, it appears that for an 800V Vp and greater, the grid bias needs to be ~ -75 to -120V for his load lines. Now of course, this for an audiophile application.

I was told that ER magazine might have some tube curves as well so I have contacted Ray for the articles that might have those.


Phil

[WD5JKO]

Jim: Cool on the 813 triode connected hand drawn curves. What is your opinion of that?  Are they close to  straightness as the tetrode connected curves?

Tom,

   It is worth looking at this full thread.

http://amfone.net/Amforum/index.php?topic=44390.50

Tucked away is a TimTron post (post 54) that summarizes 50 years of his thoughts on the subject. Worth the read, and reread.

Your 4-1000 triode connected experiment likely drove G1 and G2 together. The 813 and GM70 curves presented are with G2 tied to the plate with a resistor, such as 100 ohms. A different topology that runs as a low Mu triode and class Ab1 (no driving power). Your 4-1 triode experiment was class B (or close to it) where driving power was required. Here the tubes behave more like high Mu triodes.

So for the 813, we could have at least 3 sets of plate curves of the different configurations. Could vary the G1 and G2 drive percentage, and individual DC grid bias (G1 and G2 separate set) settings, and then have infinite possibilities. With sweep tubes using Crazy drive, works well when biasing G1 and driving it such that no grid current flows on audio peaks, and then pile on the majority of the drive to G2 where power is taken.

Jim
Wd5JKO