2x30W EL34 tube amplifier

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Introduction

Ever since building the 2W single ended tube amplifier, I wanted to build something better, something that is considered a good amplifier among tube amplifier enthusiast.

It is in no way cheap to build a tube amplifier, so this project got a good kick start a day where I got a old broken 120W bass amplifier for free, the power transformer was burned out and it had been left in a barn for about 15 years.

Considerations

When searching for a amplifier design to follow, I came across a good paper written by Claus Byrith where he took a well known Mullard design and gave it a proper discussion. Through this discussion  he came out with a improved design with valid arguments to his choices.

His design was recommended by many on www.diyaudio.com and I chose to go ahead with his paper as it was well documented compared to other older designs that mostly consist of a schematic and what forum threads you can find throughout the internet.

Specifications

Class	A
Power consumption	80 Watt, 200VAC at 0.4A
Input tube	EF86
Phase splitter tube	EEC83
Output tube	Phillips EL34
Output transformer	V78A01F, 25W 4, 8, 16 Ohm primary 5K6 Ohm secondary
Power transformer	Custom made from askjanfirst.com

Schematic

Monostage

 

Construction

4th January 2009
Obtained and disassembled a old Sound City 120 guitar amplfier, parts scavenged 6x El34 tubes, 5x ECC83 tubes, sockets, 120W mono output transformer, various jacks and potentiometers.

 

27th March 2009
Traded the 120W output transformer for 2x 30W ultra liniar output transformers.

5th May 2009
Ordered a custom power transformer.

11th June 2009
Received the custom made power transformer along with covers, and 2 EM800 ( http://www.akh.se/tubes/htm/em800.htm ) magiceye tubes for free! These will look neat used as VU meters!

 

16th June 2009
Designed power supply PCB.

17th June 2009
Designed EM800 VU meter PCB.

22nd June 2009
Etched, assembled and tested EM800 VU meter.

24th June 2009
Etched and assembled power supply.

1st July 2009
Designed mono stage PCB.

7th July 2009
Tested power supply, bias voltage is -150VDC, tweaking is needed.
Etched and assembled first mono stage PCB.

11th October 2009
Plan for the baseplate is made

12th October 2009
Construction of the baseplate begins

 

30th October 2009
Construction of baseplate is done

4th January 2010
Polishing and varnishing of baseplate

5th January 2010
Assembly of baseplate with transformers and tube sockets.

 

14th January 2010
Etched second mono stage PCB

20th January 2010
assembled second mono stage PCB

7th March 2010
Assembly on baseplate begins

16th March 2010
Assembly progress

 

20th April 2010
Assembly progress

 

25th April 2010
Assembly progress. the amount of different colour wires and insuring a proper twisting made the process take much longer than expected. But in order to get a good result, one needs to invest the necessary work in making it so.

 

26th April 2010
First power on, test and adjusting. There are problems with the negative feedback.

 

12th August 2011
I found a wooden box for the amplifier to be installed in.

10th August 2011
It is a problem having two mono stages share a power supply, negative feedback voltage needs to be adjusted for each channel. Matching of some grid resistors necessary.

17th August 2011
The amplifier runs and can play audio, but there is noise problems from ground loops.

I got the ground loops sorted out in such a manner that you have to put your ear close to the speaker in order to hear the mains hum. All ground wires goes to a star point at the power supply and all ground wires that could, is twisted or braided together. When I install the amplifier in its final enclosure, I will take additional measures to wire the ground better.

 

24th August 2011
The amplifier is complete on the baseplate, everything and in the right sizes are soldered on and only the enclosure and mounting of jacks remain the last to do. Been listening to the amplifier all day on the Isophon BS35 speaker set. wonderful.

 

Conclusion

Mono stage
Pros: Close to the speaker, easy adjustment of power supply, no critical matching with the other channel.
Cons: Need separate power supply which will increase total cost.

Stereo stage
Pros: Everything in one box, lower total cost, everything is equally coupled regarding heat and noise.
Cons: Critical matching of stages and power supply, ground loops appear easier.

It might seem that there are most cons that pros, but the fact is that the cons are much harder to deal with. For the future I would try to build mono stages to gain more experience in choosing between the two. Looking back at the process, matching the two stages to each other and the power supply took much of my time.

I have tried my Isophon BS35 speaker set on my regular amplifier for my stereo speaker set for my computer, they are miles ahead in sound quality over the stock speakers that came with this semi Hi-Fi set, Edifier S2000. The largest difference from the transistor amplifier to the EL34 tube amplifier is the bass. Its deep as the internal hell, clear like tuned piano and makes a world of difference to enjoying music. I have to be experienced as a description is far from enough compared to feeling it in your chest!

Cathode current settings are a bit on the low side, but for now they perform good enough for playing up my small apartment. Setting AC balance with a distortion meter is also on the list of future improvements.

 

I still need a proper pre amplifier that will have the ability to deal with input impedance matching, ground potentials and noise. I will properly build this myself, but for now I just use my laptop as source and pre amplifier.

Demonstration