This design is a result of a too high B+. In my case I had to deal with ~380VDC together with a pair of EL84s and a ECC83 for driver. The question was where to burn away the not needed voltage. Here, the cathode resistors (R6/R7) did the trick. The voltages over these are ca 100V, which makes it possible to do the driver stage direct coupled. The feedback is partial, ~10dB from plate back to grid at the output pentodes and ~10dB loop feedback.
Not that much more to tell -- it's just another small amp.
Mini PP schematics:
On the work bench:
Not that much more to tell -- it's just another small amp.
Mini PP schematics:
An externally hosted image should be here but it was not working when we last tested it.
On the work bench:
An externally hosted image should be here but it was not working when we last tested it.
ttan98 said:
Partsconnexion: http://www.partsconnexion.com/catalog/TerminalStrips.html
An externally hosted image should be here but it was not working when we last tested it.
Jane,
BTW what input tube would you use and what current will you run it at, just wondering..
Also you may need a balancing pot at the cathode of the input tube to balance the anode so that the EL84 can be correctly bias. I like the Jfet Yves uses for his amp. at the cathode of the input tube to determine the cathode current... Just a thought...
BTW what input tube would you use and what current will you run it at, just wondering..
Also you may need a balancing pot at the cathode of the input tube to balance the anode so that the EL84 can be correctly bias. I like the Jfet Yves uses for his amp. at the cathode of the input tube to determine the cathode current... Just a thought...
ttan98 said:
I have some ECC83s that I might use...
Since Vak is approx 100V, I suppose I have to "starve" the input tubes a little bit.
Ia about ~0.5mA seems to fit in this case.
An externally hosted image should be here but it was not working when we last tested it.
The EL84s will have quite large cathode resistors (3-4k), so a few volts out of balance will not alter the anode current too much. That is the reason why I don’t use a shared cathode resistor.
Yes, but ... no !
The apparent value of R8 (or R9) is much more lower because it receives an antiphase signal from the 6BQ5 anode.
This is good for feedback but drastically lowers the plate load of that 12AX7.
Clearly described here:
http://www.tubecad.com/march2001/2001_03.pdf
I used another approach:
http://www.dissident-audio.com/PP_ECL86/Page.html
wich certainly could be adapted to you DC coupled design.
Yves.
The apparent value of R8 (or R9) is much more lower because it receives an antiphase signal from the 6BQ5 anode.
This is good for feedback but drastically lowers the plate load of that 12AX7.
Clearly described here:
http://www.tubecad.com/march2001/2001_03.pdf
I used another approach:
http://www.dissident-audio.com/PP_ECL86/Page.html
wich certainly could be adapted to you DC coupled design.
Yves.
Yes, that’s true. The 560k load line is a static (DC) load line. The 560k tells me the target value of Rfb (R8/R9) and Ra (R4/R5) in parallel, e.g. 1.2M in parallel with 1M.
Your approach to the problem was a nice one.
How much partial feedback did you apply in the end? My target is somewhere about 6-10dB.
Your approach to the problem was a nice one.
How much partial feedback did you apply in the end? My target is somewhere about 6-10dB.
I suspect you don't really need those diode clamps, the voltage at the cathodes will just rise gradually as the tube warms up. I use EL84 in series pass regulators and the grid and plate potential are equal at power on with SS rectifiers and I have never had a problem with this in nearly 20 yrs.
Now, the circuits are recalculated and the driver/output boards are ready.
As a start I have wired one board as shunt feedback only, and the other with global loop feedback to examine the impact on Zo and distortion pattern for each circuit. In the end I will combine shunt and global feedback, and hopefully these measurements will help me to estimate the amount of global feedback needed in this set up.
Driver/output boards ready, one wired as shunt feedback and the other as loop feedback
Shunt feedback, the way it’s done here, will transform the output stage more or less into a transresistance amplifier. That implies that both output and input resistance (in theory) are approaching zero ohms. This is a good thing for driving the OPT and speaker, but low input resistance can be a challenge for the input/driver stage. A good driver in this case might be a pure transconductance amplifier. Not all tubes are linear with a vertical load line, but E80CC looks like a good candidate with a working point at Va=100V, Iq=~3.5-4mA. (A pentode will probably do better, but in this case I have to stick to dual triodes due to one noval socket only)
E80CC characteristics
The gain without global feedback will be A=Gm’ x Rm’, where Gm’ is the transconductance of the input stage and Rm’ is the transresistance of the output stage. With E80CC as driver (Gm’=~2.5mA/V) the Rm’ of the output stage, including the OPT, should be something like 4V/mA (4k ohm). That will give a total amplification for the circuit A = 2.5mA/V x 4V/mA = 10x, or 1V in for 10V out (12.5W into 8 ohms). The global feedback network will of course affect the gain.
The goal in this project is to achieve low Zo and low distortion (and good sound) with the use of pentodes (EL84) and minimum of global feedback. The practical amount of global NFB that I have to apply to achieve this goal remains to see...
