FalderGuitars 5/17/2017 12:06 PM
1 Watt Single-Ended Output Stage
Good afternoon, Like a lot of people I talk to, I'm struggling to get a decent guitar tone at home without upsetting the neighbours (or my wife). I'm currently down to an Epiphone Junior (rated around 6 watts), but even with a Bitmo attenuator between the head and speaker I can't get it down to a volume I feel comfortable playing at. Long story short, having built a half dozen of so amps - first from kits, then from schematics - I thought the best solution might be to design a 1 Watt output stage for myself. Here's the schematic: [ATTACH=CONFIG]43545[/ATTACH] Obviously, it's not the most complicated schematic ever drawn (:)), but since my experience of designing circuits has been limited to simple gain stages thus far, I'm not 100% sure I've got the maths correct. According to my calculations, it should produce 1.2 Watts, but I've been wrong before! I was hoping some of you good people out there might have the time to take a quick look and tell me if I'm at least on the right track. I've got all my calculations written down, so if there's any more data you'd like I'll be happy to supply it. If I can get my scanner to work, I'll scan a copy of my loadline :thumbsup: Thanks for looking, Matt.
 
Enzo 5/17/2017 1:36 PM
The answer to low volume is rarely watts. 1 watt should be half as loud as 10 watts. Apparently in your situation this needs to be at "mom" levels. The same level as the radio at my dentist waiting room. Really, consider just getting something with headphones, don't even try to get that eighth of a milliwatt output. The physics of your ear means the sound won't be the same at those low levels anyway.
 
FalderGuitars 5/17/2017 3:44 PM
Hi Enzo, Thanks for your reply. I get what you're saying, but I'not sure it's relevant - in this situation at least. What I'm trying to do is not so much build an amp that's quiet enough to play at home, but to make one I can push to it's limits before it gets [I]too loud[/I] to play at home. I like the sound the of power amp pushing the transformer and speaker hard. Master volume amps sadly don't cut it for me, or my problem would be easily solved. Headphones are a non-starter. At this point, I've considered and rejected all other options, having tried the majority. I appreciate that a 1 Watt amp driven to the edge will not sound - or feel - the same as a 100,50,15 or even 4 Watt amp operating in the same region, but that's no reason - in my book - not to go ahead and make one anyway...:) FWIW, I can get away with a good bit more than dentist waiting room levels, and no-ones actually complaining. It's just that at it's sweet spot, the Valve Junior makes a lot of noise for a relatively low wattage amp, and I worry my neighbours are silently grinding their teeth the whole time I'm playing, but are too polite to say so. Do you have any thoughts on the circuit itself? I've seen a few deigned for 1W, but they're alll a little hazy on either voltage or transformer type. Or both. Matt.
 
J M Fahey 5/17/2017 5:13 PM
Triodes are bland sounding as power tubes, why not try a small pentode instead? It will sound much closer to what a typical tube amp does .
 
Mike Sulzer 5/17/2017 5:14 PM
Triodes are incapable of much current with low voltage across them. A quick look at the curves, and I would expect about .5 watt with both sections in parallel, but that could be a bit pessimistic. [QUOTE=FalderGuitars;455161] According to my calculations, it should produce 1.2 Watts, [/QUOTE]
 
nosaj 5/17/2017 5:30 PM
[QUOTE=J M Fahey;455205]Triodes are bland sounding as power tubes, why not try a small pentode instead? It will sound much closer to what a typical tube amp does .[/QUOTE] How about this one? [url=http://www.lh-electric.net/projects/1312flea1a.html]Low Power Vacuum Tube Guitar Amplifier[/url] nosaj
 
FalderGuitars 5/17/2017 5:39 PM
[QUOTE=J M Fahey;455205]Triodes are bland sounding as power tubes, why not try a small pentode instead? It will sound much closer to what a typical tube amp does .[/QUOTE] Thanks for the reply. That's interesting, any recomendations for a suitable pentode? My knowledge of valves is more or less limited to what you'd find in a guitar amp. I know there are many more to choose from, but being no hi-fi enthusiast I wouldn't know where to start looking. Is an EF86 any good? That's the only small signal pentode in my bits box. Matt.
 
