2x30W EL34 tube amplifier

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.

 

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm

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 discussionย  he came out with a improved design with valid arguments to his choices. You find the instructions and original schematics here:ย http://www.lundahl.se/amplifier-schematics/30w-push-pul/

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
Power output
Adjusted for 20 Watt
Input tube
EF86
Phase splitter tube
ECC83
Output tube
Phillips EL34
Output transformer
V78A01F, 25W
5K6 Ohm primary
4, 8, 16 Ohm secondary
Power transformer
Custom made from askjanfirst.com

Schematic

Monostage

Power supply

EM800 VU meter

 

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.

13th February 2012
The best piece of wood for roof constructions that I could find at the home improvement store was dragged home, I needed some good thick and tall wood in order to make a cut with a router for the amplifier base plate to slide into.

9th February 2013
Ever since I built the amplifier, there was an issue with a small amount of 100 Hz hum from the power supply. Realizing that I did not double the capacitance in the power supply, as described if it was to be used for a stereo amplifier, I wanted to correct that mistake. With only two 220 uF capacitance on the 450 VDC rail I would see 2,3% ripple. With the recommended four times 220 uF there would be 1,05% ripple. As I had spare 1000 uF capacitors I used those to bring it further down to a mere 0,4% ripple.

While I already had the amplifier on its back I also decided to add some other modifications that I felt was necessary to give it more years to live in. A soft start circuit for the filaments and a delay on the high voltage. These steps are taken to avoid that the cold filaments with their very low cold resistance would take damage from the magnetic forces when power is applied without soft start, this will eventually break the filament as it also glows up bright yellow at first. Delaying the high voltage will prevent the tube trying to conduct from a filament that is not properly heated, this could lead to the thoriated tungsten filament losing its ability of giving off electrons as the contamination is ruined. The soft start of the filaments is done through a 1 Ohm 100 Watt resistor and after one minute the filaments get full voltage and the high voltage is also applied. All done with just one relay and one timer. A NTC resistor was also added to the primary side of the power transformer in order to soft start the five times bigger capacitance in the power supply, this should remove a great deal of stress on the rectifier diodes.

The EM800 indicator tubes are also installed and their circuits connected to the amplifier. To obtain galvanic isolation between the EM800 circuit and the amplifier I used a 1:30 current transformer on the output lead to the speakers. With a proper ferrite ring core for high frequencies the indicator tubes now show the level of the treble output.

21st December 2013

Through some time I had noticed lower sound level in the right channel and found some time to turn the amplifier around and measure what was going on.

The left channel was almost in balance at 205mV and 195mV over the cathode 10R resistors. The right channel was however at 235mV and 145mV. The bias level was also on the low side from when I first adjusted it very conservatively and was for both channels set to 350mV with balance between push-pull tubes measured to 0,005V.

After adjusting the weak tube in the right channel began red plating and the plate took enough damage/discolouring for me to change it for another from the big box of tubes. It was most likely just burned out as they are all old used tubes.

 

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. It 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 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

48 Responses to 2x30W EL34 tube amplifier

  1. Pingback: 2x 30 Watt EL34 tube amplifier done | Kaizer Power Electronics

  2. Steve Conner says:

    Sweet! Good to see you used those output transformers. I still haven’t done anything with the Sound City OPT. :-/

  3. Mads Barnkob says:

    Thanks, I will also say that of all the things I have built, this is the one thing I use the most and enjoy almost every day ๐Ÿ™‚

    You better get building that 120 Watt guitar amplifier, you already have one of the most expensive parts

  4. Andrei says:

    Hi there, nice project! i was looking to build a tube power amplifier, and after weeks of studying about them on the internet your project was the one that caught my atention the most. Can you help me please with ducumentation about the project ( schematics for the modules that u have used in the amp,) how you wired all them together, about the delays u have used, about the vu-meters, etc. . Alsow if you can give me a email adress so we could talk there it would be great.
    Thank you very much. best regards.

  5. Mads Barnkob says:

    Hi Andrei

    Thanks for the kind words, I prefer all questions to be answered here so others can benefit from the information ๐Ÿ™‚

    Building instructions, schematics and test procedure can all be found in the documentation made by Claus Byrith, hosted for Lundahl transformers: http://www.lundahl.se/amplifier-schematics/30w-push-pul/

    I used 60 seconds delay timer on the full heater voltage and high voltage supply.

