Laney Klipp 60

[jpfamps]

I expect the valves weren't matched at 600 VDC on the plates and screens!

Valves really need to be tested under working conditions. I know many vendours use 250 VDC triode strapped as their standard conditions.........

I get my EL34s tested at around 500 VDC and they are still often "unmatched" when installed in an amp.

Also mechanical disturbance will affect valve performance. I inadvertantly "unmatched" a pair of matched EL34s by letting them role off my work bench last week.

[EstynEstyn]

Thanks for the replies guys, much appreciated. But no one replied as to whether or not the Bias Adjustment schematic I came up with would work. Anyone?

[EstynEstyn]

Anyone? Please?

[Steve Conner]

Well, where is the schematic of your mod? Is that it above?

(I'm not familiar with the original amp, and the schematic you posted shows no signs of any parts added or changed. Maybe you're just really good with Photoshop.)

[EstynEstyn]

Hi Steve. Yes that is my schematic, I photoshopped it. Here is the original. Thanks.

http://www.prowessamplifiers.com/sch...s/klipp_60.jpg

[Steve Conner]

OK. So you'll need to remove the 56k resistor after the diode, to get enough bias voltage. The two 100k pots in parallel substitute for it.

And the little 2.2uf capacitors shouldn't be necessary, though I can't see them doing any harm.

Some people say that this is a bad way to set bias, because the tubes will fry if the pot wipers go open circuit. But it's the way I've always done it.

[EstynEstyn]

Excellent, thanks Steve. Out of interest, what would you have to do for a safety precaution on the pot wipers? Put a resistor in series to set a maximum draw?

[Steve Conner]

You have to either: use the pots as variable resistors instead of pots, or put resistors where you have the 2.2uF capacitors just now.

Either way, the law of the bias pots gets bent, and the two pots interact: adjusting one changes the bias voltage to the other tube too.