Tweaking My 102 Amp: A Massive Resistor Makeover

[ayan]

I know I'm going to feel stupid for asking this but what is the HF Taper and HF trim?

High frequency taper or trim is just a tone control (cap in series with a variable resistor) placed at the output of the overdrive stage, usually hanging to ground on the output of the V2B 150K resistor. Search for the term and you'll likely find a lot of info on this.

[pompeiisneaks]

3. OD HF Taper

Through the years, I went back and forth on this a few times. Early on, I used it. After getting the caps’ outer foils orientation correct, I got rid of it. When I added a larger bright cap on the Master volume, because I liked what it did to the clean, I reinstalled it and it stayed there. I had also tried using a 180K OD output resistor, instead of 150K, but I didn’t like that it decreased the available OD volume somewhat. In the end, I had opted for staying with the 150K and using the HF trim. This time around, I disconnected the HF trim and used a 180K Draloric resistor from Jelle. I will put it this way: I found that the HF trim, which for me was a .001uF cap and the 1 M trimmer dialed in at about 350-400K to ground, has more of an effect on the OD volume than using the 180K output resistor. The overdrive is on the bright side but it is very musical at the same time.

Gil

I know I'm going to feel stupid for asking this but what is the HF Taper and HF trim?

High Frequency Taper and High Frequency Trim. I think... Basically the HRM has a low mid and high frequency control on them and the taper is the capacitor used in that region, with the trim which is a pot.

Wouldn't mind someone fixing my erroneous impressions if they are

~Phil

[talbany]

3. OD HF Taper

Through the years, I went back and forth on this a few times. Early on, I used it. After getting the caps’ outer foils orientation correct, I got rid of it. When I added a larger bright cap on the Master volume, because I liked what it did to the clean, I reinstalled it and it stayed there. I had also tried using a 180K OD output resistor, instead of 150K, but I didn’t like that it decreased the available OD volume somewhat. In the end, I had opted for staying with the 150K and using the HF trim. This time around, I disconnected the HF trim and used a 180K Draloric resistor from Jelle. I will put it this way: I found that the HF trim, which for me was a .001uF cap and the 1 M trimmer dialed in at about 350-400K to ground, has more of an effect on the OD volume than using the 180K output resistor. The overdrive is on the bright side but it is very musical at the same time.

Gil

I know I'm going to feel stupid for asking this but what is the HF Taper and HF trim?

High Frequency Taper and High Frequency Trim. I think... Basically the HRM has a low mid and high frequency control on them and the taper is the capacitor used in that region, with the trim which is a pot.

Wouldn't mind someone fixing my erroneous impressions if they are

~Phil

Phil
Sounds about right however it depends on the amp. For instance in a Bluesmaster with a mega-boost when you go to overdrive and lift the main stack out with the mega-boost you have a topology much like a Marshall 2204 style Hi-gain input (however without the cathode follower driving the stack)
The HRM board now becomes the main tone stack so this OD is voiced with the main stack after V1a out of circuit. In the case of the high-frequency trim this is put there to contour the top end of a non HRM amp.

Robben Ford's 102 at one time had this high-frequency trim control. We suspect this feature was removed when Alexander up-graded 102 to a high plate Skyline which along with the LNFB on V1B has a flatter top end response so the trimmer was removed. I don't normally recommend the use of this feature as the OD tends to lose some of its harmonic content and character as well. So if you can get away with not using the trim is generally recommended.

Tony

[ayan]

Thought I'd write a quick epilogue to this thread. I had a chance to gig with the amp again this past weekend and I can call it done, it's the best it's ever sounded. I've already shared most of what I did, blow by blow, but I'll add a word to encourage everyone to spend some time on the Master volume bright cap. Being the very last thing in the preamp, it will have a significant impact on the sound. I had originally installed a 68pF cap during the makeover and wasn't crazy about it -- as I hadn't been when I'd installed it the first time a decade ago... which had prompted me to pluck it out. This time around, with the amp all done up until that point, I went through everything I had available: 10pF, 20pF, 30pF, 47pF and 68pF. With nothing in the loop, I think the amp doesn't need a bright cap at all. As we already know, adding the bright cap is likely to compromise the sound when the loop is not being used. This is a summary of my findings: (a) 10pF... I really couldn't hear anything much going on, good if you want to have a cap on the Master volume for cosmetic reasons; (b) 20pF has a small impact on the lead sound when the Master is turned low, but gives a little more note definition with the loop engaged -- I ended up with this one; (c) 30pF (COG type only), didn't like it, hurts the OD and doesn't add enough of a benefit to the clean sound; (d) 47pF (COGs and non COGs) works nicely on the clean, it hurts the OD too much for my taste; (e) 68pF (COG type only) might even be too much for the clean, and it kills the overdrive, cannot be dialed out unless the master is turned way up, and I mean almost all the way up. I know some people try different speakers, get "tuned cables sets," etc., to mitigate the effects of a bright cap in an amp. I know some claim that a bright cap and mitigation strategies downstream from it sound different than not having a bright cap in the first place. I have no reason to not believe that, it is what it is. I use EVM 12Ls almost exclusively, and these were my findings. One other thing that works really well is to play with the Dumbleator's Send and Recovery pot settings. Amp a tad bright? Turn the send up and the recovery down. Amp not chimey enough? Turn the send down... Yes, it is a balancing act, but it works really well.

Makeover for the low play Skyliner coming up next. Once again, thanks to Jelle and Tony for their help on this!

Gil

[Colossal]

This is extremely helpful, Gil. Thanks to you, Tony, and Jelle for these developments.

