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EL34 Grid 3 connection

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jbesound

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Joined 2014
#1
Modding an EL34 SE amplifier and noticed that the suppressor pin 1 was not connected. I've seen schematics with:
1. no connection
2. connection to the cathode
3. connection to the plate through a 1k resistor

What are the differences?
 
#2
Triode, pentode, ultra linear, true triode,

There are others using diodes and zeners..

The differences are what tubes are all about..Power Vs sound..or depending on your point of view measurements of distortion and feedback.

So you have asked a difficult question to just answer in a few words.
You need to research each of the above and look at the amp types and what you like...things like push pull and single ended and the effect of the output type and effect of the above types of connection used in these topologies.

Then ask why use pentodes or triodes and what are the differences with something like triode connected pentode tube vs triode tubes.
NB the different connections will probably effect the tube bias unless its done with a CCS.

Regards
M. Gregg
 
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#3
All elements should be connected to something. Leaving g3 floating is wrong.
Most likely the amp was build for a type with g3 internally connected to the kathode, like the 6L6/6550/KT88.
Connecting the g3 to anything else than cathode or ground would change the characteristics of the EL34 considerably. I have no experience with that.
 
#5
After further research, went ahead and tied pins 1 and 8, added triode / UL switches to both channels and added RL207 diodes to protect the rectifier. I have 3 sets of power tubes I swap around in my amp (Tung Sol El34B, JJ KT77, and Sylvania 6L6G's) and sometimes "short cycle" it. So far liking the KT77's in triode mode, at least with Steve Miller.
 
#6
What resistance value do you have in the G3 connection?

If you are switching then 1k in series is a must do. Also be aware that the bias will/could be different in each switch position.
So be careful you don't red plated the tubes..it will run ok in one position and cook the tube in another.
Check the voltage drop across the cathode resistor in each position and calculate the current.
As long as the "Worst case" is within tube spec you should be ok switching..

Regards
M. Gregg
 
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#7
I am curious,

could you post a drawing of the OP tube as you have it connected in each position..G2 and G3.
http://obrazki.elektroda.pl/4888110100_1372541992.gif

Reason is what is the (G3) connected to in side KT77? is it the same as 6L6? (internally connected) ie pin 1 is?
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCMQFjAB&url=http%3A%2F%2Fwww.svetlana.com%2Fpdf%2Fgenalex%2Fkt77-genalex.pdf&ei=hQ-MVJiyLsXQ7AavsoCYAw&usg=AFQjCNG9VuG5FRTK-HYldn5TgMgAbsekMA&bvm=bv.81828268,d.ZGU

EL34 can be connected as triode, true triode, or pentode etc..however how is the KT77 connected?

NB its easy to damage the OP transformer so be careful!

Regards
M. Gregg
 
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#8
My guess is..

EL34..TRUE triode, NB not triode.
EL34..strange U/L..
EL34..strange pentode..

KT77..triode
KT77..U/L
KT77..strange pentode..

If you get it wrong you will get a flash over..depends on the B+ value and grid distance.

Regards
M. Gregg
 
#9
G3 should never float. In pentode and UL connection it should be wired to the cathode. In triode connection it can go to either cathode or anode, but you probably won't notice much difference.

Note that beam tetrodes, such as KT66 and 6L6, don't have a g3 - instead they have beam plates, which should normally be wired to the cathode.
 
#10
This is close to what I have - except the 100 ohm resistor is between the plate and the switch, not the switch and G2. Plate voltage is 277v and cathode current measures 52mA. This is a relatively cheap Chinese amp from Amazon, but is a good place to start learning.
 

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#11
Plate current 52 ma isn't enough for good sound. EL34 works excellent with 75-80 ma. You have possibility to increase current to max power plate load, decreasing cathode resistor. Bias will decrease and sensitivity of Your amp will increase with 6SL7. /Careful with Your PT....Will it heat?. If yes, decrease little bit plate current to normal temp. of PT, that is about 60-70 degr.....Maybe is reason to open new theme about heating of PT?/.
 
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#13
The 200 ohm..

I would change it to 1K and fit it direct on the tube base connect the switch to the plate and U/L.
(Get the 1K as close to the tube base as possible you can use a CC resistor)

Measure the voltage across the cathode resistor then use ohms law to calculate the current..ie voltage across the resistor divided by the resistance of the resistor = current through the tube. Always put a load on the output like a resistor never run without a load or speakers connected.

Connect G3 to the cathode not to G2..
It is bad practice to use the input volume control to Gnd the grid of the input tube fit a 1Meg from input grid to Gnd
You may also get grid current which will make the volume control noisy if it is try a 22K between the pot and grid of the first tube.

Regards
M. Gregg
 
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#14
The schematic is not exactly what I have, just close. The driver tube is a 6SN7GTB and I haven't reverse engineered the rest yet. G3 is currently connected to the cathode. The next step will be putting a 1k resistor between G2 and the pole of the switch and connecting the switch contacts directly to the plate and U/L taps per M. Gregg's suggestion. I may also raise the cathode current to 60-65 mA which will put my plate dissipation around 70%. Thanks for everyone's input so far.
 
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