    The VU meters are coupled with a current transformer to the output lead of each channel.

    Kind regards
    Mads

  6. Gaurav Kumar says:

    Hi,
    I am little cofused about the connections of the 22k pots…
    Is it a stereo 22k pot with middle wiper of both connected together?

  7. Mads Barnkob says:

    Hey Gaurav Kumar

    It is two individual potentiometers. R19 control the amount of negative bias and R16 controls the balance between the upper and lower output tube.

    Kind regards
    Mads

  8. Gaurav Kumar says:

    Thanks a lot for the info. Can you also tell me the ratings of all components.
    Can you share an annotated diagram or a BoM with ratings of various components.

  9. Mads Barnkob says:

    Hi Gaurav Kumar

    Generally all capacitors are 400V or 630V rated, depeding on where in the circuit they are located, look at the supply values.

    All resistors are 1W metallized, only R22 and R23 are 5W.

    You can see the original schematics from Claus Byrith here: http://www.lundahl.se/amplifier-schematics/30w-push-pul/

    Kind regards
    Mads

  10. Gaurav Kumar says:

    In his article. he has mentioned that “The values 10 K – 100 K in the supply line for 410 V and 160 V are valid for mono blocks. ”
    I am making stereo with single power transformer with appropriate rating.
    Can you tell me how these values for resistors will change if I intend to make a single power section for both mono blocks.

    I have ordered a custom power transformer with following specs (am I right with the specs?):
    1: Toroidal Power Transformer

    Primary: 230 V
    Secondary 1: 350-0-350 CT at 0.7A
    Secondary 2: 1-130V at 50mA

    2: Toroidal Transformer
    Primary: 230 V
    Secondary 1: 0-6.3V at 5A
    Secondary 2: 0-6.3V at 5A

    Thanks a lot for your help, ping me for any help from India ๐Ÿ™‚

  11. Mads Barnkob says:

    Hey Gaurav Kumar

    I will find the power supply schematics and add them to this page, give me a little time to do so ๐Ÿ™‚
    I also made a stereo version with only one power transformer. If you look up at update from 11th June 2009 you can see a picture of the power transformer label with ratings.

    Kind regards
    Mads

  12. Gaurav Kumar says:

    Hi,
    In the supply line for 410 V and 160 V, for stereo stage, if I’m not wrong, the 2 resistors, 10k & 100K should be replaced by exactly half – 5k & 50K, am i right? Pl share your views…

  13. Mads Barnkob says:

    Hey Gaurav Kumar

    I used the original schematic of the mono block power supply to supply both channels, I do also only have the amplifier adjusted for approximately 20 Watt, so they do not load down the supply voltages.

    As you state yourself, and Claus mentioned this in his schematics on page 20 “The values 10K – 100K in the supply line for 410 V and 160 V are valid for mono blocks.”, you should adjust the values for full power, how much exactly would be up to experimentation and watching the voltages under load.

    Kind regards
    Mads

  14. Gaurav Kumar says:

    Thanks a lot, will keep you posted about experiences as I start building…

  15. JLB says:

    Hi there Mads,
    Once again cons for this project (we’ve already spoken in DIY forum :))
    I’ve got a doubt with main secondary PSU design. In your schema, you have tagged as 170, CT, 170, but it’s my assumption that voltage rate is 340V CT, so CT is 170 V, and negative end is ground. Is that assumption correct?
    Thanks in advance for your response.
    Best regards.
    JL

  16. Mads Barnkob says:

    Hey JLB

    the CT is at at 225VDC in respect to both positive and negative rail.

    The capacitors C2 and C3 are used as voltage splitters and should really have a 150K resistor across each to ensure correct voltage sharing.

    Centertapped transformers are really a old heritage from when tube rectifiers was used, there is no reason to use it today. A 0-340VAC winding going into the full bridge rectifier would be enough and capacitors would only need to be put in series if the voltage rating of them was not high enough.

