F5 power amplifier - Page 668

rikkert1978 · 17th January 2010, 04:25 PM

nope.... powersupply voltage is ok, 24 V positive and negative.

voltage over R3 and R4 is zero.. and over R11 and R12 no change in voltage by turning the potmeter (shows only 6 mV)

voltage meter is connected on both sides of R11 (and R12) parallel over R

this is the same on both channels.

thx for your reply!

bobodioulasso · 04:31 PM

Do not worry, the mosfets start turning on after several turns.
Not too many risks with a dim bulb.
You must have a voltmeter across R11 or R12 and another between output and ground. (Ofset)

rikkert1978 · 17th January 2010, 06:28 PM

We (my father and I) suspect that one or more resistors are in the wrong place.

Because turning p1 and p2 leads to nothing more then a measured value of around 8 ohms across r3 and r4.
Probably r3/r4 are of a much lower value then the 2.2k it should be, resulting in such low resistance...

We are in the procces of disasembling the amp and rechecking it. Kinda hard with the Dale resistors to see what is what sigh...

generg · 17th January 2010, 07:25 PM

p1 and p2 have really 5 kOhm? I had once 50 Ohm on a blue one. The inscription was 501 instead of 503. Good luck!

woodturner-fran · 17th January 2010, 09:37 PM

Post a few pics, that might help us diagnose your problem...

gautamz · 18th January 2010, 02:22 AM

Where I live (Bombay, India) I am unable to get the recommended input Jfets - 2sk170, 2sk370 or 2sj074. Currently I can only get the 2sj108...

Is there any other Jfet pair I could use for the inputs? What are the primary characteristics to look for when trying to source the input Jfets?

~Thanks a lot in advance.

rikkert1978 · 18th January 2010, 10:36 AM

Quote:

Originally Posted by generg

p1 and p2 have really 5 kOhm? I had once 50 Ohm on a blue one. The inscription was 501 instead of 503. Good luck!

Thanks for all the advise, the pots were 5k ones unfortunately. But on the bright side, we did find out that r 11 and r 12 were of a wrong value (4.7ohm instead of 0.47)

And we found out that alterating turns on the pots, gets the amp to play now!

So one side is luckily working

I guess I should have measured all my components before putting them in... Ah well.. Another valuable lessen learned

tinitus · 18th January 2010, 11:53 AM

Quote:

Originally Posted by rikkert1978

I guess I should have measured all my components before putting them in

Thats usually a good idea

Regarding power up with lightbulb
According to Andrew it could mean trouble if one lightbulb blows, due to classA load
It was adviced to adjust F5 into classAB only, with the lightbulb
Confirm that amp is ok
Remove the lightbulbs, and first then adjust it into pure classA

luvdunhill · 18th January 2010, 04:34 PM

Quote:

Originally Posted by mudihan

For a gain in the range of 8-12x, what would be the rough range of values for R5&R6 (leaving out R7&R8) and R1&R2? Thanks!

Did you ever find suitable values? I've been playing with the gain as well, and wanted to see if anyone else has some values they've vetted.

AndrewT · 18th January 2010, 10:47 PM

the lightbulb is there to prevent a disaster if there is a wiring fault.
Once the wiring has been proved and all modifcations completed the lightbulb should be removed and then the biasing can start.

If the light bulb is still in place when trying to bias an amp then as the bias adjuster is turned up, the bulb reduces the voltage fed to the transformer.

17th January 2010, 04:28 PM	#6672
bobodioulasso diyAudio Member Join Date: May 2008	Do not worry, the mosfets start turning on after several turns. Not too many risks with a dim bulb. You must have a voltmeter across R11 or R12 and another between output and ground. (Ofset) Last edited by bobodioulasso; 17th January 2010 at 04:31 PM.

17th January 2010, 07:25 PM	#6674
generg diyAudio Member Join Date: Jul 2009 Location: Near Frankfurt	p1 and p2 have really 5 kOhm? I had once 50 Ohm on a blue one. The inscription was 501 instead of 503. Good luck! __________________ and the First Wtt is......a really First Class PSS Idea !

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17th January 2010, 04:25 PM	#6671
rikkert1978 diyAudio Member Join Date: Aug 2005	nope.... powersupply voltage is ok, 24 V positive and negative. voltage over R3 and R4 is zero.. and over R11 and R12 no change in voltage by turning the potmeter (shows only 6 mV) voltage meter is connected on both sides of R11 (and R12) parallel over R this is the same on both channels. thx for your reply!

17th January 2010, 06:28 PM	#6673
rikkert1978 diyAudio Member Join Date: Aug 2005	We (my father and I) suspect that one or more resistors are in the wrong place. Because turning p1 and p2 leads to nothing more then a measured value of around 8 ohms across r3 and r4. Probably r3/r4 are of a much lower value then the 2.2k it should be, resulting in such low resistance... We are in the procces of disasembling the amp and rechecking it. Kinda hard with the Dale resistors to see what is what sigh...

17th January 2010, 09:37 PM	#6675
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Post a few pics, that might help us diagnose your problem...

18th January 2010, 02:22 AM	#6676
gautamz diyAudio Member Join Date: Jul 2009	Where I live (Bombay, India) I am unable to get the recommended input Jfets - 2sk170, 2sk370 or 2sj074. Currently I can only get the 2sj108... Is there any other Jfet pair I could use for the inputs? What are the primary characteristics to look for when trying to source the input Jfets? ~Thanks a lot in advance.

18th January 2010, 10:47 PM	#6680
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	the lightbulb is there to prevent a disaster if there is a wiring fault. Once the wiring has been proved and all modifcations completed the lightbulb should be removed and then the biasing can start. If the light bulb is still in place when trying to bias an amp then as the bias adjuster is turned up, the bulb reduces the voltage fed to the transformer.