6ak6-tone-experience?

[Chuck H]

I have a tiny PP output transformer that I'm itching to use for something different. I'm nonplused by the litany of PP 12au7 power tube amps because I haven't heard one I liked at all. Always shrill and blatty even with simple circuits. Looking into it, it seems that when you run triodes PP there's exceptional cancelation of the second harmonic and, if you choose to clip the tubes, a preponderance of third and fifth harmonic. Perhaps this is what I'm hearing. Also, the ratio on this little OT is a bit low for an au7 anyway. So...

How do the 6ak6's sound. Could I expect a tone more typical of the tubes we use for guitar amps? Anyone here ever built with them?

P.S. I also have a small SE OT that would be great for a parallel, single ended 12au7. And I happen to have a bucket of 12au7's that I pulled from a Conn organ. So that's a project I will get to eventually. But right now I've got a Jones to build a real, PP pentode amp, only tiny

[bob p]

no experiences with the 6ak6, though i'll throw out a couple ideas that came to mind reading your post. they won't be anything you don't already know, but they're all i can muster on one cup of coffee...

if you feel like you get too much even order cancellation, it might be worth trying to intentionally mismatch the PI outputs so it's not such an ideal diffamp. and maybe use a concertina and add a CF somewhere in the circuit. i think adding a totally unnecessary CF tends to add a little even order harmonics in those cases where they seem lacking.

my experience with small pentodes hasn't been all that rewarding. i've never been able to get tone that I really liked out of anything smaller than a 6bq5, so i use an attenuator.

[Chuck H]

i've never been able to get tone that I really liked out of anything smaller than a 6bq5, so i use an attenuator.

Hmmmm...

That's been my MO for a long time and I've actually been pretty happy with it. But what the hell am I supposed to do with a 3W 13.5k/8R output transformer? It's not like I can just leave it sittin' there

[Chuck H]

Ok, here's an idea.

The little pentode tone issue may not actually be an issue. May be more of a volume issue? That is, they probably sound like what we know pentodes to sound like, but two or three watts just sounds different than fifteen? Not often brought up, maybe speakers just don't sound right operating at five percent? So here's an idea. What if I build the little pentode amp with 6ak6's, but instead of making it a head or a combo intended for a speaker I'll run it into a reactive dummy load with a voltage divider and make a preamp that actually sounds like an overdriven amp! Or build as stated and use the divider off the dummy load to feed a 200W SS power amp.?. I could make it similar to my favorite design that has PLENTY of gain, but only when clipping the power tubes. So with this idea I can capture that tone in a preamp. Maybe include a speaker simulator so it can be plugged right into the PA!?! Imagine being able to get cranked amp tone and the only thing you need to load is a preamp! It could work. Analog modeling

[Gnobuddy]

How do the 6ak6's sound. Could I expect a tone more typical of the tubes we use for guitar amps? Anyone here ever built with them?

A couple of years ago, I found a few threads on the Internet by Printer2, describing his 2-watt, push-pull 6AK6 "mini 5E3". Here is a link to one of those threads: So your 5E3 is still too loud huh? - diyAudio

Photobucket has recently changed their terms of service, removing all the formerly freely hosted images - including the pics Printer2 posted of his 2W mini 5E3 back in the year 2014. However, he posted a sound clip of the same amp to a different hosting site, and that sound clip is still available for download (click on the one named "5E3 6AK6 Tod.mp3"): https://onedrive.live.com/?id=443A2F...3A2F0DB3904EFD

Obviously tastes differ, and I don't know if you'll like that clip or not. But to me, that clip sounds great, really nice rock guitar tone.

How quiet is it? I asked Printer2 that via PM. He measured around 103 dB SPL at 1 metre using a vintage 12" speaker. In other words, instant ear-damage loud, and if you live in an apartment, lose-your-lease loud, cops-will-knock-at-your-door loud. Only 2W? Sure, that's +3 dBW, and if you run +3dBW of power into a speaker with 100 dB SPL@1W@1m sensitivity, 103 dB SPL is exactly what you should expect to get!

