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.
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.
Attachments
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
Sorry I cannot be more helpful.
I would not however discourage you from trying it.
Andrew
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.
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.
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
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
" 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.
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.
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
Re: Instability
No, a bit of hyperbole perhaps, but I have heard the Acro 2020 which was the commercial version of this amplifier, (had a loaner for several months and have fixed a few over the years) as well as having heard several homebrews based on it. i have heard very few PP 6BQ5 amps that sounded better.
vaslejean said:
No, a bit of hyperbole perhaps, but I have heard the Acro 2020 which was the commercial version of this amplifier, (had a loaner for several months and have fixed a few over the years) as well as having heard several homebrews based on it. i have heard very few PP 6BQ5 amps that sounded better.
Thank you kevin ,
This is getting encouraging.
Do you have any recolection of the "homebrew" versions of this amplifier that you may have had in your hands ? I.E, were the transformers used in these versions acrosound units or just regular EL84 specified pp UL transformers. My comprehension when it comes to transformers is that a well designed can certainly work just as well as any "holy grail" flavoured one ...I would not replace a burnt down McIntosh output with a Hammond ...;-)
As I mentioned I would like to assemble a mono version following the information as per the Electronic Experimeter Handbook I have no data on the TO-370 OT that was used. Assuming it was anywhere close to what would be used as UL transformer for the EL84 , 7.6K to 9K primary, I am assuming I could use the Dynaco Z565 safely, to experiment at least.
thanks again
This is getting encouraging.
Do you have any recolection of the "homebrew" versions of this amplifier that you may have had in your hands ? I.E, were the transformers used in these versions acrosound units or just regular EL84 specified pp UL transformers. My comprehension when it comes to transformers is that a well designed can certainly work just as well as any "holy grail" flavoured one ...I would not replace a burnt down McIntosh output with a Hammond ...;-)
As I mentioned I would like to assemble a mono version following the information as per the Electronic Experimeter Handbook I have no data on the TO-370 OT that was used. Assuming it was anywhere close to what would be used as UL transformer for the EL84 , 7.6K to 9K primary, I am assuming I could use the Dynaco Z565 safely, to experiment at least.
thanks again
Note that the power supply in this thing is a half wave voltage doubler, better performance can be had with a full wave doubler or better still a conventional psu with a bridge and the appropriate secondary winding voltage.
Old reply to my comments about sound, the wording I used is generally intended to imply that I have not heard something better.. Whether or not I in fact have is debatable, it's a good sounding design. (And I can' remember anything that stood out as sounding better than this design.)
Old reply to my comments about sound, the wording I used is generally intended to imply that I have not heard something better.. Whether or not I in fact have is debatable, it's a good sounding design. (And I can' remember anything that stood out as sounding better than this design.)
Thanks for the info. I have made several versions of that amplifier, also with my own power supplies, and with current sources for the output stages. The thing is that all of that was ... 30+ years ago. Now, I'm thinking into re-make a new version, because I remember that the sound was very good. It is only a romantic thing, because nowadays, I use a "powerful" amp with two KR300Bs in push pull (30+ watts each monoblock)
It was very pleasant to see the article sent by planet10 (Thanks dave!!) and it's fig 3. I have done the same configuration with the same valves (ECC88), but 30 years ago. At that time, to use that valve (ECC88) in audio, was an heresy.
Surely, I will make again some of them.
Thanks again.
It was very pleasant to see the article sent by planet10 (Thanks dave!!) and it's fig 3. I have done the same configuration with the same valves (ECC88), but 30 years ago. At that time, to use that valve (ECC88) in audio, was an heresy.
Surely, I will make again some of them.
Thanks again.
Marantz 9 (that was even longer ago). Modjeski/Beveridge preamp. The Zeligman preamp featured in Audio Update. Nope, the ECC88 has had a long and honorable history in audio and rightfully so.
There is no reason that the Acrosound DC amp should not work well. As for instability, it would be helpful to know from those who encountered instability, just what kind of instability they encountered. Low frequency? The power supply is well bypassed, and the output tube cathode connection provides DC NFB to stabilize the output stage operating point, so that should not be an issue. High frequency? When using a different output transformer, the HF stabilization circuits need adjusting more often than not. In this design, that is accomplished by the cap in the GNFB circuit. The Dyna transformer provides about the best match for 6BQ5 tubes out there. It is capable of very stable operation, and should be able to operate in this circuit with minimal adjustments. Note however that in the Acro design the feedback is coming from a 32 ohm winding, while the original Dyna transformers only have a 16 ohm winding.
Dave
Dave
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