100K Drive vs 250K

[talbany]

Using my coupling capacitor calculator (compensating for frequency shifts at .01 V2a cap OD circuit Non HRM) I thought I would determine the difference in frequency response between the 100k drive pot..( V2a) and the 250k pot both set at 70K...Here are my results..
At the bottom you can see the frequency shift starting at around 15Hz to -2.77dB at 800Hz using the 250K..Clearly the 250K will allow you to set the gain higher..
The point of posting this is not to say that one is better or worse just better suited for your ears... Just that in the case of a drive or level control or OD entrance trim tone is also a factor along with gain when selecting different value pots
For those who think their OD is a bit on the bright side and want to have more gain at their finger tips the 250k maybe something to consider (personally I think this better than the treble bleed)..If you like a brighter tighter tone w/less gain the 100K is better suited..
If you want to control the highs but like the 100k Drive value you can raise the value of the OD trim but still set it a what ever resistance you like and since the pot preceed's the OD gain stages the effects will be greater..This is what I believe Dumble was thinking when scratching the trace on 124 OD entrance trim and 250k drive sorry level.. Hope this helps..

Tony

[ayan]

Using my coupling capacitor calculator (compensating for frequency shifts at .01 V2a cap OD circuit Non HRM) I thought I would determine the difference in frequency response between the 100k drive pot..( V2a) and the 250k pot both set at 70K...Here are my results..
At the bottom you can see the frequency shift starting at around 15Hz to -2.77dB at 800Hz using the 250K..Clearly the 250K will allow you to set the gain higher..
The point of posting this is not to say that one is better or worse just better suited for your ears... Just that in the case of a drive or level control or OD entrance trim tone is also a factor along with gain when selecting different value pots
For those who think their OD is a bit on the bright side and want to have more gain at their finger tips the 250k maybe something to consider (personally I think this better than the treble bleed)..If you like a brighter tighter tone w/less gain the 100K is better suited..
If you want to control the highs but like the 100k Drive value you can raise the value of the OD trim but still set it a what ever resistance you like and since the pot preceed's the OD gain stages the effects will be greater..This is what I believe Dumble was thinking when scratching the trace on 124 OD entrance trim and 250k drive sorry level.. Hope this helps..

Tony

The above must assume some kind of plate load/cathode resistors, correct? It may be worthwhile noting what those values are for those who don't have the calculator spreadsheet.

Another very simple way to address a bright OD is to raise the output cap from .0047uF to .01uF. Higher than that will not make too much of a difference, but .01uF sounds good. The treble bleed is to help impart a certain chartacter that one would be hard-pressed to achieve by tweaking before the output of the overdrive. Another common solution is to increase the output resistor to 180K, which will decrease the available overdrive volume.. or to eliminate the output resistor all together and replace the output 100K pot with a 500K pot. More volume available and less brightness. I have recommended this to some folks who have used it with success.

As far as the trimmer on 124, I saw that sucker with my own eyes. Still not sure whether Dumble had been in there and tweaked the trimmer, or simply picked a low value "350K" one from his bin of old parts.

Gil

[dogears]

Tony, in my experience, the two main differences of the 250k pot is the increased miller effect causing loss of high end and the increase in bass as the rc network created by the .01 and the pot allow more bass.

The gain observations you note are secondary IMO. Obviously the voltage divider allows for more gain with the 250k pot. At similar gain levels, the 250k is darker and deeper as noted above.

Also, note you may want to try a 150k grid resister on od2 to compensate for the miller effect.

S

[talbany]

Using my coupling capacitor calculator (compensating for frequency shifts at .01 V2a cap OD circuit Non HRM) I thought I would determine the difference in frequency response between the 100k drive pot..( V2a) and the 250k pot both set at 70K...Here are my results..
At the bottom you can see the frequency shift starting at around 15Hz to -2.77dB at 800Hz using the 250K..Clearly the 250K will allow you to set the gain higher..
The point of posting this is not to say that one is better or worse just better suited for your ears... Just that in the case of a drive or level control or OD entrance trim tone is also a factor along with gain when selecting different value pots
For those who think their OD is a bit on the bright side and want to have more gain at their finger tips the 250k maybe something to consider (personally I think this better than the treble bleed)..If you like a brighter tighter tone w/less gain the 100K is better suited..
If you want to control the highs but like the 100k Drive value you can raise the value of the OD trim but still set it a what ever resistance you like and since the pot preceed's the OD gain stages the effects will be greater..This is what I believe Dumble was thinking when scratching the trace on 124 OD entrance trim and 250k drive sorry level.. Hope this helps..

