Very low power (< 1W) amp project and questions

[Baptiste]

Hi everyone,

It's m'y first post here, after much time reading stuff here and on other forums.

I plan to start building my first guitar amp this year, my goal is both to experiment and eventually end up with a nice amp. I don't have an extended experience in this, but I'm used to design and make stuff, and I've built my hifi tube amp some years ago.

So, before the questions, here is a first recap of the project, my current specs and design thoughts :

Volume
The starting point is the volume. I'm a strictly home player, live in a flat, practice often at night once the kids sleep...
I aim at around 60-85dbA (from just enough to be heard above the unplugged sound of the strings, to a loud acoustic guitar), not interested in more for this build.
So I think of something like this:
- 100mW to 1W power amp
- low efficiency 8" speaker (like Jensen p8r)
- VVR/L-pad... if needed

Style
I don't aim at cloning a specific sound signature, I just want to tweak up to something I like.
Style-wise, I'd like to go from clean to blues/rock to moderate gain hard rock (BoC, Black Sabbath). 

Topology
I want to stay relatively close to conventional topologies, with 12a*7 preamp, and pentode power amp. Not interested in triode power stages.
I'm still hesitating between SE and PP, leaning towards SE for simplicity and epistemology reasons, and because I think if don't begin with an SE I might never try one out :-)
I plan to have an iterative breadboarding approach, starting with one tube (2 gain stages) for the preamp, then adding a second one. I guess a 4 gain stage preamp is totally enough saturation and trouble for me :-)

So, in the end, a single channel amp, something in the like :
G1(MB) > D1 > G2(MB/CC) > [D1 > G3(HB) > CF > TS] > D2 > SE > MV(VVR)

G= gain stage
D/V/MV: drive/volume/master vol. pot
MB/HB/CC : medium/hot bias, cold clipper
CF: cathode follower
TS: tone stack
[...]: bypassable sections

In the case of using VVR, I consider the VVR as MV, and the MV as a power amp drive.
I put twice D1 here, just an idea I might try, it's a stereo drive pot, like what Orange does (e.g. dark terror schematic). It seems interesting to get a wide range of clean to drive sounds on a single channel amp.
I'd also like to experiment on defeatable boost/OD/Tonestack sections.
Well, you get the idea, I want to see what I can do with 2 preamp tubes.

Now, the questions ! 

Question 1: overall topology concerns
The vast majority of SE amps use 2 gain stage champ-like preamps. Am I asking for trouble trying to put a higher-gain preamp on a SE amp ? Should I go for a PP instead ?
Other advices ?

Question 2: power tube choice
I've screened all low power amp topics I could find, here is the shortlist of tubes I found:
- 6AK6 (7pin): Pout ~1W
- 5902 (submini) : Pout ~1W
- 5672 (direct heating submini) : Pout = 80mW
- 3S4 (direct heating submini) : Pout=250mW
Low input tubes (4-5Vpp swing before cut-off) :
- 6au6
- 5654/6ak5
- 5840...
I feel the 6ak6 is a very sane candidate, it has been used in several projects. It's probably too powerful for my needs, but in SE+VVR it should be okay. If I decide to go PP, I'd rather select a lower power tube though, such as the 3S4 or 5672.
Any advice or considerations ? Is there any drawback/advantage to use lower input pentodes ? Sound-wise also, I have no idea how all those tubes would compare to conventional ones, and if I should avoid some of them...

Question 3: power supply topology
Most (very) low power output pentodes require a very much lower B+ than 12a*7. Typically 65 to 180V vs ~300V for the preamp. I couldn't find much practical answers for this, it seems everyone avoids to fall in this case.

Which PSU topology would be the best suited ? Pros & cons of those different solutions ?
2.a) Linear PSU ~300V B+, with dedicated fixed or variable voltage reduction for power section.
2.b) Linear PSU ~300V B+, with power section node at the end. Big dropping resistance.
2.c) Linear PSU, using two distinct secondaries and two separate rectifiers for preamp and power amp.
2.d) Linear PSU for preamp and heaters, separate SMPS for power amp
2.e) Separate SMPS for everyone. Heaters direct from 12v PSU.
2.f) Lower preamp B+ accordingly to the power amp restrictions. I'd like to avoid this.
2.g) replace 12a*7 with more suited tubes, such as 6112. I'd like to avoid this also, to stay closer to well known schematics.

Okay, that's more than enough for today, thanks to those who took the time to read everything ;)
Baptiste

[shooter]

that is some laundry list for <1W 

build the PA section +PS make it work just the way you want, THEN make a preamp.  the web has tons of lo-watt examples to steal from, re-inventing wheels went out of fashion just after the dark ages 

[kagliostro]

... 100mW to 1W power amp

http://el34world.com/Forum/index.php?topic=5039.0





--

http://el34world.com/Forum/index.php?topic=6759.msg59501#msg59501

Franco

[jjasilli]

low efficiency 8" speaker (like Jensen p8r)
SPL of this speaker is 91.8 dB.  I.e., 1W (i.e. 2.83V) from amp into this speaker produces 91.8dB @ 1 M.  This appears to exceed your stated design goal.  For 85dB @ 1M, you need to cut the amp output in Watts in half twice; i.e., 250mW.  Good tone probably requires pushing the amp to at least 1 or 2 0'clock on the vol dial.  So you need a power tube to meet that criteria.

