102 complete, od trim question

[duriehill]

Have finally finished a 102 build which has probably taken a couple of years on and off. There is a lot of labour in these amps! And hours of research and help from you guys.

It's my 4th amp build, and from power up yesterday, it actually worked straight off. I've never played another d style amp, but I think I nailed this one (or was lucky!). It sounds magic

Anyway, my query is that I have noticed the OD trim is rather hot. I have it set at 9am (quarter-way) which gives a usable range on the front level. But really the other 3/4 of range on the od trim is useless.

Do you guys find the same, or perhaps I need to look at something?

Thanks

[da Geezer]

Congrats on completing the build!

If you look at the 102 schem here ( http://ampgarage.com/forum/download/file.php?id=16859 ) you'll see that the OD trim on Robben's original is a 100k linear set at around 25k to ground, which would indeed be around the 9:00 mark.

On my 102-ish '74 Twin Reverb conversion, I have the OD Trim mounted on the back panel for easy adjustment, but find myself leaving it at around the 9:00 mark for most guitars....some guitars/pickups like it a bit lower (8:30) and some a bit higher, but around 11:00 on mine is the max to retain any similarity to a Dumble tone at all.

Changing the value of the OD trim pot (to say a 50k so the norm falls nearer the center of the pot travel/adjustment) changes the "feel" and tone of the OD too much for me.....there's something about that 100k (25k/75k) ratio that the circuit simply "likes"

I think you've done good and are right where you should be. It's originally intended to be a set and forget pot, internally mounted to find the best tone for that amp, then close it up and leave it.

G

[duriehill]

Thanks G

I didn't notice the note on the schem, so that's good to know my amp is set up right, cheers!

[glasman]

FWIW, I am pretty sure the Robbens amp was set to about 26.6k. What I dont know if that was when relay in or out.

I presonally like to set the internal trimmer to obtain a net gain from the input to the overdrive (220K) to the TOP of the 100K resistor (0.01uf and 100K resistor junction) to between 6 and 7 db at the center of the passband.