Hi Greg,
Not yet, but that’s coming up! I’m still working on my EL84 Baby Huey and look forward to be able to compare it to the 6V6 version. I will report my listening impressions here. (I’m also a bit distracted by the Alpha Nirvana amp at the moment.)
I actually plan to use 12xx tubes with the octal EL34 GB PCB. Of course the 12SL7 in a separate chassis mounted octal socket.
Ehmm! This happens to be a thread about the Baby Huey.
But, good to hear you like your Dynaco ST35 clone. I have a SCA35 that is rebuilt, close to ST35, and plan to compare it to the EL84 Baby Huey.
Baby Huey musings,
To reiterate, the BH scheme trades output tube gm for reduced rp which better drives the output trannie at both low and high frequencies. That and forced balance by the feedback scheme are its entire "secret" for success.
It will always work best with higher gm output tubes.
WTH is this gm thing. Well its the output tube characteristic of Anode Current (into the output tranny) swing vs grid voltage drive swing. Ideally it would be linear but in practice it is NOT. The BH scheme helps linearise it.
So a GU50 with a gm of 3 to 5.5 is problematical. Having said that I am currently running a 6SL7 + 6V6, gm = 4.5 (with 10K Raa Hamond 1609 outout trannies) on my sound system.
The problem with higher power versions is the requirement for higher output tube grid ac Voltage drive. That throws additional demands back on the 12AX7 diff amp front end.
The BH feedback scheme "anti bootstraps" the 12AX7 anode loads making them seem smaller in resistance than the actual resistor values. That means that the 12AX7 triodes are swinging more signal CURRENT to achieve the same output tube AC grid voltage drive.
This is not so important on the positive signal current swing but needs to be watched on the negative swing where low anode current leads to higher rp and lower gm in the tube. If you are swinging into this region then you are swinging into a higher distortion region.
Because it is asymmetrical then the more preferred even harmonic distortion results. this is limited by balance in the 12aX7 diff amp, and that is why the front end balance pot can be set by ear, although setting for equal 12AX7 anode voltages is a good method. WHY? because that means that you are setting the same gm at idle for the 2 sides of the 12AX7 and so AC balance is also maximised.
I have had a number of PMs and there has been some discussion in the various threads about scaling up to higher powers. I have no real reservations about Marc's EL34 iteration (with Prasi's PCBs) but must admit I have reservations about GU50 outputs, We are then in the world or needing more open loop gain and needing to add an additional stage.
Just off the top off my head I would be designing in an extra stage, probably a pentode diff amp (6CL6, 6BH7) or similar to take the shunt feedback and drive the RETAINED Mosfet Source Follower Output Tube drives with the 12AX7 (or other) diff amp kept as the input stage (with a current source tail to keep AC balance).
The EL84 + 12AX7 BH is the absolute best I have managed for this tube complement after much experimentation.
For info; the prototype stared life as a "Bevois Valley" amp which up until then was the best 12AX7 + EL84 Amp I had built (and I built quite a few).
The thing which all the folk who have listened to the BH say is "It is Fast" . Accurate phase info gives great stereo imaging and what the gurus call PRATT.
Pace Rhythm and Attack.
Enjoy your EL84 BH and be assured that the higher powered versions are no better unless you are using very inefficient speakers.
If you want to go higher power, then use Marc/Prasi's EL34 BH Boards and go KT88, KT77, 6CA7, EL34.
Lower gm tubes (6V6, 6L6, GM50 etc.) may work but be aware off the additional load you are putting on the 12AX7 diff amp, and the additional distortion that will result.
Cheers Ian
To reiterate, the BH scheme trades output tube gm for reduced rp which better drives the output trannie at both low and high frequencies. That and forced balance by the feedback scheme are its entire "secret" for success.
It will always work best with higher gm output tubes.
WTH is this gm thing. Well its the output tube characteristic of Anode Current (into the output tranny) swing vs grid voltage drive swing. Ideally it would be linear but in practice it is NOT. The BH scheme helps linearise it.
So a GU50 with a gm of 3 to 5.5 is problematical. Having said that I am currently running a 6SL7 + 6V6, gm = 4.5 (with 10K Raa Hamond 1609 outout trannies) on my sound system.
The problem with higher power versions is the requirement for higher output tube grid ac Voltage drive. That throws additional demands back on the 12AX7 diff amp front end.
The BH feedback scheme "anti bootstraps" the 12AX7 anode loads making them seem smaller in resistance than the actual resistor values. That means that the 12AX7 triodes are swinging more signal CURRENT to achieve the same output tube AC grid voltage drive.
This is not so important on the positive signal current swing but needs to be watched on the negative swing where low anode current leads to higher rp and lower gm in the tube. If you are swinging into this region then you are swinging into a higher distortion region.
Because it is asymmetrical then the more preferred even harmonic distortion results. this is limited by balance in the 12aX7 diff amp, and that is why the front end balance pot can be set by ear, although setting for equal 12AX7 anode voltages is a good method. WHY? because that means that you are setting the same gm at idle for the 2 sides of the 12AX7 and so AC balance is also maximised.
I have had a number of PMs and there has been some discussion in the various threads about scaling up to higher powers. I have no real reservations about Marc's EL34 iteration (with Prasi's PCBs) but must admit I have reservations about GU50 outputs, We are then in the world or needing more open loop gain and needing to add an additional stage.
Just off the top off my head I would be designing in an extra stage, probably a pentode diff amp (6CL6, 6BH7) or similar to take the shunt feedback and drive the RETAINED Mosfet Source Follower Output Tube drives with the 12AX7 (or other) diff amp kept as the input stage (with a current source tail to keep AC balance).
The EL84 + 12AX7 BH is the absolute best I have managed for this tube complement after much experimentation.
For info; the prototype stared life as a "Bevois Valley" amp which up until then was the best 12AX7 + EL84 Amp I had built (and I built quite a few).
The thing which all the folk who have listened to the BH say is "It is Fast" . Accurate phase info gives great stereo imaging and what the gurus call PRATT.
Pace Rhythm and Attack.
Enjoy your EL84 BH and be assured that the higher powered versions are no better unless you are using very inefficient speakers.
If you want to go higher power, then use Marc/Prasi's EL34 BH Boards and go KT88, KT77, 6CA7, EL34.
Lower gm tubes (6V6, 6L6, GM50 etc.) may work but be aware off the additional load you are putting on the 12AX7 diff amp, and the additional distortion that will result.
Cheers Ian
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Dear Ian,
I want to really thank your for all your work and for sharing all your results with us. For the time spent explaining to everyone the details, and how things can or cannot work, how can work better and how cannot.
The EL84 version is perfect for me, as my Klipsch RF82 have 97 dB @ 2.83 V / 1 m.
Just for the purpose of understanding more how to design a similar amp, can I sketch a preliminar idea of BH with GU50 and ask everyone's advice on how to improve it? Like a group design? Tell me if you prefer to have it in this thread, or to have it on another thread.
Thanks alot
Roberto
I want to really thank your for all your work and for sharing all your results with us. For the time spent explaining to everyone the details, and how things can or cannot work, how can work better and how cannot.
The EL84 version is perfect for me, as my Klipsch RF82 have 97 dB @ 2.83 V / 1 m.
Just for the purpose of understanding more how to design a similar amp, can I sketch a preliminar idea of BH with GU50 and ask everyone's advice on how to improve it? Like a group design? Tell me if you prefer to have it in this thread, or to have it on another thread.
Thanks alot
Roberto
6P15P/SV83 in BH
Gingertube,
Thanks for your “BH musings”. As alway, you provide tasty food for thought. And in this case support for my pondering of the SV83/6P15 as output tube for BH. Could SV83/12ax7 be the “new ultimate BH tube combination”? How about a PPP SV83 BH?
According to published specs the SV83 has both higher transconductance (S) and mu, compared to EL84. With similar pinout (see * below) it would drop into the EL84 BH and could be easily “rolled”. The fly in the ointment is the lower SV83’s lower screen voltage max, of 200 V.
Since SV83s are still cheap cheap, and I have many, I will just have to try it in the BH and see how long they last at “regular” EL84 screen voltage. I have seen anecdotal evidence from folks who wrote that they popped SV83s into their EL84 amplifiers who loved the sound and reported no burned-up screens. We shall see.
* (For experimentation I should caution the SV83 suppressor grid is connected to pins 1&6, rather than internally connected to the cathode as in EL84. So the caveat is: if you wire your sockets to have the suppressor grid on pins 1 or 6, double check any EL84 for internal connections to these pins before substituting it into the amp. Most times, substitutions will be fine (most EL84s work in sockets wired for SV83s), but there will always be an exception.)
Gingertube,
Thanks for your “BH musings”. As alway, you provide tasty food for thought. And in this case support for my pondering of the SV83/6P15 as output tube for BH. Could SV83/12ax7 be the “new ultimate BH tube combination”? How about a PPP SV83 BH?
According to published specs the SV83 has both higher transconductance (S) and mu, compared to EL84. With similar pinout (see * below) it would drop into the EL84 BH and could be easily “rolled”. The fly in the ointment is the lower SV83’s lower screen voltage max, of 200 V.
Since SV83s are still cheap cheap, and I have many, I will just have to try it in the BH and see how long they last at “regular” EL84 screen voltage. I have seen anecdotal evidence from folks who wrote that they popped SV83s into their EL84 amplifiers who loved the sound and reported no burned-up screens. We shall see.
* (For experimentation I should caution the SV83 suppressor grid is connected to pins 1&6, rather than internally connected to the cathode as in EL84. So the caveat is: if you wire your sockets to have the suppressor grid on pins 1 or 6, double check any EL84 for internal connections to these pins before substituting it into the amp. Most times, substitutions will be fine (most EL84s work in sockets wired for SV83s), but there will always be an exception.)
