Adventures with: The Aikido 12 Volt AC kit.
I’ve been fascinated by the Earmax headphone amp since I first saw it reviewed in HiFi News & Record Review twenty years ago.The original Earmax used two 6GM8 (ECC86) tubes for output duty and a single 12AT7 for the gain stage.The B+ supply to the output tubes was around 50 volts which made for an intriguingly simple power supply.Priced at nearly a thousand Canadian dollars it was well out of my league.
Bottlehead audio published a short lived online DIY magazine back in the early 2000s and one of the articles was titled “Batteries Not Included”.This was their interpretation of the Earmax using three 6GM8s and a combination of lantern and rechargeable batteries for power.I built two versions of this amp with two different AC power supplies about 10 or 12 years ago.It sounded OK but when I built my big Aikido about four years ago the little 6GM8 amp sound weak and un-engaging and was shoved to the back of a closet and later dismantled. See my post of Jan. 17 for a description of my “big” Aikido HPA.
audioXpress magazine reviewed the Earmax Pro back in 2002.The Pro uses 6DJ8 output tubes.Search the web for a copy of the review.I think it’s available on the “Enjoy the Music.com” web site. The HFN&RR review might be found there as well.
There have been some Chinese clones of the Earmax as well but these did not appeal to me as most used 6DJ8 output tubes.
I’ve wanted to revisit an HPA made with 6GM8 tubes as I’d acquired about two dozen of these some years ago when they were fairly cheap.
Please note that an HPA using 6GM8 tubes will only drive headphones with a 200 ohm or higher impedance.This limits the choice of HP to some of the Sennheisers, Beyer Dynamics and a few others.
Back in 2013 John Broskie made available on his tubecad.com web site a 12 volt AC powered Aikido board.At 3.6” x 7” it contains all the circuitry except for a power transformer, volume pot, input selector switch, input jacks and output jacks to make a complete line stage. At about 80 bucks, less tubes, it’s a screaming deal that includes the board, all of the board parts and user guide.You’ll also need an enclosure as well.If you’ve never seen one of Broskie’s boards they’re about twice as thick as a regular PCB, have heavy copper traces, substantial plated through holes and multiple holes for each component so you’re not stuck to one brand or style of parts.For most of the kits Broskie offers you can get just the board and user guide (if you want to source your own parts) for what seems to be dirt cheap considering the quality.If you are thinking of building one please note that you won’t get detailed step by step instructions.Be sure to assemble the tube sockets to the TOP side of the circuit board.Resistors and caps can go on either side.You’ll need to know how to solder, figure out capacitor polarities and read a circuit diagram.Lest any beginners be put off there are usually old tube and radio guys around that would be more than happy to help someone just starting out.
If I could level one minor criticism at Broskie’s boards it’s this; some of the component designations are a bit confusing at times and some part designations are duplicated between the power supply and amplifier sections of the board.As well the boards being as thick as they are (a good thing) if you solder a part in wrong it can be a real job to remove.I don’t think we can fairly level too much criticism at John Broskie.Tubecad is essentially a one man band and it must be a labour of love for all of the information he has provided the audio DIY community over the years.I’m sure he’s not making any profit helping the likes of us.
If you go to the tubecad.com web site and scroll down you will see descriptions of various Broskie circuit boards on the left side.Click on the “New Aikido 12Vac PCB” link to go to the detailed description of the 12Vac Aikido board.
Broskie publishes some of his user guides for free down load.Just scroll down while looking at the right hand side of the page and a similar amplifier circuit can be seen in the Aikido LV guide.
There are two options for the 12Vac if you order the board with parts. The 12 volt option uses four 6DJ8 tubes and a 12 volt transformer.The 18 volt option uses four 12AU7 tubes and an 18 volt transformer.The power supply (less transformer) is right on the board.It’s a combination voltage doubler to supply the filaments and voltage sextupler for B+ all from one transformer winding.Decide which version you’ll be building before ordering as some of the parts (power supply caps primarily) will be different.