FalderGuitars 5/17/2017 5:42 PM
[QUOTE=Mike Sulzer;455206]Triodes are incapable of much current with low voltage across them. A quick look at the curves, and I would expect about .5 watt with both sections in parallel, but that could be a bit pessimistic.[/QUOTE] Thanks for replying, I expect you're more clued up than myself, but if I got my calculations correct I'm expecting 1.2W. When I get a miunte I'll consolidate my workings into a readable/logical form and post them here. Matt.
 
FalderGuitars 5/17/2017 5:46 PM
[QUOTE=nosaj;455208]How about this one? [url=http://www.lh-electric.net/projects/1312flea1a.html]Low Power Vacuum Tube Guitar Amplifier[/url] nosaj[/QUOTE] Thanks for the link, I did see this circuit when I was doing my research, and was very interested. Unfortunately, the link that led me to it hadn't got the full gen on the valve choices, just the schematic. It's getting late tonight, but I'll have a look at this over breakfast. It looks at first glance like I should be able to find a suitable circuit for my needs. Fingers crossed, although I'm a little loathe to abandon my own circuit, having forced my brain to concentrate long enough to design it - maths and I do not mix! Matt.
 
Justin Thomas 5/17/2017 5:52 PM
When I looked, when I scrolled to the bottom to "Table 1," it had several potential power tubes, from 1 to 2.5 watts. Maybe a gadget malfunction? :P Justin
 
nosaj 5/17/2017 6:03 PM
[QUOTE=FalderGuitars;455210]Thanks for replying, I expect you're more clued up than myself, but if I got my calculations correct I'm expecting 1.2W. When I get a miunte I'll consolidate my workings into a readable/logical form and post them here. Matt.[/QUOTE] If you've already got an amp you like a less efficient speaker could be an answer. I took a 10inch field coil with some help from here and antiques radios forum built a speaker similar to a fluxtone Where you increase or decrease the speakers efficiency toned down a tweed deluxe from headphone level to pretty damn loud. It's not a master vol at all. Only thing I had to buy was the 50watt rheostat. nosaj
 
FalderGuitars 5/17/2017 6:04 PM
Hi, Yes, the link nosaj posted is working fine for me too. When I stumbled across the circuit a few weeks ago, the tables for valve choices wre missing - likely due to either a 'gadget malfunction' (nice phrase) or an old/dodgy link. I'll digest the options along with my porridge tomorrow morning ;-) Matt.
 
Justin Thomas 5/17/2017 6:24 PM
Porridge? Are you from the UK or Ireland? Or IN the UK or Ireland? If so: you put your location in your profile, it may help to speed things along - not everyone reads whole threads before chiming in, and you don't want someone suggesting 120V primaries or other such stuff, right? ;) Justin
 
nosaj 5/17/2017 6:29 PM
[QUOTE=Justin Thomas;455217]Porridge? Are you from the UK or Ireland? Or IN the UK or Ireland? If so: you put your location in your profile, it may help to speed things along - not everyone reads whole threads before chiming in, and you don't want someone suggesting 120V primaries or other such stuff, right? ;) Justin[/QUOTE] There sure are a lot of porridges [URL="https://en.wikipedia.org/wiki/Porridge"]https://en.wikipedia.org/wiki/Porridge[/URL] jason
 
