Bombacaototal wrote: ↑Sat Oct 06, 2018 8:40 am
And just a curious is there any difference between 1M pot and a 500k resistor across or a 500K pot and a 1M resistor across to get to 333k as far as taper, etc?
You can calculate this using (Rpot*Rfixed)/(Rpot+Rfixed):
Fe. In the 1M pot with 500k resistor if the pot is halfway: (500k*500k)/(500k+500k)=250k
In the 500kpot with 1M resistor if the pot is halfway: (250k*1000k)/(250k+1000k)=200k
Using a pot setting in steps of fe. 50k you can calculate each outcome and plot them in a XL sheet to what it does with the taper opposed to the original.
Last edited by erwin_ve on Sat Oct 06, 2018 11:35 am, edited 2 times in total.
martin manning wrote: ↑Sat Oct 06, 2018 11:16 am
On my 124, I used a 100k trim pot, 220k series in front, and 68k grid stopper, like the other dicumented ODS circuits. The 350k used on 124 is an outlier IMO. In the original, the pot was marked 500k, but appeared to have been modified. My recommendation is to go with a 100k trimmer.
That network (the two resistors and the trimmer) is shaping the frequency response in several ways. Firstly, it is part of the AC load on clean2 when OD is on. Secondly, combined with the clean2 coupling cap it is a high pass filter, and finally, when combined with the Miller capacitance of OD1, it forms a low pass filter.
That's a great explanation Martin in only a few sentences!
Charlie Wilson wrote: ↑Fri Oct 05, 2018 10:09 pm
I am curious, where does the 120k resistor in reference to #124 come from?
me too. All the layouts and schematics I have show a 220k.
Bombacaototal wrote: ↑Fri Oct 05, 2018 11:59 pm
I was actually wondering the same as well
erwin_ve wrote: ↑Sat Oct 06, 2018 5:54 am
You're right it should be 220k.
Edit my post
This is from the 124 schematic linked earlier in this thread: there is a 120k, then a 350k trimpot marked 500k, with a asterisk explaining that it's a 250k pot and a 220k resistor in series...
Schermafdruk van 2018-10-05 21-57-45.png
I'm beginning to think that the 220k combined with the 250k pot is correct... this schematic is literally saying: 120k resistor + 220k resistor + 250k pot + 68k grid stopper
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martin manning wrote: ↑Sat Oct 06, 2018 12:20 pm
Well stop that. The layout drawing and the first schematic .pdf in the “A Collection of #124 Information” thread in the files section are correct.
Martin is right. Don't be fooled. You can try it a different way and like it though. Nothing wrong with that.
"Education is what you're left with after you have forgotten what you have learned" - Enzo
Bombacaototal wrote: ↑Thu Sep 27, 2018 10:43 am
By the way, is there a consensus that the high plate overdrive (ie #102) is nicer than the low pate overdrive (ie #124). Also on the #124 it must have been a 500K trimmer to have it set at 346K, correct?
This was the original question, but looking at the 124 schematic, it hints at a 500k trimpot
Schermafdruk van 2018-10-05 21-32-48.png
Why does it say 'cmpt marked 500k'? what is cmpt? Was the board marked 500k? Was the trimpot marked 500k but in reality 350k?
But there is also an asterisk, which refers to this:
Schermafdruk van 2018-10-05 21-33-02.png
So it is a 250k pot? with a 220k in series? That's 470k
Anyway, there is a lot of confusion about these schematics and this is only natural, but it does not help to consider them the Bible.
I don't understand why all this series resistance is there, and still convinced a 500k trimpot will work too. Or a 250k. I don't think a 350k trimpot ever existed, but that's beside the point.
BTW.The layouts done in the files section here at the Amp Garage is a collection of the most comprehensive, collaborative info ever collected on the web (124 being the 1st ever documented) from both well-respected amp builders, collectors and amp technicians around the world many have either worked on owned or dealt directly with HAD in the build process or upgrading process of their amp. To also verify the authenticity of each generation ODS published. Hundreds of pictures reviewed correspondence read and verified, Some additional info was also given but wished to stay anonymous and was honoured in the process. So the info for the layouts were NOT taken from any 1 schematic,but from multiple real untouched ODS, specimens floating around the planet!.
How accurate are they?
Chances are if you have a Ceriatone amp your Overtone model was based off one of these layouts which were done way before Nik started offering these different models on his site. Other schematics posted around TAG (like the one you have) and on the net may not be 100% accurate!
Tony
Last edited by talbany on Sat Oct 06, 2018 7:28 pm, edited 3 times in total.
" The psychics on my bench is the same as Dumble'"
Structo wrote: ↑Thu Aug 30, 2012 2:10 pm
One thing that is nice about the jumbled mess the [Dumble] file[s] section is in, it prevents the paint by number guys that don't know a thing about working with high voltages from killing themselves.This also forces people to search for the information and not expect it on silver platter.
That was years ago. I’m sure it’s much more organized now. ;^) In any case, anyone who is dissatisfied can have his or her money fully and cheerfully refunded!
Structo wrote: ↑Thu Aug 30, 2012 2:10 pm
One thing that is nice about the jumbled mess the [Dumble] file[s] section is in, it prevents the paint by number guys that don't know a thing about working with high voltages from killing themselves.This also forces people to search for the information and not expect it on silver platter.
You're debating semantics with the guys that created the content you're attempting to show a strong knowledge of... may want to rethink that methodology. Most people that are training to be chefs don't go to the chef that wrote the course material they need to better understand their metier. I may be mistaking your intent, it is a forum after all, but it seems you using terms like "Mr Know It All" and trying to correct the gurus is potentially a bit offputting. If I've misunderstood the conext, then please ignore.
I think it's not the 'max voltage' but the voltage it sees. i.e. the voltage at the input of the pot may be like 350V but at the output its 330V so that is actually operating at 20V remember it's 'relative' voltage. the voltage drop over that device is pretty nominal at that point between the B+ and the other resistors. I think the most I remember seeing in many cases across an anode resistor was maybe 100V or so? but I have a limited experience with them. Most of the voltage drop happens across the tube between anode and cathode.
They have the same efficiency (99db), same weight and a somewhat similar frequency response. I think the difference between them is only at the high end.
My only concern is the 8ohm only for the 100W Celestion. I usually prefer 15ohm speakers than 8ohms (don't know scientifically why) and all of my cabs now are loaded with 15ohms. The issue may be if I want to swap speakers around I will be restricted to the difference impedance.
Would the classic lead 80 pair well with EVM-L? I know weight will be very different with the EVM being twice the celestion and 300W vs 80W as well worries me...
As a background I do not bond with the G12-65 at all (super dark and dull for me) and found the Alnico Gold better but still not great. My favourite now is the discontinued Neo Century (made in UK - not the vintage one selling now) which makes me think efficient speakers are the way to go for me.
trust me a single EVM12 cab will give you all the power you'll ever need, I would make a single, sealed ported Larry Carlton cab and stick with EVM in there.
Try that for a while and then see if you think you need a 2x12. I have the oval backed 2x12 dumble can with two vintage G12-65's it's a great cab and speaker combo for a dumble amp. it's what Robben ford has used for years. It's big and heavy though so I prefer the EVM, which is small and heavy
