Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dennyg on May 08, 2018, 11:09:55 am
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Hey guys - been away from the forum for awhile - been building an amp for my 2nd client - woohoo!
I totally get that this forum is more about classic fender designs but for you high-gainers, thought you might be interested in my latest project. Here's the overview:
* Foot-switchable dual-channel 50W amp - Uber Gain (AX84 circuit designed by Merlin and couple other gents) with stock Plexi clean channel. I sold my first Uber-gain to other guitarist in my band who replaced a Mesa 50W Dual Rect with it. That one is an SE amp with KT88 (approx 17W) and sounds amazing. We've done 25 shows with it over past year, which is how I got the order for a 50W version of same.
* Clean channel has no tone control - uses the bright/dark blend for tone. Works so well I'll be using that for clean channel on all my 2-channel builds going forward.
* Has two MV's - foot switchable - one for rhythm and one for lead (similar to JCM900 Lead Volume)
* Has cap multiplier for C2, DC heaters, and pure star-grounding scheme (i.e. only one connection to chassis at input) - there is zero hum in this amp
* I use gothic ring/term strip layout but this will be my last amp using term strips. I've designed a PCB that I can use for all four amp models - cuts down pre-amp build from 12 to 2 hours.
* Use a prototype power supply board that I've now got a PCB designed for future builds - supplies AC PA heaters, elevated DC heaters for pre-amp, regulated 5V switch relay supply, cap multiplier and choke-filtered screen and pre-amp supply, and filtered bias voltage for PA.
* external bias points and pot on rear panel - can run variety of tubes from 25-35W dissipation - EL34's, KT77's, 6L6's, KT88's
* Impedence selection on rear-panel with 2 parallel output jacks to run two cabs if desired. This thing is pure thunder with a stack of 4x12's
Hit me up if interested in schematic or any questions about this incredible amp.
Denny
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Awesome project! I love that building style, and I built a high-gain, almost hum-free amp (AX84 Lead 2) with a similar technique. I feel it has advantages over turret board by keeping components close to the tubes, with short wires or directly connected components
Mine didn't look nearly as neat as yours, though :l2:
Still, I don't blame you for moving towards a PCB. Term strips are time consuming and physically awkward to work with, since there is a limited amount of assembly you can do outside of the chassis.
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Wow, that is a style I haven't seen.
Very nice.
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Hit me up if interested in schematic . . .
Would you post it in this thread if anyone were interested?
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Hit me up if interested in schematic . . .
Would you post it in this thread if anyone were interested?
+1
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Hit me up if interested in schematic . . .
Would you post it in this thread if anyone were interested?
Gladly! I do my schematics and layouts by hand first - usually at night laying in bed with pad of paper while my wife reads on her Kindle :|
I'm just getting going with software tools but over time I'm sure they'll be more efficient. I'll take pics of the hand-drawn layouts and post this evening. For whatever it's worth, I started building amps because I couldn't get the tone I wanted (basically a JCM800 at full-throttle but at small club volumes) and I tried darn near EVERYTHING!! I built the Uber Gain to improve my build techniques because high gain amps will expose every weakness! Just so happened that it completely crushed the Mesa for pure tone, so I sold it. BTW my gigging amp is Sluckey's dual-channel plexi/800 with a high-gain switch (added stage 2 Ck bypass cap). The Lord Jesus would own that amp if He played rock (but actually think He prefers jazz).
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+2!
Very cool build! I have one more request... How about some soundclips?!
Jim
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Nice work and color corrdination is cool too. Thanks for showing it.
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Hit me up if interested in schematic . . .
Would you post it in this thread if anyone were interested?
I'd love to see this as well. Also interested in what you did with Sluckey's amp.
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Hey guys - slammed with my day job this week and owe some schematics and clips. The heart of this beast is the Uber Gain pre-amp which I've posted here to whet appetites - designed by Mason Wolak at AX84 with inputs from Merlin. I built this unmodified except dropped the 1nf anode cap in stage one (originally added to suppress parasitics) which you won't need if you have short leads in your layout. Note that there are actually only 3 gain stages plus CF but note the bootstrapping between stage3 and CF which is key to the exceptional gain. The diode/resistor circuit on the CF is simply for boot-up protection of grid/cathode arching (Merlin trick I use on all CF builds). The mid-sweep allows you to go from heavily scooped 'modern' tone to mid-focused 'classic' tone. All else is pretty straight-forward.
Clips and layouts to come...
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Also interested in what you did with Sluckey's amp.
here's the thread from my build of Sluckey's Plexi/800
http://el34world.com/Forum/index.php?topic=22892.msg244863#msg244863
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How do you like the lower-gain settings? Do you have anything to compare to?
I love the variety of gain from my AX84 Lead 2, but it's complicated and uses 3 tubes so dual-channel wasn't on the table. Unfortunately it does NOT clean up well.
In the future I'll consider dual channels, and something like this would be much easier to work with than something like the Lead 2.
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How do you like the lower-gain settings? Do you have anything to compare to?
The two gain controls provide loads of flexibility and will take you from mid-gain soldano-type gain (although never gets smooth, always has bit of crunch to it) to all out metalmania, which honestly is what it's designed to do.
If I needed heavier tone for my band, I would use the uber and dial it back for the mid-gain marshally tones but honestly can't say enough good things about the plexi/800 dual channel plus an added high gain switch - you can dial-in darn near every tone and gain level across the spectrum
I love the variety of gain from my AX84 Lead 2, but it's complicated and uses 3 tubes so dual-channel wasn't on the table. Unfortunately it does NOT clean up well.
I built Brian Schmittling's version of the Lead II w/only 4 gain stages (vs 6 in yours), and had nice smooth distortion but as you say, never did the clean-to-mean like a ken fischer amp, which after much research i think the key there is you can only have 3 gain stages to do that volume roll-back well. a fourth (or more) gain stage just won't clean up as well because the last stage will always be the first to clip. I love the simplicity of footswitching clean/crunch on stage plus a second mv for f/s lead boost
In the future I'll consider dual channels, and something like this would be much easier to work with than something like the Lead 2.