I'd like to biuld an PP amp using EL84/6bq5 tube with 12ax7 driver. I'm a beginner, still not choose any schematic for my project. Can any one help me from beginning to build this amp? Please guide me from point to point, schematic->PSU->OPT, auto bias or fix, UL or no UL ...in short everything the best for EL84 PP. Now I have 2x12ax7, 8xel84, 1x 5u4gb, 2 5z3p. PSU: propose DIY, OPT: DIY. Thanks a lot mfs!!!
this is my proposal chassis
this is my proposal chassis
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Hi BigOx!
I think you can't go wrong with EL84 and ECC83/12AX7, at least experienced people from this forum are working with those tubes and topology - and lots of innovative ideas. Gingertube has recently published a complete schematic for a PP with EL84 and ECC83 (he uses E803CC, a similar tube).
http://www.diyaudio.com/forums/showthread.php?s=&threadid=72536&highlight=
SY, another great and active member, is preparing a paper in which he describes an EL84 amp - well, that is what I think
I am making a kind of 'universal' breadboard where I will be able to test and compare the different circuits posted here. When I am finished I will post some pictures!
Erik
I think you can't go wrong with EL84 and ECC83/12AX7, at least experienced people from this forum are working with those tubes and topology - and lots of innovative ideas. Gingertube has recently published a complete schematic for a PP with EL84 and ECC83 (he uses E803CC, a similar tube).
http://www.diyaudio.com/forums/showthread.php?s=&threadid=72536&highlight=
SY, another great and active member, is preparing a paper in which he describes an EL84 amp - well, that is what I think
I am making a kind of 'universal' breadboard where I will be able to test and compare the different circuits posted here. When I am finished I will post some pictures!
Erik
ErikdeBest said:SY, another great and active member, is preparing a paper in which he describes an EL84 amp - well, that is what I think
I have a bag of red LEDs i'm laying up for one of SY's "red light district" PP EL84.
Another one to check out is Eli Dutton's El Cheapo... all different tubes but a lot of general info applicable.
dave
planet10 said:
Dave & I have built a pair of EL84 P/P loosely based on Eli's above mentioned design.
Note the El Cheapo uses 12AT7 for the LTP, and with a hot enough input or high gain linestage, you might even get satisfactory results with 12AU7.
A lengthy thread on the evolution of the design, and current schematic:
http://www.decware.com/cgi-bin/yabb/YaBB.cgi?board=Tubes;action=display;num=1126228937;start=259#259
EC8010,
In a SE amp, I agree with your advice about the 'SN7. In a PP amp, I have reservations.
The 12AT7/ECC81 is not particularly linear, but works WELL combined with PP "finals". Look at the distortion spectrum of the entire amp, not any given stage.
Like the 'T7, the 12AU7/ECC82 is not highly linear. I'd like to see a HD spectrum plot of an amp using a 'U7 LTP splitter driving PP finals. IF the "waterfall" of 2nd>3rd>4th, etc. HD is present, as is the case with 'T7s, the amp will sound GOOD.
It's NICE when the whole exceeds the sum of its parts.
In a SE amp, I agree with your advice about the 'SN7. In a PP amp, I have reservations.
The 12AT7/ECC81 is not particularly linear, but works WELL combined with PP "finals". Look at the distortion spectrum of the entire amp, not any given stage.
Like the 'T7, the 12AU7/ECC82 is not highly linear. I'd like to see a HD spectrum plot of an amp using a 'U7 LTP splitter driving PP finals. IF the "waterfall" of 2nd>3rd>4th, etc. HD is present, as is the case with 'T7s, the amp will sound GOOD.
It's NICE when the whole exceeds the sum of its parts.
Yup, and if you don't wanna go octal try a 6CG7/6FQ7, same as a 6SN7 but same base as a 12AU7.
Eli, I believe arnoldc has built a PP EL34 w/ a 6SN7 going into a 12AU7 LTP. But doing a HD spectrum plot of it...
