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CanAm Man 15th March 2011 06:27 PM

Initial setting of F5 bias--help, please!

I'm firing up my F5 amp modules today for the first time(!).

The Nelson Pass F5 Power Amp article extracted from AUdioXpress, May 08 states, under "Initial Adjustment":

"For each channel you will be adjusting P1 and P2 alternately in order to achieve 0 volts DC at the output and .59 volts across R11 and R12......

"In spite of the thermal compensation in the circuit, you should assume that there will be drift as the heat sink termerature rises, and you will need to readjust the values over the course of an hour or two. Usually it is best to start out bias adjustment low, at maybe 0.4 mV across R11 and R12 until the amp is warmed up a bit....."

I added the bold font, in the extract, above.

Question: Do I really want to initially set the R11 and R12 voltages to 0.4 MILLIVOLTS as the article suggests--or should that be 0.4 VOLTS?

I think I know the answer, but wanted to double-check before I torch my MOSFETs.



Zen Mod 15th March 2011 07:08 PM

toss that and start with using your own head ; numbers are not relevant as meaning of procedure

dial both pots to 0 ohms ( check with ohmmeter )

place one voltmeter across PSU caps ( best between + and - of PSU) to observe max voltage of PSU

place one voltmeter at output - to observe offset

place one voltmeter across one source resistors of output mosfets ; it doesn't matter which one .

for test - slowly dial up Variac ( presuming that you have one , as man with many skills) up to full mains voltage , observing voltage at PSU ....... thinking about max cap voltage ( 25V as in FW ? ) , because with 0 Iq PSU is unloaded and voltage is maxed

if nothing is smelling :rofl: - leave Variac at full mains ;

what's important - Iq must be very low , offset is irrelevant in this moment .

now turn one pot one turn ( assuming that you have multiturns )

then turn other pot one turn

observe Iq and offset

proceed one then second pot , again just one turn

observe Iq and offset

again one turn + one turn

now you are probably in range when you can see which pot is pulling offset in right direction - to 0 .

proceed iteratively with pots , while you set - say - 75% of desired Iq and zero offset

now - put lid on box and let it cook for a while - until yo get thermal equilibrium on heatsinks

it's best to use wire/clips to leave those voltmeters in place ;

open the lid , up bias to - say - 90% of desired one ,while maintaining offset

put lid on , let it cook

check ;

if all is OK - move voltmeters for Iq and offset to other channel and repeat procedure

use it few days at 90% of desired bias , then check and set to 100%

remember - temp. equilibrium with lid on is important

simple , isn't it ?

in any case - simpler than re-doing cylinder head .... :clown:

Melon Head 15th March 2011 07:29 PM

I would suggest 0.4V is the correct value.

With my version without thermistors I usually go for half the final value bias value, so for this example it would be roughly 0.3V. I usually find that it stabilises after about an hour.

AndrewT 15th March 2011 07:37 PM


start with using your own head
one of the most important statements to come from Zen Mod.

Zen Mod 15th March 2011 08:04 PM


I just meant on cylinder head ........


Melon Head 15th March 2011 08:10 PM

you are very lucky

CanAm Man 15th March 2011 08:15 PM

ZM, Andrew and Melon Head.... My thanks for the quick replies.

ZM--that's about the most detailed (and practical) info I've ever seen, on starting up an F5 for the first time..... SUPERB....! I'll pass your comments along to 6L6 and Woody, who also are close to initial testing.

I've already been up to ZM's "smell test"--no smoking and both PSUs are reading 24.7 VDC on each rail (+ and -, naturally....!) Will progress with the bias settings up to 0.4 VDC on each amp PCB (yep--multi-turn pots), and allow it to cook in......

Borrowed a large Variac from a "ham friend". That ought to give me a little more insurance, as a test further.....

Thanks, guys--I consider the three of you a few of the rare TRUE EXPERTS on the forum......who are willing to help out us newbies (or in my case, a reborn newbie to the hobby).

alazira 15th March 2011 08:15 PM

Remember Ohm's Law. I=V/R. Current= amps, V= volts, R=ohms. Recommended bias current is 1.3 amps. If your source resistor is 0.47ohm then you'll need 0.6V

CanAm Man 15th March 2011 08:29 PM

Yep. I only believe in two laws--gravity and Ohm's..... :D


Itsmee 15th March 2011 09:21 PM


Originally Posted by CanAm Man (
Yep. I only believe in two laws--gravity and Ohm's..... :D


Don't forget the wife's law, break that and the other two won't matter :rofl:

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