Converted my Bassman Hybrid-A into an HRM today...

[Normster]

I never really liked the tone of my Bassman ODS so I converted it into an HRM. Only two switches, PAB and OD. I got rid of all those wimpy 15mm push/pull pots and replaced them with 24mm Alphas. Without all that extra wire this amp is MUCH brighter than ever before.

Many thanks to Dogears for helping me straighten out the clean master issue and steering me in the right direction on the HRM adjustments. This is the first time I've ever really loved the tone of this amp! (You were right, Scotty. I DO dig the HRM sound. )

BTW, with the PAB "off" this thing is a great blues amp!!!

[Icetech]

Layout norm? wouldn't mind trying it on mine...

[Bob-I]

That made a huge difference on my Peavey transplant too. I'd do it to my Princeton transplant but there's no room.

[mat]

Hi Normster,

How much different (wiring and part wise) is the HRM to non-HRM ? As I built mine from Your Bassman-layout (and I love it) it would be nice to know if it would be possible to put a nonHRM-HRM switch in it

Thanks,

['67_Plexi]

I know many of you guys love the non-HRM older designs, but the HRM is the natural progression of the ODS. If you think about it, HAD didn't go backwards with his designs.

If you lay it out as he did, use the correct parts and tweaks as Dogears suggests and don't try to re-invent grounding schemes or relay circuits, you will end up with argubly one of the best sounding amps ever produced that is completely noise free and capable of playing any style from Country to Blues to Metal and everything in between.

Every little detail matters and just a few component diversions or substitutions can throw it right off.

My 0.02

Alan.

[Normster]

Layout norm? wouldn't mind trying it on mine...

I'll see if I can whip one up in the next few days.

That made a huge difference on my Peavey transplant too. I'd do it to my Princeton transplant but there's no room.

Yep, remarkable difference, eh? I spent the whole day playing yesterday. Man, my fingers are sore. haha

How much different (wiring and part wise) is the HRM to non-HRM ? As I built mine from Your Bassman-layout (and I love it) it would be nice to know if it would be possible to put a nonHRM-HRM switch in it.

Gary was able to add this switching to his amps, but I think it's easier to have two amps. To convert the Bassman I had to change the mid, drive, and level pots, OD input network, mid cap, cathode bypass, and of course, add the post-OD tone stack in place of OD2 coupling cap. That would be a pretty tall order for a relay bozo like me.

If you lay it out as he did, use the correct parts and tweaks as Dogears suggests and don't try to re-invent grounding schemes or relay circuits, you will end up with argubly one of the best sounding amps ever produced that is completely noise free and capable of playing any style from Country to Blues to Metal and everything in between.

Every little detail matters and just a few component diversions or substitutions can throw it right off.

I couldn't agree more. I don't think the cleans are as sweet (since there's less mid scoop) but the HRM is far more responsive. With the PAB "off" and OD at 10:00, you can go from clean to crunch just by varying pick attack. (Strat neck pickup.) What surprised me is how effortless it is to control dynamics. I can do the same thing with my Trainwreck, but I have to work at it. With PAB "on" it will definately do metal...or fusion.

[skyboltone]

<snip> don't try to re-invent grounding schemes <snip>
Every little detail matters and just a few component diversions or substitutions can throw it right off.

My 0.02

Alan.

Alan, I think you mentioned drawing up that grounding scheme. I will begin wiring this week and had intended to single star ground EVERYTHING back to one point. I don't want to do that if it's going to produce negative results.

I think HAD used the pot buss for sure, what about the board grounds? Are they ganged on the board and then led back on a single wire?

Some guidance here would be appreciated.

Thanks
Dan

[Icetech]

Thanks norm, i am starting to play again... i tend to go in spurts.. play hard for months then nothing for awhile.. comes and goes:) Probably try your new setup in another chassis...

[Luthierwnc]

I'm a big believer in star grounds. On my amps, there is a single buss wire than runs right behind the pots. I use a piece of #12 romex and sometimes leave the insulation on where it passes by HV components. It only touches the chassis at one point -- usually a power trannie bolt. The power cord ground is soldered to a terminal ring and bolted in the same place. All of the jacks are insulated and pot grounds don't touch the back of the case.

Grounds at each stage of the power supply are tied onto the buss in order. For example, the B+5 ground points are all at the end of the buss wire. When they are near each other on the board or the panel, they may be connected to each other first but they all go to about the same place on the buss. B+4 gets the next 2-3 inches on the buss and so forth until the power grounds are near the bolt.