Justin Thomas 5/17/2017 6:37 PM
Just got off three weeks in Ireland. Never heard anybody say "porridge" anywhere else! :P Either way, seems kind of "un-American" to me... paging Senator McCarthy... Justin
 
nosaj 5/17/2017 6:50 PM
[QUOTE=Justin Thomas;455219]Just got off three weeks in Ireland. Never heard anybody say "porridge" anywhere else! :P Either way, seems kind of "un-American" to me... paging Senator McCarthy... Justin[/QUOTE] It's probably not porridge but turnip greens and juice on top of some fresh honey yellow cornbread make a nice style cereal and not bad for breakfast either. nosaj
 
d95err 5/17/2017 11:48 PM
1 Watt will be way too loud to crank at home. You should look at the built-in attenuator used by the Marshall 1 Watt amps, which takes it down to about 0.1W. That attenuation was designed for the "kids-asleep-in-the-next-room" volume level. In general, I think a push-pull 1W stage with a proper phase inverter will sound better than a single-ended design.
 
FalderGuitars 5/17/2017 11:57 PM
[QUOTE=Justin Thomas;455217]Porridge? Are you from the UK or Ireland? Or IN the UK or Ireland? If so: you put your location in your profile, it may help to speed things along - not everyone reads whole threads before chiming in, and you don't want someone suggesting 120V primaries or other such stuff, right? ;) Justin[/QUOTE] Nice work, Columbo :) Duly noted and updated. I forget the majority of forums are US based, and that your National Grid privides a rather conservative 120V. Must take some of the fun out of getting a shock ;) Just one more thing...
 
FalderGuitars 5/18/2017 12:06 AM
[QUOTE=nosaj;455218]There sure are a lot of porridges [URL="https://en.wikipedia.org/wiki/Porridge"]https://en.wikipedia.org/wiki/Porridge[/URL] jason[/QUOTE] I'd think twice before saying that in Scotland ;)
 
Enzo 5/18/2017 12:42 AM
Well, if you build a 1 watt amp, no matter how loud it is, it will be no challenge to a common $6 L-pad. Dial it down as far as you like.
 
Zozobra 5/18/2017 3:12 AM
The ecc99 is a good tube to use for this too. Check these builds out by a fellow forumite: [url]http://music-electronics-forum.com/t43163/[/url] [url]http://music-electronics-forum.com/t43822/[/url]
 
mozz 5/18/2017 2:13 PM
[QUOTE=nosaj;455214]If you've already got an amp you like a less efficient speaker could be an answer. I took a 10inch field coil with some help from here and antiques radios forum built a speaker similar to a fluxtone Where you increase or decrease the speakers efficiency toned down a tweed deluxe from headphone level to pretty damn loud. It's not a master vol at all. Only thing I had to buy was the 50watt rheostat. nosaj[/QUOTE] In series or parallel with the field coil? I've got some rheostats here waiting for a project like that.
 
nosaj 5/18/2017 4:19 PM
[QUOTE=mozz;455288]In series or parallel with the field coil? I've got some rheostats here waiting for a project like that.[/QUOTE] Here's my thread [URL="http://music-electronics-forum.com/t39353/"]http://music-electronics-forum.com/t39353/[/URL] nosaj
 
Gregg 5/19/2017 1:07 AM
Since it's not a mass production amp I would recommend the 6P30B or its military version 6P30B-R. You can get those at ridiculously low prices on ebay and one of them will get you pure class A one Watt without any problems. However note that they draw like around 40mA of anode current so you would need an appropriate OT.
 
SoulFetish 5/20/2017 2:42 AM
[QUOTE=J M Fahey;455205]Triodes are bland sounding as power tubes, why not try a small pentode instead? It will sound much closer to what a typical tube amp does .[/QUOTE] Fahey speaks the truth here!! I would rather imagine a Pentode output stage , than play through a triode one.
 
