Design 'flaw' in HRM clone?

[odourboy]

Edit to say: Brain Fart, so don't waste much time reading this!

Came to a conclusion about the HRM design that I'd like to pose to the forum for your thoughts.

I recently posted a clip on an HRM build I'd done and got some detailed feedback. In each case, the reviewer remarked on weak OD mid tones - weakness in the lower mids, honking etc. So I decided I'd better dig into this and see what was going on. In another post, BobI mentioned the need to bump up the value of the cap coupling the the two OD sections to get solve a harsh constipated OD tone problem he was having with his Peavey conversion. So I had a look at that cap.

Referring to the ODS-101-HRM schematic https://tubeamparchive.com/download/file.php?id=1179
you'll see that the .0022uF cap between the two OD sections works with the drive pot to form a simple high-pass filter. Plugging the values into my trusty calculator yielded the 'knee' at about 290 Hz with the drive full open (right in the middle of the guitar range!). Notice also that as you sweep the drive back, the cuttoff frequency rises cutting off more and more mids and bass. Not very user-friendly to have a filter sweeping right through your mid range as you alter you drive eh?

I decided to pop in a .022uF cap in it's place, which moved the filter knee down by a factor of 10. Huge difference for the better. I was able to cut back my drive and input levels, reduce the mid and bass controls on the OD stack and get a smoother, less constipated OD tone - major step in the right direction into Dumble-land for this amp IMHO.

So I was wondering what you all think? Does my analysis make sense or is an HRM 'supposed' to work like this? Has anybody else had this experiece?

OB

[ayan]

Came to a conclusion about the HRM design that I'd like to pose to the forum for your thoughts.

I recently posted a clip on an HRM build I'd done and got some detailed feedback. In each case, the reviewer remarked on weak OD mid tones - weakness in the lower mids, honking etc. So I decided I'd better dig into this and see what was going on. In another post, BobI mentioned the need to bump up the value of the cap coupling the the two OD sections to get solve a harsh constipated OD tone problem he was having with his Peavey conversion. So I had a look at that cap.

Referring to the ODS-101-HRM schematic https://tubeamparchive.com/download/file.php?id=1179
you'll see that the .0022uF cap between the two OD sections works with the drive pot to form a simple high-pass filter. Plugging the values into my trusty calculator yielded the 'knee' at about 290 Hz with the drive full open (right in the middle of the guitar range!). Notice also that as you sweep the drive back, the cuttoff frequency rises cutting off more and more mids and bass. Not very user-friendly to have a filter sweeping right through your mid range as you alter you drive eh?

I decided to pop in a .022uF cap in it's place, which moved the filter knee down by a factor of 10. Huge difference for the better. I was able to cut back my drive and input levels, reduce the mid and bass controls on the OD stack and get a smoother, less constipated OD tone - major step in the right direction into Dumble-land for this amp IMHO.

So I was wondering what you all think? Does my analysis make sense or is an HRM 'supposed' to work like this? Has anybody else had this experiece?

OB

The calculation of the frequency breakpoint is not trivial, I trust you've looked up the correct equation to plug your numbers in (just about everything plays a role). I disagree with your comment about the setting of the drive pot having an effect on the filter break point: the output of V1b sees a number of things (impedance wise), one of which is a path composed of a 100K resistor + 250K pot to ground. It sees the whole value, regardless of where the pot is set at... unless you connect your drive pot "backwards," like Gibson has done in some guitars and like most people do on their "Reverb" level of Blackface amps, in which case the tube output sees a variable impedance to ground epending upon the pot's setting.

Cheers,

Gil

[dogears]

I strongly disagree. I have tried all typed of permutations at that location. Both in cap size and load to ground. I find that the 183HZ knee (your calculations are incorrect btw) is right about where it sounds best. To my ears and to the ears of the people I have either helped or conferred with. Even lowering the knee from 183HZ to 163HZ is very noticeable. The amp gets woofy on the bottom. The key to the mids is NOT this cap. Please send me pics of your build as well as the exact schematic. I know you have mentioned what you built off of, but refresh my memory... Did you exactly follow the ODS101?? What feedback circuit?

