A switchable 1.5W to 150mW PP Class AB pentode practice amp

[Graydigger]

Thought I’d post a fun little DIY project (now in rev2) for y’all’s thoughts.  (1st iteration a few months ago was a little practice amp that was designed to deliver all the goodness of a push-pull Class AB power tube section going into saturation/distortion with less than 1.5W output. Yes, there are any number of headphone, digital, and other ways to skin this cat, but its hard to beat the analog feel of a real amp, especially the power section, right?)  That still was a bit loud in real life for my goal of a real “bedroom” level (i.e. won’t bother the sleeping boss next room) practice amp, so I decided to design a switchable one that tops out at less than 150mW (i.e. half the SPL of a 1.5W amp), while still keeping the Class AB pentode yumminess.

In the ~1.5W (micro) mode, I can practice in a studio with tracks on my iPhone at 2/3-3/4 volume and it fits in nicely.  In the ~150mW (nano) mode in a bouncy den next to the bedroom, I can practice with tracks on the iPhone at 1/3 volume in the dead of night now without waking anyone up 

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[Merlin]

Beautifully made, I love it!

[Graydigger]

Thank you sir!

[Williamblake]

Very nice! Looking forward to build this.

[Graydigger]

Williamblake,

Cool beans! Just a few build suggestions:

1. Use shielded wire going into grids of preamp tubes, from second stage gain to tone stack and between R5 and R8 to gain boost resistors (R6 and R7).  The compact size makes these susceptible to picking up noise
2.  Make R6 and R7 easily accessible if you’d like to tweak preamp “baseline” gain and “boost” gain
3. Make R40 and R29 (power tube cathode bias resistors) easily accessible to tweak bias (eg PT feel) to your taste
4.  If you have room, a 500RL humdinger would probably suffice instead of the DC filament lift if you want to cut down on number of components. Lead dress and heater lead routing in the compact chassis made quite a difference in noise (should be silent unless gain cranked up)
5. If I had room, I would have doubled up rectifier UF4007s in series in case of failure

I hope you have fun with it!

[bmccowan]

Nice build and great tone. I bought some EF80s a while back but I kept changing my mind as to what to build with them. This helps me. I am keen to try that tone stack. I have used the Framus one pot stack a couple times and really like it.
The single biggest change for my sanity would be to use a larger chassis. My eyes and fingers are in their 70s and there is no way I could repeat what you accomplished without using every swear in my vocabulary multiple times. Even with that I would have burn marks on every wire and half the capacitors! It seems that the Stout chassis, or an 18w chassis would work well and avoid some of the cautions you list. I get the portability goal, but for me that equates to weight more than size.

[chaswahl]

Nice project, seems to be very well-thought-out.
What watt-value is recommended on the EF80 bias resistors R29 and R40?
And could you describe the function and operation of the CL90 thermistor and the IMOV14 varistor at the PT primary? I understand what the thermistor is for (inrush current spike), but why the CL90 rather than another rating? And what is the varistor between line and neutral doing? performing a surge-protection function? I can't remember seeing one included in this way in amp schematics.
Thanks!

[Graydigger]

Hi Chaswahl, yes - you’re all over it! 

The common cathode bias resistor for the EL80s handle very little current, even in “Hi” power (~1.5W) mode - less than 30mA max at saturation. So 1/2 watt resistors for R29 (“Hi” power) and R40 (“Lo” power) provide more than a 2X safety margin for power dissipation, even if you decide to bias very cold with a resistor twice the design value. (I used metal foil resistors as the 1% tolerance 1/2W are super cheap and I have lots of them.)

The TMOV (e.g. Littelfuse TMOV14RP130E https://www.mouser.com/ProductDetail/Littelfuse/TMOV14RP130E?qs=nXjUL3qLHyVlscyfOw9icw==   is the one I use here in the US) is a two-in-one device.  It consists of a metal oxide varistor designed to short if exposed to a current or voltage that exceeds its rating.  This creates heat, which quickly activates a thermal cutoff (fuse) molded in series to the MOV, thus opening the circuit. Because the line fuse takes a while to blow in response to sustained overcurrent, it may not protect your amp against a voltage or current surge (like lightning).

Accordingly, a good place to mount a TMOV is between the Live and Neutral of your line supply going to your transformer primary (e.g. Application 1 on the Littelfuse TMOV datasheet). When TMOVs “blow”, the color changes from shiny red to dull burnt orange and they develop a charred “blister” and a bit of smoke with that “something definitely fried” smell.  Mount it in a position where it won’t catch stuff on fire or melt something important if it blows. Using a 20mm instead of a 14mm might protect for a bit longer as TMOVs degrade from repetitive exposure to milder surges below their designed fail point.)

The Amphenol CL-90 NTC thermistor (https://www.mouser.com/datasheet/3/59/1/FA02AE58771983E28B2BE1161455825ACDF33F1FBDE5E4678E089F5FA7CD6602.pdf) is one I use for my small amps pulling less than 2A, it goes from about 120R to 0.2R on warm up (about 30 sec). Mount it where it can get some air, as it gets toasty during use.

If you’re working on, modding, or building an amp, the addition of both a TMOV and NTC costs around $5, and represent cheap and easy to install protection!