If building a line stage my preference would be the 12 volt version with Tesla ECC88s (dead cheap starting at $30 a pair from Tubemonger.com) as I’ve never been a big fan of 12AU7s for anything but phase splitter duty.
Look the 12Vac board over carefully first comparing the schematic to the board until you are familiar with the board traces and the schematic.
A couple of small gotchas:
Until you determine through trial and error the values of the power supply dropping resistors R11 and R12 don’t solder them through the holes.Just solder them temporarily to the pads on bottom side of the board.
I ordered the 12 volt version and then “hacked” the board to convert it to headphone amp duty.The output caps needed for headphone use are too big and must be located off-board.The output caps supplied with the kit are too low in capacitance to be used with headphones but might be worthwhile for bypassing larger caps.A good starting point for the large output caps is 30 uF Daytons from Parts Express.The Daytons can sometimes be purchased through Broskie as well.In the space freed up by mounting the output caps off-board I drilled holes for the caps that bypass R11 the output tube’s cathode resistor.
To create a HPA output from a LSA output in the world of Aikido the following must be done.
Have a look round the tubecad site and read some of the blog posts.You’ll probably come across the circuit I’m describing above.Broskie sometimes refers to this as the Aikido push-pull circuit.
About 30 uF of output capacitance is needed to drive 300 ohm ‘phones.For my tests I used a 20 uF Solen, a 10 uF Panasonic and a 0.1 uF Russian teflon connected up in parallel, the latter two after the picture was taken.Panasonic has some new large value 500 volt metalized polypropylene film caps, available from Mouser that I’d like to try sometime.
I used 300 ohm resistors for R2 and R4 on the board but next time I’d use something between 330 and 400 ohms to lower the current draw on the input tubes (V1 and V2) a little.
On the output tubes (V3 and V4) I used a 36 ohm resistor for R11 thought next time I’d use a 39 ohm to lower the current draw and keep it below 10 mA.
The added resistor between the anodes of the “top” output triodes (pin 1 on V3 and pin 6 on V4 and the B+ was 180 ohms, see pictures.
Power supply resistors R11 (filament supply) needs to be selected to yield between 23.5 to 24.5 volts across the series connected filaments (measure across C10).The value will vary depending on the power transformer used and your line voltage.Start with about a 30 ohm 3 watt resistor and reduce as necessary in about 2 ohm increments.
Power supply resistors R12 (B+ supply) needs to be selected to yield between 55 and 60 volts across C11.The value will vary depending on the power transformer used and your line voltage.Start with about a 2000 ohm 1 watt resistor and reduce it as necessary in about 100 ohm increments.You’ll probably need to buy a selection to set the correct voltage as the resistors Broskie supplies for R11 and R12 are for the 6DJ8 and 12AU7 line stage versions.
I used a 12 volt 2 amp toroidal transformer from Digikey and a 100K Alps blue velvet pot for the volume control.
Source was a Cambridge 840 CD player and headphones Sennheiser HD650s.
Listening Tests:
My sonic priorities are first and foremost balanced frequency response.A magic midrange is great but if the highs are rolled off, the bass is soft and lacks propulsion and percussion doesn’t have any snap and jump I can’t generate any interest in the music.That’s not to say I like harsh spitty treble and loosey-goosey slow boomy bass.Smooth frequency response, high resolution and good PRAT (pace, rhythm and timing) are what I value.
All of my tubes even those labeled GE, Motorola and Zenith are of western European manufacture.As far as I know no 6GM8s were made in the North America.The Russian 6N27P is an equivalent but now eBay crooks want as much for these as European tubes.
Sound comparisons were made directly with my “big” Aikido HPA.I’ll repeat the sound comments of my favorite set of tubes in that amp as the bench mark for evaluating the 12Vac.