A 12AZ7 would be better than an 'AT7 due to the higher permissible (200V) cathode/heater voltage, and dirt cheap right now.
Cheers
Wayne
Ok, I'll bite!
-->arnoldc...
@BigOx
Have a look around at the unofficial Eico site:
Unofficial Eico Home Page
The HF12S uses most of your tubes but is in pentode mode. I'd prefer UL or triode. No problem in changing that! So have a look around there and here (use search function) and get back with us.
With a PP EL84 output a simple split load inverter will work well. If more gain is needed, a LTP or adding a gain stage up front could be used. You could use a 6U8/6F2 (pentode/triode) to drive the EL84's Dynaco style. Many possibilities depending on what your input sources are going to be now and in the future.
The Mullard 5-10 has a lot of gain which isn't needed now-a-days. You could strap the EF86 in triode mode or possibly dropping it all together.
Next comes the design of the power supply. Once you decide on your tube compliment a suitable PT can be chosen. You say you have one?
Do you know it's specs?
Number of primary and secondary windings, current and voltage of each winding, center taps etc. And whether or not it has a CT will make no difference in the sound if the PSU is designed properly. Arnold's idea of a hybrid rectummmfrier is a good one!
And is this your first time or have you done it before? As you may know we're playing with hundreds of volts... 
Cheers
Wayne
-->arnoldc...@BigOx
Have a look around at the unofficial Eico site:
Unofficial Eico Home Page
The HF12S uses most of your tubes but is in pentode mode. I'd prefer UL or triode. No problem in changing that! So have a look around there and here (use search function) and get back with us.
With a PP EL84 output a simple split load inverter will work well. If more gain is needed, a LTP or adding a gain stage up front could be used. You could use a 6U8/6F2 (pentode/triode) to drive the EL84's Dynaco style. Many possibilities depending on what your input sources are going to be now and in the future.
The Mullard 5-10 has a lot of gain which isn't needed now-a-days. You could strap the EF86 in triode mode or possibly dropping it all together.
Next comes the design of the power supply. Once you decide on your tube compliment a suitable PT can be chosen. You say you have one?
Do you know it's specs?
Number of primary and secondary windings, current and voltage of each winding, center taps etc. And whether or not it has a CT will make no difference in the sound if the PSU is designed properly. Arnold's idea of a hybrid rectummmfrier is a good one!
And is this your first time or have you done it before? As you may know we're playing with hundreds of volts...
Cheers
Wayne
HI
hi MFS,
I am too bust to check this subject for weeks. Sry All!!!
I propose to use a design, which used 4x12AX7 4xEL84 rec. by GZ34 (attached bellow)
My Power transfo is very old, I propose to wind it again, specs are prim. 220v sec. 300v-0-300v, 6.5vx2, 5v. My friends help me in counting specific data.
OPT is very old as well but still make sound, it will be replaced later
But the problem is the center tape
!!! my OPT didn't has CT to suitable the design I prefer!!!
Could any one give comments on this??? TY
hi MFS,
I am too bust to check this subject for weeks. Sry All!!!
I propose to use a design, which used 4x12AX7 4xEL84 rec. by GZ34 (attached bellow)
My Power transfo is very old, I propose to wind it again, specs are prim. 220v sec. 300v-0-300v, 6.5vx2, 5v. My friends help me in counting specific data.
OPT is very old as well
but still make sound, it will be replaced later But the problem is the center tape
!!! my OPT didn't has CT to suitable the design I prefer!!!Could any one give comments on this??? TY
Attachments
@BigOx
Your OPT can not be used for a Push-Pull amplifier. If it has no center tap it is most likely a SE (single ended) OPT. You will need to source a suitable output transformer. A push-pull UL (ultra linear) OPT with an 8k plate to plate primary. IMHO PP-UL would be the best way to go in this case.
See post #16 and click on this link:
www.hammondmfg.com/1608.htm
You will need two OPT's something like the Hammond's in the link above.