Inspired by Printer2's amp, I designed and built my own 2W, push-pull 6AK6 amp. I used a "source-o-dyne" phase splitter made with an LND 150 MOSFET, and I deliberately unbalanced the source and drain resistors by about 12% (1 dB) to allow even harmonic distortion in the output stage to come through to the speaker. I used a selection of $1 valves from ESRC Vacuum Tubes elsewhere in the amp.

The output transformer is an OT5PP from Musical Power Supplies; the primary is rated at 22.5k anode-to-anode, or 5.625k from each end to the centre-tap. Power comes from a 48V Hammond transformer, feeding a voltage tripler to put about 225V on the 6AK6 anodes. Filaments are powered from a thift-store refugee Sony 8V switching power supply, with a suitable power resistor in series to drop voltage to 6.3V at the heaters. The 6AK6's are biased to about 10 mA quiescent cathode current each, so around 2.25 W total (anode + screen grid) dissipation, well within the 2.7W rating.

My amp was too loud also, and I ended up tinkering with small-signal pentodes and beam tetrodes in the preamp, and a master volume, so I could get a little bit of overdrive at volumes no louder than a quiet acoustic guitar, unplugged.

My amp sounds nothing like Printer2's amp, but on the whole, I like the way it sounds. Mine is geared more towards good clean, and slightly overdriven, tones, because that's what I tend to use most as a guitarist. In my own playing, I really don't care much for "power chords" and motorcycle-engine rock tones, just due to the nature of the music I enjoy most.

I'm pretty new to this business of trying to extract musically-interesting distortion from valves, so I'm still tinkering with my design. Someone like you, with so much experience under your belt, could probably get a 6AK6 amp dialed in to suit your tastes quite quickly.

-Gnobuddy

[Gnobuddy]

May be more of a volume issue? That is, they probably sound like what we know pentodes to sound like, but two or three watts just sounds different than fifteen?

I think we talked about this in an earlier thread on this forum. In addition to the Fletcher-Munson effect changing our perception of timbre at low volumes, remember the analogy I used about how an actual lion's roar scares the @#%&! out of a person standing near the lion, but if you turned it down to cat-meow volume, it wouldn't affect us the same way?

...and make a preamp that actually sounds like an overdriven amp!

This is essentially what my design has evolved into, as well. I have a small-signal beam tetrode as the last stage in the preamp, the idea being that, when overdriven, it will sound like a good single-ended 6V6 amp, except at milliwatt powers rather than several watts. That little beam tetrode feeds the master volume, which feeds the power amp section (using the 6AK6's).

Having the desired timbre come from the preamp, and a more or less clean "power" amp (two whole thundering watts!), is the only way I can think of to have valve-guitar-amp goodness at any volume I want, and not just at the usual ear-splitting levels that guitarists are forced to play at.

Maybe include a speaker simulator so it can be plugged right into the PA!?!

And now you're getting close to another idea I've been pondering lately!

I play acoustic guitar, electric guitar, and bass at the weekly jams I go to. Hauling three guitars and three amps lost its appeal very quickly, so I've been playing everything through my little acoustic guitar amp acting as a small P.A., via a small Mackie mixer, and a fistful of guitar FX pedals.

But I keep thinking about building a little valve preamp with three inputs, one for bass, one for electric guitar, one for acoustic guitar. Maybe one switchable XLR output, or perhaps three, one each ofr bass, electric, acoustic guitar. A couple of small-signal valves inside, mebbe a small beam tetrode, tone controls, some signal conditioning, some sort of steep low-pass filter/speaker emulation for the acoustic and electric guitar amps, and maybe a balanced output stage driving an XLR connector to feed the P.A.

Hauling three guitars is a pain too, so I want to try putting a piezo pickup on one of my electric guitars to see if I can get passably close to plugged-in-acoustic-guitar tones with it. But that's another project entirely!