Sometimes this means using a little extra wire. The OT might be on the other side of the amp but its ground, and the speaker jack grounds, are connected near the B+1 filter cap grounds.

If you have a ground loop and a scope, here is a handy little trick that I don't think is in the manual. Run the amp on with no signal. Ground the probe clip and point at the star and dial in the line very clearly. Leave the ground clip there and poke the other grounds with the probe. The line will get fuzzy if there is voltage potential between those two points. FWIW.

Skip

PS If you do use romex or some other commercial electrical wire, rough it with some scotchbrite and wipe it with alcohol to get the gunk off it first. Makes it a lot easier to solder.

[BobW]

I never really liked the tone of my Bassman ODS so I converted it into an HRM. Only two switches, PAB and OD. I got rid of all those wimpy 15mm push/pull pots and replaced them with 24mm Alphas. Without all that extra wire this amp is MUCH brighter than ever before.

Normster, my non HRM bassman layout/schematic, was posted in the files section last week. I've already considered another build, but based on your observations and time spent on your Bassman, should I attempt an HRM on this amp or do you think time is better spent on building another from scratch? A layout of your bassman would be wonderful. thanks, Bob

[Normster]

I treat my Bassman as a test bed so I'm not sure I would recommend doing an "upgrade." It's always better to have an excuse to build another amp.

My personal opinion is that you have to let the amp "speak" to you. I built an identical Bassman for my buddy Loren using the Hybrid-A schematic and it sounds incredible. I could never get close to that tone with mine. However, the HRM seems to suit my Bassman much better. Could be subtle differences in the iron, cap/resistor tolerance, minor pot differences, etc., but I like to do a build and then tune the build to suit the amp's tone.

If you decide to change to HRM, it looks like you're half way there already. (250kA mid pot and .01uF mid cap.) Just swap out the OD plate resistors and cathode caps/resistors (which are the wrong values anyway), OD pots, and add the post-OD tone stack in place of the OD2 coupling cap. If you put the post-OD stack on a seperate board, it's really easy to swap back if you don't like the tone.

[Bob-I]

My personal opinion is that you have to let the amp "speak" to you.

Excellent explaination. So many folks are concerned about the "right" schematic. Letting the amp speak to you is the part they're missing.

[BobW]

My personal opinion is that you have to let the amp "speak" to you.

I agree, and didn't expect a cookbook approach, was just concerned about noise/layout, yours sounds noise free. My amp is very quiet too so I'll just stick w/my layout/grounding scheme.

If you decide to change to HRM, it looks like you're half way there already. (250kA mid pot and .01uF mid cap.) Just swap out the OD plate resistors and cathode caps/resistors (which are the wrong values anyway), OD pots, and add the post-OD tone stack in place of the OD2 coupling cap. If you put the post-OD stack on a seperate board, it's really easy to swap back if you don't like the tone.

Thanks, much appreciated.

So many folks are concerned about the "right" schematic.

I can certainly agree with that one. Other than TDY work taking up about 6 months of this year, the rest of the time was spent tweaking this amp.

[Luthierwnc]

I agree too. Another big factor is how much B+ you get out of a donor. In the scratch build where you have complete control over the power stages, you can work from a level playing field. In a retro-fit, the voltages are likely to come in a bit low. The challenge here is to keep it clean longer. My experience is that you need to keep the preamp voltages pretty close to the recommended (or observed) levels, even if it means playing around with the dropping string.

Sweetness, warmth, earlier power amp distortion are often descriptions of lower power tube plate voltages. My current experimental 7591 build is a good example. It is a very sweet, bluesy sounding amp as it sits. Plate volts are around 407. On the fixed-bias side they are set for 32ma. I am redoing it now as an HRM to try and get one more number on the volume before it breaks up. I'm also 86ing the deep and mid-boost and will probably jettison the LNFB on the Dumbleator based on recent test builds.

Tommy -- I know you are reading. This is a good time to point out that sometimes you need to tear stuff up, replace unsalvagable parts and need a little extra real estate in the chassis for afterthoughts. It also underscores the idea of getting a couple extra units of cheap, common parts when you put in an order. I usually get double the spec-ed resistors and caps along with a couple nearby values for seasoning.

Well, got some soldering to do. All best, Skip

[Pete]

I am redoing it now as an HRM to try and get one more number on the volume before it breaks up.

I would like a result like that on my current 50W build. Can't for the life of me see how converting to HRM (from basically stock 1984 amp) would do this, but I'm open to anything. someone care to elaborate and/or clue me in.
many thanks
Pete