But, my shop vac does 85dB.  Running that at night in a flat would not make my family happy.

[Baptiste]

Thanks all for your quick replies

that is some laundry list for <1W 

build the PA section +PS make it work just the way you want, THEN make a preamp.  the web has tons of lo-watt examples to steal from, re-inventing wheels went out of fashion just after the dark ages 

Sure, but why should I lower expectations because of the lower wattage ? :-)

I agree with your point, I don't mean to reinvent anything, but I still want to understand and appropriate what I do.
And to follow your advice, I still need to choose a power tube, and make sure my PSU will fit the later preamp section. Hence my questions.

@Kagliostro
Thanks for the links. I had forgotten about this one. I'll have another lol at the 6au6.
The scaling on Vg2 is interesting, however, I'd rather go for regular VVR on both G2 and anode. My understanding is that lowering only Vg2 completely shifts the whole balance of the power stage, because it makes the bias point more and more assymetric in regard to Va possible swing (closer to cutoff). In regular VVR you keep better proportionality, the full power tone shaping must be much better conserved.

@jjasilli
Yes I know, 1W in this speaker is plenty loud, but I couldn't find lower efficiency. Enters the VVR or power soak... Or a lower power pentode. And at night I'm fine with cleaner sound or no power amp distortion.

[jjasilli]

I use an L-pad attenuation from Parts Express.  Haven't used it for bedroom volume.  Have you considered earphones?

[shooter]

why should I lower expectations

no reason to, the PA section is the heart, the player, guitar n to some degree the pre are the soul.  once you get the blood flowing, the player n guitar, then you add the missing spices, the pre.   

ps, still gotta do the #'s for filaments etc, I always pic 4 pre tubes - over head since I rarely use more than 3

[Big_Mike]

ECL84 tubes have a really good sound.  I built a Plexi style amp several years ago with ECL84s in PP, and it probably put out 3-4 watts.  Those tubes are true pentodes and have a surprising amount of bass.  You could probably do something single ended and it would be a watt or so.

[Baptiste]

I use an L-pad attenuation from Parts Express.  Haven't used it for bedroom volume.  Have you considered earphones?

I do use headphones, but find it cumbersome. I might add a headphone/line out tap though. And I consider L-pad to knock off just a few dB.

The PA section is the heart, the player, guitar n to some degree the pre are the soul.  once you get the blood flowing, the player n guitar, then you add the missing spices, the pre.   

ps, still gotta do the #'s for filaments etc, I always pic 4 pre tubes - over head since I rarely use more than 3

Well, about the player...  Let's say it's a work in progress
And not sure if we say the same thing, but I meant maximum 2 tubes/4gain stage pre. 

ECL84 tubes have a really good sound.  I built a Plexi style amp several years ago with ECL84s in PP, and it probably put out 3-4 watts.  Those tubes are true pentodes and have a surprising amount of bass.  You could probably do something single ended and it would be a watt or so.

It's a bit more powerful, but very nice ! I overlooked pentode/triode combos.

[shooter]

It's a bit more powerful

You could probably do something single ended and it would be a watt or so.

getting less from a SE isn't hard, more....trickyer

[Mungo Park]

Rob has a great 5E3 micro:
https://robrobinette.com/Deluxe_Micro.htm
Great directions even breadboard directions. PP or SE options. Various tube options.  Output is in your range.
I agree with above, why invent a amp that are already out there. I f you want to build it and play amps exist. If you want to go your own, go for it, it is beyond my skill level and I will watch with interest. I have built a 1 watt amp but it is your basic champ with 3-5$ tubes, 6X4 rectifier, 6mc6 and 7732. It is a dirt simple amp that sounds great and could be built in a day.

I think you should start with what sound/tone you want and build it to that end, which you stated. Tubes can sound how you want by the parts connected to them.
                                         Cheers Ron.

[Baptiste]

Thanks Ron,

Rob website and micro amps projects are great, but he uses 12au7 (double triode) for the power section, either in PP or parallel SE. I really want to use pentode(s) instead.

Are you sure you meant 6mc6 ? I googled it, it's a high power (Pa_max=33W) pentode. I didn't know the 7732 too, that's another interesting one.

Tubes can sound how you want by the parts connected to them.

I'll try to keep this in mind ;-)

[Baptiste]

Also, any advice concerning my initial questions #1 (SE vs PP for mid-gain) and #3 (PSU topology) ?

Those will help me decide on the power tube choice :-)

Thanks !
Baptiste

[Mungo Park]

Sorry double apologies.
1 I read past the pentode tube requirement.
2 A little dyslexic moment the tube is a 6CM6 it is a tetrode in the 6V6 "family". Other similar ones for future builds: 6aq5,6cw5,6005,7408,5992,7c5,12ab5,6p6s,6Ez5,6Cz5.
I will follow you build with interest, I find it great to see guitar amps come about with less "standard" tubes than in the commercially built amps.
                     Cheers Ron. 

[HotBluePlates]

... my current specs and design thoughts :

Volume
The starting point is the volume. I'm a strictly home player, live in a flat, practice often at night once the kids sleep...
I aim at around 60-85dbA (from just enough to be heard above the unplugged sound of the strings, to a loud acoustic guitar), not interested in more for this build.
So I think of something like this:
- 100mW to 1W power amp
- low efficiency 8" speaker (like Jensen p8r)
...

Have you heard and love the Jensen P8R?

Asking because a different and/or larger speaker is likely to be a more-satisfying sound, and your project is only gonna sound as good as the speaker you use.  If it were my project, I'd start with the speaker choice then look to reduce amp-power to hit a suitable volume.  And to some extent, low-volume playing will always be less-satisfying than high-volume playing (but is what it is for this project).

I've gotten some vintage amps over the past couple years, and in some cases the previous owners had swapped in new speakers to "preserve" the old ones.  Because I don't really need to worry about blowing speakers, in every case I re-installed the original speakers.  And in every case, the original speakers sounded better to me (or at least sounded like the amp's "authentic voice").

It also happened that I got a couple of mid-60's Celestion "silver" alnico speakers, and tried them in a 2x12 cab paired with a '65 AC10.  I really liked the AC10's Elac speakers (better than the Webers the prior owner installed), but became an instant fan of the T1088 Celestions after trying them.  Im fact, I liked them so much I got another pair, as well as a similar pair of T1656 Celestions.  If I were to build an amp now, even a low-volume project (and these are probably 100dB speakers like the modern Celestion alnicos are rated), I'd go out of my way to use these speakers and reduce amp-power to hit the volume target.

Food for thought...

[shooter]

any advice concerning my initial questions

#1 you want a rock solid, low ripple NO sag design similar to a SS PS.  I build 10-20W xSE, I aim for <20mVac at V1 and <2vac at the PA tube.  The OT is equally important, I like Edcor or Hammond UL rated 30 - 50% over Max plate I.  at 25W though you need 2 ppl to carry it, so I fudge there

you'll have to do your own work on PP but anything that meets min requirements for the tube(s) you choose works, sag is a thing in some PP, higher ripple is acceptable because of the natural cancelation you get.

the tubes come 1st, without that data you can't know filament draw, max current, max V etc. to buy metal

[Baptiste]

Have you heard and love the Jensen P8R?
Asking because a different and/or larger speaker is likely to be a more-satisfying sound, and your project is only gonna sound as good as the speaker you use. 
[...]
Food for thought...

Thanks for the food HotBluePlates! You have a very good point, I didn't hear it in person, just some speaker shootouts on YouTube...

I'm not settled on anything yet, I'll take the time to reconsider the question.

My initial selection was based on those points :
- at such low power, why get bigger than 8 inch ? I often read that bigger speakers needed more watts to sound good. But It's so subjective, it's hard to filter what people say on the internet...
- I'll try to limit the cab size, so I could probably get away with a 10", but a 12" is a bit too much.
- at low volume, thanks to Fletcher Munson, I guessed an American voiced speaker would sound much better than British ones.
- I read reports that the P8R behaved very well at low volume. FWIW...
- It might be naive, but I considered if I tweaked the amp tone around the speaker, maybe the speaker choice wouldn't be that critical, provided it's a good quality one... Especially since I'm never going to get any cone breakup at 0.1W

It's a lot of assumptions, so I'd be very happy to have your view on it.

In the end, I might buy 2 or 3 and decide with my ears...
 

[PRR]

A small speaker SOUNDS small at any power level. Your ear knows.

I've had hours of fun with a 12-inch RCA monitor and an amp that didn't make a whole Watt.

Yes, a Twelve and suitable box may be a bad fit in the bedroom.

[shooter]

 

go with a 2 X12 and it doubles as a work bench

[Baptiste]

any advice concerning my initial questions

#1 you want a rock solid, low ripple NO sag design similar to a SS PS.  I build 10-20W xSE, I aim for <20mVac at V1 and <2vac at the PA tube. 
...
the tubes come 1st, without that data you can't know filament draw, max current, max V etc

Thanks for the figures!

About the tubes, even without settling for a specific model, I can say that the quiescent anode current will be lower than 15mA.
For anode and grid voltage, it will depend a bit more. 6au6 might be the highest at 250V max, but for the other tubes it'd be less than 180V.
For the preamp section, I'd like to stay on conventional B+ values, let's say around 300V.
Filament heating should be less than 1A for the whole amp, but I'd like to have some margin.

So, most probable scenario : very low current requirements for the PSU, and power amp B+ at half the preamp B+.