As a start I have wired one board as shunt feedback only, and the other with global loop feedback to examine the impact on Zo and distortion pattern for each circuit. In the end I will combine shunt and global feedback, and hopefully these measurements will help me to estimate the amount of global feedback needed in this set up.
Driver/output boards ready, one wired as shunt feedback and the other as loop feedback
An externally hosted image should be here but it was not working when we last tested it.
Shunt feedback, the way it’s done here, will transform the output stage more or less into a transresistance amplifier. That implies that both output and input resistance (in theory) are approaching zero ohms. This is a good thing for driving the OPT and speaker, but low input resistance can be a challenge for the input/driver stage. A good driver in this case might be a pure transconductance amplifier. Not all tubes are linear with a vertical load line, but E80CC looks like a good candidate with a working point at Va=100V, Iq=~3.5-4mA. (A pentode will probably do better, but in this case I have to stick to dual triodes due to one noval socket only)
E80CC characteristics
An externally hosted image should be here but it was not working when we last tested it.
The gain without global feedback will be A=Gm’ x Rm’, where Gm’ is the transconductance of the input stage and Rm’ is the transresistance of the output stage. With E80CC as driver (Gm’=~2.5mA/V) the Rm’ of the output stage, including the OPT, should be something like 4V/mA (4k ohm). That will give a total amplification for the circuit A = 2.5mA/V x 4V/mA = 10x, or 1V in for 10V out (12.5W into 8 ohms). The global feedback network will of course affect the gain.
The goal in this project is to achieve low Zo and low distortion (and good sound) with the use of pentodes (EL84) and minimum of global feedback. The practical amount of global NFB that I have to apply to achieve this goal remains to see...
Looks much like this one
I once read here at the forum that Kuei Yang Wang designed a push pull DRD using the primary winding of an EL84 output tranny to load the driver stage. I think this could form a nice experiment for this design...maybe someday. Unfortunately I have never seen the schematic so I can be completely wrong (I just looked at Kuei Yang Wang profile,thinking about sending him a mail asking for the schematic - but he is away from Diyaudio because he is to busy - don't know if I can mail him)
I once read here at the forum that Kuei Yang Wang designed a push pull DRD using the primary winding of an EL84 output tranny to load the driver stage. I think this could form a nice experiment for this design...maybe someday. Unfortunately I have never seen the schematic so I can be completely wrong (I just looked at Kuei Yang Wang profile,thinking about sending him a mail asking for the schematic - but he is away from Diyaudio because he is to busy - don't know if I can mail him)
ErikdeBest said:
The similarities are that both are DC-couplet and both uses the EL84’s cathode resistors to burn away some voltage.
The difference is the way that feedback is applied. In my case shunt feedback will turn the output stage (more or less) into a transresistance stage that is driven by a transconductance driver.
The Heuvelman amplifier is conventional, voltage driven and with global loop feedback. I am trying to avoid loop feedback the best that I can.
Hi Jane
I confess I am a noob I have read some of Morgan Jones stuff on feedback, but the terms you are using here are new to me.
I gave another look at both schematics and the only difference I see is that you added R8 and R9 (resistors from plate to grid). Are those providing the feedback you mention? And what is the resistor that goes from the output to the grid of the second triode (present in both schematics) doing? I thought it was global feedback...gingertube also uses this setup in his baby huey amplifier.
You just made me curious!!
Erik
I confess I am a noob
I have read some of Morgan Jones stuff on feedback, but the terms you are using here are new to me. I gave another look at both schematics and the only difference I see is that you added R8 and R9 (resistors from plate to grid). Are those providing the feedback you mention? And what is the resistor that goes from the output to the grid of the second triode (present in both schematics) doing? I thought it was global feedback...gingertube also uses this setup in his baby huey amplifier.
You just made me curious!!
Erik
In theory a transresistance amplifier is driven by current (Ii) and output is voltage (Vo), the transresistance gain Rm=Vo/Ii. You can look at it as a current to voltage converter. Some of the theory is covered here: http://users.ece.gatech.edu/~pallen/Academic/ECE_6412/Spring_2003/L260-ShuntShuntFb-2UP.pdf
A perfect transresistance amplifier has Rin and Rout = 0 Ohm, which is hard to achieve in a stage of one tube only, the open loop gain is not high enough to make it perfect. But the input and output resistance will be approaching low resistance, which is a good thing in terms of driving a loudspeaker. R8/R9 is the trick that converts the output stage into a transresistace stage. This will lower Zo, but due to the limitations and simplicity of the circuit I suppose I will need some loop feedback (via Rfb) to bring Zo further down.
I will get the answers when I turn the power on and start measuring and listening.
A perfect transresistance amplifier has Rin and Rout = 0 Ohm, which is hard to achieve in a stage of one tube only, the open loop gain is not high enough to make it perfect. But the input and output resistance will be approaching low resistance, which is a good thing in terms of driving a loudspeaker. R8/R9 is the trick that converts the output stage into a transresistace stage. This will lower Zo, but due to the limitations and simplicity of the circuit I suppose I will need some loop feedback (via Rfb) to bring Zo further down.
I will get the answers when I turn the power on and start measuring and listening.
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