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Old July 25th, 2013, 07:28 PM   #1 (permalink)
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Design and build my own quiet amp

Hi there,

It's my first time posting here though I've been visiting for a while. I'm considering my next valve amp build and would appreciate any advice and thoughts folks here might have. My spec is as follows:

* Design, layout, source parts, & build it myself. I'm really keen to work through all the phases myself as a learning process. I realize that inevitably I will likely borrow all the parts of the design from the classics, but I'd like to try to learn why the particular parts go together as they do.

* Must be quiet! Even a Champ is too loud for playing in my flat with my wife and baby asleep. Considering something like the Firefly 12AU7 as the output. What are my options for low power output tubes?

* A bit of dirt might be nice, not totally clean.

* Tone controls

* Spring reverb

* I'll build it as a head with a separate speaker cab. Physically small a plus.

* I'm not sure what my chassis options are. I don't have access to equipment to form an enclosure but If I used a Hammond style enclosure I could punch my own holes for valves. Not sure about transformers, though.

* I'm based in UK, so will prefer UK or EU suppliers. UK power.

I have previously built a Princeton Reverb kit as part of a course and I build effects pedals so I have some experience.

Thanks for any advice that you might think of, I'll try to post updates though I can't promise they'll be frequent.

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Old July 26th, 2013, 01:06 PM   #2 (permalink)
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Sounds like a good time for subminiature tubes!

If you want to stay a little more garden-variety, push-pull triodes are a pretty good choice. 5 years back I built a super-mini version of a Marshall JCM800, using four triodes (two 12AU7, 12BH7 or ECC99) for the output section. The gain was too high, the tone stack was too much for the output section, but the output sounded quite good - especially with the ECC99. I wouldn't hesitate to use one again for a low-power amp. A couple of notes:
  • Bigger plates = bigger sound. That's why the 12BH7 and ECC99 can sound decent at very low power.
  • Use grid (fixed) biasing, otherwise you'll never get the operating range quite right.
  • Negative feedback is VERY helpful for evening out the frequency response and being able to maintain the illusion of clean headroom. :)
  • Go simple on the tone stack, one or two knobs max. There's just not enough dynamic range in a low-watt amp to make use of a lot of controls.
  • Smaller, quality iron (like the Hammond 125A) will outperform larger, discount iron.
  • Grid Stoppers are good!

Instead of transformer-coupled reverb, take a look at doing capacitor-coupled reverb instead. I have drawn up (but haven't built/tested) a little reverb circuit using a 12DW7 - low-mu side to drive the tank, high-mu for recovery. I'm not sure if I have the pin #s right or wrong though. :)

-Laird
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Old July 26th, 2013, 02:06 PM   #3 (permalink)
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Quote:
Originally Posted by laird View Post
Sounds like a good time for subminiature tubes!

If you want to stay a little more garden-variety, push-pull triodes are a pretty good choice. 5 years back I built a super-mini version of a Marshall JCM800, using four triodes (two 12AU7, 12BH7 or ECC99) for the output section. The gain was too high, the tone stack was too much for the output section, but the output sounded quite good - especially with the ECC99. I wouldn't hesitate to use one again for a low-power amp. A couple of notes:
  • Bigger plates = bigger sound. That's why the 12BH7 and ECC99 can sound decent at very low power.
  • Use grid (fixed) biasing, otherwise you'll never get the operating range quite right.
  • Negative feedback is VERY helpful for evening out the frequency response and being able to maintain the illusion of clean headroom. :)
  • Go simple on the tone stack, one or two knobs max. There's just not enough dynamic range in a low-watt amp to make use of a lot of controls.
  • Smaller, quality iron (like the Hammond 125A) will outperform larger, discount iron.
  • Grid Stoppers are good!

Instead of transformer-coupled reverb, take a look at doing capacitor-coupled reverb instead. I have drawn up (but haven't built/tested) a little reverb circuit using a 12DW7 - low-mu side to drive the tank, high-mu for recovery. I'm not sure if I have the pin #s right or wrong though. :)

-Laird
No, No, No, all wrong. Well OK, other than the sub-mini tubes, sounds like a plan.


