EF80 Small Pentode Push-Pull Power Amp

@dougsta used a pair of EF80 small, novel pentodes in his push-pull power amp design and after some discussion in that thread I decided to use the EF80 datasheet characteristics curve to design a power amp to use with my micro amp designs. New Old Stock EF80s (also called 6BX6) are inexpensive at around $6 and the supply is plentiful. The EF80 uses standard 9-pin sockets and 6.3v heaters so they are easy to implement into a modern amp build.



I decided to use 300 plate volts even though the datasheet lists the max plate and screen voltage at 250v since most great tube amps exceed these limits The Hammond 125B output transformer wired for 22.5K:8 ohm is a near perfect match for a pair of EF80 in push-pull. A 240 ohm cathode resistor is also a perfect match for a 2xEF80 amp and yields the desired 100% max plate dissipation at ide for cathode biased power tubes.



The EF80 version of the Bassman Micro LTP uses two EF80 pentodes in push-pull for true pentode power tube overdrive with 2 watts of output. Pentodes overdrive differently than triodes due to their screen so this is a way to get pentode overdrive in a 2 wattish power amp. The power amp shown below can be adapted to pretty much any preamp, including high gain amps like the Soldano SLO-100 and 50. For best results keep the plate voltage between 220 to 300 volts (design max is 250v for plate and screen).

The Bassman Micro LTP EF80 below uses 295 plate and 290 screen volts using a Hammond 269AAX 200-0-200V power transformer. The 269AAX is a good choice for any amp using an EF80 power amp but it does not include a 5v secondary so use an EZ81 (6.3v) tube or solid state rectification.



Bill of Materials


The Hammond 125B output transformer works well with the EF80. Use the orange and yellow secondary wires for 22.5K:8 ohms and 5 watts. Standard Fender reverb transformers (22.5K:8 ohms) also work well as output transformers for the EF80.

EF80s tend to be microphonic so they are best used in head amps and not combo cabs. If you do use them in a combo cab expect to have to try several tubes to find a pair that are not microphonic. Using rubber insulated sockets and silicone damping rings on the tubes may help with microphonics.

Typical screen current at 250 plate and screen volts is shown as 2.8ma in the EF80 datasheet. Using a 1k 1/2 watt screen resistor will add some screen sag distortion and emphasize the difference between triode and pentode overdrive. Since the EF80 pin 6 is not connected to anything we can use it as an anchor point for a screen resistor.

The EF80 can also be used in the preamp like the EF86 in many early tube amplifiers.

More info and the latest schematic & layouts can be found here:
https://robrobinette.com/Bassman_Micro.htm#Bassman_Micro_LTP_EF80

Rob

 

Yes, with such a small power amp current demand I doubt anyone could hear any rectifier sag so solid state is just fine. Of course you could use a B+ voltage dropping resistor of good size to drop voltage and possibly add some sag.

 

It sure is LLC, I'll make the fix.

Yep, that is a schematic error. I'm hoping the miniscule screen current combined with relatively large filter caps plus the 1k screen resistors will isolate them enough to prevent oscillation but I may have to add another power node.

 

Here's the 1 watt SLO-Nakid Micro using an EF80 power amp. It's laid out for a Princeton Reverb chassis. More info here:
https://robrobinette.com/How_the_Saldano_SLO-100_Works.htm#SLO-Nakid_Micro_EF80_Pentode

 

And the EF80 version of the single-ended Champ Micro:
https://robrobinette.com/Champ_Micro.htm#Champ_Micro_EF80

It uses a Hammond 125ASE 3 watt 10k:8 ohm single-ended output transformer.

 

The current draw is so low from this amp that voltages are higher than typical.

 

Yes. I'm planning to do a Blackvibe Micro EF86 in the near future.

 

I don't know what happened to my images here on tdpri. I'm guessing they changed some policy. Maybe my images are too large or something.