Thanks, gabdx.
At first, up to 40ma, after about 1 minute within 5ma. After 15 minutes within 1ma, 1 hour = perfect. These tubes are not matched. I've just run them for 1 hour, while adjusting the bias (it's adjustable from the top of the amp) to keep things level. I've matched each pair well enough that they both come up slowly (80ma within 1 minute, 120ma after 15). I think it's the tubes vs the circuit. It doesn't help that they are only getting 35V, either (I used 36LW6... SHHH don't tell anyone ).
I've used the auto bias boards, and while they work pretty well, they only check the bias once while at turn on. My experience tells me that after an hour, the current might be well off what it was set to. I've used VPT series transformers with 6P3S and EL34 with good performance. I've never had a magnetization issue, but they are mains transformers rated for 250VA (or 25VA. I've made 8 amps using these now)... I've had bad tubes arc over but the transformer was fine afterwards. If you live in Montréal, I am planning a visit soon. I could bring one of these amps...
At first, up to 40ma, after about 1 minute within 5ma. After 15 minutes within 1ma, 1 hour = perfect. These tubes are not matched. I've just run them for 1 hour, while adjusting the bias (it's adjustable from the top of the amp) to keep things level. I've matched each pair well enough that they both come up slowly (80ma within 1 minute, 120ma after 15). I think it's the tubes vs the circuit. It doesn't help that they are only getting 35V, either (I used 36LW6... SHHH don't tell anyone ).
I've used the auto bias boards, and while they work pretty well, they only check the bias once while at turn on. My experience tells me that after an hour, the current might be well off what it was set to. I've used VPT series transformers with 6P3S and EL34 with good performance. I've never had a magnetization issue, but they are mains transformers rated for 250VA (or 25VA. I've made 8 amps using these now)... I've had bad tubes arc over but the transformer was fine afterwards. If you live in Montréal, I am planning a visit soon. I could bring one of these amps...
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Looks really good. With mains OT, theoretically you could only get the full 75W power above 65 Hz (assuming saturation at 240V rms 50Hz = 44W into 1300 ohms), and at 30Hz saturation would be at about 16W. How does it turn out in practice? Although if you run an SS amp with a subwoofer below 80Hz then it's academic only
To be honest, I've never tried it. Most of the time <1W is loud here. It will take Energy RC22 Reference to full excursion though. I have a 12x18 room with the speakers in the narrow end, Energy RC-70. It can drive it past the point of too loud in here.
I can tell you that low bass extension is great, too.
Koda
I can tell you that low bass extension is great, too.
Koda
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12x18 foot.To be honest, I've never tried it. Most of the time <1W is loud here. It will take Energy RC22 Reference to full excursion though. I have a 12x18 foot room with the speakers in the narrow end, Energy RC-70. It can drive it past the point of too loud in here.
I can tell you that low bass extension is great, too.
Koda
I should also mention I use the following biasing circuit.
Attachments
Nice!
In class A unmatched tubes makes more 2n harmonics, benign.
In class AB one tube gets overloaded which most often results in tube failure.
In matched tubes they have the same current and operating bias.
Thanks for the answers, it is great that you achieve fix bias stability with mis-match tubes.
In class A unmatched tubes makes more 2n harmonics, benign.
In class AB one tube gets overloaded which most often results in tube failure.
In matched tubes they have the same current and operating bias.
Thanks for the answers, it is great that you achieve fix bias stability with mis-match tubes.
I can tell you that low bass extension is great, too.
Koda
Low end frequency response is a function of primary inductance (higher is better) and source impedance (lower is better). With these mains toroidals inductance is quite high, and they are usually used with low Rp tubes so f3 can be very low.
Magnetic saturation is a function of the integral of a half cycle of the voltage waveform (the area under the curve). For a lower frequency (longer cycle time) than the frequency it was designed for, you need a lower voltage in order to keep the same area under the curve. So for half the frequency can only have half the voltage which leads to a quarter of the power.
Spreadsheet is attached which shows a few useful things when using mains toroidals as output transformers.
Attachments
Magnetic saturation is a function of the integral of a half cycle of the voltage waveform (the area under the curve). For a lower frequency (longer cycle time) than the frequency it was designed for, you need a lower voltage in order to keep the same area under the curve. So for half the frequency can only have half the voltage which leads to a quarter of the power.
Using half of load resistance leads to half the power.
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