Radu’s Ultimate 300B Amplifier: Shiny Eyes 10Y/20A – 300B SET Amplifier

Ultimate 300B front overview

There is no greater satisfaction for a music lover than having an ideal audio chain. This is a 300B amplifier that will not disappoint. It is the result of many years of experimentation and R&D with DHTs ( direct heated triodes).

Why 300B?

  • Because it is one of the most praised DHTs of the past.
  • Because it can deliver enough power to drive speakers that are not super sensitive and therefore more available.
  • Because it is available as new production in many brands and “flavors”
  • Because, if well designed, an amp using 300B can satisfy an exigent audiophile

I am publishing this build after the 71A sub-watt amplifier. I love the sound of both amps, as well as few other amazing DHTs such as the 45. Each amp can offer rewarding sound experience as long as it is paired with an appropriate speaker. I want to emphasize that one can’t go wrong when using these old design vacuum tubes.

I’ve always liked the sound of a 2 stage amplifier. The best sounding of 2 stage amps have a DHT for both the final stage and the driver.  In many cases these amps have low input sensitivity and therefore need a preamp with medium or high gain. If the preamp is using a DHT then the whole audio amplification  is DHT. The great advantage of having all DHT amplification is the low distortion where the 2nd harmonic is predominant. There is no need for global negative feedback. The sound will be amplified in a way that is not loosing detail and will be extremely pleasant.

This amplifier is using either the 10Y/VT25 or the EML20A as driver. The driver is loaded on an interstage transformer Lundahl LL2756/25mA. The driver is powered through a CCS ( cascode depletion mode mosfet) that is adjusted to 25mA. Both the driver and the final stage ( EML300B) are self-biased.

The 300B is loaded on a excellent sounding output transformer Monolith Etude1. The output is switchable between 4 and 8 ohms.

Each tube filament is powered by a Coleman regulator that has individual multi-turn potentiometer for fine adjustment. As always there is no “free lunch”. The excellent sounding Coleman regulator needs re-adjustment every time one will replace a tube. Believe me, it is worth the trouble…

The power supply was built in a separate enclosure ( using a Modushop ( Italy) Pesante enclosure placed sideways to minimize footprint). It has 4 separate raw DC supplies for the filaments of the 4 DHTs. The high voltage supply is in a quasi choke input mode and it uses all film caps ASC. The rectifiers are 6CJ3 TV dampers.

The umbilical cord is permanently attached to the power supply enclosure and has a military grade Amphenol 11 pins connector at the amp end. Few years ago I found a nice 1000V NOS large Simpson panel meter on eBay. I am using it to show the B+ high voltage of about 480V. The power supply has two main switches: one for the filament supplies and the other one for the high voltage supply. They are in series and therefore one can’t turn on the high voltage before turning on the filament supply. In the past I’ve used a relay to delay the high voltage. In this case, I prefer manual switching because of the slow warm up caused by the Coleman regulators. Some of the 300B need few minutes to reach steady state on the filaments, such as the EML300B. Turning on the filament supply switch will allow the user to wait a desired amount of time before switching on the high voltage. The high voltage will slowly ( takes about 20 seconds) ramp up. This is beneficial for the life of these expensive vacuum tubes.

The input sensitivity is about 3Vrms for 8W output with the EML20A driver and about 8Vrms with the 10Y driver. I am using a switchable 01A-26 full gain  ( x8  or 18dB)  preamplifier that provides sufficient signal level for both drivers.

I will always refrain to describe how any of my builds will influence the sound. That should be a personal preference. However, the synergy amp -speaker/room is the most important. It definitely sounded noticeable better using Rullit’s field coil speakers in cabinets designed by my audiophile friend Rich, compared to my TangBand tower speakers.

Here are some pictures that will give you an idea of how this amplifier is built:


And here is the power supply: