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Old 16th April 2013, 04:07 PM   #1
6J7 is offline 6J7
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Default First little SE

This is my first SE attempt. As I have a lot of EL36, why not try them for a low power SE.. I was expecting something terrible, it seems to be not so bad.

I don't have SE OT yet, only 220v:12V standard line transformer (75VA).

What to choose ? Edcor CXSE25-8-3K ?

Correct me if I'm wrong but 3K seems to be ok for this tube when triode connected, but what about pentode, still ok ?

http://audio.ring.lt/lempos/tubes/el36_6ps31_triod.gif

EDCOR - CXSE25-8-3K
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Old 16th April 2013, 05:31 PM   #2
TheGimp is offline TheGimp  United States
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3K should be fine, however I would draw a couple of loadlines and calculate distortion for each to chose a more optimum OPT.
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Old 16th April 2013, 07:19 PM   #3
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It seems to me that the EF86 is a poor choice of driver. Your running Schade feedback, but crippling its performance by using a triode strapped pentode as the driver. It would be far better to run it pentode or drop the feedback rather than loading the triode strapped EF86 with the feedback.

A higher current pentode would be even better - something like a 6AU6 at 5-10mA.

Why not a beefy dual triode.

I am surprised that the DC in the line transformer isn't saturating badly - they can be used for response out to 5kHz, but only in parafeed configuration.

Shoog

Last edited by Shoog; 16th April 2013 at 07:26 PM.
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Old 16th April 2013, 10:02 PM   #4
6J7 is offline 6J7
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Quote:
3K should be fine
either pentode or triode connected ?

Quote:
however I would draw a couple of loadlines and calculate distortion for each to chose a more optimum OPT
I will.

Quote:
A higher current pentode would be even better - something like a 6AU6 at 5-10mA.
Cool, I have plenty of 6AU6

Quote:
Why not a beefy dual triode
12AT7 ?

Or a small power pentode like EL83 / 6P15P as voltage amp / driver ? does it make sense ?

Last edited by 6J7; 16th April 2013 at 10:04 PM.
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Old 17th April 2013, 07:13 AM   #5
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The 6AU6 is an excellent driver tube - go with that. Run it about 10mA in pentode mode and tweak your feedback resistor until you get what you like.

If you are using Schade feedback you end up with lower impedance if you run the output in pentode. The feedback resistor basically coverts voltage gain (mu) into current gain (gm), so the more gain the output tube has the more current it can push into the load.

Shoog
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Old 17th April 2013, 07:32 AM   #6
Yvesm is offline Yvesm  France
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A good OP for a trioded EL36 is around 200V / 60mA with a 2.5K to 3K load.
Bias will be around -35V, you can obtain that from a 600 ohms bypassed cathode resistor while rising B+ accordingly: some 235V.
Expect 2W at less than 5% THD (mainly H2), not so bad for such low cost tube !

The +/- 35V drive is easy to acheive from a single 6AU6, with or without shade NFB.
Listen and decide yourself !

The only point is the necessity of a clean supply as for any low rp triode in SE.
At least, you don't need for DC heater stuff.

Yves.
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Old 17th April 2013, 03:56 PM   #7
45 is offline 45  Italy
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I think that you can get about 3W (or even more) in triode connection quite easily without bothering with feedback.....
For a first SE amp I think this is the best way to start. It doesn't need more than 2.5K primary load because this is already rather high (the plate resistance being only 620-640 ohm in triode connection at 50-60 mA current).
At (conservative) 180V/50 mA with 2.2K and +/- 28V drive you already get more than 2.5W within 5% THD.
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Old 17th April 2013, 03:59 PM   #8
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This represents an excellent platform for easy experimentation. You can compare triode to pentode driver, triode to pentode output, and feedback to non-feedback - all with a few very simply modifications.

A real opportunity to learn.

Shoog
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Old 17th April 2013, 05:42 PM   #9
6J7 is offline 6J7
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Thank you all for the input.

I draw 2 loadlines, 2K3 and 3K3 (not sure if my calculations are correct).

As suggested, first I'll try 6AU6 and EL36 trioded, then pentode.

I have fullrange speakers, 2 watts seems to be quite "loud", I am surprised
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Old 17th April 2013, 08:55 PM   #10
45 is offline 45  Italy
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From the 2K3 load you will get almost 3.6W (at the primary). Second harmonic distortion seems to be less than 5%. All this assuming those curves are real....
You only need to subtract some power due to the transformer insertion loss. Let's say your OT in not particularly good and has got 0.6 dB insertion loss then you will still get more than 3W at the speakers.
With 3K3 you will get about 0.4W less and H2 doesn't seem to be different.
I would go for 2K5 max. You can buy the same Edcor transformer but 2K5 instead of 3K.
Actually you could ask if they can sell you the push-pull version with a gap for 60 mA DC current to work in single-ended. This way you have the center tap and can try the ultralinear connection at 50% for g2.....
This one for example: EDCOR - CXPP25-8-2.5K

Last edited by 45; 17th April 2013 at 08:57 PM.
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