Variable Depth on Laney GH100TI

[Dmoney]

YO!

just thought id check with some amp guys.
Just thinking variable resonance mod for a GH100TI

Im think, remove R39, Replace R40 with a 1uF cap or something,
add a 1Meg audio pot with a .0047uf bypass cap across it and a 39K resistor on the output between the points where the switch should be. and hopefully mount the pot in the hole cut for the switch where the front panel is... hopefully that'll be neat.

or use a 1Meg push/pull pot with a 50K or 100K resistor on one side, so you can switch back to one of the original switch positions, though im not sure how useful that would be if you have the variable depth anyway.

am i on the right track?

[jpfamps]

You are along the right lines.

I would remove R39, replace the switch with a pot, wiring the wires to the switch to the top and bottom of the pot. I would wire a cap across the pot and connect the wiper to the top of the pot (ie wiring it as a variable resistor).

You may need to experiment with the pot and cap values. I would probably start with say 100k pot and a 0.015 µF cap (use a non-polar film cap). You may need to experiment with both the cap value (lower values will raise the frequency response of the control) and the pot value (eg lin log, larger values will decouple the -ve feedback more effectively) to give the response you want.

You can also try reducing R40 to increase the effectiveness of the presence/ resonance controls, but would be very cautious of going below 50k (the effective value of the feedback resistor when the resonance switch is closed), as this may render the amp unstable.

[Dmoney]

cheers.

I was going to begin with using SLO depth mod values which worked really well in my 800.
The tone stack in the GH100TI doesnt use typical marshall values though, so like you say, may well be best to experiment.

Im not sure how the speaker outs are mounted. they may be pcb. would you not use a cap to prevent DC getting to the pot, or is that not really essential?

[jpfamps]

You can use a cap to prevent DC on the pot. This is not necessary for function, but will prevent any unwanted noise when the pot is rotated if this bothers you.

If you can live with the noise, then I would avoid putting an extra cap in the feedback network as this could potentially cause a phase shift that might make the amp oscillate at LF, which will depend very much on the LF time constants in the rest of the circuit. If you do want use a DC blocking cap, this will have to be non-polar.

I would also avoid increasing the total amount of global negative feedback in the amp as this may also cause the amp to oscillate, so I would not go lower than 50k for R40 (assuming of course R39 has been removed). 

[Dmoney]

cheers.
all sounds good. maybe a 100K trim pot where R40 would be to dial it in?
could be removed and swapped for a fixed resistor.

[jpfamps]

cheers.
all sounds good. maybe a 100K trim pot where R40 would be to dial it in?
could be removed and swapped for a fixed resistor.

That sounds a good idea.

[Dmoney]

Cheers  :D

[Dmoney]

got this done tonight!
I went with a 39K resistor in place of R40. I had a trim pot ready to go but mounting it easily on the PCB was a bit of an issue.

Anyway. it sounded AWESOME!
we tested it all out a few times. seemed totally fine.

when i got home I had an email from my mate saying it doesn't do much past '2' on the pot (i installed in place of the switch). I think it should be fine. unless something happened after I left.

After ideas what could cause that?

I used a 1uf cap (non directional) and a 39K resistor at R40, and a 1M audio pot with a 4700pf cap from input to wiper. The input comes from the replaced R40, and the wiper feeds the presence circuit. R39 is removed.
no grounding on the back of the pot. just some big metal washers between the pot and the chassis.