I've played around with ECC99 as a push pull pair. You'll get about 4 watts with 300ish B+ and a 22k primary. Musical Power Supplies.com has a good transformer.... or Hammond 125C or D. Be warned though that triodes do not distort like pentodes so you aren't going to get big amp tone. I've moved on to ECL82.
Brian
Brian
You are on a well-traveled road, but for most of us, a road that rarely if ever goes quite where we hope it will.
With the same "good tone at low volume" idea in mind, I built a guitar amp using push-pull pair of little 6AK6 pentodes, tubes from the era of battery-operated radios. My amp was inspired by another push-pull 6AK6 amp, built by diyAudio user Printer2. Both amps put out around 2 watts - and, guess what, Printer2 measured his amp, driving a 12" guitar speaker, at 104 dB SPL. LOUD, in other words!
Going at it another way, I worked out how much guitar amp power would be needed to match the roughly 70 dB SPL of a household vacuum cleaner, which I took as a baseline for the loudest sound that the neighbours are likely to put up with without getting too annoyed. And it turns you that you only need a few milliwatts of power (!) to get there, depending on your speaker.
When I did choke my speaker power down to a few milliwatts, I found my ears no longer heard the sound the same way (Fletcher-Munson curves, etc), so that turned out to be somewhat unsatisfying.
For me, the most successful approach - or rather, the least unsuccessful approach - has been to try and get all the tone and distortion out of the preamp, and then feed that through a low powered power stage. The 2W amp is far too loud when the output itself is distorting, but I can get it to sound decent if I put a small-signal pentode in the preamp, and basically turn the preamp into a sort of micro-Champ of sorts.
I'm using a couple of unloved (and therefore very cheap) 6JW8s in my preamp, by the way. One small-signal triode + one small-signal pentode in a single 9-pin bottle for a buck. Can't beat that!
Or maybe you can: I'm beginning to think the current best solution to the "good guitar amp tone at apartment-friendly level" is actually to buy a Boss Katana 50 (here's the 100 W version: YouTube ) and turn it down.
The Katana series are solid-state amps, but come closer to being convincingly "tubey" than the dozens of nasty-sounding modelling amps that we've all been subjected to for the past thirty odd years.
-Gnobuddy
Thats a good point about triode distortion because I dont like the way my pentodes sound in triode mode. I am trying some load boxes this week. I getting decent tone at 1/4 power with resistors but of course its still loud. I just bought an Eminence with a dial that opens the voice coil gap to drop you about 8 or 10 db, have not heard it yet. There is a $900 version of this that sounds good but, it's $900. Fletcher Munson and 65 year old ears that lived through live playing and arena rock - did you say something ? I'm going to look at some baby pentodes after I try the 12Au7 P/P.
I'm very curious what audible side-effects that might have on the sound. The bass response of a loudspeaker varies quite a lot with the strength of the magnetic field (BL product) - a weaker magnetic field will reduce damping of the fundamental bass resonance, and the speaker will grow a big(ger) bass hump there.
Also, one would expect loudspeaker transient response to become soggier; the acceleration of the speaker voice coil is directly proportional to that same BL product, so a weaker magnetic field also equates to soggier transients and lower "slew rate" for the speaker cone.
But who knows, maybe a big hump in the bass will partially compensate for the bass loss at low volume due to those Fletcher-Munson curves. And maybe reduced transient response won't be an issue when the loudspeaker cone has less distance to travel (because of the low SPL.)
IMO, the tricky part is finding baby pentodes that will work with transformer impedances that aren't impractically high.
My 6AK6 amp uses a 22.5k transformer from Musical Power supplies (I think the model name is OT5 PP.) That seems to be about the highest conveniently available primary OT impedance you can buy.
There are lots of pentodes that are even tinier than the 6AK6, but many of them will need primary impedances much higher than 22.5k, and therefore, made of unobtainium.
If I were to do it over today, I would try using a lower B+ voltage. That will make for a steeper load-line, i.e. a lower OT primary impedance, and may allow using even smaller output pentodes.
