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Man is one of the few animals that stumbles over the same rock twice... at least I am. Having tried and abandoned HRMs 8 years ago, I decided to give it another shot again in my last build. The amp started off as a full blown BM; first I changed the BM boost to something else; then I changed the preamp to Skyliner, with the convetional PAB; then I lost the BM PI. I struggled with the HRM for a few days and I finally jumped ship for the third time in eight years. But, rather that only venting my frustration, I will also try to tell you about some of the things I found along the way that you may find of interest and use.
HRM Struggles
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As you may have read in the other thread, my biggest problem has always been finding a smooth sound out of an HRM. I'll qualify my findings by disclaiming that my speakers of choice are EVM 12Ls, and that I also play a 2x12 oval-ported cabinets loaded with some kind of discontinued Eminences that sound kind of like a Celestion 90W (Boogie) speaker, except I bought them at their blow price of $24 (I think) each. Those speakers have worked well for me for quite some time.
After struggling a lot with the HRM, I was able to arrive (again) at some clonclusions. First and foremost, to get something close to that "RF" type of sound that Scott has come to master, I did have to turn the gain up quite high [Preamp volume at about 1 o'clock; OD trimmer at 25K to ground; OD gain at 1-2 o'clock] and use the amp in PAB mode. Although I realize I didn't spend enought time to adjust my approach to fit this amp, I was able to see the sound has some good qualities to it. However, it is definitely not user friendly. The amp does scream nicely on the upper register notes, but it will buzz out just as easily on the lower register notes. Double stops are somewhat amusical and have a tendency to develop that intermodulation noise (like an engine reving up) that folks are currently discussing in another thread.
My biggest gripes are: (1) When that sound was dialed in, it would only work with the bridge or bridge + neck (w/neck volume rolled off) pickups on my 335 only, the rhythm pickup didn't sound so good. Worse yet, my Tele sounded ear piering bright through the amp... and forget about the Strat, it sounded simply horrible (Strats have always been the hardest guitars to dial in with Dumble amps, in my -- and many others' -- opinion). (2) If I should dial the gain down, for a cleaner type of OD sound, the tone would become dark and the pick attack would be masked in a bad way. Brightening the amp via HRM would fix that, but then turning the gain up would make the higher-gain tone piercing bright and harsh. (3) As it is common knowledge, the sound of the HRM without the PAB on is just not happening at all. (4) I know other folks have brough this point up as well: the OD sound, dialed in, has no real ressemblence to the character of the clean channel's tone.
HRM Experimentations
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I gave the concept of HRM the benefit of the doubt, wondering whether I might have a bum amp to begin with, in which case the poor HRM should not be blamed for my less than encouraging results. So I disconnected the ground on my HRM board and basically did an A/B test: amp w/HRM VS amp with HRM board not grounded, and hanging a 100K resistor to ground at the input of the OD master (to simulate the non HRM topology). Without the HRM board, the amp was very clean and sustainy sounding, with a nice tendency to hang on to notes and start feeding back. Grounding the HRM would obviously result in volume loss, so I would up the master and listen. The tone would lose its vocal quality, it seemed as though the fundamentals were attenuated, and feedback was gone as well. I upped the gain to restore some of the sustain and the sound was workable, but I then disconnected the ground of the HRM, etc., and the tone now was like an "ebow." The amp cwouldlatch on to just about any note and sing away, the difference was night and day.
I forgot about chasing the "RF tone" and decided to dial the HRM controls so that the amp would sound as lose as possible to the non HRM case. Obviously I ended up with the midrange control cranked up and the bass and treble controls barely cracked open. The sound was pretty good that way... But this is the kicker: I would play the amp like that for a while and there were some "burrs" on the notes, like a little "crunch" [think about trying to get a pristine clean sound outof a boost type of pedal, where there will always be some stuff going on that cannot be dialed out] that was always there. Of course, lifting the ground on the HRM would get rid of that and make the sound the way it "should" be.
Conclusions
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The only valid conclusion here is that I really don't get along with the HRM... But, from a a more technical point of view, something is introduced by the HRM that is unplesant to my ears. A fellow reported in a post yesterday that when he scoped an HRM amp the observed some weird behavior at V2B. I am not sure what it is, I am not sure if it's the phase shift introduced by the additional circuitry being driven but a hgih impedance source or what. I know the HRM gets more palatable to me with higher volume but even then, it's not worth the aggravation.
I am not suggesting the HRM amps sound bad, nor that an HRM amp can't sound smooth. But I would be interested to find out whether there exists any amp that will sound "smoother" and with more sustain with the HRM stuff added to it.
Epilog
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By the end of the day yestersday, I was splitting hairs between my conversion-in-progress amp and my others. So the first thing I did is to replace the HRM board with a "conversion board." The amp sounded obviously more like I liked, but it was still not 100% right. I added the snubbers at V2's socket and things got a little better. But it still wasn't 100% right; at higher gain settings the amp seemed to be much brighter (in lead more) than at lower gain settings, something which my other amps don't really do. I fiddled with that for a while and then replaced the HRM-style pre OD network with the more conventional one for non HRM amps, and that made a hige difference. Something about having a higher impedance hanging off V1B, in addition to the obvious change in frequency shaping... Miller's capacitance perhaps?
The amps were sounding really close by then, although I never changed V2's resistors, but there was still something not quite 100%. While in OD mode, PAB VS not PAB had a difference in the feel of the highs that I was not familiar with. OK, I went for broke and added the feedback netwrok around V1B. That seemed to make the amp respond like the others... and it was late, so I have up on it for the day. In the conversion process, I left the 250KA drive pot but changed the coupling cap from .0022uF (HRM) to .005uF. The board I hung at the end of the overdrive has a .005uF coupling cap coming out of V2B, followed by a 68K resistor and a 200K trimmer to ground set about 55K to ground (this simulates the common 150 K or 180K + 100KL OD master -- set just past halfway up -- arrangement in non HRM amps), plus my trusty old 250K + .001uF cap to ground from top of the trimmer to ground. I will post pictues of "Frankenstein" later.
Bottom line, if you love your HRM, great! If you're struggling with yours and share any of the gripes I enumerated above, you may want to spend a couple of hours doind similar tests to the ones I did and who knows, you too might want to convert your amp.
Cheers,
Gil
