Another EF80 practice amp build

[GrayDigger]

I just finished a small practice amp project with a speaker defeat and headphone jack. It took a couple of weekends, was a fun project, and pretty much does what I had hoped it would do – nice cleans with decent harmonic overtones and good sustain, goes up to moderately distorted output cranked, has a small footprint, sounds and plays like a bigger amp with good touch sensitivity and pick responsive, and reproduces real push-pull class AB power tube goodness at a reasonable conversational volume (max output 1.4 watts).

I thought grafting a low output power section onto an existing high power design would be like running a big block 460 through an econo-box CVT, so (for better or worse) I designed this little amp from scratch.
It is solid state rectified, with three 12AX7 stepped gain stages into a 12AX7 cathodyne PI, and a push-pull Class AB output section using a pair of EF-80’s. Everything fit nicely in a Hammond project box (7” x 5” x 2”), and it uses commonly available parts. It’s designed around a Hammond 290AAX PT and 125A universal OT, and the desktop cab I’m making is for a 6” 4 ohm. The line-out/headphone feature works pretty well, although I did need to get a cheapo headphone amplifier from Amazon for full useability. Soup to nuts it came out to around $400, including the speaker.

I’d be happy to post a full working LTSPice schematic, some WD layouts, and any other build details if anyone is interested but didn’t want to waste board space if this is of no interest!

[GAStan]

Can't speak for anyone but myself, I'm interested. Sounds like a nice little amp. Can you convert the Spice schematic to PDF?

[GrayDigger]

Hi Glenn,

Absolutely - Let me work on it. I'm not hip much cool-guy schematic software but I'll come up with something for you in pdf.

I ended up making my own EF80 pentode model for simulation, so that is included as well as my little potentiometer model (which strangely didn't exist in my regular Spice library). I used Ayumi's 12AX7 model for simulation, but the Koren ones work fine as well. (NB - I generated my EF80 model with kip.jar off the data sheet at 200V but it seems to be a tiny bit off at lower plate voltages compared to the data sheet, probably irrelevant to this application)

For some reason, I can't seem to directly load the LTSpice file type .asc schematic or Ayumi's 12AX7.inc file to this post as an attachment, so I just changed their file types to ".txt". Before running, change their file types back: ".asc" (for the Spice schematic) and ".inc" for the Ayumi model. The switches.sub file wouldn't upload either, so that one might need to be changed back to .sub from .txt as well... hopefully that works.

Thanks for the interest!

[GAStan]

Thanks so much for posting these!

Unfortunately I don't do Spice on my phone which is all I have right now (spare time at work) so I haven't downloaded any files yet. So any downloads thus far show that others are interested but did not speak up.

Typically to get a PDF I PRINT a file and choose PDF as the printer. I haven't tried to print any Spice files so not sure if this works, Hopefully someone else will also help with this.

I built a Mojotone Studio One Watt kit that sounds really good. The Output Tube on it can be either a 12BH7 or 12AU7. The 12AU7 requires x2 Speaker Impedance of the 12BH7. I bought an extra Output Transformer for it....hmmmm

Edit: since I brought it up thought I'd post these files; these are also freely available on Mojotone's website.

MOJO1580_applications (1).pdf

Studio_One_SCH.pdf

[martin manning]

Thanks for sharing, and it sounds good!

I downloaded the files and was able to open them, but the schematic was a mess since some symbols are different from the ones I am using and the font size in the notes is too big. You are using Windows, I believe, and I am on Mac. Too much work to fix everything.

Probably the easiest way to show the schematic here is to display it in Spice on your machine, full-screen, and screen-capture the window.

[GrayDigger]

Glenn, Martin,

Per Martin’s nifty suggestion, I’ve attached a PDF screenshot of my Spice schematic with all the visual noise removed – comments, voltage measurements, op data labels, yadda yadda. I’ve moved all the notes from my Spice file into a separate document. If the PDF is unreadable, I can make you a one that is easier to read. Also, if you decide to build this, I can provide voltage readings you should be getting all around the amp, and I made a little wiring diagram for the 8 x 8 mini tag boards I used. The schematic doesn't show the rectifier - it's just a simple one with two UF4007's (center tapped PT secondary - not a bridge rectifier).

Glenn, I appreciate the Mojotone schematics – both designs appear to use a push-pull triode output section, each with different design goals. The Studio One’s FMV tone stack, for example, employs a cathode follower and has an extra dual triode for its other channel.

There are many ways to skin the cat designing a baby amp! Class AB triode power sections (and single-ended amps) are super cool, but I wanted something that didn’t feel or sound “hi-fi.” Some of my design choices might seem unconventional, so I’ll try to explain my thinking.

