Developing audio in Ireland
by Derek Walton
Walton Audio Mk2 300B
The Plitron Deluxe version

MK2 300B Amplifier

Having been totally satisifed with my last 300b I decided that some improvements could be made - namely using a better OTX and a better PSU filter. The Plitron 3035SE toriodal output transformer has amazing specifications, with a flat frequency range from 16hz to 65khz, a -3dB range from 3hz to 90khz and can be configured for 4ohm or 8ohm load. It is encapsulated in a beautiful texured finish, so what better option than to also use Plitron chokes, given their wonderful quality of build and specs. A 20H and a 10H are the basis of a double Pi filter together with A.S.C. oil caps.

This is basically the same as myy previous amp but with better PSU and not the JElabs delux version which uses 26 as the input tube.....cost has got to stop somewhere and I also went for the ease of availibility of tubes. However the PSU is that from the JEL Delux - check it out there, I will put it up here in due course. For details of the amplifier schematic see my previous 300b.


The benifits brought about by the use of the Plitron components cannot be stressed enough as well as the improved PSU.

The Plitron 3035SE OTX has a thunderous bass and what seems like an additional octave compared to the Hammond, a refined top end that defies belief and a mid range that you never thought was possible....ever! The Hammond suffered a slight ringing in the uppper octaves, giving a certain edgeiness to sibliants and the Plitron addresses all the shortcoming of the Hammond but at a price - a price that is worth the admission fee for sure and the fact that they are encapsulated and finished justifies the price.

This TX which was developed over a three year period by Plitron and Menno VanderVeen, has been designed specifically and optomised for the 300B triode with a plate resistance of 700 ohms and primary capacitance optimized for a very wide bandwidth. Primary inductance is kept constant, independant of quiscient current through the power tubes and are designed to withstand DC current and impedance mismatchs using larger diameter wire and special winding techniques.

The Plitron 20H is just so amazingly good at its job - that had I known just how good it is I would not have bought the 10H - I really didnt need it with the 20H. In fact, the 10H on its own would have been a huge improvement. I substituted it in my original amp in place of the Hammond 10H and the noise floor, residual hum just dropped outa sight and the dymanics improved by a noticable amount - so Yes choke brand does matter - it is clearly audible.

The use of a double Pi PSU allows me to have seperate B+ for the input and driver sections, with the benifits that that brings in terms of additional headroom and super dymanics. This is now so good that the first time I fired it up and played a CD the initial transient was so dymanic and powerful that I honestly thought that something had blown up, so frightening was it. We are talking scary and awesome in the true meaning of the word



*My choice of tubes remains as before despite having tried many many different input tubes - out of the dozens that I have I just love the RCA 5692 red base. A second choice, though quite different is the KenRad VT231 and then RCA6sn7GTB

*The Western Electric 300B has now replaced the JJTesla and yes, it is a silly price but believe me I would now find it very hard if not inpossible to live without it. Their customer service and 5 year warranty are beyond reproach and I have had to avail of the warranty and they were most helpful with my problem. This is a major factor when spending the sort of money that these tubes command. By contrast to that, if I had never heard it I would have forever been happy with myJJTesla.

*Rectifier - choice of three here 5u4 family, GZ34 or GZ37. All three quite different and I prefer to like the GZ37. It has the advantage of being an indirectly heated tube and the slow ramp-up of the B+. However, using the GZ34 will raise the B+ some 30v or more but can be used into a first cap of up to 60uf, whilst using the GZ37 requires the input cap to be 4uf, though I have to admit to using 7uf. knowing that many people out there are using this tube into much greater values. Even some commercial design that I have also do do, so I reckon that 7uf is safe enough and I have been using it like this for the last 9mths. The 5U4G can be used into a first cap of up to 40uf but as a directly heated rectifier means that there is no slow turn on of the B+. You make your own choice on this one.!


All the capacitors, cathode and psu resistors and ceramic tube bases were sourced from Angela Instruments again but upgraded the Black Gate ones a stage higher for this amp. For a parts list - see the original amp as everything is basically the same

tubes front front

The Schematic


All resistors I used were 2w carbon and the hum balance pot is a ten-turn type. The cathode by-pass capacitors are BlackGate FK type for the 300b tube and VK for the driver. FK being the highest grade and VK the next down. In the mk1 I used the Std type. The coupling cap is the Angela copper foil,paper and oil. Internal wire is 99.99% pure fine silver. Alps Blue Velvet pot completes the line up.

