Se EL34 cathode bias help

[kanons]

Hi. I am new here, but on my third build. I need some help, as I don`t understand electrics very much. My last build is an Ampmaker N5X 5w. I went directly for the EL34 option (TAD EL34B-STR) and put in a 270r resistor for the cathode. The Cathode cap is 47uf. Dissipation is 9,4w. Amp sounds good, but a little more focus would be good.
Question: What will changing the resistor to 220r and cap to 100uf do?

[pdf64]

I think that amp was designed around an EL84 power tube and that the octal socket was put in at customer request but that shouldn't be taken as meaning that the amp will be able to get the best out of an octal tube.
You could try to adjust the bias until the EL34 plate dissipates its rated 25W but the power supply will likely overheat and fail well before that point.
I don't think that Ampmaker advise the power tube cathode resistor be reduced below 270 ohms (stock value is 470).

That said, a 220 ohm resistor will allow the power tube to pass more current, increasing its plate dissipation.
A 100uF bypass cap on it will lower the frequency of the high pass filter formed by the cathode impedance and cathode cap.
The cathode impedance is formed by the parallel combination of the tube's cathode impedance (1/gm) and the cathode bias resistor.
From http://www.mif.pg.gda.pl/homepages/fran ... e/EL34.pdf the gm is 11mA/V so the tube's cathode impedance is 91 ohms and so the parallel combination with a 220 cathode resistor gives a total effective impedance of about 68 ohms.
The breakpoint frequency of an RC filter is 1/2PiRC so plugging 68 and 100e-6 into that gives 23Hz.
A 47uF cap would be about double that frequency.
So either are well below bottom E and therefore bass response won't be much different either way.
As for the effect on 'focus', you may have to assess that yourself.
Apologies for any errors above, hopefully someone clever will correct them
Pete

[kanons]

Great advice. Will try not to fry the transformer. Might try to up the dissipation a little bit to 12w approximately. Hope that goes well. Any other advice is welcome. Thanks.

[Stevem]

Does the amp lack focus all the time, or only when it starts to get drivin hard?

[martin manning]

I would look at what idle current it would take to get the idle point centered. The N5X schematic shows 380VAC CT power transformer, which would make plate voltage around 270, and a 5k primary on the OT. Guessing about 15-20V on the cathode (so Va-k about 250), idle current at 50mA would be about right, and put you at 50% of max dissipation (12.5W). To measure idle current measure the cathode voltage (pin 8) and divide that by the cathode resistor value.

[pdf64]

Might try to up the dissipation a little bit to 12w approximately

Due to the lowish VB+ and high (for an EL34) OT primary impedance, the EL34 won't benefit (in terms of class A clean output power) from increasing the static current / plate dissipation past a certain point.
Which is probably about where it is with a 270 ohm cathode resistor.
But different static dissipation levels may sound different.
Class A doesn't require that the power tube/s be biased to a certain degree of plate dissipation.
Depending on the tube characteristics, VB+ and OT impedance, it may be that the most class A power may only require <50% plate dissipation, or class A may just not be feasible, eg plates in p-p amp would have to run at 200% dissipation
Pete

[kanons]

Cathode current is at 49mA, screen at 4,25. Plate current is 45mA, dissipation at 9,77. B+ seems to be 245. I Get confused, as per ampmakers instructions, cath. Current is 49, but measuring at pin 8 gives 228v, divided by 270 gives 84. Which is which??

[martin manning]

So IMO you want cathode current to be ~55 mA. Voltage at pin 8 divided by cathode resistor value = 0.055.

[pdf64]

From the ampmaker site
Octal cathode current: Divide the cathode voltage (H-J) by the value of R17 (470&#8486;). A typical result is around 30-40mA, depending on the valve you are using. Contact Amp Maker if it's more than 50mA.

So is the voltage between cathode and ground 13.23V (ie 49mA x 270 ohms) or 22.8V?
Pete

[kanons]

It is 13,2. I am having trouble with different math formulas on the web. I end up with dissipation between 9,77 and 11,2. Maybe not much of a difference. all the math is telling me that I should have a 220 or 230r cathode resistor, and that would give a dissipation of 54-57-60 which presumably would be best for the el34, but the web confuses me, as some people say that this will be too hot for my amp. Can someone help me out of this mess?

[martin manning]

Plate dissipation does not matter, as you will not be anywhere near 100%. To center the operating point on the load line and maximize clean power find a cathode resistor that results in cathode current of 55mA or a bit less. Current vs. resistance is non linear, so just use trial and error to find the right value.

If your power transformer runs too hot, then you will have to back off. If you want to calculate plate dissipation it's (Va - Vk)*Ia, where Ia will typically be found as Ik - Iscreen.

[kanons]

Thank You. Great help from everyone. Will put in a smaller resistor and do the maths. I just change it back If I don't like it. Again thanks.

[gingertube]

If you leave the higher value cathode bias resistor such that the EL34 is not working very hard then it is likely that you can get some more "focus" by reducing that 2K2 screen resistor (g2 resistor). Try 220R instead BUT do not try this if working the EL34 hard with a low value cathode bias resistor.

Cheers,
Ian

[kanons]

Will try as above, but found culprit in another place: the new TAD preamp tube was too high in gain, so the distortion made the sound too loose for my taste. Much better control now, tighter. Thanks