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First little SE

emk2

Member
2009-04-16 3:56 pm
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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Shoog

Member
2002-08-15 10:16 pm
Eire
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
 
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emk2

Member
2009-04-16 3:56 pm
3K should be fine

either pentode or triode connected ?

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

I will.

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

Cool, I have plenty of 6AU6 :)

Why not a beefy dual triode

12AT7 ?

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

Member
2002-08-15 10:16 pm
Eire
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
 
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.
 

45

Member
2008-12-18 2:29 am
UK
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.
 

emk2

Member
2009-04-16 3:56 pm
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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45

Member
2008-12-18 2:29 am
UK
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
 
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emk2

Member
2009-04-16 3:56 pm
Thanks, then I will go for this turns ratio.

What is the best ? 2.5K:8 or 5K:16 (more inductance at LF ?) or this is not important in this case because of the oversized OT ?

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.....

Interesting.. but the standard SE seems to have an UL tap (43%) ?

This first SE project will be fun, 6AU6 triode or pentode, EL36 triode or pentode or UL :spin:

Now I have to work on the PS.
 

45

Member
2008-12-18 2:29 am
UK
Thanks, then I will go for this turns ratio.

What is the best ? 2.5K:8 or 5K:16 (more inductance at LF ?) or this is not important in this case because of the oversized OT ?

It is not necessarily oversized because if they do not specify the frequency for the power handling that usually means 50Hz. So, if that 2.5K transformer is rated 25W at 50Hz it can only handle 4W at 20Hz! It should be enough in your case anyway.

If you choose 5K:16R to use with 8R speakers it will be better at low frequency but you will have more insertion loss because of higher copper resistance. If you ask before buying and find that the efficiency is acceptable (i.e. about 88-90%) then it is fine.
To calculate the effiency you need to do the ratio 2.5K/(2.5K + total copper resistance). From this you can see that if you use it as a 5K transformer the copper loss wil count less (i.e. the efficiency will be higher).
The total copper resistance is the resistance of the primary + the resistance of the secondary multiplied by the square turn ratio. You can approximate the square turn ratio with 5000/16 = 312.5.
 

emk2

Member
2009-04-16 3:56 pm
I contacted Edcor... without success.

Here is a power supply proposal, drawn with what I have on hand, except the power transformer. Chokes are rated 100mA.

Off topic, this morning I received these beasts :)
 

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