Hi there,i'm having some issues with my ecc82 preamp i want to build.So,the voltage suply is 150v and what anode resistor and cathode resistor values should i use for an optimal operating point at maximum amplification factor?.What is the optimal cathode voltage for this tube at this suply voltage?..thank you!
Here's a data sheet with a chart of resistances. Looks like x14 is about what you can expect.
https://frank.pocnet.net/sheets/183/e/ECC82.pdf
https://frank.pocnet.net/sheets/183/e/ECC82.pdf
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It will run with those but with a higher impedance than my 1k and 47k.
According to LTSPICE cathode voltage is always around 0v9
According to LTSPICE cathode voltage is always around 0v9
Both(47k and1k2) and (220k and 3k9) results in 2v cathode bias for ecc82 .I checked that out in practice and the proteus simulation says the same thing🤷*♂️
In my pre amp its around 1 volt so the LTPSICE 0v9 isnt far out.
We are talking about 12au7 ?
Or have I got mixed up with 12ax7 ?
We are talking about 12au7 ?
Or have I got mixed up with 12ax7 ?
This means a plate current of about 0.5 mA and a plate voltage of about 40 V. Seems a bit at the lowish side.
Increasing the plate load resistor of a low to medium µ triode won't result in much more gain. Hence, 47 k appears to be sensible enough.
Best regards!
A ECC82/12AU7 is a high current low gain valve - up to 10mA, grid lines up to -20v, an ECC83/12AX7 is a low current high gain valve, up to 1mA,grid lines up to -4v.
Going for center biased with a Va of 150v puts you at -4v, -3v would be better perhaps. With a 220k Ra your running at under a milliamp, a rough estimate suggests 400uA,so 3/0.0004 =7k5 ; the less current we have the higher our Rk needs to be. These are rough estimates.
To get the most gain out of your valve you could try an active anode load - MJE350, 6v2 zener etc. That said a higher HT would be better and a smaller Ra,or just use an ECC83. In a linestage preamp though we don't need high gain but the opposite as only need a gain of about 4 if that.
Andy.
Going for center biased with a Va of 150v puts you at -4v, -3v would be better perhaps. With a 220k Ra your running at under a milliamp, a rough estimate suggests 400uA,so 3/0.0004 =7k5 ; the less current we have the higher our Rk needs to be. These are rough estimates.
To get the most gain out of your valve you could try an active anode load - MJE350, 6v2 zener etc. That said a higher HT would be better and a smaller Ra,or just use an ECC83. In a linestage preamp though we don't need high gain but the opposite as only need a gain of about 4 if that.
Andy.
Correct. Even a high impedance cascoded CCS as the plate load won't yield to more gain than the datasheet µ of any triode, though.
Best regards!
Best regards!
Right, the classic being an ECC88 cascode and with optimum conditions you'll get a gain of 33 for the 88 and 20 for the 82. A ECC83 has a gain of 100 and an ECC81 a gain of 70;a double triode for every application.
I'm not looking specialy on getting the most gain out of it,I'm looking to a most less distorted type who can handle maximum input signal voltage on a 150v suply..So the anode and cathode resistors must be for optimal performance at highest input level posible
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As a rule the bigger your anode resistor the less distortion. A cascode CCS or CCS loaded stage will give the least THD.
It also depends on what level of distortion you want - 0.1 or 0.01%? Your average common cathode triode gain stage can do the first quite easily. Also have a look at using local FB.
Look at the ECC83 datasheet here - ECC83 @ The Valve Museum It has a table showing Ra, Rk and THD for several anode voltages, it will give you some idea of what values to use. The best way though is to draw a load line and knock up some circuits on the bench and do some tests.
It also depends on what level of distortion you want - 0.1 or 0.01%? Your average common cathode triode gain stage can do the first quite easily. Also have a look at using local FB.
Posts crossed : )
I tried local feedback connection on an ecc83 and it changes the freqency response due to miller capacitance.I have never try it on ecc82 ...do you have a good nfb schematic for ecc82 without frequency change?
With 150V B+, I'd use 8k2 for the plate resistor and 330R for cathode. That will give you 6mA of current, 100V at the plate and 2V at the cathode.
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If maximum input voltage is the most important consideration, than 47K and 1K2 are a bit better than 220K and 3K9. From the datasheets for the ECC82 it follows that at the onset of a grid current of 0.3 μA:
47K and 1K2 give Vo = 18 V and Vi = 18/13.5 = 1.33 V
220K and 3K9 give Vo = 15 V and Vi = 15/14.5 = 1.03 V
If the lower maximum input voltage and the higher output resistance are no problem, than I think that from the distortion figures in the datasheets for the ECC82 it follows that 220K and 3K9 give a little bit less distortion than 47K and 1K2. At Vo = 18 V the distortion with 47K and 1K2 is 6.1 %. At Vo = 15 V the distortion with 220K and 3K9 is 4.4 %. Since the distortion is about proportional with Vo, the distortion at Vo = 15 V with 47K and 1K2 would be about 15/18 x 6.1 % = 5.08 %.
Note that all these figures are based on the impedance of the following stage being 150K (for 47K and 1K2) and 680K (for 220K and 3K9). Also it's only a comparison between these two possibilities.
Does your preamp have a cathode follower as the following stage or is the triode section of the ECC82 coupled straight to the output?
Thanks a lot ,you and all of you who responded me.Well the following stage is a baxandall crossover and after that is an ecc83 preamp for the final stage .The original schematic had an ecc83 instead of this ecc82,so the output impedance was and should be high .So in this case in think the 220k with the 3k9 is better.I am thinking of replacing the 3k9 cathode resistor with 4 diodes in series (1n4148) to stabilize the 2v bias..is it a good idea?
You may try some small LED’s instead of diodes. Just experiment with a few types and colors.
Regards, Gerrit
Regards, Gerrit