A small speaker SOUNDS small at any power level. Your ear knows.

OK, I'll keep this in mind, and maybe start negociating about cab size ;)

go with a 2 X12 and it doubles as a work bench

Haha, I have a workbench project I need to finish, I'll show you a picture the day I go back at it, you'll laugh. It's more of a 2X8 midget sized one :)

[HotBluePlates]

Thanks for the food HotBluePlates! You have a very good point, I didn't hear it in person, just some speaker shootouts on YouTube...

I'm not settled on anything yet, I'll take the time to reconsider the question.

My initial selection was based on those points :
- at such low power, why get bigger than 8 inch ? I often read that bigger speakers needed more watts to sound good. But It's so subjective, it's hard to filter what people say on the internet...
- I'll try to limit the cab size, so I could probably get away with a 10", but a 12" is a bit too much. ...

I'm in the fortunate position to have a '63 AC30, a '65 AC10 (which I usually play through a separate 2x12 cabinet), a '64 Deluxe Reverb, a '67 Super Reverb, and recently sold a '66 Pro Reverb.  All of those are amps with 12" speakers except the Super Reverb, but the large cabinet with 4x10" counts for this example.

I live in an apartment and sometimes play late into the night.  Between the amp & guitar volume controls, I can keep the volume down so neighbors have never complained.  But the added bass from big speakers & big cabinets help offset the fact your ear perceives low frequencies to be quieter at low volume.

Yes, all those amps will "sound better" at 105-115dB, but they all sound more satisfying than the Vibro Champ I have with its 8" speaker and small cabinet at the same low volumes.  All of those amps are clean at these low apartment volumes, but I have pedals if I want distortion.

You really owe it to yourself to hear amps/speakers in-person to get a feel for what will work for you.  Videos and sound clips online can give a misleading impression, and you won't really know what anything sounds like at the volumes you're trying to remain below without direct experience.

Or, make your best-guess but be prepared to spend more money later to get different speakers/cabinet if you find out reality doesn't match your assumptions.

- It might be naive, but I considered if I tweaked the amp tone around the speaker, maybe the speaker choice wouldn't be that critical, provided it's a good quality one... Especially since I'm never going to get any cone breakup at 0.1W

It's not just about "cone breakup" but the fact that every speaker has a different frequency response.  Yes, you can try to modify the amp to suit, but if your speaker is "the right sound" from the start everything else falls into place.

For example, the Normal channel of the AC30 and either channel of a 1962 Deluxe basically do the same thing: they amplify everything in the "mid-band" and have some amount of bass roll-off and treble roll-off due to the circuit & tubes.  They both feature 12" speakers, but they sound entirely different because their speakers sound entirely different.  The Deluxe's Oxford 12K5 sounds nothing like the AC30's Celestion Blue alnico, and neither amp sounds "like itself" when used with some other speaker.

If you must stay with an 8" speaker, the Jupiter 8SC gets high praise from folks I'd tend to trust.  However, it is louder than the Jensen 8" and probably is more expensive to get in the EU.

... So, most probable scenario : very low current requirements for the PSU, and power amp B+ at half the preamp B+. ...

Why does your preamp need to be over 150-200v?

If you're going to be using 6AK6, your preamp only needs to deliver ~9v peak to distort the output tube (when it is operating at 180v).  The R-C chart on page 2 of this 12AX7 data sheet shows it will almost manage 9v peak with a 90v supply, and can manage more than twice what you need with a 180v supply.

    -  Pick your speaker so you know its sensitivity, and the true power output you need to hit your decibel target.

    -  Pick your output tube and design just an output stage (within the tube's constraints) to hit your power target.

    -  The output tube's bias voltage will dictate the amount of drive signal needed from the preamp for maximum output power & onset of distortion.

    -  Try to design the preamp using a supply voltage lower than the output tube supply voltage (should be easy), to make power supply design easier.


Or just build the PRR 1/3w amp.  I've built one, and it's louder than what you can distort in a late-night/low-volume situation.  The screen voltage "attenuator" works for small reductions in volume, but doesn't sound terrific at the bottom end of its range.  Maybe make life easy and use a pedal for distortion when needed.

[JB]

If you want to play with low power pentodes I like EL91's.  Proper audio power pentode, used as the audio amp in Eddystone receivers so loads of mil spec versions around on the cheap.  My office noodling amp is a homebrew SE EL91 (actually switchable for the slightly beefier EL90). EF86 front end, 1 x ECC83 for gain/CF, tonestack, EL91.  Also has another '83 for reverb.   I have VVR in it which is useful.  However I run it into a Boogie 1 x 12 fitted with an EVM12L.  Don't think the EV even knows it's being spoken to at that power level!  I can run it into power amp distortion at which point it's loud enough to annoy others in the house unless I turn down the VVR.
I was swapping notes with PRR about low power amps when he sketched up the 1/3W as a suggestion.  I tried that, sounds good, still quite loud when overdriven - probably not helped by the EV.  He then added the screen grid voltage control.  You can wind it right down with that.  I found the 1/3W when clean was quieter than the acoustic sound from the strings. 