Chuck H 5/20/2017 8:56 AM
Since Matt has already stated that there isn't much satisfaction from the Valve Junior with an attenuator and would like an amp that can be cranked I will guess that he means 'without an attenuator'. My first consideration here is that, for me, when considering all the compromises of playing that quietly attenuators work pretty damn well. Perhaps it's not the attenuator or the Valve Junior that is the problem. Perhaps what you're missing in the tone is the particular sound of a push/pull amp. In which case there is no single ended micro amp that will give you what you want regardless of whether it's triode or pentode. But now we're talking about the need for a push/pull pair of pentodes producing less than a watt. That starts to nudge the ridiculous end of the micro amp fad for me. But... As a personal amp, or Rockman style playing module type thingy, you could do it by building the thing as a headphone amp. Tubes don't care a fig what the speaker load is as long as the transformer ratio mates it properly at the primary. You'll need to identify a transformer that could mate the power tubes to a 600 ohm load and provide an appropriate primary impedance. This might require repurposing a transformer typically used for something else. There is still the matter of power into the headphones, but that could be managed somewhat with a pre headphone load circuit consisting of inductors, caps and resistors to correct for the fact that headphones do not sound like guitar speakers. In the end you have an AB1 pentode amp running into headphones. At what seem to me to be far too much trouble. Perhaps try any normal push/pull amp into an attenuator that offers infinite control or has a headphone output to see if it's the single ended output clipping that's not ringing your bell. That's what I might try next.
 
Mike Sulzer 5/20/2017 1:46 PM
[QUOTE=Chuck H;455438] Perhaps what you're missing in the tone is the particular sound of a push/pull amp. In which case there is no single ended micro amp that will give you what you want regardless of whether it's triode or pentode. But now we're talking about the need for a push/pull pair of pentodes producing less than a watt. That starts to nudge the ridiculous end of the micro amp fad for me. But... [/QUOTE] Why not use a pair of EF86s? You could easily get way under a watt. And the pair would cost you only $36, so expensive, it would have to sound good. Might have to wind your own transformer, but here is another chance to lose some efficiency and get still lower power.
 
Chuck H 5/20/2017 3:36 PM
There are also some micro tubes. The ones that just have leads for soldering rather than socket pins. You don't see them in use, but they can still be found for sale if you look. You could probably find a 100-500mW power pentode in that style. As to winding a transformer... I still think you could repurpose one typically used for something else just as well as making one. Unless you're Mike, R.G., Pete or Juan (or anyone else capable that I skipped). Surely a rank amateur without all the necessary understanding would kludge a homemade job as bad or worse than using a "not quite ideal" 12V power transformer backwards or something like that.
 
Loogie 5/20/2017 6:45 PM
[QUOTE=Enzo;455239]Well, if you build a 1 watt amp, no matter how loud it is, it will be no challenge to a common $6 L-pad. Dial it down as far as you like.[/QUOTE] Ya, go for the L-pad. A few bucks. You might be surprised.
 
Gregg 5/21/2017 1:08 AM
[QUOTE]There are also some micro tubes. The ones that just have leads for soldering rather than socket pins. You don't see them in use, but they can still be found for sale if you look. You could probably find a 100-500mW power pentode in that style.[/QUOTE] Yes, go some of those for example the 5902 pentode (russian 6P25B - harder to find) or the russian ones - 6P30B or 6P30B-R. They are also cheaper. Here's a complete project with 5902 with power transformer as output transformer: [url]https://jjs.at/electronic/class_ab_subminiature.html[/url] You can make your own sockets for them or during the experiments just use a DIL-8 socket. Don't worry it will take the voltage and current - it was tested many times
 
jbltwin1 5/21/2017 2:28 AM
Don't know if you've seen this one but it's got all info for construction. [ATTACH=CONFIG]43579[/ATTACH]
 
printer2 5/21/2017 6:07 AM
On selecting your output tube I would look first at what you have foe an output transformer and get a tube that matches it. Then select the PT going by the data sheet. A one watt output would be fine (I built a push pull with the 6AK6 that sounds great, good tube.) and you can drop the voltage to it is you want a quieter output using a power attenuator. Also at less than 5W levels I find old hifi speakers from the 60-70's can sound pretty decent and they are less efficient than guitar speakers.
 
