Akai M-7 monoblocks
- Thread starter Jim Kiriakis
- Start date
analogadict
Active Member
Thank you Jack
Would this require any changes if a EL84 is used instead of the 6V6?
Regards,
Anwesh
A few more notes on this circuit...
Reverting to the 6BQ5 in place of the 6V6 will necessitate replacing the 300Ω cathode resistor with the original 150Ω value used by Akai. A 180Ω resistor will also work and might be advisable with higher modern line voltage and less robust tubes. My circuit actually has two 150Ω 1W resistors in series due to my early experiments with both tube types.
The 6X5 rectifier that I used is the octal equivalent of the 6X4 used by Akai. They are essentially interchangeable, other than the socket. Be sure to use the same 0.6A filament winding for this tube that Akai did. The 6X4 and 6X5 are known for developing heater-cathode shorts if the filament winding is not separate and floating.
The 5KΩ and 10KΩ PS resistors were taken from the original Akai amplifiers.
Designing these amps to use the 6V6 like I did might also allow using the 12V6 by connecting the two 1.8A filament windings in series (if the amplifiers are built on a common chassis). NOS 12V6 pairs or quads can still be found at decent prices on the big auction site from time to time. I stocked up several years ago, and I'll probably need to make this modification in the near future. I'm currently running on a pair of 7408s pulled from an old Stromberg amp. They were well-used even before I got them.
This was my first scratch-built audio amp after returning to VT audio about 20 years ago. It lacks much of the "best practice" grounding that I employ now in new designs to preempt hum. The RCA jacks are grounded directly to the chassis. There is no star ground. The AC mains cord runs all the way to the front of the chassis across all the audio circuitry. LOL, I even realized yesterday that I had forgotten to ground pins 2 and 7 of the EF86s, so their internal shields are simply floating. Despite all this, both channels are remarkably quiet, and I probably won't bother to make changes.
It was my intent to hide all the ugly bits under the chassis when I built this. The layout and orientation of the transformers and chokes were carefully chosen to minimize the possibility of hum and interaction. Points to note are that A) the power transformers are at the rear of the chassis, well away from the high-gain stages, B) the audio output transformers are positioned such that their laminations are at right angles to the power transformers, and C) the PS chokes, which create very little AC flux and are therefore inert, are mounted up front.
Tape decks from this era typically saw much less use in terms of hours of operation than stereo amps and consoles. The original tubes are often like new. In addition, these particular Japanese tubes are superb. The 6BQ5s are extremely dynamic, and the EF86s give up nothing to the NOS British Mullards that I've occasionally auditioned in their place.
This amp is amazingly transparent and nuanced, and I highly recommend it for either headphones or high efficiency speakers. It was instrumental as one of the "discoveries" that convinced me never to return to SS. I'm sure anyone who builds this will feel the same. Good luck and happy listening!
Jack
Reverting to the 6BQ5 in place of the 6V6 will necessitate replacing the 300Ω cathode resistor with the original 150Ω value used by Akai. A 180Ω resistor will also work and might be advisable with higher modern line voltage and less robust tubes. My circuit actually has two 150Ω 1W resistors in series due to my early experiments with both tube types.
The 6X5 rectifier that I used is the octal equivalent of the 6X4 used by Akai. They are essentially interchangeable, other than the socket. Be sure to use the same 0.6A filament winding for this tube that Akai did. The 6X4 and 6X5 are known for developing heater-cathode shorts if the filament winding is not separate and floating.
The 5KΩ and 10KΩ PS resistors were taken from the original Akai amplifiers.
Designing these amps to use the 6V6 like I did might also allow using the 12V6 by connecting the two 1.8A filament windings in series (if the amplifiers are built on a common chassis). NOS 12V6 pairs or quads can still be found at decent prices on the big auction site from time to time. I stocked up several years ago, and I'll probably need to make this modification in the near future. I'm currently running on a pair of 7408s pulled from an old Stromberg amp. They were well-used even before I got them.
This was my first scratch-built audio amp after returning to VT audio about 20 years ago. It lacks much of the "best practice" grounding that I employ now in new designs to preempt hum. The RCA jacks are grounded directly to the chassis. There is no star ground. The AC mains cord runs all the way to the front of the chassis across all the audio circuitry. LOL, I even realized yesterday that I had forgotten to ground pins 2 and 7 of the EF86s, so their internal shields are simply floating. Despite all this, both channels are remarkably quiet, and I probably won't bother to make changes.
It was my intent to hide all the ugly bits under the chassis when I built this. The layout and orientation of the transformers and chokes were carefully chosen to minimize the possibility of hum and interaction. Points to note are that A) the power transformers are at the rear of the chassis, well away from the high-gain stages, B) the audio output transformers are positioned such that their laminations are at right angles to the power transformers, and C) the PS chokes, which create very little AC flux and are therefore inert, are mounted up front.
Tape decks from this era typically saw much less use in terms of hours of operation than stereo amps and consoles. The original tubes are often like new. In addition, these particular Japanese tubes are superb. The 6BQ5s are extremely dynamic, and the EF86s give up nothing to the NOS British Mullards that I've occasionally auditioned in their place.
This amp is amazingly transparent and nuanced, and I highly recommend it for either headphones or high efficiency speakers. It was instrumental as one of the "discoveries" that convinced me never to return to SS. I'm sure anyone who builds this will feel the same. Good luck and happy listening!