Use an inefficient speaker, I have a bunch of old 8" paging speakers that I sometimes use. They are about 90 dB or there abouts. You can also get a wall volume control/rheostat to knock down the level at the amp output if it is still too loud.
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Old July 29th, 2013, 06:19 PM   #4 (permalink)
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Thanks for the tips and ideas, I'll have a look at those tubes. I've never heard of capacitor coupled reverb, will look into that too.
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Old July 30th, 2013, 10:18 AM   #5 (permalink)
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Go to a music store and try out 5 watt tube amps. Hopefully you can get the amp into a small room and play them throughout their volume ranges. 5 watts can be exceedingly loud depending on the speaker sensitivity/effeciency (db) rating it's coupled with. The problem with most twin triodes as output tubes is usable clean head room and they generally sound thinner than pentodes. Most AX84 firefly end up using an ECC99 (different bias value) as the output tube to increase output and headroom.

You could opt to install a VVR http://www.ampmaker.com/store/VVR-ki...mplifiers.html in any amp you build to play at lower volumes. You could save yourself some cash and install a VVR in your champ. There's VVR schematics on-line if you want to build your own too.
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Old July 30th, 2013, 05:46 PM   #6 (permalink)
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... The problem with most twin triodes as output tubes is usable clean head room and they generally sound thinner than pentodes. Most AX84 firefly end up using an ECC99 (different bias value) as the output tube to increase output and headroom.

You could opt to install a VVR ...
If I want dirt and low volume, isn't that the opposite of clean headroom? I'm not being sarcastic, I just wonder if that means a Firefly type design isn't exactly what I'm after!

I already have a power soak on the Princeton I built which gets it down to a manageable volume level. I suppose I could build another into my new amp design and even add a switch to activate or deactivate it. That would mean I could use a more conventional type of output stage.
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Old July 30th, 2013, 10:15 PM   #7 (permalink)
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Think about it for a minute, you want clean headroom at low volume right? Well, your champ should provide that easily at it's lowest/low volume settings. Look around the internet and you will most likely find that most low output tube amps (3 watts and less) are designed for medium to high gain. The Firefly's designer Doug Hammond wanted a high gain marshall sound at low volume.
I am not saying that you can't have both clean and dirt at low output, what I am saying is there will most likely be a compromise towards either clean or dirt as you won't get both at comparable volume levels. That's why a distortion pedal is often times a guitarist best-est friend.

If you want good clean and good distortion at low watts you may want to consider building a two channel amp. You could follow any Fender two channel amp design such as a twin reverb and use low watt output tube(s) and/ or lower B+ (eg: 250v). Unfortunately, two channels adds extra expense and chassis space needed. I don't mean to deter you or sound negative and I hope whichever build you choose turns out great.
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Old July 31st, 2013, 12:10 AM   #8 (permalink)
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Although I have heard some of the triode output based amps sound good at the higher gain stuff they never did it for me at the edge of breakup range. I like the sound of pentodes. Here is one man's take on a roughly 1W amp. I could see doing a single 6AK6 output. Some good sound files.

http://home.polstra.com/amps/littlerock/
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Old July 31st, 2013, 01:32 PM   #9 (permalink)
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Although I have heard some of the triode output based amps sound good at the higher gain stuff they never did it for me at the edge of breakup range. I like the sound of pentodes. Here is one man's take on a roughly 1W amp. I could see doing a single 6AK6 output. Some good sound files.

http://home.polstra.com/amps/littlerock/
I agree with you P2. I owned a firely for a while and tried it with both a 12au7 and ecc99. The ECC99 improved clean-room and output compared to the 12au7, but IMO both had a tinny sterile sound that lack character. Rolling the preamp tubes with a 12ay7 for V1 and 12bh7 in V2 helped, but I ended up tearing it down and built a low wattage champ-ish type that I am much happier with. Pentodes are fuller sounding and their distortion is more gradual.

A single-ended 6BM8 (ECL82) Triode/Pentode with 250 plate voltage would put out 3 watts. Using a 7-pin single triode preamp (e.g, 6AV6, 6SF5, 6C4) would keep in low to medium gain territory.