Mick Bailey 5/21/2017 8:51 AM
I've built quite a few low-wattage amps and an ECC82/12AU7 in self-split mode sounds fine - much better than SE, which always sounds like an overdriven hi-fi amp when using a triode. Take a listen to the Firefly Mk3 clips (the one with boost). Here's one to be going on with; [URL="https://www.youtube.com/watch?v=u9_WM7Wdtys"]https://www.youtube.com/watch?v=u9_WM7Wdtys[/URL]
 
FalderGuitars 5/23/2017 1:04 AM
Morning all, Thanks to everyone for their responses, they've given me plenty of food for thought - alongside my morning porridge :thumbsup: As usual, just when I thought I knew what I wanted, a host of options present themselves. Still, we live and learn... A couple of points I should have probably made clear at the head of the thread: 1. I like the sound of a single-ended amp, which is why I'm planning to build one. 2. I've no interest in using any kind of L-pad/attenuator etc. (I'm actually currently using one, but at the volume levels I need the tonal response goes all screwy). 3. I'm aware there are other attenuators available, but see point 2 :) The advantage (for me) of the circuit I drew was that I have all the parts to hand. There are a couple of single ended amps out there using (ECC82) triode output stages (the Palmer Eins and Hughes and Kettner Something-Or-Other), so I'd assumed they make a decent choice. However, since there seems to be a consensus here that triode power amps sound like the proverbial 'bee in a jam-jar', so I'm beginning to have second thoughts. Time to print out some pentode datasheets? Could be...:thumbsup: Thanks again to everyone for helping out, Matt.
 
Gregg 5/23/2017 2:48 AM
Check out the schematics in the attachment. 50EH5 is basically a 5902/6P30B with a 50V filament and different cathode resistor. It doesn't get more simple than that. You get ~0.5W from one tube only. If you run it at higher voltage like 150-180V you can get maybe 1W clean. The russian 6P30B-R can take safely up to 220-230V (tested) and it costs only couple of bucks. A reverb OT won't cut it though and will saturate at that anode current so you'll need an appropriate one. [ATTACH]43600[/ATTACH]
 
Enzo 5/23/2017 9:59 AM
Not to mention a 50v heater supply.
 
Mick Bailey 5/23/2017 10:44 AM
L-pads can be excellent in a small amp. But, if you're using one 'straight' your amp will lose a lot of apparent top-end definition when turned down. This is partly due to psycho-acoustic effects (Fletcher-Munson, etc.). I always use a non-polarized (polyester) HF bypass capacitor placed across the input/output of the L-pad. Ideally, make it switchable - 2.2uf/none/4.7uf as a suggestion. My own EL84 SE amp uses 3.3uf. It makes a great deal of difference to the padded sound.
 
Gregg 5/24/2017 1:09 AM
[QUOTE]Not to mention a 50v heater supply. [/QUOTE] That's not such a problem. You can use a PT with one secondary only like ~50V and derive anode voltage from there via doubler or tripler but it's not even necessary because as you can see several other tube options are available.
 
Mick Bailey 5/24/2017 1:27 AM
The catch with doublers or triplers is that the smoothest supplies are from full-wave circuits, but these don't reference ground directly from the transformer secondary. You can end up with hum problems due to the potential difference between the heater ground reference and signal ground. IMHO this complicates a simple SE amp into having a high component count. FWIW, the ECL82 makes a fine SE low-wattage pentode amp. You also get a triode in the same envelope. That means for a two-tube amp you have the added benefit of three preamp stages, so either switchable gain or a CF to drive the tone stack. They also sound nice at relatively low voltages. I really like these tubes - they're very much underrated.
 
benito_red 5/24/2017 5:34 AM
I built several SE amps, some of them with EL84 other with 6V6. The results were very good, but too loud at the "sweet spot", with problems with neighbors that you can imagine. I tried to reduce the power using different tipes of power attenuators: Lpad with resistors, Lpad with resistors and capacitors (as in VOX AC4TV), then with a voltage reductor, but I did not like the results. So I started to build 1watt SE tube amp (using 6BM8 = ECL82) but the volume was still too high and I didn't like the sound. My conclusion after all those tests was that at bed-room volume level a decent solid state amp + a good overdrive pedal does the job better then tube amp. Also amp modelling (f.i. Roland microCUBE) can be a good alternative.
 