My experience of 8" speakers is limited to a 70's silver face Champ I've just repaired for a friend.  Whether it's the (original) speaker or the cab it just sounds small and boxey, particularly when driven into distortion.  Not so bad clean.  Probably fine in a band mix but if you're playing on your own at home I'd find irritating. 

[shooter]

looks like the data points are all there, out in the sticks where I live, the next step would be Shite or get off the pot 
plug in the weller n make smoke

[Baptiste]

...
It's not just about "cone breakup" but the fact that every speaker has a different frequency response.  Yes, you can try to modify the amp to suit, but if your speaker is "the right sound" from the start everything else falls into place.

If you must stay with an 8" speaker, the Jupiter 8SC gets high praise from folks I'd tend to trust.  However, it is louder than the Jensen 8" and probably is more expensive to get in the EU.

Thanks for all the advices about speakers. Very much appreciated.

Why does your preamp need to be over 150-200v?

If you're going to be using 6AK6, your preamp only needs to deliver ~9v peak to distort the output tube (when it is operating at 180v).  The R-C chart on page 2 of this 12AX7 data sheet shows it will almost manage 9v peak with a 90v supply, and can manage more than twice what you need with a 180v supply.

I just thought that by staying close to conventional values for preamp B+, it would be easier to reuse existing schematics of well known amps and stay close to the expected tone/behaviour.
I get your point, and it's reassuring to know I can go with that kind of voltages for the preamp. However, if I go for a VVR on the power section anyway, is there any point to try to keep voltages lower on the preamp ?

    -  Pick your speaker so you know its sensitivity, and the true power output you need to hit your decibel target.

    -  Pick your output tube and design just an output stage (within the tube's constraints) to hit your power target.

    -  The output tube's bias voltage will dictate the amount of drive signal needed from the preamp for maximum output power & onset of distortion.

    -  Try to design the preamp using a supply voltage lower than the output tube supply voltage (should be easy), to make power supply design easier.

That's pragmatic. I'm not going to choose the speaker immediately, but I'll make a not to constraining choice on sensitivity and go from there.

If you want to play with low power pentodes I like EL91's.
...
I found the 1/3W when clean was quieter than the acoustic sound from the strings. 

Thanks JB for the input and feedback. I had not shortlisted EL91 because it was a bit higher output, but not so much indeed, and as Shooter already stated, it's easier to get lower power from a tube...

I found the 1/3W when clean was quieter than the acoustic sound from the strings. 

I definitely need to put a VVR on this project. It will compensate for the wrong estimation I'm likely to make regarding clean vs overdriven volume.
About this, I have a very neophyte question : what kind of overdrive level do we get from the power amp at full swing, before clipping ? I know it's about 10%THD, but have no clue of what it translates to in regard to sound. Is it still a harmonically rich clean, or already overdrive territory ? And on the other side, is there a rule of thumb about what % of swing level is the limit a truly clean sound ?

looks like the data points are all there, out in the sticks where I live, the next step would be Shite or get off the pot 
plug in the weller n make smoke

Well, thanks for the help and for pushing me, but I don't know exactly when I'll start. It might take take a while on your Shooter scale ;)

[shooter]

It might take take a while

just holl'r when it's up n you're ready to tweak it 

[Big_Mike]

Rob Robinette's Micro Bassman sounds really good, and it can be a micro JTM45 with just a preamp tube swap. 

[Baptiste]

Hi there,

I made a little table to quantify power VS clean/OD volume, and to estimate the kind of flexibility I could expect from a power amp section with VVR and post-OT attenuation.
I made a few reasonable assumptions in the parameters, I might be a bit off. Comments are welcomed.

It confirms without surprise that something around 1/4W would provide quite a lot of flexibility for my situation. Of course, it doesn't take into account the added flexibility of the preamp itself.

As a reference, here is an interesting video to illustrate what a 0.25W amp gives, with a Celestion V30 (100dB sensitivity), without any power scaling.
https://www.youtube.com/watch?v=8ZN4_kwi9gU&list=PLrJmIrbvx2T0STffCJtr2pW_ncq7K8nL1&index=7

I'll try to document the math and the design choices as I progress on the conception, it might interest people like me who don't have much first hand knowledge but want to understand the underlying logic...

Cheers,
Baptiste

[Baptiste]

Also, I began to check bias points and loadlines for the tubes we discussed.

There is something I don't understand on the 6au6 datasheets, such as this one :
https://frank.pocnet.net/sheets/093/6/6AU6A.pdf

The typical operation values for Class A amplifier are totally off compared to the average plate characteristics figure.
typical operation @Vg2=150V : Ia = 10.6mA, Vg1 = -6.5V
It makes no sense to me, at Ia=10mA, Vg1 should be at -1.0V. -6.5V is not even on the graph...

What did I miss ?