Dear Roberto,
Wavebourn, who is the resident expert on GU50, has experimented with and built good GU50 PP amplifiers, believes that you need 800vdc on the plates and 100w transformers of 8k+ of input impedance before the GU50 begins to sing.
(See post #3 in GU-50 PP amp project - Any favs? and also his development of his Pyramid amplifier here: www.wavebourn.com • View topic - Pyramid-VII: home stereo vacuum tube amp )
I am quite interested in building a good GU50 PP amplifier since I have 20 tubes in storage, but in my view this tube is a different animal than those living in the present Baby Huey patch. My suggestion would be to start your own thread for a new group design of a GU50 PP amplifier unrestrained by the 12ax7 BH topology. And, I admit I would personally like this thread to keep focus on low-power amps and EL84 type tubes as the thread title says: “EL84 Amp - Baby Huey”.
With Ian’s permission you could call your thread “Baby Huey’s Big Brother”
. In the BH EL34 thread Marc had posted his design for a PPP KT120 amp based on the BH design philosophy, and Ian expressed an interest in that, so perhaps that could also be further explored in a new Big Brother thread. (Incidentally, Edcor still sells the 100w output transformers Wavebourn “designed” for his Pyramid GU50 amplifier - IIRC he ended up using this transformer: EDCOR - CXPP100-10K)This is Ian’s thread, so let’s wait for his view.
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Ian, thank for sharing your musings on the BH.
In your experience, what is the primary impedance of the output transformers that worked well for this combo?
In your experience, what is the primary impedance of the output transformers that worked well for this combo?
As per your suggestion, I've opened this thread with a more standard design to be defined, then if possible be developed for GU50.With Ian’s permission you could call your thread “Baby Huey’s Big Brother”
Baby Huey enters into puberty: 6550 KT88 possibly GU50?
6P14P-EB
In the 6P14P-EB speec. , I find that G2, like Anode, is 250V.
Any recommendation (increase R14 and 15 maybe?), my power transformers are the max. 250V AC, along with the change in R13, any other suggestions?
Best regards
Fernan
In the 6P14P-EB speec. , I find that G2, like Anode, is 250V.
Any recommendation (increase R14 and 15 maybe?), my power transformers are the max. 250V AC, along with the change in R13, any other suggestions?
Best regards
Fernan
Regarding Raa, toroidy TTG-ECL86 PP 10K, have a specification of 25VA (equal to the missing alternative 8K PP Toroidy for EL84) there would be a small loss of power vs Toroidy TTG-EL84PP 50VA, (wiht both in my stock) recommend comparing them or directly go to the current 8K, leaving 10K to play with 6P1P-EB (I like the appearance of these 6P1P)
Fernan,
Sorry, but I’m not sure I understand the above correctly. Shorter sentences might be clearer. Are you just saying you plan to use Toroidy TTG-EL84PP for your BHEL84 and 6P1P-EB output tubes with your toroidy TTG-ECL86 transformers? I think that is an excellent plan!
Regarding G2 for 6P14P-EB, I have seen mostly 300 V listed as max for G2. However, people had been using them as 7189 substitutes in demanding amplifiers like Scott 222C without trouble. In Scott 222C the Plate voltage is 420V and G2 is 345V in pentode mode. I have also not heard of any BH EL84 builders that experienced trouble with 6P14P-EB tubes.
What is your B+ voltage after the large capacitor (C6)? If too high you might consider using a small value for C7 to tune the B+ if you have a choke. Try to simulate your power supply using Duncan Amp’s PSUD2 simulator. Anyway, also use 1 k for R14&15 as recommended by gingertube.
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Thanks Francois, sorry for my translations with google, they can really be very confusing.
It's true, I've always thought about them with 7189
the Post #2293 has confused me, a little.
The voltage after C6, I still don't know, I just received the PCBs today
I have thought about the possibility, of C7, to go down to 22uF, maybe like this, don't raise the voltage too much, am I correct?
YES! the 6P1P-EB tube is so ugly that I find it very beautiful !!! .........
My small apartment is not prepared for great powers, nor do I want problems with the neighbors
The BH EL34 is postponed for this reason, due to my stock / quality of tubes and OPTs, it is the maximum I will use.
All the best!
Fernan
Fernan,
I think post #2294 referred to 6P15P, not 6P14P-EB. I find the specifications for G2 of the 6P15P tube conflicting, but it appears the recommended operating G2 is 150V, while the maximum might be higher. See for example G2 max =330V (Not sure I can trust it) here: Tube Tester Files - 6П15П - 6Pi15Pi (SV83)
PSUD2 is your friend if you want to tune C7 for a reduced B+. Let us know how that goes.
6P1P-EB tube are beautiful, especially to my wallet
. I plan to build a BH using them too, with 10k output transformers.