“Big” Aikido LSA sound:
Mullard Blackburn 6DJ8 Ring Getter plus shield (NOS +100 hours)
Sound: WOW!Even after 1 hour of use these sound very promising, after 25 hours even better.At 100+ hours my favorite tube.This tube gives me the “lock” I’m looking for.Deep, extended, forceful, articulate bass.Lovely textural, articulate midrange that is especially nice with piano and female vocal.Detailed treble that might just sound a bit too “hot” on sub-optimal source material.Great PRAT.Macro dynamics that give great snap to percussion.A great boogie factor that had my wife bobbing her head and tapping her toe to the music 3 seconds after I handed her the headphones.These tubes also do subtle too; the micro dynamics show subtle shifts in the music exquisitely.These tubes in concert with (pun intended) the GE Hot Wire 6CG7s are the magic for me.
Aikido 12Vac listening tests:
I’ll start with my favorite tubes first and list their place of manufacture and build details not the name on the tube.
Valvo Hamburg, A-Frame with “pizza-pan” getter (NOS +50 hours)
Sound: WOW!A lot like my “big” Aikido with GE hotwires and Mullards.This really surprised me.The two amps are so much different but sound so similar.Different tubes, different power supplies different output caps and different volume pots.I had to tear my mind away from just enjoying the music to objectively listen for differences.Keep in mind that it was not a blind test but if it had been the results could have almost gone either way.Overall the 12Vac was slightly less resolving than the “big” amp though this might improve with more time on the amp.The bass of the 12Vac was a little less extended and not quite as forceful.Not surprising when you consider the over-built regulated B+ supply of the big amp but surprising how close to the big amp you do get with the simple supply in the 12Vac.The 12Vac was not quite as dynamic, both micro and macro, as the big amp.I could go on but it would be splitting hairs.Given the choice I’d rather listen to the “big” Aikido than the 12Vac but is was not a hardship listening to the “little” amp.That the 12Vac with these tubes comes so close it is amazing!
If you have a bunch of these tubes in your NOS stash it would be a shame not to put them to work.If you don’t fancy an HPA just forgo the mods I’ve suggested and build a line stage. The B+ voltage could be reduce to between 40 and 50 volts as not much current is needed for a line stage compared to an HPA. The values of R2 and R4 on V1 and V2 could be adjusted for 2.5 to 3.5 mA of current draw.R8 and R11 on V3 and V4 could be adjusted to draw 4 to 6 mA.
Valvo Hamburg, ring getter with shield (NOS +50 hours)
Sound: Sort of a lite version of the tubes above.A little less bass and a little less dynamic.Nice but once you’ve heard the best it seems a bit of an anti-climax.Might be the better choice for use with bassier ‘phones like the Sennheiser HD650s.These might make great line stage tubes.
Telefunken Germany (NOS +50 hours)
The typical Telefunken sound: nice midrange but a little weak in the bass and treble.These might suit some but don’t really cut it for me.These might make good line stage tubes as well depending on your sonic preferences and matching components.
Amperex Holland ring getter with shield (NOS +50 hours)
These just sound bland.Nothing really grabbed me about these tubes.Not very dynamic and the bass and treble seem a bit rolled off.These were the tubes I tried first and made me think the 12Vac might just not make a very good HPA.Tube rolling in my favorite Valvos sure changed my mind quickly.
So you ask, where does that leave me if I want an inexpensive Aikido HPA and don’t want to spend money and time chasing down rare 6GM8 tubes?Here’s my plan using inexpensive and more or less readily available tubes.
First we should note that as the filaments in the 12Vac are series connected they must all be rated at the same current draw but not necessarily the same voltage.You can’t use 6CG7’s and 6DJ8’s as they have different current draw.We might however use 12BH7s and 7DJ8s.I got excellent sound from Electro-Harmonix 6CG7s and Tesla ECC88s in my “big” Aikido HPA.I’d be willing to bet the cost of a pair of E-H 12BH7s that they might sound as good as their 6CG7 siblings.