Other question : G3 is not internally connected to the cathode, and the data provided suppose Vg3 = 0V. Does Vg3 have a noticeable impact on the graphs ?

Thanks,
Baptiste

[1blueheron]

It sounds to me as if your objective is more "low volume" than it is "low power".  I think it is important to make the distinction.

Have you considered going in a little different direction and building a tried and true moderate powered amplifier but using a variable efficiency speaker?   

What is a variable efficiency speaker?  One that you can modulate the magnetic flux and thus control the efficiency and overall volume output.   In basic terms, instead of using a high power rare earth magnet with fixed field, you replace it with an electromagnet that you can control the current through and therefore the magnetic flux.  In the old days this was a neccessity as permanent magnets were notoriously weak.  They were called feild coil speakers back then.

Fluxtone is selling a modern version of field coils at an outrageous price.   Not sure what magic they put in the box but it sure is expensive. 

Might be worth playing with a homebrew version?

As for an output pentode...  I have been playing with a little SE  amp design that so far is pushing all our buttons the right way. It uses the 6AQ5.   It can be played at bedroom level with one gain stage for very clean and a second gain stage for a nice distortion.  Pick attack is very good and it is very dynamic, has great harmonics and is easy to get along with.  It is a little sensitive to speaker selection though.

[Baptiste]

You're right, I aim at low volume.
Low power is just the consequence.

I've read about fluxtone speakers before, but they are crazy expensive. What do you mean about a Homebrew version ? It doesn't look like an easy task to convert a regular speaker...

I know 2 models of affordable variable sensitivity speakers : the eminence Maverick and reignmaker. They allow to adjust the magnet position (I think), and the sensitivity ranges from 91 to 100dB. Less range than fluxtone, but not bad. They are around 130€ in Europe.

[PRR]

> -6.5V is not even on the graph... What did I miss ?

"Grid Voltage for Ib= 10 Microamps"

The Typical grid voltage is about -1. Do the math on Plate+Screen Current and Bias resistor.

If you want to turn the tube OFF, you will need about -6V.

[1blueheron]

You're right, I aim at low volume.
Low power is just the consequence.

I've read about fluxtone speakers before, but they are crazy expensive. What do you mean about a Homebrew version ? It doesn't look like an easy task to convert a regular speaker...

I know 2 models of affordable variable sensitivity speakers : the eminence Maverick and reignmaker. They allow to adjust the magnet position (I think), and the sensitivity ranges from 91 to 100dB. Less range than fluxtone, but not bad. They are around 130€ in Europe.

3 options:
1.  Find an old piece of gear that used a field coil speaker, salvage the speaker and the coil,  and supply a current to the coil as your needs dictate
2.  Remove the magnet structure from a speaker and replace it with a coil of wire to which you apply current as needed.
3.  Wrap wire around the magnet and use it as a hybrid with the wire coil reducing the power of the magnet by reversing the field.

If you try this experiment, be aware that with a very inefficient speaker, there will be a temptation to turn the volume up and burn up the voice coil winding of the speaker rather than modulating the current in the field coil.  You will need to create a safe guard to prevent this. 

[Baptiste]

"Grid Voltage for Ib= 10 Microamps"

The Typical grid voltage is about -1.

Of course !
I expected the bias voltage, overlooked the current information.
Oh, and I missed the line saying the suppressor grid is connected to the cathode also... It answers my other question I suppose.

This typical point of operation seems quite hot biased, isn't it? Is there a reason for this ? Other tube datasheets I have seen generally provide a neutral bias basis.

3 options:
1.  Find an old piece of gear that used a field coil speaker, salvage the speaker and the coil,  and supply a current to the coil as your needs dictate
2.  Remove the magnet structure from a speaker and replace it with a coil of wire to which you apply current as needed.
3.  Wrap wire around the magnet and use it as a hybrid with the wire coil reducing the power of the magnet by reversing the field.

Wow. I'm not ready for options 2 and 3, but I'll have a look at field coil speakers by curiosity.

[PRR]

> This typical point of operation seems quite hot biased, isn't it?

DataSheets are normally *show-off* conditions. The 6AU6 is a radio IF tube where we need LOTS of gain to pull in weak stations. Except in all normal use, a "normal" station would overload the radio so the AVC system de-biases the IF tubes to a lower gain. How much AVC voltage might you need? Probably at-most roughly that "6.5V" which makes current (and gain) very-very-small compared to the "typical" (idle, no signal) 5mA-10mA condition. (However reducing gain in a tuned-circuit amplifier is very different from reducing gain in an Audio amplifier.) (The -6V might be a reference IF you were building a fixed-bias 6AU6 amplifier, say a push-pull Power amplifier, because that is about the most your bias supply needs to deliver.)

[HotBluePlates]

It sounds to me as if your objective is more "low volume" than it is "low power".  I think it is important to make the distinction.

Have you considered going in a little different direction and building a tried and true moderate powered amplifier but using a variable efficiency speaker? ...

Fun for thinking about, but very impractical for a sub-watt (or a "sub-80dB SPL") amp project.

