Allright, an uncle of mine has found back his old guitar speaker with a Goodmans Axiom 301 and he asked me for an amp to go with it. The guitar itself is an acoustic steel string guitar, with a passive magnetic pickup in the sound hole (dating back to the late sixties) and a basic volume control and eq onboard (all passive).
The amp will be mostly used at home and in pub/small parties, for folk and soft rock music. With a 100db sensitivity rating, the driver doesn't need much power.
Requirements are basic, no need for elaborate effects. It should sound "clean but warm" (sic). Cost is an issue though, the lower the better but it has to be decent. An additional mic input for voice would be appreciated.
I know next to nothing about guitar amps... What would you suggest ? SS, tubes, hybrid ? 😕
Thanks for any pointers on this one.
I've got a small power transformer I could put to use, it has beefy heaters secondaries but the B+ isn't that strong: 260-230-0-230-260 @100ma but with 175r dcr on each side of the CT.
The amp will be mostly used at home and in pub/small parties, for folk and soft rock music. With a 100db sensitivity rating, the driver doesn't need much power.
Requirements are basic, no need for elaborate effects. It should sound "clean but warm" (sic). Cost is an issue though, the lower the better but it has to be decent. An additional mic input for voice would be appreciated.
I know next to nothing about guitar amps... What would you suggest ? SS, tubes, hybrid ? 😕
Thanks for any pointers on this one.
I've got a small power transformer I could put to use, it has beefy heaters secondaries but the B+ isn't that strong: 260-230-0-230-260 @100ma but with 175r dcr on each side of the CT.
Have a long, hard look at the $100 amp thread.
You need less than 10W for clean on 100db/w but for "warm" you need gentle clipping (generating 2nd harmonic and compression rather than 3rd then fizz)
SE 6L6 + 12A*7 w/ a 12V halogen tranny (or some other power transformer) as the OPT (e.g. Grant Will's Solo)
or
PP 6V6/EL84 + 12A*7 w/ a 100V PA tranformer as the OPT (so many to choose from but there's simple stuff like this GA-40 based example - just avoid the high gain ones)
are the two easy answers.
Any 5W triode or pentode can probably be used. Parallel 12AU7 might even be loud enough in PP mode.
260CT@100mA is 26W. More than enough power. What heaters does it have?
A slightly more complicated answer is: add a laptop power supply to any 15W single supply "chip" amp and find a nice sounding preamp from runoffgroove. e.g. Ginger, Peppermill or Supredeux (for the rockier end). There's sound clips here.
You need less than 10W for clean on 100db/w but for "warm" you need gentle clipping (generating 2nd harmonic and compression rather than 3rd then fizz)
SE 6L6 + 12A*7 w/ a 12V halogen tranny (or some other power transformer) as the OPT (e.g. Grant Will's Solo)
or
PP 6V6/EL84 + 12A*7 w/ a 100V PA tranformer as the OPT (so many to choose from but there's simple stuff like this GA-40 based example - just avoid the high gain ones)
are the two easy answers.
Any 5W triode or pentode can probably be used. Parallel 12AU7 might even be loud enough in PP mode.
260CT@100mA is 26W. More than enough power. What heaters does it have?
A slightly more complicated answer is: add a laptop power supply to any 15W single supply "chip" amp and find a nice sounding preamp from runoffgroove. e.g. Ginger, Peppermill or Supredeux (for the rockier end). There's sound clips here.
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Ah, too late to edit my post. I was going to say that early amps are probably the best place to look - e.g. the Supro/Valco/Gretch (here's some schematics)
If you're really lazy, the Vox Pathfinder 15R, with or without the recommended mods (boost control and LED disable switch) is a pretty good place to start for little $ but I don't think you'd be able to fit the Axiom in (10" yes. 12" maybe not)
If you're really lazy, the Vox Pathfinder 15R, with or without the recommended mods (boost control and LED disable switch) is a pretty good place to start for little $ but I don't think you'd be able to fit the Axiom in (10" yes. 12" maybe not)
Thanks a lot for all the suggestions. I'm going to go through that.
