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Old 29th October 2007, 03:59 AM   #1
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Default 5W EL84 drive EL84 SE

Hi,
Need some input before I build one.

I recon this is nothing new, but I did not see threads in this forum in this.

While re-looking into EL84's datasheet, it looks promising to drive a EL84(output) in ultra linear mode with a trode connected mode EL84.

Also, read from somewhere that small pentode with a small Ra gives very pleasing results.

Why 5W? I happened to have a pair of hand wound 50mA max. 5.6k OPT with a 4.2k tap.

Has anyone tried this arrangement?
Please comment.
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Old 29th October 2007, 06:04 AM   #2
18thell is offline 18thell  United Kingdom
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Default EL84 drive EL84

It seems to be redundancy, due to the same capacity tube drive the same tube, as we know, EL84 ( 6BQ5 ) is a sensitivity tube, the neg. bias only 7.2V. Do you need to have a million dollars to settle a thousand dollar bill ???

The correct way of driving an EL84 is to use a 12bh7 or similar power tube, excess driving power will bring you distortion. consider 12AU7, or 6FQ7 ---- etc.

If U use EL84 for driver, what loading will U use, and also the coupling method.
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Old 29th October 2007, 06:42 AM   #3
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Hi 18thell,

Thanks for your respond.

I'm thinking of about -17.1~17.5V-ish bias for the power tube, so that that if 1Vrms Vin could use another EL84 set to ~15x gain to drive the output stage to near full power.

I would like to have fairly high cathode current (~10~20ma) on the driver.
Load wise, I haven't really decided yet, maybe I'll just relate to what the power tube want...

Could you elaborate on how distortion will be introduce to the power tube?

Cheers
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Old 29th October 2007, 06:46 AM   #4
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Hi 18thell,
I left this out ...

I'm looking at simple RC copling for now, but I'm tempted to Direct Couple em... as I have some suitable power trans serving as heavy duty door stopper in the house...

Definately not considering IT.

Cheers
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Old 29th October 2007, 07:19 AM   #5
18thell is offline 18thell  United Kingdom
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If you use RC couple, that would be fine, but if you use direct couple, the voltage between the driver and output stage must be very high in order to satisfy the tube requirement.

If using trans. for inter coupling, the quality of the coupling trans. must be very high, otherwise poor sound reproduction U will get.

for EL84 plate loading, the ideal is 4.3K and for grid input is not less than 15K. How and where can U get such 4.3K: 15K ratio tranny. it would be a problem. The answer should be use RC couple, the C must be certain high in order to pass low frequency and enough driving current!!! consider this problem.
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Old 29th October 2007, 07:50 AM   #6
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Default It's been done

Here you go

http://www.diyparadise.com/variation.html
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Old 29th October 2007, 08:46 AM   #7
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Default 5W EL84 drive EL84 SE

Hi 18thell,
I agree that IT can be very expensive and frustrating when quality is not to the mark...

I like DC on paper, but when comes to really building one, I'll park lots of safty and hence slow down the prgress... It has always been a tempting ides....

Maybe we can monkey

Coupling caps is not a problem, I have quite a few to play around with.

The Mullard datasheet did shows 4.5k load for pentode and 3.5k for triode. But could you kindly elaborate how the 15k grid input is determined?

Woud love to hear some comments on choke loading the plate too.

BTW, need to make correction on the gai, theoritical gain of EL84 is 19x, so I don't think clipping the output tube is a big problem If we are careful with our volume pot.

Cheers

Simpleton,
Thanks for the link, but tht is for a pp amp, I'm more interested in a simple and good sounding SE amp.


Cheers
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Old 29th October 2007, 11:53 AM   #8
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Yes I know, I was just refering to the driver stage being a triode strapped EL84 driving the next EL84 stage (here, PP), as it seems to be your requirement (but driving SE power stage).
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Old 29th October 2007, 06:12 PM   #9
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Quote:
Thanks for the link, but tht is for a pp amp, I'm more interested in a simple and good sounding SE amp
It has been observed that this amp should sound very SE in characture. This is because it forgoes the error correction of a normal PP amp.
It could also be argued that it could easily be cheaper to build than an SE amp as PP transformers are cheaper and sound better pound for pound than SE transformers. You also get improved bottom end and more watts to boot.

Worth considering at least. If you do - make shore and use the CCS.

Shoog
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Old 29th October 2007, 06:20 PM   #10
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Indeed, after building mine all I can say it that it sounds great with excellent bass and detail. So I second Shoog's opinion.

Again concerning this circuit, please note the in this self-split configuration, the grid is polarized at aproximately the same level as of a regular SE EL84 stage (7~8V), which again is a good basis of study/inspiration for a SE EL84-EL84 drive-like-drive topology.
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