HRM clean stack sub

[Smokebreak]

Im working on a straight OD HRM, otherwise known as the minimalist.

I've got the HRM stack on the front panel, as well as the 250K drive. No switching. It sounds great, but I'm wondering what the best way to approximate the clean stack, in PAB mode always. The minimalist has a 330K/150K divider, which doesn't make a whole lot of sense to me, other than just being a voltage divider to knock down the gain.

I want to approximate the clean stack loading in PAB with with a simple network.

I've got the mid network approximated with, coming of CL1 plate : 150K>.01>50K>gnd, to approximate the mid pot at half rotation.

Also off the plate I have .001(somewhere inbetween 330p and .002) to a 100k(treble pot around half?) to the 1M volume pot. I imagine there should be a divider there, with the 100K and it's lower leg in // with the pot, but but it would start with 44M before it sees anything else, so I just left this off.

Is there a better way to do this? I'm not sure if I have the treble pot action correct.

BTW 5K6//.68 or .47 sounds great on OD1 to lose a bit of the smothering thickness. I've also got a 82K slope right now! I like that combo with buckers.

[UltraHookedOnPhonix]

How lossy is the Skyline stack in PAB mode? I think around -6dB? In that case the 330K/150K divider in the "Minimalist HRM" schem has a slightly steeper attenuation of -10dB. Either bump the value of the 150K to 330K, or better yet, use a A500K "gain" pot with a 470K resistor feeding it.

I don't know if there's a better way to do it but if your mimicking the clean stack in PAB, shouldn't you also add the Bass network components? In PAB mode the Bass wiper is broken leaving a 510K load to ground from the 100nF cap. In addition, all this is in // with the Mid components!

Some more things to keep in mind for an OD-only, minimalist amp:

If you want ODS-correct loading on the plate of CL2, don't forget to add a 1M resistor to ground after the 47nF plate coupler. This resistor simulates the 1M clean master pot and will bring the load down to approx 410K (if using the "low" or "high"-gain OD network). Without this resistor, you'd be seeing a total load of 695K there which would also affect the frequency at which your lows roll-off. Sure, very slightly, thanks to that big honking 47nF coupling cap which still ensures a roll-off just below 20Hz either way.

With the low-gain network, most guys like cranking the 25K trimmer. You can use a fixed network to simulate this degree of attenuation and still omit the 1M CL master pot substitution resistor. Put a 390K resistor in series with the 47nF cap and at the junction of it and the following 68K grid resistor, tie a 15K resistor to ground. Voila, -28.6dB of attenuation and a correct load of 405K!

Also note something Dogears mentioned many moons ago... the 47pF bright cap on the CL master gives it some variable LPF action in OD mode. For instance, if the CL master is at it's min setting, you're going to see a HF roll-off at 3.4kHz going into the OD section. So, with the CL master set to "4" (around where a lot of guys park theirs), you get the following diagram. Of course with a 68K tail, you're practically lifting the entire LPF network anyways so that's a moot point...I'm convinced that HAD added the 47pF cap in // with the 470K resistor as a way to compensate for the aforementioned scenario. Makes sense no?

[Smokebreak]

Thanks a bunch for the thoughts. I had the 1M hanging after the 47n coupler, but the network the considers the 47p bright cap makes total sense.

I had the bass pot network drawn out, then started thinking that it was getting to be so many components in there, I might as well make a little board with trimmers for the clean stack in PAB mode!

I'm a little foggy on the description of the 25K cranked trimmer network...so for the 470K//47p>220K, and the cranked 25K+4k7....which would be a 690K/30K divider, ...you'd sub a 390K/15K divider?
I do see the rolloff that the 47p bright cap on the MV has, and then the subsequent 47p // with the 470K to compensate, so basically if you're not rolling off with the MV network, there's no need to compensate with the peaker cap?

So basically 47n>390K/15K divider>68K stopper and boom that subs the whole network?

I built this up in another amp, and had it voiced well for buckers, then a tele, and settled for something inbetween, then some players came over to try it out, and out comes a strat(which I do not own). Sounded pretty bad! I pulled the slope back down to 33K and everything started to snap back into shape. I guess I have an excuse to buy another guitar now.

[martin manning]

I think this is more complicated than it needs to be. I played around with sims of the whole Skyliner TS in PAB vs a simple replacement. In PAB the TS is basically just a HP filter, and it looks like all you need coming from the plate of V1a is:

1) A 1n6 and a 280p for the mid boost and treble cap, or just use a 240p if no Mid Boost is needed.
2) Pick a resistor to simulate the treble pot, 0-250k. This only means +/- a dB up on the flat, so it doesn't do much.
3) a 1M volume pot to ground, with a bright cap if you want it.

There is just a bit more bass in this, but it only amounts to about 1 dB at 100Hz.

