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.
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.
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.
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.