Jack
Jim Kiriakis
Active Member
Taking the plunge and starting the Kyoto EL84 Mono Blocks build. 99% of parts acquired, will start another thread to document. I decided against deconstructing my Akai M-7's as they now producing pleasing sounds. I cut out sections of circuit, replaced a few critical capacitors (though not power supply) per Dave Gillespie's list. So instead will mimic the Akai power transformers, chokes and OT's with new parts. I will use the NEC EF86's from the M-7's and also try its original EL84s. Should be fun and interesting and see how it compares to recently completed RH84 and KG84. Perhaps the Kyoto can be coined "TL84" with a nod to Jack's handle: TriodeLuvr?
Well, I appreciate the sentiment, but there's no reason to name it after me if all the iron will be different. Besides, "TL" doesn't seem very appropriate considering there aren't any triodes in it. 
It might actually be better to start a new thread at this point if the major components won't be Akai. It will be more like a modified Mullard than anything else. Incidentally, the original Japanese tubes are very good, and they typically have much less use than tubes from that era removed from amps and preamps.
Jack
It might actually be better to start a new thread at this point if the major components won't be Akai. It will be more like a modified Mullard than anything else. Incidentally, the original Japanese tubes are very good, and they typically have much less use than tubes from that era removed from amps and preamps. Jack
analogadict
Active Member
I came across the following link in a discussion on another forum.
http://www.bbceng.info/ti/eqpt/AM8_2.pdf
So, the BBC used two modified versions of the Mullard 3:3. One a/c coupled and the other d/c coupled. The EF86 heaters were not subjected to starvation mode. Sharing it here as it could be of interest and I would like to know what Jack and others think of this.
Regards,
Anwesh
http://www.bbceng.info/ti/eqpt/AM8_2.pdf
So, the BBC used two modified versions of the Mullard 3:3. One a/c coupled and the other d/c coupled. The EF86 heaters were not subjected to starvation mode. Sharing it here as it could be of interest and I would like to know what Jack and others think of this.
Regards,
Anwesh
Anwesh, thanks for the link. I had not seen that document previously. It's interesting that Mullard designed a version connecting the anode and screen grid of the EL84 with a 47K resistor. Considering that the screen is also connected to AC ground through a 150 ohm resistor, it's not clear to me how the 47K could produce a significant effect. I have not tried this myself though.
Other than that, the most significant difference between all these circuits lies in the operating points of the EF86. It is evident from the many circuit variations employed over the years, as well as tube charts and data, that the EF86 is a tube that performs exceedingly well without the need to maintain a critical set of parameters or an operational "sweet spot."
For my purpose, operating the EF86 in starvation mode was not acceptable. I abandoned Mullard's suggested configuration in this regard due to the volatility of the operating points as a function of variabilities in tube type (manufacturer) and age. It was my goal to be able to use a number of EF86 types without the need to re-bias or make other changes to accommodate differences. I have since used Mullard, NEC Japan and Russian 6J32P, and I feel confident that the sonic differences between them are not the result of shifting operating conditions.
I will just add that the values and topology I used have created a circuit that responds very precisely to the characteristics of the tubes themselves. It is possible to select preamp and output tubes, and combinations of same, to produce much better distortion characteristics than is possible by simply using random samples. Ironically, this has been the one amplifier that has challenged my general aversion to “tube rolling.”
In any event, if the circuit is built as I described, it would be a simple matter to use it as a test bed for other component values and techniques. I don’t personally have an interest in doing that, but it might be of value for historical purposes.
Jack
Other than that, the most significant difference between all these circuits lies in the operating points of the EF86. It is evident from the many circuit variations employed over the years, as well as tube charts and data, that the EF86 is a tube that performs exceedingly well without the need to maintain a critical set of parameters or an operational "sweet spot."
For my purpose, operating the EF86 in starvation mode was not acceptable. I abandoned Mullard's suggested configuration in this regard due to the volatility of the operating points as a function of variabilities in tube type (manufacturer) and age. It was my goal to be able to use a number of EF86 types without the need to re-bias or make other changes to accommodate differences. I have since used Mullard, NEC Japan and Russian 6J32P, and I feel confident that the sonic differences between them are not the result of shifting operating conditions.
I will just add that the values and topology I used have created a circuit that responds very precisely to the characteristics of the tubes themselves. It is possible to select preamp and output tubes, and combinations of same, to produce much better distortion characteristics than is possible by simply using random samples. Ironically, this has been the one amplifier that has challenged my general aversion to “tube rolling.”
In any event, if the circuit is built as I described, it would be a simple matter to use it as a test bed for other component values and techniques. I don’t personally have an interest in doing that, but it might be of value for historical purposes.
Jack
LOL, I've managed to confuse two similar threads. If any of the above doesn't make sense, that's why. Sorry for the confusion, here's the other discussion:
https://audiokarma.org/forums/index.php?threads/kyoto-el84-mono-block-build.999572/
Jack
https://audiokarma.org/forums/index.php?threads/kyoto-el84-mono-block-build.999572/
Jack
analogadict
Active Member
Hi Jack,
Thank you for the lucid explanation. I posted my query here as your previous posts about 6V6 circuit are in this topic.
Regards,
Anwesh