The 12BH7 draws 0.3 amps (input tubes V1 & V2) and 7DJ8s (PCC88s) also draw 0.3 amps (output tubes V3 & V4).Adding up the series connected filaments (2 x 12.6 volts) + (2 x 7.5 volts) = 40.2 volts.This voltage requirement is too high for the version powered by a 12 volt transformer but might just work with the version using an 18 volt transformer.We’d have to use an 18 to 20 volt transformer with at least a 2 amp rating.A wire wound resistor for R11 would burn off the surplus filament voltage to drop the raw voltage down to the 38 to 40 volts we need.We should wind up with about 150 volts of raw DC but drop this to no less than about 130 volts at C11 by playing around with the value of R12.The advantages of this configuration are: 12BH7s are currently available new from Electro-Harmonix (lots of NOS are available too) and (though not as plentiful as 6DJ8s) NOS 7DJ8s are fairly easy to get and relatively cheap.Tubemonger.com sells Tesla PCC88s for about 19 dollars each.There are also NOS Tungsrams and Amperexs around as well.
The other option would be to use four 12DJ8 tubes.This a bit problematic as they are fairly rare and the only ones I’ve come across are the US made ECG/Philips/Sylvania.The construction appears the same as the ECG 6DJ8, 6922 and 7308 tubes.My sonic impressions of all the ECG built tubes were not positive.
One of the beauties of the Aikido circuit is that we are not stuck to only one tube.Using the 12BH7s as V1 and V2 I’d start out with about 1K ohm resistors for R2 and R4 but adjust as necessary for about 3 to 4 mA of current draw through the tube.For R11 (cathode of V3 & V4) I’d start off with 100 ohms but adjust as necessary for 10 mA of current draw.
Using the formula from one of the Aikido user guides we can calculate the anode resistor for V3 and V4. Ranode = (rp + 2Rload) /mu. rp for a PCC88 is about 2600 ohms and the mu is 33.Rload is the impedance of your ‘phones.Ranode =97 ohms for 300 ohm ‘phones.
When time permits I’m going to try it myself.I’ll post my results.
So have I finally gotten the Earmax out of my system?I think so.If I had another set of the metal work from my Earmax homage HPA build would I build one using 6GM8s?Tough call but as I don’t have another set of metal work is guess not. My Earmax homage is compact and light weight, not including the power supply, but it’s somewhat fragile so it wouldn’t make a good amp to be carting about.So no, I wouldn’t build another Earmax homage even one with 6GM8s and a much smaller power supply.One Earmax pretender kicking around the house is enough.
What I might do given the time is make a point to point wired version in a low profile case with tubes mounted horizontally inside. I think I could keep the size about the same as the power supply on my Earmax homage.A case about 2¼” x 5¾” x 11¾” including some jazzy looking black cherry side panels might do the trick nicely.If I do go ahead with this I’ll post pictures.
Cheers, Steve
I’ve been fascinated by the Earmax headphone amp since I first saw it reviewed in HiFi News & Record Review twenty years ago.The original Earmax used two 6GM8 (ECC86) tubes for output duty and a single 12AT7 for the gain stage.The B+ supply to the output tubes was around 50 volts which made for an intriguingly simple power supply.Priced at nearly a thousand Canadian dollars it was well out of my league.
Bottlehead audio published a short lived online DIY magazine back in the early 2000s and one of the articles was titled “Batteries Not Included”.This was their interpretation of the Earmax using three 6GM8s and a combination of lantern and rechargeable batteries for power.I built two versions of this amp with two different AC power supplies about 10 or 12 years ago.It sounded OK but when I built my big Aikido about four years ago the little 6GM8 amp sound weak and un-engaging and was shoved to the back of a closet and later dismantled. See my post of Jan. 17 for a description of my “big” Aikido HPA.
audioXpress magazine reviewed the Earmax Pro back in 2002.The Pro uses 6DJ8 output tubes.Search the web for a copy of the review.I think it’s available on the “Enjoy the Music.com” web site. The HFN&RR review might be found there as well.
There have been some Chinese clones of the Earmax as well but these did not appeal to me as most used 6DJ8 output tubes.
