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vaslejean 1st April 2009 02:31 PM

ACROSOUND direct-couple EL84 amplifier question
 
2 Attachment(s)
Hi Everyone,

Does anyone have any experience with such an amplifier. It is a direct coupled EL84 amplifier using a ECC83 as voltage and phase splitter.
I am being asked by a friend if it could be possible to assemble such an amplifier nowadays. I have on hand litterature describing the process to build such am appartus. It was conceived by ACROSOUND somewhere around 1962 and proposed as a project in an issue of Popular Electronic eperimenter handbook.

Now as it is direct coupled the voltages involved are different from a regular amplifier.

In the assembly manual published by ACROSOUND (which I do have as well) the B+ voltage is indicated a 440 V. dc. The el84 output tubes voltage , as indicated would be for the anode 430 V. dc. , screen would see 433 V. dc. The cathode is set at 88 V. dc.

In the article the voltage given are to those of a DYNACO sca-35 (or st-35 if you please)

Now while these figures seems awfully high for the EL84 tubes does the following math formula apply to this case:

Anode voltage (430) - cathode voltage (88) = 342 V.dc above ground ?
figures that would then bring the operating voltage parameter closer to those suggested by EL84 manufacturer.

The direct-coupled article in which this amplifier is described the author makes great fuss about an amplifier upon which he pretend his amplifier is inspired , the 1930 Loftin White. The arcticle is signed by Herbert I Keroes a name that has left some trace in the audio world back then ...

Sorry for this too long preambule, back to my questions

Anyone has heard or build this amplifier ? Would it be safe to reproduce it around EL84M (super EL84). I would use a dynaco Z565 transformer as output, and possibly the power transformer from that same amplifer that already pull out about 380 to 390 volts dc while driving two chanel , hopping that for a mono amplifier I could get close to 420 to 440 V. dc.

Andrewbee 1st April 2009 03:06 PM

I built an Acrosound EL84 PP circuit similar to this some years ago, I cannot remember if it was indeed this one but it too came from an old Acrosound catalogue and I also used opt's from an SCA 35. I could not get the amp stable. Eventually I got one channel conditionally stable but never both. I eventually gave up and chalked it up as a learning experience. Perhaps there is something particular with the Acrosound opt's that it requires.

Sorry I cannot be more helpful.

I would not however discourage you from trying it.

Andrew

Alex Kitic 1st April 2009 03:48 PM

Quote:

Anode voltage (430) - cathode voltage (88) = 342 V.dc above ground ? figures that would then bring the operating voltage parameter closer to those suggested by EL84 manufacturer.
That is correct. But frankly, more than 300 V is not advisable with EL84s.

I would also not want to discourage you, but there is quite a possibility that you will not achieve stability with this amp.

vaslejean 1st April 2009 04:40 PM

Instability
 
Thank you,
it just makes one wonder how could Keroes in the introduction of his afordmentioned article states his pretention as follow:

..." Here is a modern version of the Loftin-White amplifier that is free of distortion, is exeptionally stable, and as exellent frequency response..."

Commercial pitch of an over exited engineer ?

Now for all I know of Loftin-White amplifiers, is that when properly made, they do sound superior to quite a few , but then they may have more diy amateur swearing at them than swearing by them as it appears to be a difficult topology to masters.

For now I would be tempted to experiment on a mono layout, hoping to (Am I being naive ?) reduce the conditions that could lead to instability ?

As for the EL84 operating voltage, the only amplifiers respecting the tube manufaturer specified data that I know of seems to be the Leak TL12+ at around 290 or 300 V. Dc. Some other french made amplifier that I know of seems to do the same.

My mass producted Heathkit runs the output tube well over 360 V.dc actualy closer to 385 V. (and it never had problem with that) on a nice day. Dynaco SCA-35 are running them just a little lower.

Thanks for the input.

bear 1st April 2009 04:51 PM

I'm looking for the DC reference for the cathodes of the output tubes and not finding one? Did I miss it?

Edit: forget that... I see it now, through R3 & R4...

Should work nicely... no reason not to try this circuit.
I'd use poly/film caps for the electrolytics if possible, or else bypass the electrolytics with film caps for best results.

The schematic is calling for about 250vdc B+, if you build it with a higher value, you'll have to adjust the rest of the resistors to make it work and bias properly - IF it can be made to do so.

Btw, there are other DC coupled tube amps to consider...

_-_-bear

Andrewbee 1st April 2009 05:59 PM

l
Quote:

Btw, there are other DC coupled tube amps to consider...
E.G. http://www.wimdehaan.nl/tubeamps/el84dc/index.html

edit : http://www.wimdehaan.nl/tubeamps/el84dc/index.html

vaslejean 1st April 2009 06:41 PM

" The schematic is calling for about 250vdc B+, if you build it with a higher value, you'll have to adjust the rest of the resistors to make it work and bias properly - IF it can be made to do so. "

Do you mean that the voltage operating potentials are too high ?

Both power supply as per the one in the article I mentioned, and the schematic posted are respectively providing a B+ of 390 V dc. and 440 V. dc. The figure regarding the cathode voltage is taken from the various voltages operating points provided as a guide in the commercial assembly manual. The operating voltages provided in my initial post are for the commercialised acrosound amplifier version of the amplifer. They should be lower as per the amplifier described in the articles. Also, the commerial Acrosound amplifier is stereo. The "project" one is mono.

The amplifier as described in the article uses a ps made of a ss voltage doubler with 165 Ac V. at 200ma with a resistor inserted (22 Ohms 2 W)in serie before the rectifier. The commercial version uses a different transformer with 175 Ac.V.

Thanks to you and to "Bear" who posted the links to the electronic world file containing valuable infos on direct-coupled el84 pp.

vaslejean 1st April 2009 06:46 PM

Thank you andrewbee for the links, I am reading the file just now ...And I apparently gave credit for sending the links to Bear who is helpfull just as you anyway.

Have you build any of the schematics proposed in that Electronic World issue ?

Andrewbee 1st April 2009 07:11 PM

Quote:

Have you build any of the schematics proposed in that Electronic World issue ?
No, at least not yet. I almost built the one with the 6DJ8 but never got around to it.

I am busy enjoying Tripath amplifiers so have not done very much with tubes for the last 18 months or so. I do have four tube amplifiers in various stages of completion, none of which are direct coupled, one is EL84PP, one is 2A3SE, one is a 3 stage 300B SE and the other a EL84SE. Also have in my head a single end 2SK2221 or 1058 SS amp. So many projects and so little time:D

bear 1st April 2009 08:58 PM

oops gone batty... that is a v doubler supply, so double...

I'd try to not build a doubler myself for my own amp, but it will make sound... :_)

_-_-bear


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