I’m building a spare parts amp, roughly based on the Zen, EL84 single ended, 6sn7 driver.
I’m realizing, the choke I have on hand is slightly mismatched, it’s 80 ma and I think I’m going to be in the 110 ma range.
I also realized my plate voltage is going to be a bit high.
The original zen uses a 1k resistor, rather than a choke.
I figured, with my power transformer, I’d need around 400 ohms to get the plate voltage where I want it.
My question is, if I put a resistor in series with the choke, say a 250 ohm resistor in series with a 150 ohm choke, in addition to lowering the plate voltage, will it also reduce the current going through the choke? I’m thinking most of the current would be dissipated in the voltage dropping resistor.
Thanks!
I’m realizing, the choke I have on hand is slightly mismatched, it’s 80 ma and I think I’m going to be in the 110 ma range.
I also realized my plate voltage is going to be a bit high.
The original zen uses a 1k resistor, rather than a choke.
I figured, with my power transformer, I’d need around 400 ohms to get the plate voltage where I want it.
My question is, if I put a resistor in series with the choke, say a 250 ohm resistor in series with a 150 ohm choke, in addition to lowering the plate voltage, will it also reduce the current going through the choke? I’m thinking most of the current would be dissipated in the voltage dropping resistor.
Thanks!
That'll work.
One more trivia on the choke: the max current spec is not just for it to get too hot, that normally is not the issue. But if you exceed the max current spec, the flux in the core gets so high that it (the induction) collapses, and then you have nothing left than a length of wire and no induction ... ;-)
Jan
One more trivia on the choke: the max current spec is not just for it to get too hot, that normally is not the issue. But if you exceed the max current spec, the flux in the core gets so high that it (the induction) collapses, and then you have nothing left than a length of wire and no induction ... ;-)
Jan
Last edited:
The inductance falls off very slowly in over-current. Heat is often not terrible. I would just build it, check the ripple, and in an hour check the heat.
If it isn't happy, and you are limited to spare-parts, the alternative is to back-down the main load (power tube) to nearer 80mA, and suffer the slight loss of power output.
If it isn't happy, and you are limited to spare-parts, the alternative is to back-down the main load (power tube) to nearer 80mA, and suffer the slight loss of power output.
Perhaps a network can reduce ripple current on the choke. https://www.electronics-tutorials.ws/inductor/ac-inductors.html
Attachments
That’s an interesting circuit.
I finished the amp and I think the choke is working fine. Total current draw is approximately 90 ma, in triode. I put a triode/pentode switch in, but didn’t measure the current in pentode. Voltages are very close to optimum.
I biased the 6sn7 at 5 ma, which looked like just above the knee in the curves, appr 28k anode, at just under 300 volts b+.
I’m letting it burn in before making any final judgements. This one is my first single ended amp. I’ve been a push pull pentode/ultralinear type.
I finished the amp and I think the choke is working fine. Total current draw is approximately 90 ma, in triode. I put a triode/pentode switch in, but didn’t measure the current in pentode. Voltages are very close to optimum.
I biased the 6sn7 at 5 ma, which looked like just above the knee in the curves, appr 28k anode, at just under 300 volts b+.
I’m letting it burn in before making any final judgements. This one is my first single ended amp. I’ve been a push pull pentode/ultralinear type.
There's a distinction between triode and penthode mode wrt current draw, as in penthode mode current varies more than in triode mode. If the choke saturates at peaks, more IM distortion will be noticable. For excellent results it's common practice to make a stiff power supply, allowing for double the current rate needed.
Ok, there is definitely something wrong. Some recordings sound fantastic and some sound terrible. I was listening to some Meters albums, in honor of the late Art Neville, and the heavy bass seemed to be throwing the amp off. Also, compressed loud recordings. Something very bad occurs right in the lower midrange which I’ve never heard before. It may well be IMD. Seems to be just as bad in pentode as triode.
The output transformers are from a Magnavox console amp that I fished out of my neighbor’s trash can. I was thinking that they are saturating. The zen has a 1000 mf bypass capacitor on the power tubes. I used a 470, but I read on this site that running too large a value can lead to transformer saturation. So, my plan is to reduce that cap to as low as tolerable.
The current draw in pentode was slightly less. But, I may have measured it with signal. Here’s my power supply. The power transformer is 265volts/120 ma. The filtering is CLCRC split out to 2 RC’s to each side of the 6SN7. The values are 48mf/8H at 80 ma, 240 ohm/51mf/4.9k/27mf/ 1.6k/22 mf x 2. Voltages are around 376/343/300/289. I connect the screens to the 300 in pentode with 1k screen resistors. I have to say, the amp is dead quiet...no hum at all. If I put my ear right up to the horn, there’s a bit of hash, but I think that’s my dac operating off usb power.
Do you think there could be a power supply problem?
The output transformers are from a Magnavox console amp that I fished out of my neighbor’s trash can. I was thinking that they are saturating. The zen has a 1000 mf bypass capacitor on the power tubes. I used a 470, but I read on this site that running too large a value can lead to transformer saturation. So, my plan is to reduce that cap to as low as tolerable.
The current draw in pentode was slightly less. But, I may have measured it with signal. Here’s my power supply. The power transformer is 265volts/120 ma. The filtering is CLCRC split out to 2 RC’s to each side of the 6SN7. The values are 48mf/8H at 80 ma, 240 ohm/51mf/4.9k/27mf/ 1.6k/22 mf x 2. Voltages are around 376/343/300/289. I connect the screens to the 300 in pentode with 1k screen resistors. I have to say, the amp is dead quiet...no hum at all. If I put my ear right up to the horn, there’s a bit of hash, but I think that’s my dac operating off usb power.
Do you think there could be a power supply problem?
Hi Jan. My reasoning is pure intuitive
Because of the low PSRR rectification ripple will be mixed into the audio signal, hence the intermodulation. When PS ripple voltage rises because of decreased inductance at choke saturation, so will the IM rise at loud passages.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.