I’ve wanted to revisit an HPA made with 6GM8 tubes as I’d acquired about two dozen of these some years ago when they were fairly cheap.
Please note that an HPA using 6GM8 tubes will only drive headphones with a 200 ohm or higher impedance.This limits the choice of HP to some of the Sennheisers, Beyer Dynamics and a few others.
Back in 2013 John Broskie made available on his tubecad.com web site a 12 volt AC powered Aikido board.At 3.6” x 7” it contains all the circuitry except for a power transformer, volume pot, input selector switch, input jacks and output jacks to make a complete line stage. At about 80 bucks, less tubes, it’s a screaming deal that includes the board, all of the board parts and user guide.You’ll also need an enclosure as well.If you’ve never seen one of Broskie’s boards they’re about twice as thick as a regular PCB, have heavy copper traces, substantial plated through holes and multiple holes for each component so you’re not stuck to one brand or style of parts.For most of the kits Broskie offers you can get just the board and user guide (if you want to source your own parts) for what seems to be dirt cheap considering the quality.If you are thinking of building one please note that you won’t get detailed step by step instructions.Be sure to assemble the tube sockets to the TOP side of the circuit board.Resistors and caps can go on either side.You’ll need to know how to solder, figure out capacitor polarities and read a circuit diagram.Lest any beginners be put off there are usually old tube and radio guys around that would be more than happy to help someone just starting out.
If I could level one minor criticism at Broskie’s boards it’s this; some of the component designations are a bit confusing at times and some part designations are duplicated between the power supply and amplifier sections of the board.As well the boards being as thick as they are (a good thing) if you solder a part in wrong it can be a real job to remove.I don’t think we can fairly level too much criticism at John Broskie.Tubecad is essentially a one man band and it must be a labour of love for all of the information he has provided the audio DIY community over the years.I’m sure he’s not making any profit helping the likes of us.
If you go to the tubecad.com web site and scroll down you will see descriptions of various Broskie circuit boards on the left side.Click on the “New Aikido 12Vac PCB” link to go to the detailed description of the 12Vac Aikido board.
Broskie publishes some of his user guides for free down load.Just scroll down while looking at the right hand side of the page and a similar amplifier circuit can be seen in the Aikido LV guide.
There are two options for the 12Vac if you order the board with parts. The 12 volt option uses four 6DJ8 tubes and a 12 volt transformer.The 18 volt option uses four 12AU7 tubes and an 18 volt transformer.The power supply (less transformer) is right on the board.It’s a combination voltage doubler to supply the filaments and voltage sextupler for B+ all from one transformer winding.Decide which version you’ll be building before ordering as some of the parts (power supply caps primarily) will be different.
If building a line stage my preference would be the 12 volt version with Tesla ECC88s (dead cheap starting at $30 a pair from Tubemonger.com) as I’ve never been a big fan of 12AU7s for anything but phase splitter duty.
Look the 12Vac board over carefully first comparing the schematic to the board until you are familiar with the board traces and the schematic.
A couple of small gotchas:
- Comparing the schematic on page 6 of the user guide to the board will be confusing until you change V1 on the schematic to V2 (and vice versa) and change V3 to V4 (and vice versa).
- There are three R10s on the board (and on the schematic).The parts list (page 5) shows 2 different values for R10.The R10 close to capacitor C2 is 100K ohms and the two R10s close to tube sockets V3 and V4 are 300 ohms.
- Resistor designation R11 is used three times on the board as well.Twice near tube sockets V3 and V4 and again in the power supply section near C10 and C11
Until you determine through trial and error the values of the power supply dropping resistors R11 and R12 don’t solder them through the holes.Just solder them temporarily to the pads on bottom side of the board.
I ordered the 12 volt version and then “hacked” the board to convert it to headphone amp duty.The output caps needed for headphone use are too big and must be located off-board.The output caps supplied with the kit are too low in capacitance to be used with headphones but might be worthwhile for bypassing larger caps.A good starting point for the large output caps is 30 uF Daytons from Parts Express.The Daytons can sometimes be purchased through Broskie as well.In the space freed up by mounting the output caps off-board I drilled holes for the caps that bypass R11 the output tube’s cathode resistor.