Incidentally, diyAudio user DJGibson posted some details of an amp he built using a pair of truly tiny 6AK5 pentodes. IIRC they are limited to half the peak current of 6AK6's, and so would need roughly twice the load impedance if fed the same B+. But with half the B+, and half the peak current? The same OT impedance might work. Output power would only be a fraction of a watt, probably still enough to be "too loud"!
I'm no longer convinced that a truly flea-power output stage is necessarily the best way to go, though. You may not share that opinion, of course.
-Gnobuddy
Edit: I forgot to mention, I found that getting the guitar speaker off the floor, and placed up close to my ears at head-height, made an enormous difference. This lets you get a bit more SPL at your ears, while still being quiet for anyone ten or fifteen feet away.
Last edited:
After I built the amp, I connected an 8 ohm dummy load to the speaker outputs, and measured the frequency response of the power amp section. To my surprise, bass response went down far below guitar frequencies - I think it was (-3 dB) at something like 27 Hz, and this included that OT5 PP output transformer!
I was half-convinced I must have made some sort of mistake, but I had other things on my mind, so I forgot about it. Some months later I had the amp back on the workbench, so I measured the frequency response again - and got the same result (within measurement errors.)
I did measure at low power output, not at the full thundering 2 watts.
Perhaps the transformers's full-power bandwidth might have been a bit less extreme.At any event, I agree, that the Musical Power Supplies OT5PP is a good OT - too good for a guitar amp, really!
-Gnobuddy
Which means there's some scope for mis-matching the OPT, e.g. connecting a 16ohm load on the 4 ohm tap which should push the 3db point up to 108Hz (if my brain is working this morning
)That's an interesting thought - and would make for an insanely high 90k anode-to-anode primary impedance, if it really does work.Which means there's some scope for mis-matching the OPT, e.g. connecting a 16ohm load on the 4 ohm tap which should push the 3db point up to 108Hz (if my brain is working this morning
I keep coming back to the thought that the best guitar tones I've ever heard have not been live, but from recordings, played back at household-friendly loudness levels. If David Gilmour's guitar from a stadium concert can be filtered through the entire recording and playback chain, and emerge sounding great at 70 dB SPL from my studio monitor loudspeakers, surely we can build a guitar amp that sounds equally great at the same 70 dB SPL?
My hunch is that such a guitar amp will have to be very different from the beasts that Fender and Marshall and Hiwatt offered up in the era of rock. I don't think simply scaling the amp down from 200 watts to 200 milliwatts is going to do the trick.
Recorded guitar is post-processed - there is EQ, reverb, and who knows what other studio tricks applied after the sound leaves the speaker - and I think that has to be duplicated in our hypothetical amp. I think we should be looking at a preamp that generates signals similar to the ones that would normally be fed to a guitar speaker, but at milliwatt or microwatt levels. That would be followed by speaker / cab emulation filtering. And that would be followed by EQ, delay, reverb, etc, of the sort used when recording and mixing guitar tracks.
Recently I've been using my long-neglected Zoom G3, combined with a DIY powered speaker, as my living room guitar amp. The G3 offers three individual (digitally simulated) "stomp boxes", which can include amp simulations. What I've been using lately is an amp simulation first in the chain, followed by a reverb/delay "pedal", and finally an arena reverb, before the signal exits the Zoom and goes on to my powered speaker (a pair of thrift-store boombox speakers glued together and wired to a class-D audio amp module and a 24V SMPS.)
The G3 was a budget-priced digital modeller from several years ago, and its built-in amp simulations will never be mistaken for a real valve guitar amp. Still, once I've chained in the right post-"amplifier" effects, its not bad as a living-room guitar amp. If the crude digital amp model in the Zoom were replaced by an actual valve preamp, I expect it would sound considerably better.
I've used the same powered speaker with a Joyo American Sound (supposedly a Sansamp Blonde copy) as well. It wasn't bad, but sounded better when the Joyo was followed by an (actual) reverb pedal. It never occurred to me until this moment to try chaining the Joyo into the Zoom, using the Joyo as the amp simulator, and the Zoom for post-amp EQ and effects. But I'm definitely going to try that as well.
-Gnobuddy
Attachments
Last edited:
That’s an intriguing thought. It might tie in with how a lot of live players set up their rig these days:
Guitar signal – choice of 2 or 3 selected overdrive pedals (which attempt to simulate tube power stage distortion) – delay – reverb – tube amp (set up clean, but loud enough to give some acoustic feedback to the guitar if required) – guitar speaker - microphone - PA system.
The main difference here is the subtle distortion of a clean tube amp is introduced too late in the chain, perhaps.
Post recording processing is part of the tone that we ultimately experience but there is another thing to consider. Many of the seminal recordings did not use 100 watt unruly Marshall or Hiwatt stacks.its 18 watt Marshals, Supro’s, Tweed Deluxes, etc. I think that we can get decent tone into our recording software with low wattage but at least for me, it’s the live playing in my garage that suffers at low SPL output. My neighbors are happier but then I’m left flat.
George
George
Fair enough. But even the "little" amps of yesteryear probably were putting out 100 - 110 dB SPL - it only takes ten watts to get to 110 dB with a 100 dB@1W@1m speaker, after all.
So we know things can sound pretty good at 100 dB onstage. It's the yawning gap between 100 dB and 70 dB that seems to be the killer!
The other thing about post-speaker processing is that, properly done, it takes away a lot of the harshness that the actual guitar amp produces. Personally, I like that less-harsh sound more than the raw sound straight from a guitar amp.
I do understand.
I was once visiting a zoo, and standing outside the enclosure only a few metres away when the lion decided he'd had enough, and let out a full-throated, unbelievably loud, roar.
The shattering emotional impact of that sound was incredible - for a second, there was no doubt where I fitted into the food-chain, and where the lion did. A lifetime of modern civilized life fled from my brain, and for a moment, I became one of our ancestral humans on the African savannah, thinking only of avoiding imminent death at the lion's jaws.
I'm quite sure that if you recorded that lion's roar, and played it back at 70 dB SPL, it would not have anything like the same emotional impact.
Unfortunately, there is probably no technical fix for what evolution has wired into our brains: loud sounds automatically have much more emotional impact. They have to, otherwise our ancestors wouldn't have used every heart-pounding, lung-burning, fingernail-splitting shred of their strength and speed to run desperately from the hungry lion, and the human race wouldn't exist today.
Have you tried getting the guitar speaker up to head-height and within a couple of feet from your ears? If you haven't, do try it - it makes a huge difference for me, without most of the annoyances that go with headphones (headphone cord or wireless latency, "closed in" feeling, etc.) With your head in the speaker's near-field region, you can get a lot more SPL at your ears, without bothering the neighbours.
-Gnobuddy
It is hard to get punchy, creamy distortion from guitar to amp to speaker real time with Ted Templeton nowhere in sight. I am not giving up on that any time soon and have been swapping parts in my test bed every day now for weeks getting closer.
BTW, I am pretty satisfied with 1/4 power using a high wattage resistor divider. I thought that being a simple resistive load that it would suck tone more but its not bad. I am able to run a SE octal or 9 pin output tube at full output. If I use an EL84 I get full tube distortion at less than full pre amp drive so that helps a little as well.
George
BTW, I am pretty satisfied with 1/4 power using a high wattage resistor divider. I thought that being a simple resistive load that it would suck tone more but its not bad. I am able to run a SE octal or 9 pin output tube at full output. If I use an EL84 I get full tube distortion at less than full pre amp drive so that helps a little as well.
George
Glad that's working for you!
Of possible interest: a while ago there was a discussion on the output impedance of (valve) guitar amps. One person measured two of his DIY guitar amps, one SE, one push-pull, both with pentode output valves (6V6 and 12AB5 IIRC), and no overall negative feedback. In both cases, the output impedance was in the ballpark of 50 ohms.
The interesting thing is that when you feed a guitar speaker from such a high impedance source, the rise in speaker impedance due to voice coil inductance, causes a matching rise in the amount of power fed to the speaker. So you get a treble boost that starts somewhere above the speakers bass resonance, brightening the sound considerably.
The high impedance drive also causes a bass "hump" around the speakers fundamental resonance frequency, which may perhaps matter as well (though a typical open-back guitar cab will do a lot to remove most of the bass response of the speaker anyway.)
