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300B PSE MK2 - Parallel Single Ended

                     (the reference amp. with fixed bias)

by Andrea Ciuffoli

contact me for any problem or question:
  I am still working to give you more information ...

Here some photos of my test version



This project born because I need a reference amplifier.

The main results should match these items:

With the fixed bias option the tipical noise (of the amplifiers with direct heating tubes and ac filaments) at the switch-on is skipped!


All the audio transformers used in this project are Lundahlproducts:

Lundahl is one of the world´s leading manufacturer of audio transformers used in many professional studio recording and radio stations so He have a very big experince and all these products use a custom audio C-core.

I have used the Lundahl transformers also in my new transformer coupled headphone amplifier and in the DAC getting in both case incredible performances.

The total price of the transformers is about 700$.

About input stage I have tested many combination starting from parallel 6SN7 to the 6H30 russian incredible tube but with
the 5842 Raytheon the sonic result is very more higher.

The other vantage using the 5842 is the voltage gain that allow to connect the amplifier directly to the DAC chip so we will have only two tubes on the signal. Infact I am proposing in this same page also a digital input.

I am using a DACT stepper attenuator instead of the normal ALPS or NOBLE solution, because the the sound is clearer. A good review of these components can be found on the article published in Hi-Fi World.

In this schematic also there are no passive components on the signal!

The output tubes used are the Svetlana 300B.It is one of the lowest-distortion tubes ever made, and is a close duplicate of the original Western Electric 300B.
TheSvetlana 300B uses ultra-pure carbonized nickel plate material and a proprietary oxide coating on the filament to give Western Electric-like performance.
The filament is center-tapped to insure low hum. The Svetlana 300B is manufactured in Russia at the
Svetlana factory in St. Petersburg.
The strict manufacturing and quality controls at the Svetlana plant assure functionality and sound as good as Western Electric manufactured products.

The tube diode rectifier GZ34 should be the Mullard version. Some DIY don't like the sound of the GZ34 because consider this to much hard near solid state diodes but this is not true because depend by the power supply design and with inductive power supply we will have the best result.

About passive components types,  I don't leave many choices:  Holco or Caddock resistances on cathode, ELNA Cerafine capacitors on cathode and power supply.
For all my new projects, I am using the very good ELNA Cerafine capacitors that I love and which can be find at good price by
Welborne Labs
The ELNA Cerafine capacitors contain super fine ceramic particles which, through chemical reaction, improve the discharging speed between the anode and electrolyte with very low distortion.
Any other electrolytic or polypropylene capacitor give very lower sonic performances introducing some alteration. Only the Blackgate WKZ could give better sound, but I have not tested it and I know of some problems during startup phase.

About wire forget the teflon ! use only stranded tinned copper wire with pvc isulator and for loudspeakers I am testing the elevator cables (paralleled wire with a good section).

A very good quality stranded tinned copper wire could be buy directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.

The my custom transformer for the filaments has been make by F.I.A.T s.r.l.
via refrancore, 63
00166 Roma
tel. 06 61561705 - 06 61567539   fax 06 61550871


This project born without any simulation because the sonic performances are the main target so only listener test will give the right way to optimize the schematics (bias points).

The main schematic

The LL1660/10mA in the 1:2.25 configuration have a right gap for 20mA.

If this amplifier will be used with a tube preampl. use a 100K 1/4W Caddock resistance instead the stepper attenuator.

If this amplifier is used after a CDplayer use the DACT 100K +100K stepper attenuator.

Power supply

The Hybrid Graetz Bridge concept after a Fulvio Chiappetta paper, published by the Italian Magazine 'Costruire HiFi' a reference for the DIYers.
He tested and comparated this topology versus a pure tube vacuum state power supply design with a 300B output tube amplifier and there was no difference both while listening and measuring the output noise spectrum.

I built this power supply and the sonic result is very high even if it uses semiconductors the sound depends on the rectifier tube.

Infact the vacuum tube rectifier is in series with the solid state diode and the switch characteristic is of the lower speed device so the tube.

The 0.56 ohm resistors could be adjusted to have correct 5V on filaments of tubes and a value range from 0.47 ohm to 0.68 ohm should be good to test and match any rectifier.

I have used 2 x 1Kohm MP816 Caddock resistors for the automatic bias of the 300B but a normal 470ohm 25W could be used.

This resistors could be adjusted to have the perfect bias current 140mA.

Digital Input stage

This project give a first class sonic performance but this can be easily compromised by a  bad input signal so I suggest you to don't use preampl. and use one of my DAC projects.
Using one of my top DAC projects all the DIYer's will reach the same sonic result.
The DAC normally included into commercial CDPlayer (even these of significant price) have a rather low performance because the common usage of high NFB operational amplifiers "destroy" the sound given compression effects and poor soundstage stability.

Sonic results

click here

Layout views

click on the images to get real scale


Layout size

Attention:    some 100uF+100uF 500V ELNA Cerafine capacitors are higher so you should use an higher chassie!

click on the images to get real scale


3 mm aluminum with external size 490 x 220 mm


3 mm aluminum with external size 490 x 120 mm