J M Fahey 5/24/2017 7:08 AM
Amen brother ... although many won´t like that result ;)
 
PeanutNore 5/25/2017 1:55 PM
[QUOTE=FalderGuitars;455634]There are a couple of single ended amps out there using (ECC82) triode output stages (the Palmer Eins and Hughes and Kettner Something-Or-Other), so I'd assumed they make a decent choice. However, since there seems to be a consensus here that triode power amps sound like the proverbial 'bee in a jam-jar', so I'm beginning to have second thoughts. [/QUOTE] I've built 4 low power triode power amps at this point, and I have zero doubt that they can sound good when done right. Since you're looking for single ended ones, you might be interested in [URL="https://soundcloud.com/sam-brown-225/6505-open-112-gibson-lp-studio"]this one (audio clip)[/URL] that uses both triodes of an ECC99 (basically a beefed up ECC82) in parallel as a power amp after a preamp that clones the 5150 lead channel. Since [URL="http://music-electronics-forum.com/t43822/"]posting about it here[/URL] I realized that without a phase inverter it can't drive the power tube grid enough to get full volume, so I added another ECC83 after the tone stack as a gain recovery stage w/ cathode follower, with a DPDT to bypass it for a low volume mode. It's basically a 0.1 watt / 5 watt switch. Ok, so that's a super high gain circuit where the tone is almost all in the preamp, what about a simple low gain amp and some ECC82 power tube distortion? [URL="http://imgur.com/a/wJvcw"]This amp[/URL] is a miniaturized clone of an Orange Pics-only head with an ECC82 push-pull fixed bias power amp, and [URL="https://soundcloud.com/sam-brown-225/ogre-1h-open-112-sm57-dean-vx"]this[/URL] is what it sounds like.
 
FalderGuitars 7/29/2017 4:15 AM
Morning all, Since I'm gearing up to start a new build (more of that elsewhere), I thought it only fair - after all the advice you gave - to let you know how I got on with the build. The answer is, I didn't... Before you all have me (rightly:)) stoned for wasting your time, I'll quickly point out it was a suggestion made by nosaj - namely that I try a less efficient speaker - which solved the problem. I'd been using a Celestion G12M, simply because I had one. I knew in principle that it was too much speaker for a so small an amp, but I hadn't realised just how much difference changing down to a smaller, less efficient speaker would make. Printer2 also suggested trying hi-fi speakers, which I'll admit sounded like a terrible idea, before I heard how it actually [I]sounded[/I] (does that make any sense?). I found an old Telefunken hi-fi speaker hiding in the dustiest corner of the workshop, after removing the blown tweeter and cross-over, it's found a new lease of life as a 6" guitar cab. [ATTACH=CONFIG]44279[/ATTACH][ATTACH=CONFIG]44278[/ATTACH] The bottom end is a little flabby for humbuckers, but it takes my Strat wonderfully. I'll probably upgrade to a Jensen Alnico when finances permit, but for now I'm a happy camper, and can hit the sweet spot at 'bedroom' levels - let the 4th rate Hendrix impersonations commence...:thumbsup: Thanks again to everyone for helping out, Matt.
 