Tony

The above must assume some kind of plate load/cathode resistors, correct? It may be worthwhile noting what those values are for those who don't have the calculator spreadsheet.

Another very simple way to address a bright OD is to raise the output cap from .0047uF to .01uF. Higher than that will not make too much of a difference, but .01uF sounds good. The treble bleed is to help impart a certain chartacter that one would be hard-pressed to achieve by tweaking before the output of the overdrive. Another common solution is to increase the output resistor to 180K, which will decrease the available overdrive volume.. or to eliminate the output resistor all together and replace the output 100K pot with a 500K pot. More volume available and less brightness. I have recommended this to some folks who have used it with success.

As far as the trimmer on 124, I saw that sucker with my own eyes. Still not sure whether Dumble had been in there and tweaked the trimmer, or simply picked a low value "350K" one from his bin of old parts.

Gil

Gil
Thanks for the response and clarification of the OD trimmer on 124..The calculations were derived from the following..
Output impedance of the driving stage
Size of the coupling capacitor (.01)
Size of the Drive pot
Setting of the pot (in this case 70K)
Using a 1K signal

The above must assume some kind of plate load/cathode resistors, correct?

Understanding this is a fairly crude model that considers the fundamental frequency response of the cap itself in a static model in this case stock Non HRM..Everything else being equal..Or am I wrong in assuming this..

The findings do collaborate with what my ears tell me..(High and low plate)
Although the model doesn't take into account the Miller effect The size of the pot or( load) and where the pot is set does effect the input impedance of the following driving stage effecting tone..(Like Scott mentions) Will try 150k.. Thanks!!
FWIW.. I was NOT in any way saying that this method was any better or worse than the treble bleed.. Just different.. The treble bleed to my ears does help tame the highs no doubt but to me smooths the top end to much for my taste where I like that bit edge/crunch on the top end that the higher value pots give me.. That's it just taste..Keep in mind the 250k mod I've done on 100w only.. For those building a 70's version the 250K Overdrive control can also help especially if you are using the funky OD entrance which can get be beamey and lack some girth..

FWIW.. I have had great success with removing/lowering the output resistor and raising the pot 500k as well!!..PUNCHY

Tony

[ayan]

FWIW.. I have had great success with removing/lowering the output resistor and raising the pot 500k as well!!..PUNCHY

Tony

Tony, you are a genious. Really, I think that is a pretty good place to make a lot of chance for not a lot of work.

Cheer,

Gil

[talbany]

Tony, you are a genious

Yeah genious.. Want to see my list.. 5000 ways to make your amp sound like ass and 3 ways to make it sound good ..

T

[Dr d]

Thanks for the post Tony.... your points of discussion are always of interest and usually address those nigling questions that are difficult to get your head around. Thanks also to Gil and Scott for your input. Tuning the OD on the non-HRM can be a bit of a nightmare and it is therefore useful to know the different ways of tackling frequency response. Personally, I like the "gravity" that the 250k pot allows.....especially useful for me at lower playing volumes. Thanks again guys.

[wjdunham]

Helpful analysis Tony. I have not played much with the values on the drive and level pots for the OD, all my amps have 100K in both. I hooked up a decade box in place of the 180K resistor on the output of OD2. I didn't hear much of a shift in tone (playing with values between 100K and 200K), just gain. I've read here people think 150K vs. 180K makes a difference in the thickness of the OD, but I really didn't hear it.

One thing you've mentioned here and also in another thread is the smoothing effect of the larger value pots. Can anyone comment on what's going on here, as far as the higher resistance carbon trace bleeding some of the highs. Where are they going? There must be some capacitive coupling to the case (ground) or some other similar effect?
Curious minds need to know.
Thanks
Bill

[wjdunham]

After reading further in this subject (always helps to do that BEFORE posting), I see Scott's explanation of the increased Miller effect. Having not done the calculations, I had always assumed that with the typical values that the cutoff was way above the audible range of frequencies (ie milller effect was used to prevent radio frequency coupling, not to roll off the highs in audible range). Certainly the high frequency loss due to miller effect would explain the situation more than loss due to the traces in the higher value pots?

