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:

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

  1. Radu,

    Thanks so much for the pics … they are an inspiration for my 300b amp project which will be based on Tom Christensen’s PCB design with a number of my additions. I suspect it will not be as awesome looking as your efforts , as my efforts and knowledge over the years is really all about circuit stuff, not chassis work and finish skills. I did talk to Jack Elano, and l have purchased a pair of his favorite 300b SE opt’s from him.

    I also talked to Kevin Carter, but the small Lundahl C Cores and required covers are a bit more work ( more money too. ) I am thinking the big Electra Prints will look better with the 300b’s IMO. I wish I could afford a set of the opt that Ale Moglia likes… I see that you used a pair recently in your 300b build. I recently rebuilt a McIntosh MC240 with a set of fresh GE 7027 tubes, and a modernized ( no crummy twist locks ) power supply , this MC240 is really outstanding if you like PP amps. I have perhaps serviced over one hundred of these amps having started in a hi end audio service shop at 16 ( am 65 now ).

    In the meantime, I am righting all the wrongs made by a over zealous internet educated owner of a Threshold S500. This has been quite the project and a major expense for the owner who could not keep his hands out of this unit. Hopefully it will behave for me. This working on tube stuff one week and solid state stuff the following week can be a source of wtf moments.

    Cheers ,

    Herb Ward Norwalk

    On Tue, Jun 4, 2019, 8:14 PM simplepleasuretubeamps wrote:

    > raductar posted: ” 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? Becaus” >

    • Herb,
      Glad to hear that you are building a 300B amp. I am building now a PP EL34 amp and as soon as I’m ready it will be posted.
      I am favoring the SETs, but I like a good PP amp on the right speaker.
      Good luck lifting that 92 lbs S/500 monster…;))
      Let me know if I can be of any help.
      I’m just around the corner.

      • Hi Radu,
        Very nice work indeed. And thanks for sharing.
        Any preference between the VT25 and the EML20A soundwise, the VT25 being the thoriated type. EML is very expensive but at least available.
        Is there no need for a SUT?
        BTW I learned a lot from your 01A build.
        Thank you.


      • Hi Nash,
        I prefer the EML20A mesh. It seems to be in more synergy with the 300B and needs a average gain preamp to drive it.
        The 10Y driver will benefit from a SUT, but will ask for a low output impedance preamp that is also capable of driving a higher capacitive load. Not a big fan of SUT.

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