To create a HPA output from a LSA output in the world of Aikido the following must be done.
- The resistor between the two halves of the output tube must be replaced with a jumper. (R8 on the Aikido 12Vac board)
- The circuit trace between the anode to the “top” output triode and the B+ line must be cut and a resistor inserted, see pictures.
- The cathode resistor (R11) on the “bottom” output triode needs to be bypassed with a 1000 uF, 6 volt electrolytic capacitor and a film capacitor between 0.1 and 1.0 uF.
- R13 on the board needs to be replaced with a jumper.
Have a look round the tubecad site and read some of the blog posts.You’ll probably come across the circuit I’m describing above.Broskie sometimes refers to this as the Aikido push-pull circuit.
About 30 uF of output capacitance is needed to drive 300 ohm ‘phones.For my tests I used a 20 uF Solen, a 10 uF Panasonic and a 0.1 uF Russian teflon connected up in parallel, the latter two after the picture was taken.Panasonic has some new large value 500 volt metalized polypropylene film caps, available from Mouser that I’d like to try sometime.
I used 300 ohm resistors for R2 and R4 on the board but next time I’d use something between 330 and 400 ohms to lower the current draw on the input tubes (V1 and V2) a little.
On the output tubes (V3 and V4) I used a 36 ohm resistor for R11 thought next time I’d use a 39 ohm to lower the current draw and keep it below 10 mA.
The added resistor between the anodes of the “top” output triodes (pin 1 on V3 and pin 6 on V4 and the B+ was 180 ohms, see pictures.
Power supply resistors R11 (filament supply) needs to be selected to yield between 23.5 to 24.5 volts across the series connected filaments (measure across C10).The value will vary depending on the power transformer used and your line voltage.Start with about a 30 ohm 3 watt resistor and reduce as necessary in about 2 ohm increments.
Power supply resistors R12 (B+ supply) needs to be selected to yield between 55 and 60 volts across C11.The value will vary depending on the power transformer used and your line voltage.Start with about a 2000 ohm 1 watt resistor and reduce it as necessary in about 100 ohm increments.You’ll probably need to buy a selection to set the correct voltage as the resistors Broskie supplies for R11 and R12 are for the 6DJ8 and 12AU7 line stage versions.
I used a 12 volt 2 amp toroidal transformer from Digikey and a 100K Alps blue velvet pot for the volume control.
Source was a Cambridge 840 CD player and headphones Sennheiser HD650s.
Listening Tests:
My sonic priorities are first and foremost balanced frequency response.A magic midrange is great but if the highs are rolled off, the bass is soft and lacks propulsion and percussion doesn’t have any snap and jump I can’t generate any interest in the music.That’s not to say I like harsh spitty treble and loosey-goosey slow boomy bass.Smooth frequency response, high resolution and good PRAT (pace, rhythm and timing) are what I value.
All of my tubes even those labeled GE, Motorola and Zenith are of western European manufacture.As far as I know no 6GM8s were made in the North America.The Russian 6N27P is an equivalent but now eBay crooks want as much for these as European tubes.
Sound comparisons were made directly with my “big” Aikido HPA.I’ll repeat the sound comments of my favorite set of tubes in that amp as the bench mark for evaluating the 12Vac.
“Big” Aikido LSA sound:
Mullard Blackburn 6DJ8 Ring Getter plus shield (NOS +100 hours)
Sound: WOW!Even after 1 hour of use these sound very promising, after 25 hours even better.At 100+ hours my favorite tube.This tube gives me the “lock” I’m looking for.Deep, extended, forceful, articulate bass.Lovely textural, articulate midrange that is especially nice with piano and female vocal.Detailed treble that might just sound a bit too “hot” on sub-optimal source material.Great PRAT.Macro dynamics that give great snap to percussion.A great boogie factor that had my wife bobbing her head and tapping her toe to the music 3 seconds after I handed her the headphones.These tubes also do subtle too; the micro dynamics show subtle shifts in the music exquisitely.These tubes in concert with (pun intended) the GE Hot Wire 6CG7s are the magic for me.
