i guess i should qualify my results.... i almost exclusively use the amp in jazz mode. and with this slope resistor change i am extremely happy. and i still have plenty off bass and mids in my controls anf if i need more i use the mids boost switch)

i previously found the mid boost and rock mode way to much for my likings and so always used the amp in jazz. but after this mod..it seems to have tamed these setting to make them more useable and controllable

fab

ps, if there are and members here in melbourne australia, feel free to get in touch to come and try it out.

yes there are a few of us about!!

no i havent, i dont want to take it too far as you also pay with extra insertion loss in the tone stack (ie more loss)

i may bring it up a tad, say to 170k...just to hear the difference. my simulations indicate approx 1db more scoop in the mids

yes it can sound thin depending on your speakers.... my amp is set up with 2 x thiele boxes with ev12l.... bottom end is way huge....and punchy. like a quadbox with clarity. i have tried the amp with an open back cab and i agree it can sound thin.

this amp really needs a special cab with it, i also found the bottom end too loose and flabby in an open cab (rock mode), but now a totally different animal with the thiele cabs.

blueprints for ev thiele cabs can be found on the web and in my opinion are simply fantastic, they also work really well with my 69 fender dual showman

yes thats the one

i have to say after a few gigs that the amp sounds much better with the resistor change to 180k

pedals  sound much better, reverb more lush in the loop
and the clean channel is bouncier *ie more scooped

thanks dr ika
yes music and sound is such a personal think and we all have our own perceived conceptions of what a great tone is

its is wonderful that Nik has buils such a wonderful platform upon which with a little experimentation and tweaking you can make the amp sound perfect to your own tastes.

i reckon if we compiled a list of all the mods done by various people in this forum, it would read like a nice little cookbook of dumlesque tones

a couple of points:

1. placing a resistor in parallel will always decrease resistance. therefore to get an increase in resistance you either need to  change the resistor or add another resistor in series
2. a correction on the parallel resistor formula posted earlier:

 1/R1 (Existing Slope) + 1/R2 (secondary test value) = 1/Total R.

lowering the slope resistance will decrease the overall insertion loss of the tonestack and therefore provide more signal to the overdrive section. hence giving more bloom, however also with an increase in bass and mids
increasing the slope resistor has the opposite effect, it increases the insertion loss and there the result will be a more pristine signal to the overdrive section, with less bass and mids, but the overall tone is much more natural. to compensate for the loss, i have turned up the overdrive somewhat when needed, however the sound is tighter and clearer.

each to his own.... however a customer has heard my amp many times before, however after hearing it last week with the resistor mod, has decided to place an order.

i cannot stress enough for those using EV12L's, putting them in a tuned box such as the TL606 makes the world of difference to the overall sound and response too ....tightens the bass up without loss of highs

i am happy with the 180k mod and am not keen on trying the 68k

however i have simulated it... you get more signal to the rest of the amp and more bass and mids making the sound less scooped

overall i would say it is not the direction i want to take it; and if anything try slightly increasing the 180 k to 190 or even 200k

however being cautious, i want to try it in a live situation as it is now

the overdrive is much smoother and refined, especially with a les paul and the PAB engaged

i have performed a detailed circuit analysis on the skyline eq, using different resistors.

results are as follows:

for a given eq setting (ie bass middle treble), and varying the slope resistor only, the effect is to slightly reduce the bass, and make a larger skoop in the mids, dropping approx 1db at 1.5 kHz,

... hence, the more fender like tone!!

i may try increasing it to 200k and see what happens.................

hi everyone
i just tried the 180k mod and found it very good. up till now i had hardly used the clean channel as it found kind of "dry" or boxy as other have put it, only to be exacerbated with the PAB.

the 180k mod definitely makes the clean channel more fender like; although with a slightly higher insertion loss to the tone stack.

as for others who mention flabby bass, yes i know of the problem. since i got the HRM some years ago i have almost exclusively used it in jazz mode as i found the bass in rock mode too flabby and unfocussed. this was using the amp as a combo with an EVM12L. recently, i made some Thiele boxes for my HRM each with EVM12L. the difference is extreme, bass is tight , mids much more focussed and sweet highs.

i was hoping the thiele boxes would rectify the issues i had with the clean channel and it did to some extent, however the fundamental tone did not change. but the bottom end issues were none the less resolved.

BUT, with the 180k resistor and the thiele boxes, the clean channel is now much more useable, and the deep switch works really well with it.

i am of the opinion that the deep bass of the HRM was made for a tuned cabinet which really makes a HUGE difference.

thanks for the mod suggestion, and i like it.

PS: in earlier threads pickmaster suggested LOWERING the slope resistor to 82K, i cant recall exactly the improvments he got, i will find it and report back

i have the same problem.. the mid boost is just too much....i did some calcs and simulations and worked out a pot value that allows you to sweep the midrange....using a push-pull pot still gives the switching... but allow one to vary it from 0 up to full

just one of those little jobs i keep promising myself to so next time open up the amp...

try the amp in jazz mode when using the OD channel
turn up the bass and mids, treble to taste....
you will find the OD tight and punchy and singing when PAB is engaged

i find also my EV12l copes better with the huge bottom end than my G12-65........i do use both together, but thank god the EV dominates

yep... exactly. i use jazz mode 100% of the time because of that... notes are clear, the OD is tight and defined, bottom end is punchy. sounds absolutly fantastic with my LP with duncan antiquities and is prfect for rock and blues.

nuxuk
i got your email the other day,,, however did not get a response...

i love mine stock and have so for the past 2 years. however i dont use the mid boost switch as it is a bit too much for me. i am planning to replace the switch with a push pull pot and make the amount of mid boost controllable.