Fuchs ODS Internal Trim Pots - What Do They Do?

I'm sure Andy or Scott can fill you in more completely, but I've tweaked every single one of my TDS internal trimmers by ear looking for some tonal changes I wasn't getting from the chassis knobs and found the audible changes to be very subtle, except the internal Channel 'B' gain. But, I do know that some are set up by measurement (voltage and/or resistance), so it probably makes sense to get the scoop from Andy regarding which ones you're allowed to do by ear. Once again, most of them seem to make almost no audible difference, so it's not worth messing with them until you know exactly which one(s) will produce the effect you're after.

 

hardys: There have been three versions of the ODS circuit/motherboard since 2000.

Earliest units had only one internal OD trimmer. Eventually we added an HFOD (which regulated the amount of highs INTO the overdrive). Then an LFOD was added (adjusted lows INTO overdrive), which was discontinued after a year or two, since most people left it full-on.

The majority of ODS's have two OD trimmers going into the OD: All are located on the main board next-to the OD relay. Factory setting is approx 220-K (around midway) on the hfod, and approx 50% rotation on the OD gain, depending on tubes installed etc.

In 2007 we added a post OD HF trimmer and an internal reverb dwell adjustment, which has remained "standard" since.

 

Remember, they work backwards - ccw for more gain.

 

Are you sure of that ?

 

Bump !

 

Tom, maybe on the mods....

Production amps are clockwise for more gain and/or brightness. The two are related. If you bump the hfod (ie, reduce its resistance), you get more gain too.

 

I have a 2007 ODS 50 I purchased in july. Is there any resources available to point out where these trim pots and what they do. I have learned where v1 2 3 4 are ect. from Andys post. The effects loop Scott L. helped get me started. It would really be helpful to have all this info somewhere. The manual from Fuchs website is very old and not helpful. Perhaps a NEW manual Andy..(hint)...-) The posts here at TGP are very helpful as well. Thanks

 

maybe I'm wrong.....

 

So regarding the 3 trimmer version vs the latest does that mean that on the latest boards it will have the maximum amount of LF built into the OD circuit?
That could explain some of the difference I have noticed between my ODS30 modded to SLX and a new model I tried out last month.

Thanks

Dan

 

RE: Tom Gross' comment: I believe the mods do work in reverse. They only have one OD trimmer btw. The production trimmers work in "normal fashion".

Danel59: Yes, the amps have no low frequency limiting or rolloff between clean and dirty channels at this time.

 

Guess I better stop buying mods and get a production Fuchs

 

Ya-know, if everyone did, I'd be retired by now......

 

Andy - Do adjusting these trim pots affect your warranty on the amp?

 

First of all, one interesting note, brought up by one of the shop guys. Depending on the brands of trim pots, they may work in reverse...lol. I noticed a few different brands do that. Weird.

Anyway, no, it would not void warranty, unless you messed something ELSE up while inside.

Depending on how old the unit(s) are (some dealers insist on NOT rotating stock), you should have two pre OD trims, one post OD trim and the reverb dwell.

Reverb dwell sets the amount of drive INTO the pan. Not unlike the dwell control on an outboard Fender reverb. Depending on how this is set, you can get surfy to smooth, which is cool. The current boards show a date of 2007 on them. The earlier amps did not have this.

The manual does show the board, I will check if it's the current version. If not, we will upgrade it. We are in the midst of writing a new site, and I hope to have a tech page, which can cover all this fun stuff.


We evolve always.....

 

So with the HF Trimmer is it possable to get (on the OD) a way darker solo tone?
Looking for low fizzys with a Tele Bridge Pup.

 

I recently learned a few useful things about the internal trim pots in my recently acquired TDS, some of which knowledge came after implying to Andy that one of the pots (the HFOD) was functioning in a way opposite what was intended. As Scott Lerner pointed out, the HFOD increases gain along with treble content as the resistance on the pot is lowered, which made me think at first that the bass was being boosted (which it was, but not as much as the treble frequencies were boosted). So that is one thing learned - the HFOD will bring about an increase in overall gain (thanks to Scott for setting me on the right track) along with an even greater increase in treble frequencies. It will conversely bring about a decrease in overall gain along with a decrease in treble content.

I found that the main thing that helped the sound of the amp's overdrive was the AC Balance control. The amp's overdrive sounded "OK" before, but had some "grittiness" to it, which I was at first thinking might be due to the selection of preamp tubes (is there a chart showing which preamp tubes do what in a TDS or ODS?). I read in the TDS manual that the AC balance was factory set so that the overdrive would produce "linear clipping at maximum power output", and this was done with an oscilloscope (which I don't have). I wasn't sure if the AC balance pot would affect the overdrive at less than maximum power output (at basement studio levels, which is where the amp is being used). I do know that sometimes people can hear things that can't be seen on scopes or measured on voltage meters, and I can still hear pretty well in spite of having tinnitus, so I decided to try adjusting the pot. The amp's adjustment might have shifted over time, or perhaps somebody else adjusted the controls before I purchased it. In any case, by turning the AC Balance pot slightly and listening to the results, I was able to get rid of pretty much all of the grit in the overdrive tone, so that now the overdrive has a singing tone quality to it (like Larry Carlton's or Robben Ford's tone) rather than a raspy sound. I was able to dial this in by ear (to find the optimum sound for the volume levels I am playing at), and would recommend that any Fuchs ODS or TDS owners who have an amp with similar symptoms (gritty sounding overdrive) try tweaking the AC balance control while listening carefully to the results. It might save your relationship with your amp (which this probably has done for me and my TDS).

Sorry for being long winded about this.