My goal was to fully drive the power section while keeping the preamp very clean. I love the sound of great cleans – they’re trickier to design, but easier to add good sounding distortion to than the other way around. I really like pushing pentodes and all the IMD that comes with it. I also enjoy how a cathodyne phase inverter reacts when pushed, so I staged the gain here accordingly: early power tube distortion, followed by the cathodyne, with the preamp distorting last as the volume increases. Of course, it doesn't unfold linearly in reality, but that was the intent.

Designing around 12AX7s posed a challenge for me because of their high mu. To prevent them from dominating this amp’s distortion profile, I staged the preamp gain using a warm/cool/warm bias strategy while still ensuring enough got to the cathodyne to push it hard. I opted for a single-knob tone control, choosing a high-impedance design I learned from Merlin Blencowe. In hindsight, I guess I could have used a two knob voltage-wasting FMV to tame interstage gain.

The EF80s added another little challenge since they only need about 6Vpp to distort (given my choices for Zpri, Ebb, Eg2, and bias). This explains the small (33K) anode and cathode resistors in the phase inverter, along with the voltage divider before the EF80s. I used 2.2M resistors on the cathodyne output (plus large grid stoppers) to prevent grid current from getting back to the phase inverter and affecting its anode load. With 220K EF80 grid reference resistors forming the bottom of the divider, this setup kept the PI output down while keeping the grid reference within EF80 specs.

Martin, I checked the functionality of the "stripped down" Spice schematic in case I accidentally cut out something important. It seems to work for op, tran and Bode simulations

Let me know what you think!

[martin manning]

I’ve attached a PDF screenshot of my Spice schematic with all the visual noise removed – comments, voltage measurements, op data labels, yadda yadda. I’ve moved all the notes from my Spice file into a separate document.

Much better! The screen capture and separate notes are easy to read.
Sharing LTSpice files is clunky due to the issues I mentioned above. You can reduce some of that by using the popular models and symbols that can be found on the web, and/or zipping everything required for the model together.

[GrayDigger]

Thank you for the tips Martin. Zipping everything together! Brilliant.

I’m guilty of modifying my Spice libraries to the point where I can’t keep track of what is “standard” and what’s not.

[GrayDigger]

Hello Martin,

Here's a PDF from KiCAD (My first time - please forgive rookie effort). I appreciate your gentle prodding to get "with the program".

EF80 Mini Practice Amp schematic.pdf

and here is the KiCAD .sch file (zipped)

EF80 Mini Practice Amp.kicad_sch.zip

If anyone has any interest in actually building this one, I annotated the KiCAD file with my measured voltages around the circuit - extremely close to LTSpice predictions.

Thanks again!

[GrayDigger]

I just finished the little table-top cabinet for this little fella. I made some minor adjustments based on how the speaker and cabinet sounded. All in all pleased that this didn't sound nearly as tiny and boxy as I thought it could. Thanks Amp Garage community for all the inspiration.

[Bergheim]

I really like the micro pentode push-pull concept. Bet it's still pretty loud when cranked?

I plotted the tonestack values in a tonestack calculator to see the frequency response (without the resistors in series with the caps, don't know exactly how much they affect the overall response), and it's got a mid hump pretty much all the way through the sweep. Maybe that's intentional? The reason I ask is because IME a mid hump contributes to a boxy sound, so if that's still an issue, a little mid scoop might help. It does have an in your face lo-fi overdrive, and probably cuts very well through the mix though.

Pretty cool build overall

[GrayDigger]

Hi Bergheim,

Right on. Yeah, I guess there’s different scales of loudness, right? 100W - everyone leaves. 50W - drummer says you’re too loud. 20W - singer says you’re too loud. 10W - singer says you’re still too loud. 5W - singer still says you’re too loud. < 1.4W - with the door to the den closed, wife in bedroom texts - “can you turn it down?”. Can’t win for losing, right?

If I get what you’re asking, I modeled this tone stack (the original is from valvewizard) with a reasonable mid-scoop - here is the tone stack alone with the coupling cap from the prior stage:

Tone Stack Schematic.pdf

Here is the tone stack modeled (in LTSpice) by itself:

Tone Stack.pdf

And this is the tone stack modeled through the entire amp (to speaker):

Whole Amp.pdf

I WAG’d on the scoop frequency and about how much scoop, based on a tiny speaker. I used my coupling capacitors and (to a lesser extent) downstream cathode bypass caps to fine tune the frequency response for the whole amp, making the mid scoop a little pronounced and rolling off stuff over 7K and under 100Hz (no bass out of a 6” speaker anyway).

If your tone stack modeler can run this circuit, you’ll find you can do all sorts of cool stuff by simply tweaking the components. For example, C1 controls the amount of mid scoop/boost you get (here with tone knob at noon, measured at speaker out):

Influence of C1 alone.pdf

FWIW, ‘scope tracings agree with the LTSpice simulations to a scary degree. You can experiment and get pretty much whatever your heart desires (especially if you combine with a bright bypass)…

Thanks a bunch