nosaj 7/29/2017 7:22 AM
[QUOTE=FalderGuitars;461038]Morning all, Since I'm gearing up to start a new build (more of that elsewhere), I thought it only fair - after all the advice you gave - to let you know how I got on with the build. The answer is, I didn't... Before you all have me (rightly:)) stoned for wasting your time, I'll quickly point out it was a suggestion made by nosaj - namely that I try a less efficient speaker - which solved the problem. I'd been using a Celestion G12M, simply because I had one. I knew in principle that it was too much speaker for a so small an amp, but I hadn't realised just how much difference changing down to a smaller, less efficient speaker would make. Printer2 also suggested trying hi-fi speakers, which I'll admit sounded like a terrible idea, before I heard how it actually [I]sounded[/I] (does that make any sense?). I found an old Telefunken hi-fi speaker hiding in the dustiest corner of the workshop, after removing the blown tweeter and cross-over, it's found a new lease of life as a 6" guitar cab. [ATTACH=CONFIG]44279[/ATTACH][ATTACH=CONFIG]44278[/ATTACH] The bottom end is a little flabby for humbuckers, but it takes my Strat wonderfully. I'll probably upgrade to a Jensen Alnico when finances permit, but for now I'm a happy camper, and can hit the sweet spot at 'bedroom' levels - let the 4th rate Hendrix impersonations commence...:thumbsup: Thanks again to everyone for helping out, Matt.[/QUOTE] You are not wasting our time if you are experimenting and reporting back. You become part of the community when you shares your trials and tribulations. great that it worked out for you. Sometimes better is cheaper or simpler. nosaj
 
Gnobuddy 7/29/2017 10:53 PM
[QUOTE=FalderGuitars;455634] Time to print out some pentode datasheets? Could be...:thumbsup: [/QUOTE] I went tilting at the same windmills you're headed for. Inspired by Printer2's 2W 5e3, I designed and built a 2 W push-pull amp using a pair of 6AK6 pentodes, and added an L-pad. It turned out to be loud enough for jam sessions with half a dozen other acoustic guitarists and a fiddle, but sounded as lifeless as a solid state guitar amp when dialled down to midnight neighbour-friendly levels with the L-pad. Then I designed a new preamp for it, using a 6JW8 triode-pentode valve. You could think of the preamp itself as a milliwatt-power Champ, if you like, triode input stage, driving the pentode (preamp) output stage, with the tone stack after the pentode, rather than between triode and pentode. There is a gain control between triode and pentode, and is also a master volume after the tone control, and before the 2W power amp. This idea worked out much better than the L-pad speaker attenuator. The preamp generates genuine single-ended pentode distortion, because that's exactly what the last stage of the preamp is! I can dial the SPL down with the master volume to fairly low levels, and still get a somewhat "valvey" sound out of it, probably 99% of it generated from the preamp. Remember, this is not the annoying, fizzy 12AX7 preamp distortion you're probably hearing in your head - there are no 12AX7s in this preamp! If you decide to just go with your original plan - a small single-ended pentode driving your speaker through an output transformer - the tricky part becomes finding a suitable transformer, with a high-enough primary impedance (and inductance) to work with a flea-power tube. The Fender reverb transformer (or Hammond equivalent, etc) is about as high as you can find off the shelf - 22.5k or so primary Z with 8 ohms loading the secondary side. There seem to be a number of low power valves that can work with this sort of impedance, examples being the 6AK6, the even smaller 6AK5, 6DX8 triode-pentode, 6LY8 triode-pentode, et cetera. You will probably be on your own with the design to a considerable degree, though you may get lucky and find spring-reverb driver circuits that can easily turn into your power amp design. For example, draw a 22.5k loadline - say crossing the voltage axis at 450 volts and the current axis at 20 mA. For single-ended operation, the anode will then need to sit at half that voltage and half that current - 225 V on the anode and 10 mA quiescent anode current. That's 2.25 watts dissipation on the plate, so look for a valve that can handle a bit more than that (6AK6, for instance). You are now well on your way to a practical small SE design, using a transformer that actually exists. One of the nice things about the "small pentode in the preamp" approach I ended up taking, is that you don't have to scour the planet for tiny output transformers with improbably high primary impedance and inductance. You can just use an ordinary 1/2 watt anode resistor instead! Oh, one more thing. I once asked Printer2 about the SPL levels from his little 2-watt 5e3 amp. At full blast into a 12" guitar speaker, it was over 100 dB @ 1 metre. :eek: (Which makes perfect sense if you have a 97 dB/W@1m speaker, and feed it 2 watts, which is +3dBW.) For reference, a typical home vacuum cleaner - the loudest sound most neighbours will tolerate during the daytime - is around 70 dB SPL @ 1 metre. :exclaim: -Gnobuddy
 
