Low plate classic entrance!

[Dr d]

I'm having a hard time dialing in the OD entrance trim. At the moment I have it set at @37ohms to ground because any higher seems to be a little gainy.....other times it feels like its not gainy enough!!. Should it be set a little higher at say 50ohms? Any experience/input on this one would be greatly appreciated.

[Structo]

If you are talking about the 100K trimmer on the OD entrance, you measure the resistance to ground from the wiper to ground.

Do this with the amp on and OD switched on. (Be careful!)

This was something that was not widely known for quite a while and once the word got out it helped a bunch of us.

The recommended setting is around 22K-25K (that is ohms) but it is subjective and depends what kind of OD tone you are after.

[Dr d]

Thanks for the reply, Tom. My understanding/findings for the the low plate design is that the setting needed to be higher....just how high is what I am uncertain about. Thanks all the same.

[greiswig]

I'm a bit puzzled by the volts (30V?) reference, but I'll assume you're referring to kilohms?

Elsewhere I remember several people saying that they preferred settings higher than 25k ohms to ground also. This struck me as odd, because on my own amp (low plate classic), anything over about 18k is too much gain.

PS - to add to what Tom said, measuring the resistance on my amp is a bit tricky: if I remember right, it is only accurate if the amp is all the way off or if the amp is all the way on, i.e. not on standby. If you have standby on, the reading will be different. But the reading with the amp off is good as well as safer.

[Dr d]

Thanks Greiswig.....I meant ohms The thing that puzzles me is the knowledge that #124 was set at @ 100ohms. I know it is a different entrance etc but being low plate leads me to think it might be roughly twice the setting than for a high plate. I'm probably barking up the wrong tree!! Thanks for your input G.

[Structo]

Actually the notes from #124 say that the OD trimmer was marked as a 500K trimmer, but in fact measured at 350K.
It also says that it was 100K to ground.

All that is conflicting with the majority of other amps that have been looked at.

Dogears has stated more than once that the typical 220K feeding the 100K trimmer is the more common setup and that it should be set with the amp on and standby set to play, measure ohms from the wiper to ground.

It does make a difference if the amp is in standby or in play
The OD switch must be turned on and the normal range of settings is 20K to 30K.
But that is not set in stone.
Now where you set that is very subjective and there is no set rule for that.
If you like a bit more overdrive then set it higher.

Before most of us were setting it with the amp off and that is not a real world condition.
I know it sounds weird to have the amp on and measuring resistance but that is the way to do it.

Just a matter of trying a few different settings to see what you like.

Also, have the amp volume up where you normally would play.
If you gig outside of home, then make sure you are setting the trimmer at the volume you gig at.

[greiswig]

FWIW, I have measured repeatedly with amp off and fully on (standby in "Play" position), and I don't measure any difference between those two settings using my Fluke. Hence my comment that it might be safer to measure it with the amp off.

[steve_p]

Has anyone used a 250k OD trimmer on a 124 build? I am working on one now and that is what I currently have installed.
I figured that 250 was closer to the 350k measured value of the actual amp vs a 100k that is seen elsewhere.

What difference can I expect to see between the two values?
If it is suggested that I change it out, I am at a good point where it wouldn't be too tough to take the board out and change the pot.

Thanks,
Steve

[Dr d]

Thanks for the input, guys. It is clear that for high plate designs the ball park figure for setting the trigger is @ 20-30ohms. I was just looking for a guide for the low plate. Thanks all the same.

[martin manning]

The setting to ground will depend upon the value of the trimmer as well as the amount of overdrive distortion desired. This network is a voltage divider that attenuates the signal from V1b going into V2a. ODS #124 had a 220k and a 345k (measured value) trimmer and was set at ~100k to ground for ~82% attenuation. Amps with 100k trimmers seem to be set ~25k to ground for ~92%, and I don't think it will be much different for low- vs. high-plate, but you can see that there is a little less attenuation in #124. That could be because the low-plate circuit has a little less distortion to begin with, but the total resistance (the 220k plus the trimmer element) also plays a role in setting the AC load on V1b, and will affect its distortion characteristics too. I believe that in #124 the 565k total resistance is uniquely high.

The issue with measuring the resistance to ground is that with the OD relay "off" the 220k plus the upper part of the trimmer element will be paralleled with the lower part of the element (as determined by the wiper setting). This will make a couple of k-ohm difference in the measurement, and isn't a big deal. Set the trimmer by ear so that you can get a good range of overdrive tones with the drive control and/or to get the distortion tone you find most pleasing at a particular distortion level.

Thanks for the input, guys. It is clear that for high plate designs the ball park figure for setting the trigger is @ 20-30ohms. I was just looking for a guide for the low plate. Thanks all the same.

[tubedogsmith]

Maybe I'm missing something but it would seem to me that the best way to set the OD trimmer is by listening and adjusting. Once you like what you hear you can measure it with the amp on or off for reference.

[greiswig]

Maybe I'm missing something but it would seem to me that the best way to set the OD trimmer is by listening and adjusting. Once you like what you hear you can measure it with the amp on or off for reference.

I think this is a key point. Use the "measured values" things as guidelines, but only that. Let your ear guide you.

That said, it really threw me to find that my amp distorts plenty with such a low trimmer setting, and that fact made me wonder if I had something else wrong with the circuit somewhere. In the end, I stopped worrying about it because it sounds pretty darn good.

[Dr d]

Thanks again for your input (especially you, Martin). I agree, use your ears!! However, it just seemed a little less obvious with the low plate!

[marcos]

FWIW,
I remember a post by Dogears a while back where he reported
that the real Dumbles with the low plate configuration had the trimmer set
between 40 and 50k to ground , probably to compensate for the lower gain of the lower plates. The attenuation will be similar to #124 in this case. BTW at least two more amps have the 500k trimmer, #121
and #123, so that seemed to be HADs favourite setup for a while.
As always, trust your ears
Marcos

[martin manning]

Thanks for the additional info, Marcos!