Which tube for OPT 7,5k SE low watts???

[Klimon]

Hello,

I have a pair of 7,5k to 8ohms opts I'ld like to build an amplifier with; se triode (-mode) and < 5watts. I have been looking for suitable tubes, (al1, al4, e(c)l11, ecl86), but they don't look very nice candidates (Cannot find any modern schematics with these tubes nor do they have a good reputation)

Could anyone tell me how I can see on a tube datasheet which opts (primary impedance) are suited for that particular tube - in SE? Sometimes the desired Zout (that's it, no?) seems to correspond with the Ra value, but sometimes not... Or maybe a suggestion for a superb tube that would fit my opts?

Thanks!

 

[Bas Horneman]

I believe it is a rule of thumb to use a tranformer that has a primary impedance of 3 times the Ra....So a 300B for instance is often specc'ed for 2k5...ie 3*800 (almost anyway)

A little higher or lower is not a real problem...so an EL84 amp might be a very good candidate I'd say.

Plenty of schematic around for those tubes

Regards,
Bas

 

[Alain Dupont]

Klimon,

There was in AudioXpress February 2005 a beautifull article
on minimalist SE amplifiers ; I have just found it:

A PCL82 tube with a 7K/8Ohms output transformer... about 2watts output!

The PCL82 is a Triode + Pentode
goto: tubedata.org for datasheet

Regards.

PS: I can make a photo of the schematic, I have no scanner...

Alain.

 

[dhaen]

Hi,

What Bas says is quite right for valves in triode mode. In pentode mode things are less simple.
Have a look at some output transformer manufacturer's websites. They often list transformers by application, that will give you some idea.

 

[Bas Horneman]

In pentode mode things are less simple.

Thanks for the help John....by the way that's why I use triodes...nice and simple.

 

[Klimon]

Hello,

Alain, please send me a photo of that schematic! It looks perfect for this job; Here is my email address: Simon.laevers@student.kuleuven.ac.be
Many thanks!!!

Bas, the opts actually come from an old el84 amp, but are made for 4ohm speakers (7,5k to 8ohms = 3,8k to 4ohms). If I used this combination on my 8ohm speakers the high tones were too present...I did some research on the net and found that when connecting 8ohm speakers to a 4ohm tap you can expect too prominent highs, when you connect 4ohms to a 8ohm tap the sound will be dull. So I ordered a pair of 4k to 8ohms opts for the little el84 se and the older opts have come available for a better suited amp.

Best greetings,
Simon

 

[Klimon]

John, that's indeed the way to go. Most manufacturers have 5.2k opts for el84, some 4k opts. For my 7.5k opts I only found those rather old and unatractive tube-types mentioned. Does anyone know why they make opts for el84 se with these two different Zout? Is it to suit two types of circuits (triode and UL)?

 

[Klimon]

Alain, no need for a photo anymore, the article is on the internet both on AudioXpress' site and on the authors homesite. I also found another Italian (rather renowned there) amp with the Plc82. Thanks for the tip

 

[Eli Duttman]

Don't worry about the extra impedance. You sacrifice some power for improved damping factor. The DECWARE SE84 does exactly that and sounds fine.

Triode wired EL84s will be fine. If you have a desire to use Mullard tubes without spending too much, triode wired EL821s will work nicely for you.

 

[planet10]

I've seen EL84 OPTs from just under 5k to as high as 8k. In general these are OK for a range of tubes including EL84, ECL82/6BM8, ECL86, 6V6. Morse did a nice little 6BM8 amp (search foe no flash, no bang, just music). From our experience it is well worth running these tubes in "RH" mode (search the forum for RH84). The 6BM8 is a good candidate for this.

dave

 

[Klimon]

Well, I'll let you know when I've listened to the 4k opts if the too prominent highs have vanished, but for now I'm still convinced they are caused by 7,5k being to high!

 

[Yvesm]

Hello,
...
Bas, the opts actually come from an old el84 amp, but are made for 4ohm speakers (7,5k to 8ohms = 3,8k to 4ohms). If I used this combination on my 8ohm speakers the high tones were too present...I did some research on the net and found that when connecting 8ohm speakers to a 4ohm tap you can expect too prominent highs, when you connect 4ohms to a 8ohm tap the sound will be dull
....
Best greetings,
Simon

Yeap !
This does not work satisfactory because the Lp (primary inductance) remains the same and thus has heavier effect on shunting low frequency range. In short, less basses.
The reverse (loading a 8 Ohms output with a 4 Ohms load) sounds better, just losses incresase.

Yves.

 

[Klimon]

Just read the articles of the two most interesting PCL82 se amps: Scherzo and the one featured (Dutch design) in AudioXpress. I must say the scherzo does seem the more attractive option of the two, but here we go again : de designer suggests using opts with primary 3,5 to 5k whereas the Dutch design states 6K is optimal, but he uses 7,5k himself ('cause he had that value lying around).

Why again such a difference in recommended primary impedance for two schematics that use the same tubes and are quite alike??? (I'ld prefer to build the scherzo, but wouldn't bother if I knew it would have the same high-tone overprominence as my el84 if I used those too high 7,5k opts)

 

[skyraider]

6Bm8 is a nice candidate for a classic tube sound without much compromise.

However, I think the limitation is in the triode driver. I plan to mod my current 6bm8 to SRPP->parallel.

6bm8 is cheap as a bonus.