[Chris333]

I converted a high-plate skyliner to a no-clean-side HRM a couple months ago. Briefly considered building a clean tonestack with trimmers so that when I bypassed it with the PAB, all the little details would be right. That seemed overly complicated. Instead of looking at the clean tonestack/PAB as a necessary part of the OD sound, I began to think maybe it's more of a compromise necessary to have a decent clean sound. Anyway, the clean TS is replaced with a .002 uf cap with a 150K to ground. The rest is of the preamp is basically HRM 101. I've experimented with coupler and tonestack caps (haven't messed with the slope yet) but I always end up pretty close to the schematic for the best sound, YMMV. I love the tone, and it's super-fun to play, but has a lot of gain. Reminds me of an old Marshall I got rid of long ago , the way it seems to sound best with the controls on the cranked side, instead of trying to throttle it back. If you come up with a way to get a decent low to mid gain sound out of yours, please post -- I'm too lazy to do the leg work!

[Luthierwnc]

for laughs, hang a PAB resistor off the mid tail with a p/p pot. 68k should be about there but a 100k trimmer on the p/p pins is a better idea for tweaking. sh

[Smokebreak]

This is great getting everyone's takes on this deal.
I finally just started making it sound best to my ears, and ended up, in order, with 2K2/.68, 3K3/22u, 5K6/.68 , 1K5/1u for the cathodes.
On the first plate 2n>470K/150K divider>1M pot . I snipped the 47n over the 470K.
I also changed the 47n to 22n. Overall it knocked the gain down a bit, chilled out the honky mids, tightened it up and made it meaner. Kinda like a Marshall;)

[UltraHookedOnPhonix]

I'm a little foggy on the description of the 25K cranked trimmer network...so for the 470K//47p>220K, and the cranked 25K+4k7....which would be a 690K/30K divider, ...you'd sub a 390K/15K divider?

I do see the rolloff that the 47p bright cap on the MV has, and then the subsequent 47p // with the 470K to compensate, so basically if you're not rolling off with the MV network, there's no need to compensate with the peaker cap?

So basically 47n>390K/15K divider>68K stopper and boom that subs the whole network?

Close on you first question...the low-gain OD network is: 470K//47pF-->200K-->25K pot to ground (with or without 4K7 tail). The net resistance of this network is 695K. Parallel that with the clean master and you get 410K. Cranked trimmer gives a voltage gain of .036 (-28.9dB). I sub a 390K/15K divider to where the load is very close (405K) and the attenuation is about right .037 (-28.6dB), plus you can do away with the 1M to ground after the 47nF coupler.

Yes for both your other questions. Spot-on!8)

[Smokebreak]

Awesome. Got it!
So this amp is smokin, for my tastes, at this point. To me it's kinda in that gnarly, ballsy, dark Naylor territory. honestly, I didn't intend to even take it this far..
The dude that brought the strat over also brought a Bandmaster and a Bassman, which I'll be putting new OTs in. He was interested in some high gain mods, and sent me this video of Joe B, and said I want THAT tone: https://www.youtube.com/watch?v=Oq1FfGN7H3Q&spfreload=1

I should have asked you guys first, but after some very hasty research, I figured it was an HRM. Honestly, I have no idea what amp that is, so if someone knows I'm all ears.
So I built up the minimalist in my Country gigging amp(poor thing), just to offer him a different flavor than the other stuff I have around here.
He didn't fall in love with it, but did with one of the 70's funky circuits. So that's what's going in the Bandmaster.
So, a bit frustrated that the HRM wasn't lovable, and being that I don't have a country gig this weekend, I delved into it a bit, so I appreciate the words of wisdom!

Chris, the way I outlined in my last post knocks a good bit of gain out of it, even with volume and drive cranked. BUT, now the 25K trim(which i put on the front panel, just because a had a spot) totally sounds good cranked down a bit. I got the MV up about halfway, which was very,very loud it seemed, for a 50 watter, and sounded very tough, somewhere between a cranked plexi/800 level of gain. This was with the 25K less than halfway up!

[Smokebreak]

I made a demo of the amp with a doubled rhythm guitar and 2 leads. I got the drummer and bassist off the internet, but they were solid.

https://soundcloud.com/jeremyslemenda/hrm1

[Luthierwnc]

Yup! That's an HRM! I've only made two of them but on both I set the OD button on the footswitch to automatically activate the PAB -- if it wasn't already on.

Do try the resistor on the tail of the OD stack. It's overdrive for the overdrive. You don't need as much resistance as on the V1a stack.

The attachment was a rack preamp module I made last year designed to make any relatively clean power amp section of another amp into two different D-amps as long as it had passive send and return jacks. On this one, the PAB worked independently since it needed to control both the HRM and non-HRM (and HRM into non-HRM) modes. It worked great but I sold my larger non D-amps so it was just sitting in the rack. sh

[Smokebreak]

Thanks for the confirmation! That's a nice looking design. I'll lift the HRM stack a bit when I get back to the amp and see what I get.

[Chris333]

Clip sounds great!

[Smokebreak]

Thanks, Chris! That clip was with humbuckers in a LP copy at pretty low volume.
A tele sounds really good with this new configuration, too, with the treble rolled down quite a bit. It's not harsh, just a little bright. The presence has been parked at 9 o'clock for days, so I may make that fixed, and put a variable treble bleed in it's place. I've got a loud rehearsal tonight, so I'll bring it and see where the treble sits, as that may be all I'll change at this point.