I have experimented with the ECC83 as an amplifier and I have found that there is a harmonic distortion minimum when the ratio ov the DC voltage across the tube is about 1.2 to 1.5 times the voltage across the anode load resistor. You can achieve that by changing the value of the 1k8 resistor in the cathode of the triode connected EF86 to something like 2k2 or 2k7.
I have not tried that, so this is nothing more than an educated guess.
Hans J Weedon.
I have not tried that, so this is nothing more than an educated guess.
Hans J Weedon.
On the pole zero of the LTP, I was mistaken, the 1 Meg 100nF RC does indeed represent a pole inside the feedback loop. That may be one reason that in a typical setup there is a subharmonic peak in the frequency domain for small signal tests. I sort of wondered why that was, and now I know why. Thank you so much for correcting my understanding of the circuit.
Hans J Weedon.
Hans J Weedon.
That's a good idea or just build the Mullard 5-20 as is. No mucking about...
Hi I have studied the various versions of the ECC88 dual triode.
The original version of the tube was intended for use as a low noise cascade preamp at RF, and they did not pay much attention to the internal mechanical construction. At RF frequencies nobody cares if the elements vibrate a little bit. Phillips/Mullard discovered that some people tried to use ECC88 in Audio circuits, but complained about microphonics and filament hum injection. The military wanted to use this dual triode too, and defined a dual triode with ECC88 characteristics, but militarized as far as acceleration and shock were concerned. They defined a new high reliability version of the tube with an average life greater than 10,000 hours operating life.
This became the E188CC version (and other 4 digit numerical names) of the design. In this version the elements were held solidly in place to survive the mil-spec. A new cathode with reduced interface problems with the tube off most of the time. They also “spiraled” the filament to improve lifetime. Fortunately the mechanically more solidly assembled electrodes reduced the microphonic effects enough to now OK the E188CC as a low level microphone preamp as well. The twisting of the heater construction reduced the hum pickup too. If you are building an Audio Amplifier specifying the ECC88, try switching to the E188CC as a substitute. You will like it if you do.
Hans J Weedon.
The original version of the tube was intended for use as a low noise cascade preamp at RF, and they did not pay much attention to the internal mechanical construction. At RF frequencies nobody cares if the elements vibrate a little bit. Phillips/Mullard discovered that some people tried to use ECC88 in Audio circuits, but complained about microphonics and filament hum injection. The military wanted to use this dual triode too, and defined a dual triode with ECC88 characteristics, but militarized as far as acceleration and shock were concerned. They defined a new high reliability version of the tube with an average life greater than 10,000 hours operating life.
This became the E188CC version (and other 4 digit numerical names) of the design. In this version the elements were held solidly in place to survive the mil-spec. A new cathode with reduced interface problems with the tube off most of the time. They also “spiraled” the filament to improve lifetime. Fortunately the mechanically more solidly assembled electrodes reduced the microphonic effects enough to now OK the E188CC as a low level microphone preamp as well. The twisting of the heater construction reduced the hum pickup too. If you are building an Audio Amplifier specifying the ECC88, try switching to the E188CC as a substitute. You will like it if you do.
Hans J Weedon.
Varying the first stage anode voltage can unbalance the LTP. Let me explain it again - I seem to have to do this every few months in different threads.Kay Pirinha said:
The cause is grid current. If the first stage anode voltage is high then the LTP draws more current through the tail resistor. This also means a lower anode voltage on the LTP. Lower anode voltage and higher anode current means smaller negative grid-cathode voltage. The ECC83/12AX7 has high mu so there is only a small window of bias between cutoff (too negative) and grid current (insufficiently negative, typically from around -1V and up). So high current in the LTP means both triodes start to draw a little grid current. This doesn't affect the first triode much as that can draw on the first stage anode. The second triode gets its bias via a 1M resistor, so the grid current drops some voltage and the result is imbalance.
Glad to be able to help. This issue is missed by most people. I noticed that the Mullard amplifier book itself shows a small LF peak and started wondering why.HJWeedon said:
I have the intention to build a 5-10 with the EF86 triode strapped as done and described by Claus Byrith. I just found this topic and I intend to have a look at the Elektor circuit posted here (thanks).
All the information you need to recalculate the feedback loop can be found under the next URL:
Designing for Global Negative Feedback
You can put it in an excel and play around.
All the information you need to recalculate the feedback loop can be found under the next URL:
Designing for Global Negative Feedback
You can put it in an excel and play around.
Hi
I know this is a an old thread but I just have one question concerning modification of the EF86 to triode mode in the Mullard 5-10.
Is it necessary to change the value of the cathode bypass capacitor? I note that Elektor reduce it from 50mfd to 10mfd
Thanks
Malcolm
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