F5Turbo Illustrated Build Guide

to Phoenix NJ #1178/1179

Good Morning Phoenix NJ,


I would bias your amp to 300mV (as a starting point). Let it cook for minimum
30 minutes and then check your temperature at the heatsink (outside between the
fins where your Mosfets are mounted from inside). You shouldn't measure more than 50° C. Then I would run it for a week.Listen to some music...
If you are below 50° C you can carefully increase bias. Perhaps to 330mV or 350 mV. Check temperature again.
I never shorted my inputs at any amp for the biasing-procedure. Never short the input of your amp if it is switched on! Dangerous :redhot::bomb:

Cheers
Dirk
 
Got other channel licked. I didn't realize guy who worked on first opened up hole for PS on front end board. (12GA wire. ) I think it should be maybe 18? But i think all the issues resulted from none connections . I have all resolved. Maybe get to spend time saturday and do proper bias and try to pass some audio.
Going back to questions i had earlier. in first bias instructions 6L6 said it might be beneficial to short input during bias procedure (IIRC comment was not necessary)
the following bias instructions made no mention of shorting input during procedure.
Is there a better method?
also is it normal for the DC offset to wander around rather than stay pretty solid (the bias stays rock solid)?
Probably Saturday will have time to do slow cook in.
 
to Phoenix NJ #1182

Good Morning Phoenix NJ,


the bias of a ClassA - amp is in most cases not 'rock-solid'. That is my experience.

My target for biasing an class-a - amp is, that if it has warmed up, the DC-offset at speakeroutput should be very low. Below 50mV DC-offset is good, below 10mV is really good, below 1mV is excellent.
IF your amp has reached its working temperature, then it should have the low
DC-offset.

If your amp is cold it will have a much larger DC-offset (when you switch on) as
the DC-offset after circa 30 minutes. This is normal. Why? Because all the
electronic components in the circuit have to warm up and are influenced by temperature. Especially your Mosfets have to heat up the heatsinks -
this takes some time (20-30 minutes).
The fluctuation of DC-offset also depends on the design of the circuit.
If you build a BA-3 frontend it is very normal, that it has a slightly higher
drift up and down. But correctly biased it stabilizes pretty well. But I would
never expect a 'rock-solid' DC-offset from it. And as long it is not in a closed case - each windblow over the JFets can be seen on the DC-offset.



Cheers
Dirk :D
 
Good morning cubicincher,
I appreciate you explaining the drift. The heating of components explains why it seems more stable when you're first biasing ( lower current flow).
I'm still wondering if it's better or worse to bias with input shorted. Based on your comments I won't change that condition while its powered. The observation that the bias and offset changed ( the one time I did it) makes me think you have to have a different final operating point if it's biased shorted or open. Carrying the thought one part further, it'd seem to follow that it should be biased with an impedance that closely matches the input impedance of the preamp one would use with it. I suspect I'm overthinking this point but this is somewhat uncharted territory for me.
 
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Congrats on sorting out your non-working channel! :)

re: above. Cool that you're thinking through the situation. Think about it perhaps from a different angle. This is the way I might think it through...

Why would the input impedance of the pre-amp affect what's connected to your power amplifier? Is the output impedance of your pre-amp generally considered very high or very low in relative terms? Which would be closer to the power amplifier's operation when connected to your pre-amp outputs; an open circuit, or a short? Why else might one short the inputs of the power amp while biasing or checking for offset?
 
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Still a little unhappy with the DC offset in one channel. The good channel is rock solid. The iffy channel the DC offset wanders. If i put the cover on and watch the meter it will go up then down, settle some then wander more. I think i can get it to wander between +/- 50mv . The good channel sits dead steady on 0 with maybe a 1mv twitch every now and again. Any thoughts.
 
For sake of discussion, if i set to +200mv (moving slightly) set cover on and watch over a 5 minute period it will sink below -75mv. If i let it go it will continue. I tried lowering N side bias pot to +50mv reading replace cover and it starts falling again. Just drifts negative with cover on. never really settles. I'm not even sure if its better shorted.
 
Alright , I'm convinced the DC offset wander is over. I changed all the outputs on that channel. Then i couldn't bias at all. The ground wire from the front end board which physically was still in place, just not making connection on the N board. Reflowes the joint. Double checked my power. Biased up nicely with rock solid DC offset. While i don't know for sure, i suspect that ground wire was the issue all along. Making just enough to seemingly function. Likely heating up with current flow and creating higher resistance. Just my theory. In any event, its been cooking at a hair under 350mv not more than a few thousandths of a volt apart channel to channel at ov offset. I think tomorrow i do some housekeeping and try and pass some audio through this.
 
Thank you
Still biasing (with dc offset). I'm near operating point. The pots are very touchy. once cover is on and hot values change. When cover off it cools. Then you have to wait for it to bake again. So close. I have some cable dress things to do and i see it really has to heat up for close to an hour before its at operating point. Probably wont get to listen until tomorrow. I think i'll have some questions about that 3rd pot which (if i understand correctly changes the distortion characteristics).
i'll start center and work from there.
Thanks again