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DIY All-in-one Experimental Amp
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Old 1st November 2019, 07:43 PM   #1
KarenColumbo is offline KarenColumbo
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Default DIY All-in-one Experimental Amp

Dear DIYaudios,


May I submit a project to your watchful eyes? It's something I've been planning a long time, and I think it's time I actually gather my wits and try to build it.


I want this to be the base for twiddling as many knobs and switches as possible to be able to experiment with voicing and sound generation in an amp. In the case of a good outcome I want to build a few stripped-down versions of this with what I learned from this particular monster as background.


I attached the current schematic for this.


There's some points I should make clear beforehand: The "BOOST" double-throw is actually a relay, all other switches are the common kind, the "BIAS" switch will only be toggled when power is off. The base for all this mayhem are Merlin B.s excellent Universal PCBs since I come from stompboxes and so I'm used to PCBs. I'm willing to expand my preciously little knowledge in the fields of point-to-point and other advanced techniques, but for this I feel I'm safer with those I feel comfortable with.

If you would cast a scrutinizing eye on my project to help me find fatal flaws in the contruct I'd be very glad! Thank you in advance whoever gives this a once-over!
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File Type: jpg Berlin_v4.jpg (471.3 KB, 268 views)
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Old 1st November 2019, 08:12 PM   #2
Tubelab_com is offline Tubelab_com  United States
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I went down a similar road only to find that I put too much gain in one box. The amp was so microphonic that you could talk to the input tube and hear your voice in the speaker, and I only had 4 gain stages before the PI, you have 6.

I didn't use 12AX7's either, I used a cheap tube with about half the gain of a 12AX7.

I would build the back half of this gain monster first and get it working by plugging the guitar in where the FX return jack is now. Once that is stable and you are happy with it add one more gain stage at a time, tame it, then go further.

See if you can Google up the schematic of the Soldano SLO 100. It has a bunch of gain stages, but tricks like 470K grid stoppers are used to tame the total gain and roll off the highs to kill the hiss. Each stage had different tricks used to make the whole amp stable, and LDR are used in place of the relays to bypass gain stages as needed.

I wound up doing things to reduce the gain of three stages and totally bypass another to get an amp that I liked. There were two versions of this amp. Note that I had to wrap the input tube in styrofoam to keep the microphonics down on the EL34 version. I could get a monster feedback without even plugging in the guitar.
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File Type: jpg EL34_A.jpg (241.5 KB, 280 views)
File Type: jpg BoardTop_A.jpg (235.7 KB, 257 views)
File Type: jpg UnderTheDeck_A.jpg (242.1 KB, 256 views)
File Type: jpg ToneTest_A.jpg (229.4 KB, 250 views)
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Old 1st November 2019, 09:26 PM   #3
KarenColumbo is offline KarenColumbo
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Quote:
Originally Posted by Tubelab_com View Post
I went down a similar road only to find that I put too much gain in one box. The amp was so microphonic that you could talk to the input tube and hear your voice in the speaker, and I only had 4 gain stages before the PI, you have 6.

I didn't use 12AX7's either, I used a cheap tube with about half the gain of a 12AX7.

I would build the back half of this gain monster first and get it working by plugging the guitar in where the FX return jack is now. Once that is stable and you are happy with it add one more gain stage at a time, tame it, then go further.

See if you can Google up the schematic of the Soldano SLO 100. It has a bunch of gain stages, but tricks like 470K grid stoppers are used to tame the total gain and roll off the highs to kill the hiss. Each stage had different tricks used to make the whole amp stable, and LDR are used in place of the relays to bypass gain stages as needed.

I wound up doing things to reduce the gain of three stages and totally bypass another to get an amp that I liked. There were two versions of this amp. Note that I had to wrap the input tube in styrofoam to keep the microphonics down on the EL34 version. I could get a monster feedback without even plugging in the guitar.

Thx for the reply! Well, actually there's only 3-4 gain stages here, one can be switched in or out, one is a cathode follower before a quite lossy tone stack, and then there's just the FX loop which SHOULD have unity gain. Still too much? I could purge the FX loop, only wanted to plug in some reverb based on the FV-1 spinsemi ...



I like the idea of going "backwards" step by step. Food for thought, thank you!
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Old 1st November 2019, 11:40 PM   #4
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by KarenColumbo View Post
I like the idea of going "backwards" step by step.
I build that way, too. Power supply first, then power amp, then preamp. I test (and trouble-shoot if necessary) as I go.