Now, depending on the design of the fixed-resistor speaker attenuator, the attenuator itself may present a much lower driving impedance to the speaker than the raw guitar amp did on it's own. And so it's likely that inserting such an attenuator will change the frequency response of the amp enough to be heard, and possibly enough to make the sound less attractive.
Some years ago I wired up some 8 ohm power resistors to make a 1/4 power attenuator for my Superchamp XD, but at the time, I had no idea that valve guitar amps had such high output impedances, and my DIY attenuator had an 8 ohm output impedance.
The 1/4 power attenuator didn't sound bad, but the amp was still absurdly loud, so I built a second 1/4 power attenuator and chained the two (to give me a 1/16th power attenuator.) That was quieter, but also sounded pretty "blah".
Since then, I had some success using an 8 ohm dummy load instead of a speaker, wiring a pair of resistors across the dummy load to get a line-level signal, feeding that into a Danelectro Fish-n-Chips 7-band graphic EQ pedal, and feeding that into a small acoustic guitar amp (Acoustic AG30.)
It felt rather ridiculous to use two guitar amps - one silenced via a dummy load - just to be able to get low-volume guitar tone, but this approach did work pretty well for me. The EQ pedal was a major factor, letting me put back some of the EQ that a real guitar speaker would otherwise have imposed on the sound.
-Gnobuddy
No. The transformer *reflects* the tube plates to the speaker at a lower impedance.
Best-loading for a pentode may be 5K. But the pentode actually acts-like (in its active zone) maybe 50K. The impedance seen by the 8 Ohm speaker is maybe 80 Ohms. (This is the reverse of most Hi-Fi amps, where the amplifier acts-like <0.1 Ohms to an 8 Ohm load.)
I see PRR has already explained things, but there is a little more I can add.
If you want to see the effect of output impedance of your guitar amp for yourself, what you do is first connect, say, an 8 ohm dummy load in lieu of an (8 ohm) speaker. Then you drive the amp with a steady, small signal (such as from a sine-wave generator, or you could play back a sine wave recording). Pick a frequency near the middle of the guitar frequency range, say between 300 and 500 Hz.
Now you measure the voltage across the 8 ohm dummy load with an oscilloscope, turning down the signal level if necessary to make sure the amp is only putting out a small signal, say 1 volt RMS or less.
Now use clip-leads to add a second 8 ohm dummy load in parallel with the first one. The combined load is now only 4 ohms, not exactly what the amp is expecting, but not enough of a change to damage it, particularly when operated at low power.
When you add the second dummy load, you will see the output voltage of the amp decrease.
The amount of decrease tells you how much output impedance the amp has. A big drop tells you the amp has a high output impedance. A small drop tells you the amp has a low output impedance.
There is an exact mathematical formula you can use to work out the output impedance, given the measured amount of drop in voltage. But in simple terms, if the voltage drops to nearly half when the second 8 ohm dummy load is paralleled with the first one, then the amp's output impedance is much more than 8 ohms.
And if the amp's output impedance is much more than 8 ohms, you get all those effects we have just been talking about: a big(ger) bass hump around the speaker's fundamental resonance frequency, and a treble boost that matches the rise in speaker impedance at higher frequencies due to voice coil inductance.
You can read a somewhat opinionated version of the effect of amplifier output impedance on loudspeaker frequency response here, if you like: Amplifiers: Solid State amps verses Valve amps
A similar discussion to this one came up on another diyAudio thread a year or so ago, and diyAudio member Printer2 measured two of his own DIY valve guitar amps, coming up with output impedances in the region of 50 ohms in both cases.
This output impedance comes from the anode resistance (plate resistance) of the output pentodes, and not from the speaker itself.
-Gnobuddy
From the little investigating I did, in the valve Hi-Fi era, it seems that ultralinear-mode is (was) the bees-knees: output impedance almost as low as triode mode, power efficiency almost as high as pentode mode. You still need negative feedback to lower the impedance well below the speaker impedance, of course, but at least you're starting out with a more reasonable anode impedance.
But there are not many guitar amps using ultralinear output stages (I'm guessing they may not take well to heavy overdrive?), so that may not be very relevant here.
-Gnobuddy
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.