In the diagram you posted that circuit has quite a bit of gain, too much by today's standard. Also the global (NFB) feedback has been 'tuned' for that particular OPT. So unless you can find the original OPT I would not 'copy' the schematic exactly but use it only as a guide. One way to get the gain 'down' would be to use lower mu tubes i.e. 6FQ7/6CG7 which would be in keeping everything 9-pin.
Did you check out the
Unoffical Eico Home Page
for some ideas??
Also do a search(s) on this forum for EL84, 6BQ5, EL34, Dynaco, ST35, Stereo 35, push pull, etc. You will find quite a bit of information here on this very same subject. It seems you may have to go to 'school' on the subject/project you have in mind. Please don't take offense as we all here have been to school at one time or another! I'm still learning myself after all these years! The same could be said for most of us here... 
If we know a little more about you and your skills; which will in turn then help us point/guide you in the right direction!
Cheers
Wayne
Your OPT can not be used for a Push-Pull amplifier. If it has no center tap it is most likely a SE (single ended) OPT. You will need to source a suitable output transformer. A push-pull UL (ultra linear) OPT with an 8k plate to plate primary. IMHO PP-UL would be the best way to go in this case.
See post #16 and click on this link:
www.hammondmfg.com/1608.htm
You will need two OPT's something like the Hammond's in the link above.
In the diagram you posted that circuit has quite a bit of gain, too much by today's standard. Also the global (NFB) feedback has been 'tuned' for that particular OPT. So unless you can find the original OPT I would not 'copy' the schematic exactly but use it only as a guide. One way to get the gain 'down' would be to use lower mu tubes i.e. 6FQ7/6CG7 which would be in keeping everything 9-pin.
Did you check out the
Unoffical Eico Home Page
for some ideas??
Also do a search(s) on this forum for EL84, 6BQ5, EL34, Dynaco, ST35, Stereo 35, push pull, etc. You will find quite a bit of information here on this very same subject. It seems you may have to go to 'school' on the subject/project you have in mind. Please don't take offense as we all here have been to school at one time or another!
I'm still learning myself after all these years! The same could be said for most of us here... If we know a little more about you and your skills; which will in turn then help us point/guide you in the right direction!
Cheers
Wayne
I was also just wondering about that OPT, whether it is the time of night or the temperature here 2 degrees outside).
But my contribution would refer to the use of a common cathode resistor for all 4 EL84s in the given circuit. This was a (to my mind stupid) saving move in certain Dynaco's. I would really not couple the 2 stereo channels that way for the saving of one resistor. The bypass capacitor is not enough to ensure decoupling, albeit that there is not stereo info in low frequencies. This is the opposite than to use a separate resistor for each EL84, which can sometimes cause time constant problems.
So BigOx, if you plan to go this way, do use a separate cathode resistor for each channel, twice the value now given (I did not jot the value down).
And may I also add as someone who has been round the block a few times, to echo Wayne, keep at it, ask questions - there are not stupid questions, only stupid answers. You would really not like to know how many circuits I built in the beginning before the first one worked! Yes, we are all still learning. (If that should stop for anybody, it will not be because he finally knows everything. It will only be because he is stupid.)
Regards.
But my contribution would refer to the use of a common cathode resistor for all 4 EL84s in the given circuit. This was a (to my mind stupid) saving move in certain Dynaco's. I would really not couple the 2 stereo channels that way for the saving of one resistor. The bypass capacitor is not enough to ensure decoupling, albeit that there is not stereo info in low frequencies. This is the opposite than to use a separate resistor for each EL84, which can sometimes cause time constant problems.
So BigOx, if you plan to go this way, do use a separate cathode resistor for each channel, twice the value now given (I did not jot the value down).
And may I also add as someone who has been round the block a few times, to echo Wayne, keep at it, ask questions - there are not stupid questions, only stupid answers. You would really not like to know how many circuits I built in the beginning before the first one worked! Yes, we are all still learning. (If that should stop for anybody, it will not be because he finally knows everything. It will only be because he is stupid.)
Regards.
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