[M Fowler]

I 2nd that, thanks for all your help and that's coming from a guy that hates tinkering with a completed build.

Mark

[jelle]

Awesome, glad to hear!

Jelle

[stelligan]

Interesting and thorough investigation of master bright cap and loop. I have a 102 with a master bright cap and t-filter at the loop that switches in when no cables present. May take the bright cap off master and the filter off the loop. Less is more....
Thanks

[ayan]

Interesting and thorough investigation of master bright cap and loop. I have a 102 with a master bright cap and t-filter at the loop that switches in when no cables present. May take the bright cap off master and the filter off the loop. Less is more....
Thanks

If you use an external Dumbleator, you will probably want to play with the settings on it, especially if you're running long cables to your FX -- whether they be in rack or on a pedalboard. That pronounced interaction between the Send pot (assuming it's the stock 250K value) and HF roll off has been probably my most useful "discovery" this year. I do wonder whether Dumble planned it that way, of it if just happened. Either way, I think of it as a very powerful tone shaping feature.

[JD0x0]

I've been omitting the master volume bypass cap in my builds. I don't like what it does, especially when it's turned down more. Doesn't quite just balance out the Fletcher-Munson curve IMO. Plus, you always have the actual 'bright' switch in the preamp, which can more or less do the same job (not to mention, the presence circuit..). It may even be preferable, as the HF accentuation earlier in the circuit has more of a chance to get smoothed out by the snubbers and compression of the clipping stages. Accentuating highs right in front of the PI, probably compound on the highs added by the presence circuit, and there's no snubbers or Low pass network to smooth out added peaky or fizziness.

[ayan]

I've been omitting the master volume bypass cap in my builds. I don't like what it does, especially when it's turned down more. Doesn't quite just balance out the Fletcher-Munson curve IMO. Plus, you always have the actual 'bright' switch in the preamp, which can more or less do the same job (not to mention, the presence circuit..). It may even be preferable, as the HF accentuation earlier in the circuit has more of a chance to get smoothed out by the snubbers and compression of the clipping stages. Accentuating highs right in front of the PI, probably compound on the highs added by the presence circuit, and there's no snubbers or Low pass network to smooth out added peaky or fizziness.

I totally agree with what you say. Now, after the makeover, the regular bright switch and the presence yield a very smooth and chewy sound. I had been told by Jelle that would be the case, and right he was. If you’re looking for a crisp edge, like the master bright would give, all I can say is that it’s not happening in my amp with those two options. Some might want that; I don’t need that option. My 102 amp is now super smooth.

[jam-mill]

Thought I'd write a quick epilogue to this thread. I had a chance to gig with the amp again this past weekend and I can call it done, it's the best it's ever sounded. I've already shared most of what I did, blow by blow, but I'll add a word to encourage everyone to spend some time on the Master volume bright cap. Being the very last thing in the preamp, it will have a significant impact on the sound. I had originally installed a 68pF cap during the makeover and wasn't crazy about it -- as I hadn't been when I'd installed it the first time a decade ago... which had prompted me to pluck it out. This time around, with the amp all done up until that point, I went through everything I had available: 10pF, 20pF, 30pF, 47pF and 68pF. With nothing in the loop, I think the amp doesn't need a bright cap at all. As we already know, adding the bright cap is likely to compromise the sound when the loop is not being used. This is a summary of my findings: (a) 10pF... I really couldn't hear anything much going on, good if you want to have a cap on the Master volume for cosmetic reasons; (b) 20pF has a small impact on the lead sound when the Master is turned low, but gives a little more note definition with the loop engaged -- I ended up with this one; (c) 30pF (COG type only), didn't like it, hurts the OD and doesn't add enough of a benefit to the clean sound; (d) 47pF (COGs and non COGs) works nicely on the clean, it hurts the OD too much for my taste; (e) 68pF (COG type only) might even be too much for the clean, and it kills the overdrive, cannot be dialed out unless the master is turned way up, and I mean almost all the way up. I know some people try different speakers, get "tuned cables sets," etc., to mitigate the effects of a bright cap in an amp. I know some claim that a bright cap and mitigation strategies downstream from it sound different than not having a bright cap in the first place. I have no reason to not believe that, it is what it is. I use EVM 12Ls almost exclusively, and these were my findings. One other thing that works really well is to play with the Dumbleator's Send and Recovery pot settings. Amp a tad bright? Turn the send up and the recovery down. Amp not chimey enough? Turn the send down... Yes, it is a balancing act, but it works really well.

Makeover for the low play Skyliner coming up next. Once again, thanks to Jelle and Tony for their help on this!

Gil

If Jelle will send me a complete set of these "special" resistors for free, I'll perform the same experiment to validate your findings.

[ayan]

If Jelle will send me a complete set of these "special" resistors for free, I'll perform the same experiment to validate your findings.

I’ve already started modifying my low plate skyliner amp with parts I bought from Jelle. I will start a separate thread on that when I’m done with that amp, probably in a couple of weeks, to document my findings. As with the majority of my posts, I truly hope it will help some folks in this community.

Gil

[Abe5150]

When I built my 102 back in July I thought the amp was to bright, so I tried the HF taper and I clipped the 68pf cap. I played it like that for a while and it sounded good then I went back and put it back to stock and it sounded better. These amps change a lot overtime you have to keep on playing them. I wonder if Dumble knew that too and burned them in for a really long time, he definitely took his sweet time. What do you think?

[Reeltarded]

Ok, now I'm excited, again.. just like yesterday, and last week.. and last year.. and a decade ago.

I'm not as excited as I was in 87. You can't touch that!