Aikido 12Vac listening tests:
I’ll start with my favorite tubes first and list their place of manufacture and build details not the name on the tube.
Valvo Hamburg, A-Frame with “pizza-pan” getter (NOS +50 hours)
Sound: WOW!A lot like my “big” Aikido with GE hotwires and Mullards.This really surprised me.The two amps are so much different but sound so similar.Different tubes, different power supplies different output caps and different volume pots.I had to tear my mind away from just enjoying the music to objectively listen for differences.Keep in mind that it was not a blind test but if it had been the results could have almost gone either way.Overall the 12Vac was slightly less resolving than the “big” amp though this might improve with more time on the amp.The bass of the 12Vac was a little less extended and not quite as forceful.Not surprising when you consider the over-built regulated B+ supply of the big amp but surprising how close to the big amp you do get with the simple supply in the 12Vac.The 12Vac was not quite as dynamic, both micro and macro, as the big amp.I could go on but it would be splitting hairs.Given the choice I’d rather listen to the “big” Aikido than the 12Vac but is was not a hardship listening to the “little” amp.That the 12Vac with these tubes comes so close it is amazing!
If you have a bunch of these tubes in your NOS stash it would be a shame not to put them to work.If you don’t fancy an HPA just forgo the mods I’ve suggested and build a line stage. The B+ voltage could be reduce to between 40 and 50 volts as not much current is needed for a line stage compared to an HPA. The values of R2 and R4 on V1 and V2 could be adjusted for 2.5 to 3.5 mA of current draw.R8 and R11 on V3 and V4 could be adjusted to draw 4 to 6 mA.
Valvo Hamburg, ring getter with shield (NOS +50 hours)
Sound: Sort of a lite version of the tubes above.A little less bass and a little less dynamic.Nice but once you’ve heard the best it seems a bit of an anti-climax.Might be the better choice for use with bassier ‘phones like the Sennheiser HD650s.These might make great line stage tubes.
Telefunken Germany (NOS +50 hours)
The typical Telefunken sound: nice midrange but a little weak in the bass and treble.These might suit some but don’t really cut it for me.These might make good line stage tubes as well depending on your sonic preferences and matching components.
Amperex Holland ring getter with shield (NOS +50 hours)
These just sound bland.Nothing really grabbed me about these tubes.Not very dynamic and the bass and treble seem a bit rolled off.These were the tubes I tried first and made me think the 12Vac might just not make a very good HPA.Tube rolling in my favorite Valvos sure changed my mind quickly.
So you ask, where does that leave me if I want an inexpensive Aikido HPA and don’t want to spend money and time chasing down rare 6GM8 tubes?Here’s my plan using inexpensive and more or less readily available tubes.
First we should note that as the filaments in the 12Vac are series connected they must all be rated at the same current draw but not necessarily the same voltage.You can’t use 6CG7’s and 6DJ8’s as they have different current draw.We might however use 12BH7s and 7DJ8s.I got excellent sound from Electro-Harmonix 6CG7s and Tesla ECC88s in my “big” Aikido HPA.I’d be willing to bet the cost of a pair of E-H 12BH7s that they might sound as good as their 6CG7 siblings.
The 12BH7 draws 0.3 amps (input tubes V1 & V2) and 7DJ8s (PCC88s) also draw 0.3 amps (output tubes V3 & V4).Adding up the series connected filaments (2 x 12.6 volts) + (2 x 7.5 volts) = 40.2 volts.This voltage requirement is too high for the version powered by a 12 volt transformer but might just work with the version using an 18 volt transformer.We’d have to use an 18 to 20 volt transformer with at least a 2 amp rating.A wire wound resistor for R11 would burn off the surplus filament voltage to drop the raw voltage down to the 38 to 40 volts we need.We should wind up with about 150 volts of raw DC but drop this to no less than about 130 volts at C11 by playing around with the value of R12.The advantages of this configuration are: 12BH7s are currently available new from Electro-Harmonix (lots of NOS are available too) and (though not as plentiful as 6DJ8s) NOS 7DJ8s are fairly easy to get and relatively cheap.Tubemonger.com sells Tesla PCC88s for about 19 dollars each.There are also NOS Tungsrams and Amperexs around as well.