Here are the specs of the transformer:
100W power transformer Recommend for Single Ended tube power AMP or Mono power AMP.
Primary 0-220-230-240V
260-230-0-230-260V 100mA
0-5V 3A with CT
0-6.3 3A with CT
0-6.3 3A with CT
Here are the specs of the transformer:
100W power transformer Recommend for Single Ended tube power AMP or Mono power AMP.
Primary 0-220-230-240V
260-230-0-230-260V 100mA
0-5V 3A with CT
0-6.3 3A with CT
0-6.3 3A with CT
OK... I did some readings. Here's what I'm considering.
* PSU
- entering the transformer data into psud, I get about 315Vdc with a 70mA loading, 300Vdc with a 100mA load.
- assuming two EL84 (2x750mA) and 2 small signal tubes (2x300mA), one of the 6.3Vac, 3A secondaries is sufficient.
- I've got two free low voltage secondaries, 5V and 6.3V. That makes up to a 11,3Vac winding free for aux supplies.
* Amp
- output stage: considering that both guitar and voice could go through, the amp output section has to be quite clean. With the power transformer on hand, the el84 datasheet offers some guidance. In pentode mode, with a 300V b+ and a 8K CT opt, a push pull amp can easily offer 4W before hitting 1% thd. See this article
- output transformer: I need a 8K:16r with ct here. The easiest to get are the hammond classic which are around 40€ at tube-town.net. I'm considering either the 1750PA or 1760E. They seem much better than the slightly cheaper 125E ?
- gain and phase splitting stage: unless gnfb is needed, we don't need tons of gain at this point.
* Mic preamp
- The esp66 is a good starting point. Ultra low noise being unnecessary, I'll probably replace the 2N4403 by bc560 or the like. It will be mixed with the guitar section right at the input of the amp.
* Questions left pending
- open loop or nfb ? If I want to extract the most clean power possible, nfb is necessary. But is that desirable ? That would determine the input stage of the amp.
- guitar input section: I'll need a tone stack and some more gain at least... Maybe a section generating some 2nd order distortion for that "warm sound". Maybe that supreaux deux ?
I'm going to draw some schematics to make things clearer...
* PSU
- entering the transformer data into psud, I get about 315Vdc with a 70mA loading, 300Vdc with a 100mA load.
- assuming two EL84 (2x750mA) and 2 small signal tubes (2x300mA), one of the 6.3Vac, 3A secondaries is sufficient.
- I've got two free low voltage secondaries, 5V and 6.3V. That makes up to a 11,3Vac winding free for aux supplies.
* Amp
- output stage: considering that both guitar and voice could go through, the amp output section has to be quite clean. With the power transformer on hand, the el84 datasheet offers some guidance. In pentode mode, with a 300V b+ and a 8K CT opt, a push pull amp can easily offer 4W before hitting 1% thd. See this article
- output transformer: I need a 8K:16r with ct here. The easiest to get are the hammond classic which are around 40€ at tube-town.net. I'm considering either the 1750PA or 1760E. They seem much better than the slightly cheaper 125E ?
- gain and phase splitting stage: unless gnfb is needed, we don't need tons of gain at this point.
* Mic preamp
- The esp66 is a good starting point. Ultra low noise being unnecessary, I'll probably replace the 2N4403 by bc560 or the like. It will be mixed with the guitar section right at the input of the amp.
* Questions left pending
- open loop or nfb ? If I want to extract the most clean power possible, nfb is necessary. But is that desirable ? That would determine the input stage of the amp.
- guitar input section: I'll need a tone stack and some more gain at least... Maybe a section generating some 2nd order distortion for that "warm sound". Maybe that supreaux deux ?
I'm going to draw some schematics to make things clearer...
You don't want the most clean power: you want it to clip nicely. The seminal paper for preamps is Hamm, R. O. (1973). Tubes Versus Transistors-is there an Audible Difference. Journal of the audio engineering society, 21(4), 267-273. It is available on-line (e.g. here ).
Which pretty much rules out NFB, as normally implemented.