Fluxtone products are extraordinarily expensive.  A Fryette Power Station 2 costs less than half-price.  The Universal Audio OX is similar-price but can be used with any speaker, allows headphone-out, and is a recording/effect solution.  The same money buys so much more elsewhere.

And Baptiste already mentioned the Eminence adjustable-magnet speakers. Much more cost effective, but you get a 7dB range of adjustment; about the same as reducing amp-power to 1/5 its original value.  70dB SPL from a Maverick on the low-end of its sensitivity requires amp-power of ~5.5 milliwatts.

The end-result of this project may be finding that low-dB is unsatisfying, and requires a headphone output for quiet playing (that's pretty much where I landed).  It may turn out that a reactive load is the best low-volume option (and perhaps most cost-effective, though building is usually more fun).

[Baptiste]

The end-result of this project may be finding that low-dB is unsatisfying, and requires a headphone output for quiet playing (that's pretty much where I landed).  It may turn out that a reactive load is the best low-volume option (and perhaps most cost-effective, though building is usually more fun).

Thanks for the feedback. I'm sure you are objectively right.
The thing is that I'm not used to playing loud, so I'm not convinced I have the same expectations as someone who would.
To rephrase my initial objectives, my goal  is not as much to get nighttime full power amp distortion, as it is to make sure the range of usable volume fits as much as possible to my needs. If I built, say a 5W champ, I'd seldom get the opportunity to turn the volume up to any kind of sweet spot.
OTOH, with a 1/4W amp with VVR and attenuator, I should be in the ball park for day use. It's a best effort approach.

At night, well I'll see how it behaves. I can play cleaner, or with preamp only overdrive, or plug a loadbox/cabsim for headphones or to go straight to my hifi.

However, in any case, I need an amp to start with. Well, of course I don't need to built an amp, but I want to. And half of my goal is just to build it ;-)

[JB]

OTOH, with a 1/4W amp with VVR and attenuator, I should be in the ball park for day use. It's a best effort approach.

You need to aim higher and attenuate/VVR reduce power to 1/4W or less. If you ever want to play it clean, with the amp louder than your unplugged guitar, then 1/4W isn't enough.   

I think you want minimum 1W and reduce from there.  That could still be a 5W Champ and you'd be permanently running it at low HT.  6V6 (or EL90 miniature version like I use) will last forever like that. 

Choose your output bottle, get yourself a suitable OT (Hammond 125A,B,C little single ended trannies are ideal for playing with a variety of low watt devices) and PT, build a basic pre-amp to drive it and play around from there.  Starting with one small pentode and one 12AX7 in a Champ circuit with VVR will give you lots of information to move forward with your project.

[Baptiste]

You need to aim higher and attenuate/VVR reduce power to 1/4W or less. If you ever want to play it clean, with the amp louder than your unplugged guitar, then 1/4W isn't enough.   

I don't doubt your experience on this, but are you sure about the numbers ? If you take a non MV amp as a reference (such as a champ), it might be true because you quickly get a mix of preamp and power amp overdrive. It doesn't mean you can't get clean from a 1/4W amp with master volume.

Here is my calculation :
1/4W into a 95dB efficiency speaker gives 89dB with full swing at power amp input. Let's say I divide by 2 the input swing to stay clean. It's a 6dB knock off, I end up with 84dB of clean sound, which is a (very) loud acoustic guitar.
I might be wrong with the required input swing at power amp for clean sound, but even if I divide by 4 the voltage input, I get 78dB clean. Enough for me.

Choose your output bottle, get yourself a suitable OT (Hammond 125A,B,C little single ended trannies are ideal for playing with a variety of low watt devices) and PT, build a basic pre-amp to drive it and play around from there.  Starting with one small pentode and one 12AX7 in a Champ circuit with VVR will give you lots of information to move forward with your project.

I totally agree with you on this. I'm shortlisting the tube choice right now, once I choose it and the corresponding OT I'll be able to follow this road.

[JB]

You need to aim higher and attenuate/VVR reduce power to 1/4W or less. If you ever want to play it clean, with the amp louder than your unplugged guitar, then 1/4W isn't enough.   

I don't doubt your experience on this, but are you sure about the numbers ? If you take a non MV amp as a reference (such as a champ), it might be true because you quickly get a mix of preamp and power amp overdrive. It doesn't mean you can't get clean from a 1/4W amp with master volume.

I'm basing it on running PRR's 1/3W amp into my EVM12L (100 dB for 1W/1m).  With the volume reduced until PA distortion stopped it was too quiet.  I didn't measure power output though, may have been way under 1/3W.

[DummyLoad]

https://www.ebay.com/itm/6G6G-NOS-vacuum-tube-RCA-Victor/183847129586?hash=item2ace2491f2:g:fTcAAOSwRjxdAsa4

6AK6 descended from 6G6G.

as you already know, ≈1W with 10K load (hammond 125ASE) with 180V anode and g2 supplies. cool coke bottle octal - same pinout as 6V6. flip a switch makes ≈1/3W in triode mode. no need for VVR.