The other option would be to use four 12DJ8 tubes.This a bit problematic as they are fairly rare and the only ones I’ve come across are the US made ECG/Philips/Sylvania.The construction appears the same as the ECG 6DJ8, 6922 and 7308 tubes.My sonic impressions of all the ECG built tubes were not positive.
One of the beauties of the Aikido circuit is that we are not stuck to only one tube.Using the 12BH7s as V1 and V2 I’d start out with about 1K ohm resistors for R2 and R4 but adjust as necessary for about 3 to 4 mA of current draw through the tube.For R11 (cathode of V3 & V4) I’d start off with 100 ohms but adjust as necessary for 10 mA of current draw.
Using the formula from one of the Aikido user guides we can calculate the anode resistor for V3 and V4. Ranode = (rp + 2Rload) /mu. rp for a PCC88 is about 2600 ohms and the mu is 33.Rload is the impedance of your ‘phones.Ranode =97 ohms for 300 ohm ‘phones.
When time permits I’m going to try it myself.I’ll post my results.
So have I finally gotten the Earmax out of my system?I think so.If I had another set of the metal work from my Earmax homage HPA build would I build one using 6GM8s?Tough call but as I don’t have another set of metal work is guess not. My Earmax homage is compact and light weight, not including the power supply, but it’s somewhat fragile so it wouldn’t make a good amp to be carting about.So no, I wouldn’t build another Earmax homage even one with 6GM8s and a much smaller power supply.One Earmax pretender kicking around the house is enough.
What I might do given the time is make a point to point wired version in a low profile case with tubes mounted horizontally inside. I think I could keep the size about the same as the power supply on my Earmax homage.A case about 2¼” x 5¾” x 11¾” including some jazzy looking black cherry side panels might do the trick nicely.If I do go ahead with this I’ll post pictures.
Cheers, Steve
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Hi Steve,
Great posts. I am a total newbie but took the plunge with some courage to build the 12vac. my goal was to simply build a linestage to add to my 2 channel setup. I went with the 18v version, not sure why but probably because of the availability of cheap 12au7 incase if i had to tube roll. probably not a very good decision in hind sight. Well its water under the bridge now.
Now the tube rolling bug has hit me and i wanted to get some inputs and was wondering if you can help me. I know the 12vac uses 4 tubes. whats the logical tube roll combination
a) all 4 replaced by another set
b) can we replace two (one in each channel) or
b) replace two (both in one channel)..probably this is not a valid option
Also, you mentioned to do some trial and error on the values of the power supply dropping resistors R11 and R12. I just took middle of the road values and went ahead. What should i be measuring to see if i am not hurting the tubes/ getting the max out of my preamp.
Thanks for all your help
Great posts. I am a total newbie but took the plunge with some courage to build the 12vac. my goal was to simply build a linestage to add to my 2 channel setup. I went with the 18v version, not sure why but probably because of the availability of cheap 12au7 incase if i had to tube roll. probably not a very good decision in hind sight. Well its water under the bridge now.
Now the tube rolling bug has hit me and i wanted to get some inputs and was wondering if you can help me. I know the 12vac uses 4 tubes. whats the logical tube roll combination
a) all 4 replaced by another set
b) can we replace two (one in each channel) or
b) replace two (both in one channel)..probably this is not a valid option
Also, you mentioned to do some trial and error on the values of the power supply dropping resistors R11 and R12. I just took middle of the road values and went ahead. What should i be measuring to see if i am not hurting the tubes/ getting the max out of my preamp.
Thanks for all your help
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