For the same reason 6V6s (beam tetrode with undersized heater) would be preferable to EL84s (notoriously "chimey" pentode)
On the mike side of things consider a soft limiter such as in the late Fred Nachbauer's "Dogzilla" amp project, more extensively described here: DOGZILLA - Active Tone Control / Limiter
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What amplifier was used with this beloved speaker "before"?
In guitar work, the amplifier matters.
A particular concern is damping. Hi-Fi fad is high damping, which means heavy NFB. Guitar amps have varied all over the place. Ampeg VT40 had no damping which suited the particular speakers and cabinet it had. Fender tended to around unity damping, and picked speakers to work that way. When Marshall took Fender's plan, but used UK speakers (also the music changed), they increased damping. Sunn sold a lot of highly damped amps. Classic transistor amps are highly damped, but some designers are reducing damping.
So find the beloved amp, or whatever scraps of memory remain, and ponder that.
In guitar work, the amplifier matters.
A particular concern is damping. Hi-Fi fad is high damping, which means heavy NFB. Guitar amps have varied all over the place. Ampeg VT40 had no damping which suited the particular speakers and cabinet it had. Fender tended to around unity damping, and picked speakers to work that way. When Marshall took Fender's plan, but used UK speakers (also the music changed), they increased damping. Sunn sold a lot of highly damped amps. Classic transistor amps are highly damped, but some designers are reducing damping.
So find the beloved amp, or whatever scraps of memory remain, and ponder that.
From the scraps of memory I gathered, a few different amps were used. Mostly early solid state, from local small sellers here in Belgium. Those are long gone without leaving a trace.
Honnestly, I got the impression that the driver was bought on an impulse because it was the "best" they could get and the band had a good deal on it. But they never got an amp of matching quality and used whatever they could get their hands on.
There's a fair bit of nostalgia in this project...
Honnestly, I got the impression that the driver was bought on an impulse because it was the "best" they could get and the band had a good deal on it. But they never got an amp of matching quality and used whatever they could get their hands on.
There's a fair bit of nostalgia in this project...
Goodmans Axiom Brochure
Massive magnet. Impressive speaker. Asking big bucks on eBay.
Not too sensitive to amp damping because the bass resonance is far below the guitar range.
Overlook the "40W in USA" thing. At the time, UK amp makers were very honest, and some USA makers were pushing IHF numbers from transient tests at high distortion.
However for *distorted* electric guitar, I would not go over 10 honest Sine Watts, because in heavy clipping this can be 20 Watts of heat in the coil. And with older shellac, that was enough for a 1.75" coil.
What about a Fender AA Champ clone? It was good enough for Clapton. With the stock small speaker it plays very small clubs; with a big-magnet Twelve it should play many venues fine. The parts are readily available, as are layouts.
Massive magnet. Impressive speaker. Asking big bucks on eBay.
Not too sensitive to amp damping because the bass resonance is far below the guitar range.
Overlook the "40W in USA" thing. At the time, UK amp makers were very honest, and some USA makers were pushing IHF numbers from transient tests at high distortion.
However for *distorted* electric guitar, I would not go over 10 honest Sine Watts, because in heavy clipping this can be 20 Watts of heat in the coil. And with older shellac, that was enough for a 1.75" coil.
What about a Fender AA Champ clone? It was good enough for Clapton. With the stock small speaker it plays very small clubs; with a big-magnet Twelve it should play many venues fine. The parts are readily available, as are layouts.
For an Acoustic Guitar go SS.
Way cleaner/smoother, you won´t justify getting into way more complex, heavy and expensive tubes since you will NOT make the amp clip.
Build the datasheet example LM3886 amplifier, 50W into 8 ohms , which you will play at an average 15 to 25W, the rest is clean headroom.
You can easily get kits for them, including PCB and power supply, maybe the heatsink, you might have to get the power transformer on your own but being very popular it´s available from multiple sources ... which might include the kit supplier.
FWIW ***ALL*** Acoustic Guitar amplifiers are SS, just check major Music Shops, or catalogs from Peavey/Crate/Fender/Marshall/Laney/Trace Elliot/Yamaha/Roland/Fishman/etc. , that should tell you something.