-Gnobuddy

[Chuck H]

Wow! I really like the 5e3 clip you posted. Because it sounds like a 5e3, but if it were running at lowish voltage and a little hint of pentode, rather than only beam tetrode in the tone. But the bottom line is that it sounds like a guitar amp Not "my" sort of amp, but I can hear past that. I'm feeling pretty positive right now. Thanks.

P.S. Italia is a company making some mediocre electric guitars (but they play and sound good) that include a resonant box with a piezo pickup. Mine is surprisingly decent. By that I mean "for an electric guitar". So it's not going to stun you with acoustic guitar tone accuracy. But then I've never strung my acoustic guitar with 10 gauge nickel strings and I don't have it setup with a 5/32nds action! Still, way better than other efforts I've heard to accomplish the same thing. Another nice feature is that the piezo is a separate jack, not a switch. So you can A/B switch with your foot on the fly. I'd use it without reservation for a live gig where I needed to switch acoustic/electric within the same song. Certainly before resorting to one of those stand up guitar stands. That $h!t looks cumbersome as hell.

[Gnobuddy]

Wow! I really like the 5e3 clip you posted.

I'm glad you liked it! I also think it has a nice rich rock tone. No shortage of bass, either, unlike many little DIY amps. Probably because of a good choice of output transformer, and the 12" speaker. IMO, Printer2 really created a masterpiece there.

P.S. Italia is a company making some mediocre electric guitars (but they play and sound good) that include a resonant box with a piezo pickup. Mine is surprisingly decent.

Thanks for the tip! I'll look into that possibility, but the budget is tight, and I may end up having to use a fifty-cent piezo disc and a home-brewed preamp. I know how to make a good accelerometer out of a cheap piezo disc, let's hope that also works as a good acoustic guitar sensor!

I actually do have 10 gauge strings on one of my acoustic guitars. Heresy, I know! I use a pretty heavy-gauge pick, which helps compensate for the thinner and brighter tone of the thin strings. Some aspects of my playing technique with that guitar are also geared towards preventing excessive "jangle" from the thin strings. Plugged in, all it takes is a graphic EQ pedal to make those thin strings sound as beefy as a set of .012's. I love the power a graphic EQ has to transform your guitar sound!

-Gnobuddy

[Gnobuddy]

I just checked, I have some of Printer2's missing "2W 5E3" pics saved on my computer. With the understanding that this entirely Printer2's work, and not mine, I could probably post some of them here...I have a feeling he wouldn't mind, these are all pics he already posted for the world to see, until Photobucket took them away.

Lemme know if you want me to post any of them, and I will.

-Gnobuddy

[Chuck H]

I just checked, I have some of Printer2's missing "2W 5E3" pics saved on my computer. With the understanding that this entirely Printer2's work, and not mine, I could probably post some of them here...I have a feeling he wouldn't mind, these are all pics he already posted for the world to see, until Photobucket took them away.

Lemme know if you want me to post any of them, and I will.

-Gnobuddy

I don't need to see the pics. But thank you. I know what a build looks like I haven't read his build article yet. Going there now.

[bob p]

the problem with low-wattage amps is that they still produce a decent amount of power, as far as getting too-loud-for-the-neighbors goes. as much as we like to crank amps, we have to admit that if you've got efficient speakers it only takes milliwatts to make something "too loud" for someone else's ears.

it's an interesting idea to build something smaller and smaller until you find that magic micro-amp. and i understand how hard it is to not build something when you've got an oddball piece of iron there, just taunting you...

for me an attenuator has been a good enough solution to the too much power problem. let us know how it plays out, chuck.

[Chuck H]

I agree about the volume. Doing some short math I figure my attenuated practice level right now is about one watt (probably just shy of that). So two watts (probably a hair more) is going to be comparatively loud. But there's that level of volume that allows for some acoustic feedback which I would like to get. It's probably good that I've developed a skill for dodging flying objects and I keep a pillow in the shop.

