Lab Series L3: S.S. techie question - anyone help?

Tony474 · April 9th, 2008, 03:49 PM

I just picked up a 1979-ish L3 at a very reasonable price indeed. It's not the best but far from the worst amp I ever heard, though clean headroom is limited, but it is in great condition and even came with a cover and also the original schematic diagram and circuit description. From the latter, this caught my eye:

"The reverb and input controls feed the mixer-distortion amplifier Q9, which is biased by R35 so that 5 VDC appears across it. This maximises the gain concurrently with lowering headroom, producing a defined distortion threshold. Furthermore, the gain is somewhat more predictable with device variations than if the transistor were biased with a voltage source. The drains of Q9 are maintained at approximately half of the 5 VDC by feedback resistor R45."

Q9 is specified as "Transistor, PNP, Power, TIP30, Lead Form" and is mounted to a heat sink;
R35 is 22K ohm, ±5%, 1/4W;
R45 is 4.7 Megohm ±5% 1/4W.

What I'd like to know is this: since it seems from the above that headroom is deliberately limited, would altering the type or value of any of these components increase the headroom available without adversely affecting anything else in the circuitry? Or is there more to it than that?

Tony474 · April 10th, 2008, 05:26 AM

Hello?

del · April 10th, 2008, 07:02 AM

hi, i cant help u with the technical side, but i have exactly the same amp, and from my personal experience and from what i ve read from others, u get the nicest sounds just by turning the master all the way up to 9, and then adjust volume with the volume control.
they are fun amps, incredible light and mine never needed work.
good luck.

April 9th, 2008, 03:49 PM	#1 (permalink)
Tony474 Friend of Leo's Join Date: Apr 2007 Location: North Bushey, near London, England Age: 61 Posts: 2,385	Lab Series L3: S.S. techie question - anyone help? I just picked up a 1979-ish L3 at a very reasonable price indeed. It's not the best but far from the worst amp I ever heard, though clean headroom is limited, but it is in great condition and even came with a cover and also the original schematic diagram and circuit description. From the latter, this caught my eye: "The reverb and input controls feed the mixer-distortion amplifier Q9, which is biased by R35 so that 5 VDC appears across it. This maximises the gain concurrently with lowering headroom, producing a defined distortion threshold. Furthermore, the gain is somewhat more predictable with device variations than if the transistor were biased with a voltage source. The drains of Q9 are maintained at approximately half of the 5 VDC by feedback resistor R45." Q9 is specified as "Transistor, PNP, Power, TIP30, Lead Form" and is mounted to a heat sink; R35 is 22K ohm, ±5%, 1/4W; R45 is 4.7 Megohm ±5% 1/4W. What I'd like to know is this: since it seems from the above that headroom is deliberately limited, would altering the type or value of any of these components increase the headroom available without adversely affecting anything else in the circuitry? Or is there more to it than that? __________________ *Proud to be The Man From Uncool.* I cried because I had no shoes - until I met a man who had no feet...

April 10th, 2008, 05:26 AM	#2 (permalink)
Tony474 Friend of Leo's Join Date: Apr 2007 Location: North Bushey, near London, England Age: 61 Posts: 2,385	Hello? __________________ *Proud to be The Man From Uncool.* I cried because I had no shoes - until I met a man who had no feet...

April 10th, 2008, 07:02 AM	#3 (permalink)
del Tele-Meister Join Date: Nov 2006 Location: antwerp, belgium Age: 28 Posts: 106	hi, i cant help u with the technical side, but i have exactly the same amp, and from my personal experience and from what i ve read from others, u get the nicest sounds just by turning the master all the way up to 9, and then adjust volume with the volume control. they are fun amps, incredible light and mine never needed work. good luck.