The voltages shown are only a guide and will vary with different tube line up and also mains supply variations, which makes these impossible to be exact about.


The Power Supply Unit


The value of the first cap can always be dependant upon the type of rectifier tube one uses and the required B+. I chose to install on the chassis, two 15uF caps and three 50uF caps. Depending on how the pair of 15uF are connected I therefore have the choice of 7.5uf / 15uF / 30uF.

In the case of using a GZ37 which according to Mullard Data sheet states that the max input cap value should not exceed 4uF, I am choosing to use 7.5uf, well lets call that 7uF, yes I know it is inexcess but when I see what value others use in this position - up to 50uF - I think I am being conservative! In using two 15uF caps in series to get 7uF one must use a voltage divider ( two resistors of equal value) to give equal voltage across each capacitor.

Using just a single 15uF, as I did in the original amp with say, a 5U4G/GB is also fine with only a couple of extra B+ volts (due to the increase in capacitance) as the GZ37 and the 5U4 have similar voltage drops.

Using a GZ34 however hikes up the B+ considerable as this typically drops 17v as against 45-50 for the 5U4 - but the GZ34 can be loaded with up to 60uF, which will also raise the B+. Using two caps in parallel to attain a value will result in a faster time constant, though in this case it is probably just an acedemic exercise! One could substitute two Angela 30uF caps in place of the Angela 15uf in order to get 60uF, as they are the same diameter cans - only higher.

So you can see with the options here we can have a choice of tube, with a suitable cap value and variety of B+ all built in. If one of the 15uF are not being used then we can put that in one of the other sections in the filter..so there are no redundant caps however we connect them. Everything is there future-proof and flexible so to speak.

The resistors in the rectifier plate circuit are to provide the correct load for the tube, a matter that is rarely ever mentioned or addressed. In the case of the GZ37 , it requires a plate load of 75ohm min and a GZ34 this should be 150ohm. Using the Sowter transformers, they have a low reflected secondary impedance of about 24ohm, hence the inclusion of the additional resistors. They are also current limiters at switch-on.

The 50K bleed resistor is placed after the second choke where it draws a standing current through the chokes and aids regulation.

The Plitron OTX and chokes are available in Europe directly from the designer, Menno van der Veen, in the Netherlands at Amplimo, or in Canada from Plitron. Eitherway you get the very same product. Power and heater transformers are available to your specifications from Sowter - an alternative to this would be the Angela Universal PTX which incorporates both heater and HV windings in one.








Hum levels...

Using AC on the heaters and with a multi-turn hum pot and attention to detail the residual hum is approx 1 - 2 mv. This is only audible on my 98dB Lowthers with my head a few inches away. Any speaker with lower sensitivity , well its just not audible at all. Input tubes also effect the results here.

Will it work with low sensitivity speakers...well, see how it performed at the 2003 Dublin Hi-Fi Show *HERE*

Currently I am using it to drive Spendor BC1, which is 84dB, but then it is not in a large room.

How much did it cost...

How long is a piece of string or how deep are your pockets? - impossible to answer, so I can only answer for myself especially as I am not USA based but live in Ireland and get stung by duties, taxes and freight. More money than sense? No - I sold everything that I wasnt using to pay for this over a year period - I just wanted to make that very best that Icould, once and for all . - Ha!....I will do better the next time......I will refine this amp further later this year but will require a new chassis

However, in round figures:

2 Plitron OTX = $450
4 Plitron chokes = $900
4 Sowter PTX = $650
10 Angela Cap = $170
2 Angela coppercaps = $50
4 BG FK = $80
2 Alps pots = $60
Western Electric 300B = $850
2 RCA 5692 redbase =$160
2 Mullard GZ37 = $100
Metalwork = $200
Sundries = $200

Total approx $3870 .....not including taxes, duty and freight.....but dont tell my wife that - I want to live to build another amp!

Where did I get the chassis...

The chassis is made by myself from nothing more than 4mm aluminium sheet and channel extrusions that I got from a metal supplier. All the cutting, drilling was done with nothing more than a couple of hand files and a hand drill....just takes care and patience and can be finished with watchmakers accuracy despite what might be seen as rather crude tools. See also JELabs for a similar and more detailed description of this method.

If you have any questions please ask