EL84 driver stage

[catalin gramada]

Hello. found a bunch of el84 I can use to drive a pair of kt88. I understand can be a very good candidate for that instead a 12au7.Unfortunately I did not found in any datasheet design refereces how to implement it in triode mode.I expect a moderate gain around 20 into 100k load with ca. 3v at input.
Can I conect screen grid to plate and use the graph caracteristics and design like any regular triode?I don.t know how to design the stage. Can You help me with one sugestion.please? Thanks.

 

[rayma]

Can I conect screen grid to plate and use the graph caracteristics and design like any regular triode?

http://www.4tubes.com/DATASHEETS/SCANS-Original/E/EL84-TRIODE-MODE.gif

 

[catalin gramada]

Thank you. The grid leak should be max 220k as for pentode or can I do it as big as 1M.for instance. Please? I.m not familiar to use EL84 like a regular gain stage so a lot of questions goes through my mind please understand.thanks.

 

[rayma]

The grid leak should be max 220k as for pentode or can I do it as big as 1M.for instance.

The GE data sheet says 300k with fixed bias, and 1M with cathode bias, as the maximum Rg values.
These should still apply.

 

[gingertube]

Patrick Turner often uses EL84 as drivers of power tubes.
Here is just one example from his website.
80W-AB1-amp-2014
and another for a 300W Amp
http://www.turneraudio.com.au/300w-1+2-schem-input-driver-output-jan06.html
Note he uses centre tapped choke loads but that is not really required.
Cheers,
Ian

 

[kstagger]

I used a SV83 - Russian relative of the EL84 - to drive a 300B. Not my idea - Thorsten Loesch did this with his Legacy amplifier.
http://community.fortunecity.ws/rivendell/xentar/1179/projects/legacy/Legacy.html

Most of my builds have used pentodes - either the EF86/EF40 or a 6J7/6SJ7 - for driving the output tube. Simple... and plenty of gain for negative feedback.

 

[Nailbunny7]

You could try it like this if you're feeling lazy (like I often am). Lovingly drawn in paint
Don't forget to add your grid leak, input cap, etc. I would keep the grid leak at 300K (or lower, if you want) and the tube should last a long time.

 

[catalin gramada]

Thank you for kind replies. One more question please...Saw in datasheet max g2 voltage to 300v. That means max peak voltage.please? I wonder for what consideration the limited value is 300v? Maybe for triode conection the same demands will not be applied here and can raise the g2 max peak voltage up to 560v.exactly like for plate as time are connected together. I asked cause a supply 450-500v is more convenient for me.with a 10-12k at ca.15mA draw will put the anode arround 300v at idling what should be max acceptable .Thanks

 

[Chris Hornbeck]

The datasheet voltage rating is for standing ("DC") voltage. You'll be fine.

All good fortune,
Chris

 

[catalin gramada]

Thanks. One more question please...Thinking to add a tail resistor for a better common mode rejection can I drop 100V on it at DC idling as max acceptable limit for EL84 please?

 

[Jaap]

http://web.archive.org/web/20150831185716/http://www.pimmlabs.com/web/drivers.htm

 

[gingertube]

Peak Heater Cathode Voltage = 100V
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/6BQ5.PDF

So you would most likely need to elevate the heater supply.

Cheers,
Ian

 

[catalin gramada]

Hi. I did not found any info how much plate current should I draw for el84 in triode mode to keep internal resistance into a optimal 2-3k and how much total distorsoon should I expect from a differential without a ccs compensation in a tail.please? Thanks.
What should be considered a good point to start.please?

 

[jazbo8]

Why would 2-3k be considered the optimal load? Where did that figure come from?

 

[catalin gramada]

Sorry. Mean 3.5k from ge datasheet. I read it diferent chart .my mistake....

 

[jazbo8]

May be I am not understanding your design, isn't this thread about using the EL84 as a driver for the KT88, if so, where does the 3.5k fit in?

 

[catalin gramada]

Hi. Yes it is. I have to chose a plate load for el84 in triode configuration. From GE datasheet 3.5k it shows to be optimal
Still don.t know will perform with 10mA or 20 mA on the plate?to can choose the operation point

 

[jazbo8]

You are not designing a power amplifier so the load for the EL84 is the recommended grid resistor of the KT88, which is 100k for fixed bias and 470k for cathode bias. The distortion will be very low because it is very lightly loaded.

 

[catalin gramada]

Thanks.And for DC condition .please?

 

[jazbo8]

The 10mA~20mA (per tube) you showed earlier is just fine.