[dogears]

Miller Effect reaches deep into the audible range when using bigger grid resistors.

The grid resistor on the preamp stages typically ranges from 0 to 68K, although very large values, such as 470K, are sometimes used in high-gain preamps to shape the frequency response and prevent "blocking" distortion in the preamp section under heavy overdrive conditions. The Miller capacitance of a typical 12AX7 is around 151pF, so the upper frequency response -3dB cutoff point of a stage using a 68K grid resistor is around 15.5kHz. The frequency response drops to around 2.2kHz if a 470k grid resistor is used. This "free" response rolloff can be used to tame the "buzziness" of high-gain preamp stages without having to add additional rolloff capacitors.

After reading further in this subject (always helps to do that BEFORE posting), I see Scott's explanation of the increased Miller effect. Having not done the calculations, I had always assumed that with the typical values that the cutoff was way above the audible range of frequencies (ie milller effect was used to prevent radio frequency coupling, not to roll off the highs in audible range). Certainly the high frequency loss due to miller effect would explain the situation more than loss due to the traces in the higher value pots?

[mr_hankey]

How would one go about making the OD's high end smoother, without decreasing high end? My #124-ish build sounds good with lower gain; but when I turn it up it gets too crunchy. Smooth high gain just isn't happening.

[dogears]

Change the plate loads to 220/150 and cathodes to 3.3 and 2.2. 4.7uf on all four stages. Set gain trimmer at 24-26K to ground, per taste.

Use 100K drive pot. Not 250K.

Use standard 220K into 100K OD entrance.

She'll be smooth as silk....

How would one go about making the OD's high end smoother, without decreasing high end? My #124-ish build sounds good with lower gain; but when I turn it up it gets too crunchy. Smooth high gain just isn't happening.

[talbany]

After reading further in this subject (always helps to do that BEFORE posting), I see Scott's explanation of the increased Miller effect. Having not done the calculations, I had always assumed that with the typical values that the cutoff was way above the audible range of frequencies (ie milller effect was used to prevent radio frequency coupling, not to roll off the highs in audible range). Certainly the high frequency loss due to miller effect would explain the situation more than loss due to the traces in the higher value pots?

Bill
Actually both..
Grid stoppers act as a very high frequency low-pass filter in conjunction with the input capacitance of the triode which is the sum of the grid-to-cathode capacitance and the Miller capacitance, and can get as high as 100pF or more...68K is really all you need to block out stray RF.. Anything above this and your now entering tone shaping as well as surpressing any oscillations that might be generated.. although very large values, such as 220K, are used in high-gain preamps to shape the frequency response and prevent "blocking" distortion in the preamp section under heavy overdrive conditions. The Miller capacitance of a typical 12AX7 is around 151pF, so the upper frequency response -3dB cutoff point of a stage using a 68K grid resistor is around 15.5kHz. The frequency response drops to around 2.2kHz well within the audiable range like Scott mentions.. if a 470k grid resistor is used. This "free" response rolloff can be used to tame the "buzziness" of high-gain preamp stages without having to add additional rolloff capacitors.

Miller Capacitance. The grid of a tube and the plate form a teeny capacitor, but when the plate is changing voltage relative to the grid, as happens in a triode, the small amount of capacitance is multiplied by the tube’s gain. That teeny capacitance is now real significant. When the pot is of high value and the position is down at low volume, there is a large series resistance (the carbon track) trying to drive the input capacitance of the tube and loosing treble in the process.

Some of the info above Randall Aiken

Hope this helps.............Tony

[wjdunham]

I tried a few different OD entrance trimmers today on a 50W low plate skyline amp with EL34's. I tried 100K, 250K and 500K. I settled on the 500K as the best of the three, mainly because I could finally get the amp to bloom and feed back with the higher value. I had the 100K in there since I built the amp, the tone was fine, it just wouldn't bloom until I made this change. I had also tried a few different PI tubes, I have about 6V higher on the negative input side. I'm really happy with the direction that the 500K trimmer took it. The amp is still plenty bright, and I didn't notice a big difference in the highs between the three.
I forgot to take a measurement before I buttoned the amp back up for more testing, but I had it set just more than 1/3 of the way up. This has really been the last mystery for me, getting the amp to bloom at reasonable volumes, and where the tuning points are to make the amp respond.
Bill