Thoriated Tungsten 7/30/2017 5:53 AM
Guys, Try designing a classic output amp while using regular power tubes run at reduced voltages yet regular output impedance. PP 6V6 run at 100V anode/g2 into 8k looks interesting, about 0.5 W output power. Cathodes run at +5.0V via 2x 330 ohm cathode resistors. Resting plate current 15mA per tube. Reduce DC HT supply voltage as much as required if this is still too much power. Also the perceived loudness of the different frequencies may account for the 'lifelessness' of low power amps. You need increased bass and treble to make music sound natural at low perceived volume, something old tube radios usually take into consideration in the design. This may also explain why the reactive power attenuators are popular, as the designs I have seen precisely enhances bass and treble. Thus low volume may require a completely different tone control circuit, compared to your usual classic tone stack. Edit: Corrected estimated output power.
 
Gnobuddy 7/30/2017 11:51 AM
[QUOTE=Thoriated Tungsten;461111] You need increased bass and treble to make music sound natural at low perceived volume, something old tube radios usually take into consideration in the design. [/quote] I came to the same conclusion, the Fletcher-Munson effect becomes very significant when the volume is turned down very low. I think there is also an evolutionary/emotional component at work. If you've ever been near a lion when it roared, you will know how powerful the emotional impact of a loud sound can be. For a second, you stop being a human being, and instead simply become terrified food on the lion's menu. If you recorded that roar and played it back at kitten-miaow SPL levels, it simply would not have the same psychological impact. I think one of the reasons why millions of people have damaged their hearing by listening to stupid-loud music, is this powerful emotional component that goes with loudness. Loud music draws you in and gets you emotionally involved. Whether good or bad, it cannot be ignored. (This might also explain why so many loud bands had successful careers making really lousy music. :) ) [QUOTE=Thoriated Tungsten;461111] This may also explain why the reactive power attenuators are popular, as the designs I have seen precisely enhances bass and treble. [/quote] Now that is interesting, and may explain the obsession with boosted bass response. As for treble boost, you get that anyway from a high-Z amp driving voice coil inductance - so the question becomes, do the reactive attenuators increase that boost beyond what you'd normally get from a high-Z valve amp output impedance? The best-sounding DIY attenuators I've come up with, all involve a graphic EQ. The simplest is an 8 ohm resistor dummy load, a voltage divider across it to get the signal down to line level, a graphic EQ to shape the frequency response, and clean, full-range flat-frequency solid state amplification & speaker after that. I've also used an active DI box, fed by the guitar amp's speaker level signal. The rest of the chain is the same as before, i.e. graphic EQ, followed by flat, clean solid-state amplification and speaker. The DI box provides ground isolation, converts the unbalanced signal to balanced, and also provides signal attenuation via a couple of built-in pads. In either case, to my ears, the graphic EQ plays a significant role, as it lets you shape the frequency response until it has some life to it, even at low SPL. IMO that's a lot more versatile than fixed L-C-R filtering built into an overpriced reactive speaker attenuator. -Gnobuddy
 
Chuck H 7/31/2017 9:05 PM
[QUOTE=Gnobuddy;461129] If you've ever been near a lion when it roared, you will know how powerful the emotional impact of a loud sound can be. For a second, you stop being a human being, and instead simply become terrified food on the lion's menu. If you recorded that roar and played it back at kitten-miaow SPL levels, it simply would not have the same psychological impact.[/QUOTE] I'm experiencing analogy envy ;)