Good morning guys, I have a quick question, since I’ve eliminated the diodes, could/should I bias up to 400mv/425mv? I have not changed the source resistors, they are still 1ohm/.5ohm in parallel and I do have the 1nf caps c3 & c4 installed in the output boards?
Could you? Yes. Should you? Not really. It doesn’t make much sonic difference and does not change the power output.
Give the amp a cuople of days before you adjust the final bias.
When the final bias should be set, adjust it acording to heatsink temp. You should be within 25-30c above ambient. And also stay within around 35W dissipation pr output device for a long healthy life.
When the final bias should be set, adjust it acording to heatsink temp. You should be within 25-30c above ambient. And also stay within around 35W dissipation pr output device for a long healthy life.
Got it, thank you! Wish me luck, I’m going to give it a shot a bit later. Hopefully this is not a rerun of what happened last weekend ��
I vote yes, if your heatsinks are very comfortable with the existing bias.
Amplifier is currently biased to 1.2A (at 300mV), and this is lower Class A envelope than even the vanilla F5.
For the best sound from these vertical MOSFETs you should be at least 750mA per device, preferably 850mA. 850mA through the device is 425mV across source resistors (they will run hot, which they didn't do before).
At 1.7A bias for two pairs, your Class A envelope is now 45W into 8 ohms, a much more comfortable region for the MOSFETs.
I would aim for 2A bias or as close to as you get with the heatsinks you got.
That is 500mV across the source resistors.
1A pr output and 32V rails =32W dissipation pr device. And 128W dissipation pr ch.
That is 500mV across the source resistors.
1A pr output and 32V rails =32W dissipation pr device. And 128W dissipation pr ch.
Thanks guys for the information. In general, does my bias go up as I start giving an input signal? That is what I’m noticing and before I let the magic smoke out again, I thought I would ask. My amp draw on the main is not changing though.
It is normal for the current draw per rail to move around when the output is drawing current (when you provide any input) for a push-pull amp like the F5/T. The total current drawn from the supply will still be the same as the bias current while the amp stays in Class A. It's just being re-routed via the load into the common return between the two supplies. It is not advisable to run signal tests into the F5 with no load. It stresses the output stage.
Be absolutely sure your system can handle the intended level of bias. You will be putting an additional 35W into the sink per channel, so you should do this only if your target bias is not causing thermal stress.
At a very basic level, your sink temperature at 1.2A should be about 45 degrees (thermal rating 0.25C/W) to be within 55 degrees if/when you crank up the bias to 1.7A. If you're planning on 2A bias, that's a thermal rating requirement of 0.2C/W. Your sink temperature should be around 40 degrees at 1.2A to satisfy this requirement.
This assumes ambient temps of 25 degrees. If you have higher ambient temperatures, sinks will need to be uprated accordingly. Also the insides of the amp will run hotter. The source resistors will be quite hot in particular, enough to cause a minor blister. You can expect surface temperatures of over 80 degrees on the MOX resistors used in the F5T kits.
Be absolutely sure your system can handle the intended level of bias. You will be putting an additional 35W into the sink per channel, so you should do this only if your target bias is not causing thermal stress.
At a very basic level, your sink temperature at 1.2A should be about 45 degrees (thermal rating 0.25C/W) to be within 55 degrees if/when you crank up the bias to 1.7A. If you're planning on 2A bias, that's a thermal rating requirement of 0.2C/W. Your sink temperature should be around 40 degrees at 1.2A to satisfy this requirement.
This assumes ambient temps of 25 degrees. If you have higher ambient temperatures, sinks will need to be uprated accordingly. Also the insides of the amp will run hotter. The source resistors will be quite hot in particular, enough to cause a minor blister. You can expect surface temperatures of over 80 degrees on the MOX resistors used in the F5T kits.
Sangram, thank you for the information. I’m still running around 330mv bias but I noticed as I gave a small input signal, my bias was starting to jump up. I wanted to be sure this was normal after the 2 previous smokings. I do have an 8-ohm dummy load connected as well.
How much is it going up? A few mV? No big deal. Remember this is an entirely DC coupled amp, so things are going to wiggle around a teeny bit as you change the input.
Thank you for jumping in 6L6, it jumped about 10/15mv and that’s where I shut the signal down and came to the forum. I had a 1k test signal at about 2.8v
What is your sink temperarure (top of sinks) at your current bias? In my experience, the 5U 400 can not take 125w per sink without increasing over 30 degrees above ambient. That’s with keratherm. I have done this until recently, but have gone down a bit now, to 115w per side, but that is also a bit high for the ventilation I have right now.
I know Audiosan and I disagree about this. But maybe you run the amps in a fridge, Audiosan? 😀
An then, something I have pondered upon since reading one of 6L6s many sublime guides:
6L6: In one of the guides (Aleph J?) you mention something along the lines of the sink temp being less important with Keratherm. Or put another way: with Keratherm one can use higher bias. Maybe I got you wrong, dunno, but could you elaborate on this?
I guess Keratherms give lower case temp per degree increase on the sinks, due to the low thermal resistance, and that that is part of the explanation, if I got you right that is.
Andy
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andy. I don't 🙂 i've seen alot of people get nice temp readings from this sinks with pretty high amount of dissipations. I have not used this sinks for class A yet. Only class A/B.
4U300mm sinks at 30c ambient(sinks inside the chassis), 22W dissipaten =37c at the paseplate of the sinks 🙂
And thats where temperatures should be measured. Not on the top of the sinks.
I will do soon. A M2X in a 3U400mm chassis. And a BA3 running +/- 34ish Vdc and 6 pair outputs at around 2A. That one will go in a 4U500mm chassis.
4U300mm sinks at 30c ambient(sinks inside the chassis), 22W dissipaten =37c at the paseplate of the sinks 🙂
And thats where temperatures should be measured. Not on the top of the sinks.
I will do soon. A M2X in a 3U400mm chassis. And a BA3 running +/- 34ish Vdc and 6 pair outputs at around 2A. That one will go in a 4U500mm chassis.
Do you mean baseplate, as at the bottom of the sinks?
Cheers,
Andy
PS: aonce this pandemic calms down, I suggest a Class A beer!
Cheers,
Andy
PS: aonce this pandemic calms down, I suggest a Class A beer!
Yes. baseplate. Seems i turned the b upside down😀
Class A bear would be great. but that depends where you live 🙂
Class A bear would be great. but that depends where you live 🙂
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2 volts gets you where you wanna be, Lucrockin. But I agree, time to use some music.
Audiosan: I live in the heart of corona in Norway.
Audiosan: I live in the heart of corona in Norway.