Hello guys. I had the opportunity of listen to one of these amps the other day and loved the sound. I want to build one and have the output and power transformers as well as some new EL84 that I can use.
I assume everybody is familiar with the schematic but I include it here for quick reference.
My question is: I want to use an ECC83 in lieu of the 7247 because I have several Ecc83 and 7247 is hard to come by and expensive in my neck of the woods.
I am aware of the second triode of the 7247 having a gain of 20 as opposed to the 12AX7 triode which would be 100.. It is safe for me to assume that I can adapt R6 and R7 to lower the gain? What other changes will you recommend?
Thank you in advance
Mario
I assume everybody is familiar with the schematic but I include it here for quick reference.
My question is: I want to use an ECC83 in lieu of the 7247 because I have several Ecc83 and 7247 is hard to come by and expensive in my neck of the woods.
I am aware of the second triode of the 7247 having a gain of 20 as opposed to the 12AX7 triode which would be 100.. It is safe for me to assume that I can adapt R6 and R7 to lower the gain? What other changes will you recommend?
Thank you in advance
Mario
Attachments
Agree with audiowize and gingertube, or if you have any 6GH8A tubes, you can always try Dave Gillespie's design, see page 6 in this link:
http://tronola.com/PC-10A_Assembly.pdf
Best,
Anand.
http://tronola.com/PC-10A_Assembly.pdf
Best,
Anand.
There is no way you can replace the 7247 with ECC83.
But, 7247 is a selected variant of 12dw7, and those are in no way scarce. In
addition there is also new production : ECC832 from JJ.
As you will spend quite some money on other parts , don't sneak out on one of the cheapest part of the amp !
Last edited:
Shielded cables can work for some things.
But they can also work as low pass filters for other things.
I see 1 Meg ohm pots driving shielded cables, OK for Guitar amps, maybe.
Not good for Hi Fi.
The JJ ECC832.
JJ also makes a different pinout version, the triodes are swapped.
I recommend going to Eurotubes.com.
They re-test the JJ tubes that they get from Slovakia.
Eurotubes tests for current, noise, balance when applicable, etc.
But they can also work as low pass filters for other things.
I see 1 Meg ohm pots driving shielded cables, OK for Guitar amps, maybe.
Not good for Hi Fi.
The JJ ECC832.
JJ also makes a different pinout version, the triodes are swapped.
I recommend going to Eurotubes.com.
They re-test the JJ tubes that they get from Slovakia.
Eurotubes tests for current, noise, balance when applicable, etc.
Last edited:
There is a schematic out there for the diytube PCB , no longer produced that replaced the 7247 with a 12AX7 and 12AU7 (can sub in 12BH7 for improved sonics). and you could add Dave Gillespie's EFB mod to it for all it's great advantages. Triode Electronics sold the PCB. I will try and find the schematic later.
Another way to go is with a 12AT7 . The Tubecad or tubelab.com schematics would work there to make it work.
Another way to go is with a 12AT7 . The Tubecad or tubelab.com schematics would work there to make it work.
Last edited:
What make and model are your output transformers?
Are they the re-make of the original ones.
Those are available, perhaps slightly spendy.
But they are well worth it.
Otherwise, you may have to adjust the negative feedback network, to keep the amplifier from oscillating.
C6, C7, and the resistor that is across C7 too.
Even if the amplifier does not oscillate, a different output transformer can affect the sound and performance.
And do not use another tube.
You like the sound of the one you heard, so use a 12DW7 or the JJ ECC823.
I believe Eurotubes.com does a noise test on the ECC823 (ask them, they will get back to you).
Changing to other tubes, or changing to a different output transformer, may change the sound character.
(even though you have a fair amount of global negative feedback).
6BQ5 Beam Power tube, or EL84 Pentode are both OK.
JJ makes a quality EL84 (Eurotubes re-tests the JJs too).
Are they the re-make of the original ones.
Those are available, perhaps slightly spendy.
But they are well worth it.
Otherwise, you may have to adjust the negative feedback network, to keep the amplifier from oscillating.
C6, C7, and the resistor that is across C7 too.
Even if the amplifier does not oscillate, a different output transformer can affect the sound and performance.
And do not use another tube.
You like the sound of the one you heard, so use a 12DW7 or the JJ ECC823.
I believe Eurotubes.com does a noise test on the ECC823 (ask them, they will get back to you).
Changing to other tubes, or changing to a different output transformer, may change the sound character.
(even though you have a fair amount of global negative feedback).
6BQ5 Beam Power tube, or EL84 Pentode are both OK.
JJ makes a quality EL84 (Eurotubes re-tests the JJs too).
Last edited:
Thank you very much for the advise. The transformers are custom made by hand by a renowned maker here in Argentina: index en actualizacion
he made them to spec for the EL84 and swears we will be happy with them
Gracias!
mario2,
Those look like very well engineered and well built transformers. Quality.
My Spanish is terrible, but I read the specs.
If I had the time and money for EL84 amplifiers, I would be more than willing to try both the Single Ended, and Push Pull models of those transformers.