[mozz]

Make a 6cl6 pp amp. I'll mail you a few, i have a 100 and will never get to that project until about 2025.

[Gnobuddy]

I don't need to see the pics. But thank you. I know what a build looks like I haven't read his build article yet. Going there now.

Oh, I'm sure you know what a traditional build looks like!

But I should have been clearer - one of the pics (he posted a GIF file) is a schematic (without part values, but may be useful). Another pic is a screen capture of a spreadsheet bill of materials (which, of course, does include component values).

Some of the build pics are also revealing, for example, you can see that the output transformer is actually one of those inexpensive 70V audio line transformers. Given the strong bass in the MP3 clip, I wouldn't have guessed that - those little audio line transformers often have very low primary inductance compared to "real" valve OPTs, and there are frequent mentions online of thin bass in DIY builds because of that. But it seems the combination of OPT and speaker that Printer2 used has no shortage of adequate bass.

-Gnobuddy

[Gnobuddy]

...we have to admit that if you've got efficient speakers it only takes milliwatts to make something "too loud" for someone else's ears.

A few years ago, I discovered that I could hear ten microwatts of 1 kHz sine-wave into an old Weber speaker. The speaker was on my workbench, and I was sitting on a chair pulled up to the same bench, with my ears two to three feet away from the speaker.

I was living in a rented suite on a 20-acre farm at the time, and it was a very quiet environment. Still, I hadn't expected to be able to clearly hear microwatts of power into a speaker. I thought I must have made a mistake in my AC voltage measurement, or my math. But when I calculated the estimated SPL, it all made sense. Ten microwatts - that's 10^(-5) watts, or (-50) dBW. Put that into a speaker with 95 dB@1W@1m sensitivity, and it will make an SPL of +45 dB. According to typical SPL tables (like this one: Noise Level Chart: dB Levels of Common Sounds), that is somewhere between "babbling brook" and "light traffic". Quite audible, and in fact, that 1 kHz test tone was loud enough to be annoying after a few seconds!

That led me to sit down and do some calculations on guitar amp power vs expected SPL. I decided on 70 dB SPL (about as loud as a vacuum cleaner) as the loudest sound that would be acceptable in an apartment. How much power does it take to generate 70 dB SPL at 1 metre distance from a speaker with 95 dB@1W@1m sensitivity? Three point two milliwatts!

As another data point, I once briefly owned a VOX AC4TV, and found it too loud to use at night on the 1/4W (250 mW) setting.

So there you go, with a typical guitar speaker, the "right" amp power for overdriven guitar tones in an apartment setting is somewhere between three and three hundred milliwatts!

it's an interesting idea to build something smaller and smaller until you find that magic micro-amp.

Been there, failed to do that...when the output power was the limiting factor, either my overdriven guitar sounds were too loud, or my clean tones were too quiet. And other physiological (Fletcher-Munson) and psychological (loud sounds have more emotional impact) factors came into play. I never was able to get guitar sounds I really liked with that approach.

So I re-thought the plan, and decided to have a preamp that generated the timbres I wanted, and a master volume, and a mostly "clean" power amp with a whopping two watts power capability. For me, that approach has been a lot more successful. I can get guitar tones I enjoy at SPL levels comparable to playing on an unplugged acoustic guitar.

I understand how hard it is to not build something when you've got an oddball piece of iron there, just taunting you...

I have a few 6LY8 triode-pentodes and a couple of 70V audio line transformers lying around. I keep thinking I have to turn those into a two-bottle mini "18 watter" some day. The triodes are the same as the ones in the 12AX7, so just two 6LY8s should be enough for both a push-pull phase splitter, and the output pentodes. Add a 6JW8 in the preamp and you can have both a triode and a pentode channel...no overpriced EF86's necessary!

(Actually I think both the 6LY8 and 6JW8 have little beam tetrodes in them, not true pentodes.)

-Gnobuddy