Thanks

[odourboy]

Sorry Gents - I take most of this back. I found I had an error in the simulation I was using. The frequency does not sweep as I was thinking it might (my initial thinking was that the 'filter' was being formed between the cap and 'tail' of the drive pot, and a messed up simulation supported my own faulty reasoning!) I still have a descrepency in the -3dB point, but my model isn't totally complete, so I'm willing to concede that one as well.

Anyway, I did want a second opinion which is why I posted this as a question!

So I think my amp is sounding better when I change that cap, but it's not for the reason I thought. Hmmmm....back to square one.

Think I'll just order a kit.

[ayan]

Sorry Gents - I take most of this back. I found I had an error in the simulation I was using. The frequency does not sweep as I was thinking it might (my initial thinking was that the 'filter' was being formed between the cap and 'tail' of the drive pot, and a messed up simulation supported my own faulty reasoning!) I still have a descrepency in the -3dB point, but my model isn't totally complete, so I'm willing to concede that one as well.

Anyway, I did want a second opinion which is why I posted this as a question!

So I think my amp is sounding better when I change that cap, but it's not for the reason I thought. Hmmmm....back to square one.

Think I'll just order a kit.

Ordering a kit is a way to go, but the schematics floating out there are trued and tried and sound good -- in different ways depending upon which one you use. If you want to chase the last 5% of "great tone," you may spend a lifetime at it, but a well-built amp built to those schems will get you there almost all the way, guaranteed. Don't give up quite yet, once you get the amp dialed in, you may never look back -- then again, it may be the beginning of the end for you and your free time.

Gil

[Tonegeek]

Ditto on not giving up! I was alternately frustrated and elated for 2 months trying to dial my amp in. Ignoring the fact that I have a built in Dumblelator, I ended up mostly returning to the values in ODS-101 after wandering around for a while. (I too needed more lower mids and tried upping the OD coupler but returned back to .0022 when it sounded just like what Dogears described). While trying for more fats, a while back I put the input to the PI at .02 and my cathode cap on OD2 to 5uF. Just last week I lowered the two caps to .01 and 1uF respectively (both ODS-101 recommended values). That in combo with reajusting the HRM was the magic it needed. This next statement is a bit counter-intuitive given my cap changes but it makes sense when you analyze it: My bass was actually tighter and I got the lower mids I was needing after the changes. BAsically lowering the bass (just getting rid of junk below useable guitar frequencies) brought out the lower mids and the real bass of the guitar. Anyway to each his own. If your ears are happy with the change then that's what matters most. I would bet money there are some clones out there that strayed way off ODS 101 and sound great (LOL groovetubin )
good luck!
whit

[groovtubin]

Ditto on not giving up! I was alternately frustrated and elated for 2 months trying to dial my amp in. Ignoring the fact that I have a built in Dumblelator, I ended up mostly returning to the values in ODS-101 after wandering around for a while. (I too needed more lower mids and tried upping the OD coupler but returned back to .0022 when it sounded just like what Dogears described). While trying for more fats, a while back I put the input to the PI at .02 and my cathode cap on OD2 to 5uF. Just last week I lowered the two caps to .01 and 1uF respectively (both ODS-101 recommended values). That in combo with reajusting the HRM was the magic it needed. This next statement is a bit counter-intuitive given my cap changes but it makes sense when you analyze it: My bass was actually tighter and I got the lower mids I was needing after the changes. BAsically lowering the bass (just getting rid of junk below useable guitar frequencies) brought out the lower mids and the real bass of the guitar. Anyway to each his own. If your ears are happy with the change then that's what matters most. I would bet money there are some clones out there that strayed way off ODS 101 and sound great (LOL groovetubin )
good luck!
whit

OLOLOLO!! If they only knew...