EROS 500 Select Class A Monoblocks

Experience one of the most musical, powerful Class A tube amps available!

EROS 300

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Downloadable: Eros 500 Select manual

Read full Press Release

Eros 500 Select Key Features

  • Sextet of KT150 output tubes create 250 watts of Class A musical power.
  • Class A power is fast for a more lively, immediate response to your music
  • The Bias Select switch is engineered with 3 perfect auto bias settings for KT88, KT120, KT150 and KT170 output tubes.
  • The Bias Select switch is safe and easy to use, change on the “Fly” so you can hear the difference.
  • 4 Ohm and 8 Ohm taps gives you options for your speaker preference. 4 Ohm may give you more base and the 8 Ohm tap may sound more open.
  • The optimized frequency response provides the best musical range to benefit the natural harmonic structure of the instruments
  • The SpeakOn cable connector provides most reliable hookup for your subwoofer.
  • Transformer balanced true differential XLR input offers noise cancelling and rejection, as well as providing ground isolation from your preamp.
  • Should you need to eliminate a hum? The Eros 500 has a XLR ground switch installed for your safety. This eliminates the dangerous temptation to lift the ground from the mains.
  • The combo Push-Pull and Ultra Linear circuit design provides lower distortion, more power, and a better performance out of your tubes.
  • No Negative Feedback in the circuit design provides better dynamic range because it doesn’t compress the music.
  • Non-inverting polarity retains the same phase as the input (so your trumpets blow and don’t suck)
  • Large Custom Toroid power transformer provides plenty of available power.
  • Toroids have lower noise because they concentrate the magnetic field and are very efficient in the 50 to 60Hz mains range.
  • Choice of high quality gold plated Single Ended RCA or Balanced XLR connectors to suit your needs or cable preferences
  • Gold pin sockets are more reliable because gold doesn’t oxidize when it heat cycles.
  • An extensive grounding scheme is designed into the power supply, wiring and all tube circuitry.
  • Elegantly designed, heavy duty 14 & 16 gauge zinc plated, fully grounded steel enclosure isolates external noise and vibration.
  • All fuses are easily accessible
  • It’s ready to enjoy with 50 hours of factory burn in on all circuits and vacuum tubes.
  • Each unit is hand built “Made in the USA”

Spec Comparison between the new EROS 500 Select vs EROS 300

Feature EROS 500 Select Mono Amp EROS 300 Mono Amp Discription
Watts per channel 250W Class A 150W Class A More power controls your speakers better
Power Tubes KT88, KT120, KT 150, KT170 KT88 More choices to customize your sound
Auto Bias 3 position Bias select Switch 1 Bias setting Easy, safe and changes can be done on the fly
Tube Watts per Channel KT88 = 150 WPC KT88 = 150 WPC Many varieties of this tube available
Tube Watts per Channel KT120 = 150 WPC n/a Has it’s own unique signature sound
Tube Watts per Channel KT150 = 250 WPC n/a Different bias settings create a different sound
Tube Watts per Channel KT170 = 250 WPC n/a Same power as KT150 but very different sound
Signal tubes 12AU7/ECC82 12AU7/ECC82 Hardest working tubes in the amp, unsung hereos
Frequency response 20Hz to 30Hz 20Hz -50Hz Bandwidth limited for better stereo imaging
Total Harmonic Distrotion ~ 0.22% at 1W into 8 Ohms ~ 0.8% at 1W into 8 Ohms Allows a more precise, clearer musical signal
Gain 27dB 29dB Significant increase in power requires less gain
Noise level > 4 mV RMS into 8 Ohms > 5 mV RMS into 8 Ohms Quieter with darker darks
Custom Toroid 15 lbs 10 lbs Creates additional power to better drive the Tubes
Power Supply New custom design choke based design Quieter with a higher output voltage
Power comsuption rated output  756 watts 360 watts Much more powerful Class A amplifier
Weight 79 lbs. 59 lbs. More robust components to create the power
Shipping weight 96 lbs 79 lbs Custom foam and packaging for a safe delivery
Mains fuse 10 Amp Slow Blow 6.3 Amp Slow Blow More power requires a bigger fuse
Output Speaker fuses 10 Amp & 7 Amp 8 Amp and 5 Amp Protects the from accidental speaker cable shorting
Output Taps 4 and 8 Ohm taps 4 and 8 Ohm taps 4 Ohms controls the base better, 8 Ohm more open
Inputs SE & Transformer balanced SE & Transformer balanced Balanced is preferred but gives you a choice
Output Design Push-Pull, Ultra-Linear Push-Pull, Ultra-Linear Lower distortion with more power
Grounding Independent Grd. Swtches Independent Grd. Swtches helps to eliminate the hum from ground loops
Design No Negitive feedback No Negitive feedback No compression from the amplifier