160-170V to preamps works better. 

120V bias/filament PT w/ FWB would work well - ≈170V B+ or for slightly more a hammond 269AX. i'd probably go with the hammond 261C6 paired with a hammond 125ASE. 

honestly for this stuff, a line6 pod and headphones...i do that (ducks and runs), even a 1/2W-1W op-amp rig would likely work and sound just as good with a lot less headache and bulk.   


happy hacking! 


--pete

[shooter]

happy hacking! 

he won't plug in a solder Iron, engineers 
(disclaimer: tongue in my cheek  )

[Big_Mike]

OTOH, with a 1/4W amp with VVR and attenuator, I should be in the ball park for day use. It's a best effort approach.

You need to aim higher and attenuate/VVR reduce power to 1/4W or less. If you ever want to play it clean, with the amp louder than your unplugged guitar, then 1/4W isn't enough.   

I think you want minimum 1W and reduce from there.  That could still be a 5W Champ and you'd be permanently running it at low HT.  6V6 (or EL90 miniature version like I use) will last forever like that. 

Choose your output bottle, get yourself a suitable OT (Hammond 125A,B,C little single ended trannies are ideal for playing with a variety of low watt devices) and PT, build a basic pre-amp to drive it and play around from there.  Starting with one small pentode and one 12AX7 in a Champ circuit with VVR will give you lots of information to move forward with your project.

Really interested in this thread.  I would like to build a Plexi style based around either EL91 or ECL84.  Which tube would be closer in tone to an EL34?  And does anyone have a schematic based on either of these tubes?  Looking for a good 3-5 watt amp in push-pull mode.

[sluckey]

Look at the core series at ax84.com. There's a nice 2 watt pp amp. Plenty of preamps to mate with it or use your own design. Or use Doug's plexi preamp.

     https://ax84.com/archive/ax84.com/coreprojects.html

[JB]

I’ve not tried ECL84. A pair of EL91’s will do about 4W. Check the data sheets, I think that’s at around 250V with 20K a-a load. I’ve tried lots of different preamp styles with them, currently a 5E3 arrangement.

[Baptiste]

Thanks JB and DummyLoad for the feedback and references.

I was leaning towards 6au6, but thanks to JB I'm hesitating with 6AK6/6G6/EL91 on the other side (EL91 is about similar to 6AK6 at the same B+) 
So, here is a quick recap.

Option 1 - 6au6
About 0.3-0.4W at 160V B+ with a 20K (10K Hammond ASE with 8Ω speaker on the 4Ω tap) or 25K load (fender reverb tranny)
Pros
- power is close to my theorical needs, minus JB warnings.
- very easy to drive. Could be perfect for a passive effect loop. Might even be used without any preamp section.
- very large range of bias (compared to the other ones). Maybe not useful, but room for experimentation...
- very low current draw. Not very important, but makes VVR or fixed voltage reduction (to get a bit higher B+ for preamp) a non issue in terms of power dissipation.

Option 2 - 6AK6/...
As DummyLoad stated, about 1W at 180V B+ with 10K (Hammond 125ASE)
Pros
- more clean headroom
- more conservative load values => maybe better bass response ?
- higher B+, not much difference though.

Also, looking at the plate characteristics of both tubes, I have a feeling the 6au6 has more distortion than the 6au6.

Basically, I think the choice resumes to clean VS overdrive priorities.
It's probably not a big deal though, I could always change my mind after experimentation.

Also, if I go 6au6, is the 125ASE a good option for 20k load, compared to the reverb tranny ? The reverb tranny is cheaper and the 25k load is fine, but the Hammond is reusable for other tubes if needed...
The power transformer should not be an issue, there is at least one seller in France that makes low cost power transformers with bespoke secondary voltages.

he won't plug in a solder Iron, engineers 
(disclaimer: tongue in my cheek  )

I will. But I have holidays coming first 

[JB]

I tried using a reverb transformer early in on low watt experiments. They have limited bandwidth. Go for a Hammond - sounds better, more loading options.

[Baptiste]

Okay, I'll do that.

[shooter]

6au6

built a 3SE outta those, get crunch, finicky, good volume.

But I have holidays coming

what better way to relax than building an amp in the back-country 

[HotBluePlates]

... Also, if I go 6au6, is the 125ASE a good option for 20k load, compared to the reverb tranny ? The reverb tranny is cheaper and the 25k load is fine, but the Hammond is reusable for other tubes if needed...

I tried using a reverb transformer early in on low watt experiments. They have limited bandwidth. Go for a Hammond - sounds better, more loading options.

Hoffman's reverb transformer is ~$18, where Mouser's Hammond 125ASE is $46.  I thought the reverb trafo sounded great with my 6AU6 1/3 watt build.

[shooter]

I thought the reverb trafo sounded great with my 6AU6 1/3 watt build.

+1
3tubes got me closer to 2W+ usable audio, 4W squared n screaming