Starting new build 2nd gen 100watt HRM...(Finished!!)

[ER]

Starting my new build finally! I would really appreciate a second set of eyes on this from you more experienced guys 'cause I'm still in a bit of a brain fog from meds and this is a bit of an ambitious build for me.

Build is early style values (dual bright, deep, Rock-jazz +PAB) with a 100 watt power section and precision power supply ala 183 w/6ca7's, liftable HRM on the OD but as a hiwatt stack at the entrance, foot-switchable FET/OD/PAB/HRM-PAB. Manual OD and FET (plus level) on the back panel, and tone stack lifts with on/off switches at the 100K mid pots. Oh yeah, and dual masters (clean and OD). All edcor iron. JCM800 chassis tubes down. Main guitar is a 70's hardtail strat and fender (white) lap steel.

Front panel controls are; Input, clean gain, bright, deep, Rock/Jazz, t, m, b, clean master, OD gain, t, m, b, OD master, Presence.

So before I start cutting and soldering, are there any changes I need to make? (I know I need 180K to ground at C39 just don't have the part on hand yet, and I decided to omit the 100/50 watt switch shown on the schematic). Sorry for the sloppiness on the schematic, I think I need to redraw the whole thing before continuing, just want to make sure it will do what I think it will do before I redraw.

Thanks, Erik

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[steve_p]

How do you plan on incorporating the tube screamer into the build?

Looks like a cool project. Can't wait to see the results.

Steve

[ER]

I thought all dumbles had a tube screamer inside?

[noworrybeefcurry]

Its interesting that the second tone stack is right before the OD gain stages, Id love to hear how that blends with the 100k/1.5k setup.

[Structo]

Its interesting that the second tone stack is right before the OD gain stages, Id love to hear how that blends with the 100k/1.5k setup.

That's what I was wondering.
Is that your intention to put the HRM tone stack before OD1?

[ER]

I was going to try it there to make it more like a Hiwatt with the PAB engaged on the clean channel. I always preferred my hiwatts to my marshalls, if it doesn't work like I'm hoping it will, I'll put it post OD or just remove it.

The true plexi tone stack runs off the cathode, but the hiwatt is off the plate and in the middle of the 4 stages and has the 220K master in the same position as the usual OD entrance trimmer and hoping it will be close enough to a regular dumble OD with the HRM PAB engaged, so I though it was worth trying. Not sure if it will mess with the overdrive being at the entrance but I figured it was worth trying and easy enough to change if it doesn't have the vibe I'm looking for.

My other line of thinking was that with 2 gain stages post EQ on the OD it may retain more of that early generation rawness compared to the later HRM amps.

Here's the hiwatt pre schematic for comparison;
http://www.hiwatt.com/manual/DR103_PDF/ ... lifier.pdf

Thanks, Erik

[ER]

Dumble inspired, but very much becoming it's own thing.

Made some good progress on the amp, front half of main board and all pots are in ands hooked up, FET board footswitch and relays. Will tackle the back half of main board and head into the power section and power supply and see how far I get today.

Need to order a few more parts, cooked an LED in the footswitch during mock up, and I decided to drop the power cord out the bottom of the chassis and put a 120v socket in the existing hole in the back panel to plug in the future loop that will have reverb/buffered loop and SSS style tone filters. I've been overly ambitious doing this build from scratch with mix and match design features, no reason to stop now.

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You guys that do this for a living earn every penny and have my upmost respect!

Coming into the home stretch with the wiring, then I'll move on to making saw dust. Digging through my junk boxes in the garage and found a nice brushed aluminum handle from an old tube tektronix scope that will go real nice with the brushed aluminum chassis and trim. Got some wheat grill cloth and walnut wood for the cab.

Peace- ER

[ER]

Also some tips that came in handy for me with the 12v relays;

If you use the 3 lug terminal at the relay supply rectifier, you can set a 9 volt battery upside down on lugs 2(-/ground) and 3(+) and you can easily test all your relays and foot switch with a multimeter to make sure they're working as planned before moving forward.

I used a 5 din socket for four relays (FET, cleanmegaboostPAB, OD, ODmegaboostPAB) and used the socket body base as ground and the top pin for 12v+. I used cat 5 (grey to match my grey washing machine power cord) to the foot switch and solid core 24AWG radio shack (HAD approved) telephone wire inside the amp.

Did I mention I'm a cheapskate? Equal parts necessity and Scottish DNA.

