Laney L20 schematic

[MarkusBass]

I'm looking for schematic of Laney L20 guitar amp. I have a problem with the TONE pot. It works only when turned up 90-100%. And there is nothing wrong with the pot. If this is by design, I'd like to modify the curcuit.

Mark

[J M Fahey]

What does the tone pot do?

Maybe you can lift just that small piece of schematic by hand, shouldn't be too hard .

Might need good light and a pair of +4 reading glasses used as binocular loupes.

There's not much a single tone pot can do, but I'm interested to suggest proper taper.

For now, and based on your description, you might need a same value, reverse Log/"C" taper one.

[MarkusBass]

Here is a very similar amp (look at VR3 in the middle of the schematic). The TONE pot cuts high frequencies in the same way as in guitar the Tone pot does. The pot is 220kB which seems to be linear. I think that I need audio. The problem is that the pots are not typical. Also the input gain pot seems to be incorrect taper. On "2" the amp has almost full power. This is the same problem as with many Fender amps.

Mark

[J M Fahey]

Thanks Markus
The one you posted looks like the Laney version of classic VOX "cut" control, a plate to plate PI treble cut, and indeed it needs a Log pot for smooth operation.

The other option is that it's a plain, guitar type control, bleeding highs to ground.

It again would prefer a Log pot.

Same with the Gain control, if it's wired as a regular Volume one.

[MarkusBass]

Yes, I already simulated the frequency response of the circuit in SPICE. With linear pot it looks very bad but with log pot it's OK. See below.



I don't know why they decided to use incorrect taper. The value of the pot and its taper are printed on the board so it is not a mistake. They designed it like this but in this case the pot works firm "9" to "10" only.

Mark

[MarkusBass]

Just for reference, here is the frequency response of the circuit with linear pot. It looks very bad.



Thanks Juan for your answers.

Mark

[J M Fahey]

I'm also amazed.

Nowadays many over the counter or mail suppliers only carry linears, and Log/Audio if available only on a few values; what amazes me is that cost is the same so it's not a price problem (and even if it were ..... it would be a couple cents difference) .

Even more amazed by the fact that:
a) manufactureres buy OEM in bulk, so they can have anything custom made
b) they do spend extra money on way more complex and expensive mechanical differences, such as different shaft length and shape, support wings, longer threaded bushings, ball/spring mechanisms to provide "stepped" response or click center ... but they won't worry about the main function of the pot, its resistive track?
Everything else is enclosure and looks.