A bonus is that you can plug in a guitar and a couple of FX pedals (acting as a crude preamp) into the power amp as soon as it's built, and hear your creation come to life, without having to wait until the preamp is built too.


-Gnobuddy
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Old 2nd November 2019, 03:17 AM   #5
PRR is offline PRR  United States
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> find fatal flaws

Not "fatal", but.... I don't see NFB from the speaker-end, and without that your "resonance" control can't do anything.

This is a big HIGH gain plan, and layout will be critical. Not a project for the inexperienced.
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Old 3rd November 2019, 06:33 PM   #6
Gnobuddy is offline Gnobuddy  Canada
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This is a big HIGH gain plan
IMO it looks a lot like a Marshall JCM800 50W model 2204 (attached), with an FX loop added.

The FX loop presumably will have roughly unity gain end-to-end, so I would think overall gain would be about the same as the 2204.

While most of the 2204 demos I found had the gain turned up to "classic rock" levels, the following demo by a really good musician shows that clean tones are still accessible, and very usable: YouTube

It seems to me these 3-gain-stage Marshalls had high enough gain for classic rock and maybe the earliest metal precursors (Deep Purple, etc). But gain was still low enough to allow some playing dynamics, and you could still tell the instrument was a guitar.

Three gain stages doesn't cut it for what the kids call "high gain" in recent decades, amps like the Peavey 5150, with enough gain stages to make your head swim, and your guitar sound like a power tool grinding on a tin roof: http://www.4tubes.com/SCHEMATICS/Mus...0/evh_comp.gif

-Gnobuddy
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Old 3rd November 2019, 08:43 PM   #7
JMFahey is online now JMFahey  Argentina
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I don't see NFB from the speaker-end, and without that your "resonance" control can't do anything.
This tells me you have no clue and are not actually *designing*, just copypasting random bits from different schematics, with no clue as to actual function.
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Old 4th November 2019, 01:56 PM   #8
Tubelab_com is offline Tubelab_com  United States
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That schematic looks like it was drawn in Eagle. OPT's are not a simple library pick, and as such may not bee included in the schematic. They are not shown in many of my schematics either, but a simple pad labeled "from OPT secondary" would be possible.

It was stated that it was a breadboard, or some type of prototype. I often build something like this in stages, and the OPT feedback is left for near the end of the process.

Want some truly crazy sounds, stick a tone stack with volume control in the feedback loop. WARNING, instability and oscillation is possible. Some neat screaming sounds are available right on the edge, but make sure the output stage, OPT and speaker can deal with it before going there.
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Old 4th November 2019, 03:00 PM   #9
jjasniew is offline jjasniew  United States
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I wrote an article - 20 + years ago, still floating about the internet - about taking an existing tube amp and rewiring it to have multiple cascaded gain stages. I've made one recently with 3, but this time put a fixed tone stack (ala Mesa Maverick) in between stage I and II, with the first volume control between stage II and III. This "shapes" the frequency response before intentionally overloading stage III. I believe careful shaping of frequency response stage to stage is key to a decent sound.

It's for sale on ebay right now - no one wants it. At least the lesson repeats that I'd never make a living at this... But it's still fun! Tubelab's efforts are impressive to see.

30 years ago I made an all differential amp by putting the phase splitter as the very first tube. Double everything - tone, volumes - all the way to the output tubes. I tried to sell it and one player noted "you got it to sound like el34s using 6l6s!" I didnt know what that meant nor could hear what he was hearing. He never came back with the money - which was a little heartbreaking - and I dont recall what finally happened to that amp...
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Old 4th November 2019, 08:18 PM   #10
Gnobuddy is offline Gnobuddy  Canada
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...He never came back with the money - which was a little heartbreaking...
I feel for you. Electric guitar players are incredibly conservative as a group, particularly the ones who are still interested in tube amps.

If it's any consolation: pity today's most talented violin-makers. Carefully conducted double-blind testing ( Million-dollar Strads fall to modern violins in blind ‘sound check’ | Science | AAAS ) has shown quite conclusively that the best modern violins are preferred by top-notch violinists over 350-year-old Stradivarius and Amati and Guarneri violins.

But as soon as you take away the blindfolds, violinists and millionaires alike are desperate to get their hands on the multi-million-dollar Strad's and Amatis, while the better-performing modern instruments are treated as second-rate, and second-rate by a a long, long, long way at that.

It must be heart-breaking for these contemporary violin-makers. They're creating instruments today that are better than the best ever made in history, but they're viewed as vastly inferior by everyone.

-Gnobuddy
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