But I have moved on to 7591, KT77, KT66, and 6BX7 low power amplifiers (I do not use those tubes anywhere near to the power levels they can produce, I run them much cooler).
My very first amplifier was a Knight kit push pull EL84 in Pentode Mode. I built one in about 1959 for my brother, then built one for me in about 1962, then we had stereo on Roberts Headphones, and a pair of Electro Voice loudspeakers. Just a simple Gerrard changer, and a Pickering phono cartridge. Those were fun listening days.
Perhaps someday I will get the time to do a low power EL84 push pull amp too.
The push pull models from Argentina are 43% UL tap.
I have heard that somewhere near 25-30% UL taps are also good for EL84 tubes too.
If I ever build a push pull EL84 amplifier again, I would probably purchase 2 sets of output transformers (different UL tap %) and check out the difference in performance and sound.
I am pretty sure that with what I now know, the new EL84 amplifiers would far surpass those old Knight kits.
Happy building and happy listening!
Those look like very well engineered and well built transformers. Quality.
My Spanish is terrible, but I read the specs.
If I had the time and money for EL84 amplifiers, I would be more than willing to try both the Single Ended, and Push Pull models of those transformers.
But I have moved on to 7591, KT77, KT66, and 6BX7 low power amplifiers (I do not use those tubes anywhere near to the power levels they can produce, I run them much cooler).
My very first amplifier was a Knight kit push pull EL84 in Pentode Mode. I built one in about 1959 for my brother, then built one for me in about 1962, then we had stereo on Roberts Headphones, and a pair of Electro Voice loudspeakers. Just a simple Gerrard changer, and a Pickering phono cartridge. Those were fun listening days.
Perhaps someday I will get the time to do a low power EL84 push pull amp too.
The push pull models from Argentina are 43% UL tap.
I have heard that somewhere near 25-30% UL taps are also good for EL84 tubes too.
If I ever build a push pull EL84 amplifier again, I would probably purchase 2 sets of output transformers (different UL tap %) and check out the difference in performance and sound.
I am pretty sure that with what I now know, the new EL84 amplifiers would far surpass those old Knight kits.
Happy building and happy listening!
Last edited:
Thank you 6A3summer, here are some pics of the second tube amp I built, it is stereo with the ECL82 tube and uses a couple of Hammond Organ transformers with hum taps that I bought from the late Gary Schneider.
This is a fantastic sounding amp. I used the typical schmatic for record players and it sounds great. This amp was a birthday present for my nephew who is also an electronics enthusiast.
Now onto the PP EL84!!
Mario
This is a fantastic sounding amp. I used the typical schmatic for record players and it sounds great. This amp was a birthday present for my nephew who is also an electronics enthusiast.
Now onto the PP EL84!!
Mario
Question on the B+
Quick question on B+
I see this circuit runs with +370V on the plates, 375 on the screens due to the UL taps. Isn't this too much for the Electroharmonix and JJ EL84? Manufacturer of course mentions 300 as limiting values but I am never sure whether this is for designers or if actually affects tube life.
As you can see on the circuit, Vk is noted to be 13.5V which for a 95 ohm resistor comes out at 142 mA total for both channels (all 4 tubes), so if I do 35.5 mA for the plate voltage which is 370 volts, I end up with a 13.1 Watts for the plate dissipation, or slightly above the maximum dissipation of 12W... I can't make sense of this..
I have no reference on this circuit on tube life, and since I have a 180V tap on the power transformer, I can also try to lower B+ to under 300 (changing some components) if worth the effort? This amplifier I am building it for my brother and will be used for weekend audiophile listening.
Any feeback is appreciated. Thanks in advance
Mario
Quick question on B+
I see this circuit runs with +370V on the plates, 375 on the screens due to the UL taps. Isn't this too much for the Electroharmonix and JJ EL84? Manufacturer of course mentions 300 as limiting values but I am never sure whether this is for designers or if actually affects tube life.
As you can see on the circuit, Vk is noted to be 13.5V which for a 95 ohm resistor comes out at 142 mA total for both channels (all 4 tubes), so if I do 35.5 mA for the plate voltage which is 370 volts, I end up with a 13.1 Watts for the plate dissipation, or slightly above the maximum dissipation of 12W... I can't make sense of this..
I have no reference on this circuit on tube life, and since I have a 180V tap on the power transformer, I can also try to lower B+ to under 300 (changing some components) if worth the effort? This amplifier I am building it for my brother and will be used for weekend audiophile listening.
Any feeback is appreciated. Thanks in advance
Mario
Yes, you can put a 12AX7 in that hole. No changes. The driver stage runs unity gain. The 12AX7 is hot enough to pull two 30k loads series.
Yes, you can run EL84/6BQ5 considerably past the 330V and 12W in the specs, unless they are very expensive or hard to replace (dog-sled to the north pole radar station). Who wants to live forever?
Last edited:
That's what I suspected since this circuit has been around for such a long time. I was concerned that newer tubes are not so resilient, thanks for the good info.
Oh yes, right! Thanks, doing the calculation with 356V and 30mA I have 11 Watts and less than 1W for the g2
Gracias!
Mario
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.