Specifications per amp (based on KT150 tubes)


  • Pure Class A, Push-Pull Ultra Linear design
  • 250 Watts per amp
  • 4 Ohm and 8 Ohm non-differential output taps
  • Three 5-way binding posts
  • 10 Amp and 7 Amp Slow-blow speaker fuses (6mm x 32mm)
  • Non-inverting output polarity
  • SpeakOn connector for a subwoofer
  • SpeakOn subwoofer cable specs: Pins +1 and +2 go to the 8 Ohm output and pin -1 goes to ground

Power rating for Auto Bias Select Switch Settings

Left Position:

  • KT88 = 150 Watts per channel
  • KT120 = 150 Watts per channel
  • KT150 = 150 Watts per channel
  • KT170 = 150 Watts per channel

Middle switch position:

  • KT150 = 200 Watts per channel
  • KT170 = 200 Watts per channel

Right switch position:

  • KT150 = 250 Watts per channel
  • KT170 = 250 Watts per channel


  • Transformer true balanced differential input
  • XLR gold pin balanced input with pin 2 hot, impedance 12K Ohms
  • RCA gold pin single ended input, impedance of 12K Ohms
  • XLR ground lift switch
  • 3.3 Volts RMS input to reach rated output

Detailed Specs

  • Monaural
  • Auto bias
  • No negative feedback
  • Frequency response 20Hz to 30Khz ~ 3dB
  • Total Harmonic Distortion ~ 0.07% at 1W into 8 Ohms
  • Gain of 27dB
  • Noise level > 4 mV RMS into 8 Ohms with input shorted
  • 1% metal film resisters throughout
  • Polypropylene capacitors throughout the signal path


  • Large custom Toroidal power transformer
  • 120 Volts AC mains with 10 Amp 5mm x 20mm slow blow fuse
  • Mains fuse located in drawer compartment of IEC power connector
  • 3 pin 15 Amp IEC power inlet connector
  • Optional factory installed 220V, 230V/240V 50/60Hz with 6.3 Amp slow blow fuse
  • Power consumption at rated output 756 Watts
  • On/Off switch conveniently located on front panel
  • Amber LED low voltage and green LED high voltage indicators

Active Components

  • Six (6) KT150’s vacuum tubes
  • Two (2) ECC82 (12AU7) vacuum tubes

Other Features

  • Each unit is made by hand
  • 50 hour factory burn in on all circuits and vacuum tubes
  • Dimensions 17″ W X 20″ D X 10″ H (43.1 cm x 25.4 cm x 50.8 cm)
  • Weight 79 lbs. (35.8 Kg)
  • Shipping weight 96 Lbs. (43.5 Kg)
  • Shipping box 27″ W X 27″ D X 13″ H (68.5 cm x 68.5 cm x 33 cm)
  • 16 and 14 gauge Zinc plated steel enclosure
  • Two year limited warranty
  • Six months warranty on 12AU7 Vacuum Tubes, 90 day warranty on KT150 power tubes

World Premier – Zesto Audio Eros 500 Select Class A Monoblocks

01-04-2023 | By Robert H. Levi | Issue 125

I am at a loss for words to describe my excitement listening to the groundbreaking Eros 500 monoblocks. I was expecting excellence, but not ball-busting greatness.”  Read entire review

Thousand Oaks, CA, December 8, 2022 – Zesto Audio is pleased to announce the launch of their new Eros 500 Select Monoblocks, quite possibly the most powerful Class A tube amps available. Each amp has 250 watts of dynamic Class A power. The Eros 500 Select ships with KT150 output tubes, but has the perfect auto bias settings to accommodate KT88s, KT120s and KT170s.

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