Oh yea, as a side note the "Boogie" clip in the link you posted is Johnny Winter's: Mean Town Blues ... dig it.
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Old August 2nd, 2013, 01:29 PM   #10 (permalink)
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You got me curious and I had to try it. You might find this of interest.

http://www.tdpri.com/forum/shock-bro...ml#post5198492
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Old August 22nd, 2013, 06:06 PM   #11 (permalink)
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p2, that is an interesting post, thank you. One thing I don't understand, though: the .022u cap coming of the plate on the second half of the 12AX7 is unconnected - where does that go?
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Old August 22nd, 2013, 06:09 PM   #12 (permalink)
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I'm going to continue my studies on amp design a bit more before really digging into this project further. I've got an opportunity coming up to collaborate on an amp cabinet with my father, who is a really good woodworker, and I'd like to use a tried and true circuit. So I'm going to build a 5e3. I'll likely post some questions in another thread and revisit this later. Thanks for all your thoughts and see you 'round!
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Old August 22nd, 2013, 06:18 PM   #13 (permalink)
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p2, that is an interesting post, thank you. One thing I don't understand, though: the .022u cap coming of the plate on the second half of the 12AX7 is unconnected - where does that go?
Sorry, basically what I did was try two preamp sections. One with the 12AX7 to the output tube (536k/220k junction) and then the 6AU6 to the output tube.
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Old August 22nd, 2013, 06:25 PM   #14 (permalink)
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Thanks for the chuckles, Tim. You had them thinking there for a while before you hit that U-turn. FWIW, I have a 'quiet' amp here....a Silvertone Md. 1431 made by Danelectro with a single 6V6....it won't wake anyone up andyet it has great sounds. The OT is one of the smallest transformers I have ever seen in a guitar amp, and the 6" speaker has to be one of themost inefficient that I have heard. AS printer2 ntoes, a few watts can be really loud unless you tame it with the speaker..or an attneuator, l-pad, rheostat or whatever. I prefer inefficient speakers to other devices.
How are you going to keep that 5E3 quiet....and then teh question arises....why build an amp that really needs to be loud in order to sound good whe you are looking for 'quiet'? (;^)
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Old August 24th, 2013, 10:45 AM   #15 (permalink)
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You got me curious and I had to try it. You might find this of interest.

http://www.tdpri.com/forum/shock-bro...ml#post5198492
Printer2: Just checked out your link and although I haven't built a low watt 2 channel amp yet, it's been on my mind for a while. I like the 6AU6 (EF86 alternative) as well as for the output. What does the overall output level at full volume sound like comparing 12AX7 vs. 6AU6? My assumption is the 12AX7 channel is louder and obviously more distorted.

If I end up building one I'd like to incorporate a foot switch to switch between channels.
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Old August 24th, 2013, 11:46 AM   #16 (permalink)
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Yes the 12AX7 had more gain and you could get more area under the power curve making it louder with added distortion. I liked the 6AU6 sound a little better at its full volume, a mix of clean and distortion, the edge of breakup thing.
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Old August 24th, 2013, 01:18 PM   #17 (permalink)
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Out of the three amps I've built so far I don't have a 12ax7 in any of them. I don't like how they boost the preamp signal so quickly and the power tube(s) distortion tone doesn't shine through as much at higher volumes.
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Old August 24th, 2013, 03:44 PM   #18 (permalink)
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Out of the three amps I've built so far I don't have a 12ax7 in any of them. I don't like how they boost the preamp signal so quickly and the power tube(s) distortion tone doesn't shine through as much at higher volumes.

Well.... It all depends on how you setup the gain structure of the preamp. You won't get distortion from the power tubes unless you overdrive them. That means you need a certain amount of power amp input signal, which depends on the poweramp setup.

Consider a Trainwreck. The preamp and phase inverter stay clean until after the powertubes overdrive. This is due to how the gain is balanced at each preamp stage. Each preamp stage is not overdriving the next, until past the point where the total gain out of the preamp is overdriving the poweramp.

It's not as simple as "12ax7's breakup before the poweramp can distort." Every amp is different and it's all how you want to design for what sound you want.
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Old August 24th, 2013, 04:32 PM   #19 (permalink)
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Michael, Ken Fisher was quoted as saying that his philosophy is for all stages to hit that OD level at the same time.
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Old August 24th, 2013, 10:33 PM   #20 (permalink)
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You can look at each stage on a scope, or do the math. They don't all overdrive at the same time.

http://home.polstra.com/amps/wreck1/scope/

It's beside the point though. You can design for whatever gain you want in each stage, to overdrive or not each following stage. It's not just the idea that one tube distorts and one doesn't
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