You can build the Rod Elliott Guitar preamplifier , (not fitting the clipping diodes), sound is "Fendery clean" and will be perfect for your guitar.
If you want something more sophisticated, I might send you some commercial Acoustic Guitar schematic, which typically include some notch filter, or an extra channel for an Acoustic/Voice microphone and even an extra Microphone channel, if you need one, so you can sing and play at, say, some Family party without needing extra amplification.
Way cleaner/smoother, you won´t justify getting into way more complex, heavy and expensive tubes since you will NOT make the amp clip.
Build the datasheet example LM3886 amplifier, 50W into 8 ohms , which you will play at an average 15 to 25W, the rest is clean headroom.
You can easily get kits for them, including PCB and power supply, maybe the heatsink, you might have to get the power transformer on your own but being very popular it´s available from multiple sources ... which might include the kit supplier.
FWIW ***ALL*** Acoustic Guitar amplifiers are SS, just check major Music Shops, or catalogs from Peavey/Crate/Fender/Marshall/Laney/Trace Elliot/Yamaha/Roland/Fishman/etc. , that should tell you something.
You can build the Rod Elliott Guitar preamplifier , (not fitting the clipping diodes), sound is "Fendery clean" and will be perfect for your guitar.
If you want something more sophisticated, I might send you some commercial Acoustic Guitar schematic, which typically include some notch filter, or an extra channel for an Acoustic/Voice microphone and even an extra Microphone channel, if you need one, so you can sing and play at, say, some Family party without needing extra amplification.
In the line of "keeping it clean", would the following schematic be ok ?
Starting from the top left:
- input is a simple gain stage with some LF and HF roll-off. A switch (or a hi-lo jacks setup) pads the input if needed.
- then comes a classic Baxandall EQ, followed by a buffered volume control for the guitar section (pots are r5-6, r13-14, r1-19).
- then a virtual earth summing amp.
- then a relatively soft limiter section, to avoid uncontrolled clipping at the output.
- then the usual lm3886. Assuming +/-32Vdc supplies and with the limiter in place, it will give about 15 clean watts into a 16r speaker.
- at the bottom, the mic preamp, quite the usual stuff. Gain control is r30-31.
- PSU is not shown but I plan on using two 50Va 2*12Vac transformers, giving me +/-24Vac. Either I can use the 12V taps for the opamps or add another small transfo.
So the questions:
- what kind of output does a magnetic pick up usually give ? What kind of gain do I need at the input, considering the limiter has to kick in at around 1.2V (considering the minimum gain of the lm3886 for stability) ?
- is a master control volume necessary ? Where best to put it ?
- would it be a good idea to add a pair of big caps at the output, to protect the driver against a DC fault ?
Thanks again for all the advice 🙂
Starting from the top left:
- input is a simple gain stage with some LF and HF roll-off. A switch (or a hi-lo jacks setup) pads the input if needed.
- then comes a classic Baxandall EQ, followed by a buffered volume control for the guitar section (pots are r5-6, r13-14, r1-19).
- then a virtual earth summing amp.
- then a relatively soft limiter section, to avoid uncontrolled clipping at the output.
- then the usual lm3886. Assuming +/-32Vdc supplies and with the limiter in place, it will give about 15 clean watts into a 16r speaker.
- at the bottom, the mic preamp, quite the usual stuff. Gain control is r30-31.
- PSU is not shown but I plan on using two 50Va 2*12Vac transformers, giving me +/-24Vac. Either I can use the 12V taps for the opamps or add another small transfo.
So the questions:
- what kind of output does a magnetic pick up usually give ? What kind of gain do I need at the input, considering the limiter has to kick in at around 1.2V (considering the minimum gain of the lm3886 for stability) ?
- is a master control volume necessary ? Where best to put it ?
- would it be a good idea to add a pair of big caps at the output, to protect the driver against a DC fault ?
Thanks again for all the advice 🙂
Attachments
Classic Fenders, full-up, can reach clipping with 20mV input.
Classic Fenders, turned-down some, will accept 400mV "clean" (5%THD in the first stage).
Classic Fenders, turned-down some, will accept 400mV "clean" (5%THD in the first stage).
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