[ER]

Everything done except hooking up the power supply board and the rectifier board.

I like the narrow chassis, short hookups. If I was doing it again I'd probably move the HRM circuit (the mallorys) on the main board just before the PI and there's plenty of room for a loop and reverb (well not plenty but enough).

This is a 27" JCM 800 chassis, just to put into perspective how huge the edcor trannies are! I put all the OPT secondaries on their own jacks with a parallel set on the 4ohm so I can do 2x 8ohm cabs if I want. Presence comes off the 8ohm tap for now as shown on #183 schemo.

Waiting for 120v outlet from mouser to mount on back panel, don't want to get the power supply in there and have to do any cutting or filing, hopefully it will come today.

Very wabi-sabi compared to most of the builds on here.

Curious to hear how this thing will sound...

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[ER]

And a little wood porn for those so inclined;
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[ER]

Done! well sort of...[img:500:375]http://farm9.staticflickr.com/8149/7206 ... 737cec.jpg[/img]
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It's alive! Took some fiddling before I could put tubes in, relay power cap was grounded on eyelet and causing some weirdness and making the limiter bulb bright.

The good;
Clean sounds awesome. Switching working as planned. Tone controls, bright, deep, rock/jazz, presence- all work wonderfully and good useable range, nice tones. The 2 way bright has nice steps in brightness with 100pF up and 500pF down-middle off. Dual Master is really nice, works well. This thing has some serious cajones.

The bad;
FET doesn't work- bit of hum and signal drops way down when engaged, level control makes little difference. HRM tone controls don't work, bypass works and gives a decent boost but turning the TMB controls makes no difference in the tone. Sounds good when bypassed. Any ideas?

The ugly;
Some slight audible grunge on the OD as notes trail off. Need to dial in the OD trimmer, you have to get the gain controls up pretty high to get the overdrive going. Will most likely sort out with the HRM stack fixed and trimmers adjusted. Any ideas?

The JJ's and edcors sound nice, really full bodied. 477v on the plates and biased @35.5mA. Other voltages are right in the ball park, once I sort through the bad and ugly (see above) I'll do some 12ax7 tube rolling and get everything dialed, PI etc. and post voltages.

Then some clips and out to the woodshop!

[mhartman]

Nice! Great looking build.

[ER]

Well well, the Hiwatt schematic (as posted above) from the factory is wrong...

The resistor linking the treble pot to the mid pot is actually 22K, not the 220K shown! Chango-presto it works! I have to say I'm really happy with how it turned out. I also moved the OD in at the relay to the plate of the clean out skipping the coupling cap (it still goes to the clean master though).

Sounds like a dumble clean, turn on OD and HRM-PAB, and it sounds like a dumble OD, turn on clean PAB and OD with the HRM stack engaged and it really does sound like a Hiwatt , OD with both PAB's and it's rock city.

Now to figure out what's going on with that FET circuit...

[Structo]

Wow, that Edcor OT is huge!

Nice looking build!

Where did you hide the HRM tone trimmers?

Regarding the FET, you may have inadvertently fried it.
They are pretty sensitive.

Adjust the source bypass resistor so you get about 10v at the Drain.

And adjust R5 to get about 20v at the junction of R4 and R6.

What jack did you end up using for the FET input.

When I first built my I used the Switchcraft 14B and had nothing but trouble.

I ordered a Kobicon 161-0084-E

But I was later told that the Calrad 30-385 is higher quality.

[ER]

Thanks Structo-

The HRM controls are hidden on the front panel cleverly disguised as tone controls.

Yeah the edcors are massive, I can put it tubes down and there's about a 1/2 inch clearance for the 6ca7's!

I used a single jack on the input, a cliff style. The FET has it's own footswitch relay and a level control on the back panel, I wanted to be able to a/b between the guitar and a lap steel and also engage the FET. I used a calrad on the loop return, found one in my junk box still in the radio shack packaging. Switchcraft style on all the rest.

Thanks, I'll check those voltages. Have to go through that whole board again. I'm pretty sure I fried it, I had the board all laid out and the FET that came had a 4rth pin for the can, so I had to rebuild the board and I can never tell if the pinout on those things is looking at the bottom or from above so it got soldered more than once. It may not even be in there right...

I'd like to thank you and the rest of the contributors here on amp garage, I never would have been able to do this without soaking in all the wisdom and collective knowledge over the last few years.

-ER