    Kind regards
    Mads

  17. JLB says:

    Thanks again Mads. I’m afraid my knowledge about electronics are not so deep as yours so I’m going to beg you a little of patience with me ๐Ÿ™‚
    I’m in the process to order power supplies (my intention is to build 2 mono blocks) so I want to fully understand what is required. In the original CB’s schema main secondary is noted as 370v , 0.2 amp CT, so does it mean that wave is 370V, CT, -370V (and there is an additional end for ground which is called CT), or the source is 370V, 0, and the CT is 185V? It’s my understanding that 370 v are needed after rectifying to achieve 370 x square-root(2) volts. So, how in the world you have used only 170v on your design? Does it mean that you can use only 170volts and the res until 420 volts are supplied by caps?
    By your previous answer is my understanding too that I can’t get rid of CT power source and just use a regular power supply. In such case, should it be a power source with 370, -370 volts? What happens with resistor R52 (the one connected to CT) from the original circuit? Should it be removed? If so, just the resistor or the wire between capacitors too?
    Could you please help me with these questions?
    Thank you.
    Regards.
    JL

  18. JLB says:

    Hi there Mads. Forget about my last question: finally I figure it out how this power supplies and rectifier estate work.
    Best regards.
    JL

  19. Mads Barnkob says:

    Hey JLB

    I am sorry that I did not get back to you fast enough, I have been on vacation ๐Ÿ™‚

    Kind regards
    Mads

  20. Gaurav Kumar says:

    Hi, in the power supply schematic you showed the optional resistor in the Vvg , did you use a pot here?

    I am using a single power supply stage for my set up. Did you also use the same design? Any comments on that.

    I request a conversation, can you share your contact, I’ll give you a call whenever you like. Thanks!

  21. Gaurav Kumar says:

    Need your advise on Volume control also, where have you put in, i mean at which stage, I have attached the diagram, please share your opinion on that.
    Thanks!

  22. Mads Barnkob says:

    Hi Gaurav Kumar

    -Vvg resistor is optional and depending on the amount of needed negative bias it can be adjusted with adding this resistor to raise the voltage in a positive direction. You should use two separate resistors, one to each stage, this makes it much easier to adjust each stage individually, else one stage will go out of balance as you adjust the other. If your want to use a potentiometer, be sure to use one with a high enough wattage rating.

    I did not build in a volume control, I use my preamp for volume control. If you want to put volume control directly into this amplifier you should have it at the signal input to the first tube.
    Volume control should always be applied to place where the signal has the lowest amplitude, else you have a much higher risk of inducing a high amplitude source of noise. Either add potentiometer to the 4K7 rid resistor at the EF86 or pull the signal to ground with a potentiometer, I am not sure which method works best here.

    Kind regards
    Mads

  23. Gaurav Kumar says:

    Thanks for your Reply.
    One more query, I’m using single supply stage and have reduced the Resistor values by half at 410V/160V taps, considering that for both channels current requirement will double, so to maintain the same potential, resistors needed will be half of what is prescribed in the schematic.
    Am I right in doing so?

    Also, pl tell me what is the range of current to the grid bias which determines the rating of pot for bias adjustment. will a normal 16mm Alpha pot do the job here?
    I extremely appreciate your time and effort for sharing your experiences and advice.
    Will share my build report with you.

  24. Mads Barnkob says:

    Hi Gaurav Kumar

    Claus Byrith describes using half the resistor values in the power supply in order to supply two mono stages. I did not do this, however I have only adjusted my amplifier for 2x 20 Watt, so the power supply can still supply enough current for two mono stages.

    The negative grid bias works from the voltage applied to the grid repelling electrons at the grid, this does not require much current, typical 1-2 mA for EL34, to be sure double the figure when selecting components.

    Kind regards
    Mads

  25. Gaurav Kumar says:

    Thanks a lot! I’ll soon share my experiences as I start testing which I’m expecting this week. I’m using point to point construction method rather than PCB, also integrated Baxandall tone control as the first stage based on single 12AX7 per channel.
    Layout resembles the diagram with a thick COPPER WIRE for GROUND BUS connected to chassis (Aluminium sheet) at one end.

  26. Mads Barnkob says:

    It sounds like you are on your way to have a working amplifier, did you see the prototype wiring I made for the 100W 6P45S tube amplifier that you can find in the menu?
    It sounds like you are taking the same route on the wiring, just be careful not to add any ground loops from input signal wire to ground wire to star point to chassis.

    Kind regards
    Mads

  27. Gaurav Kumar says:

    I guess I’m taking full care on that front, thanks for raising the concern.
    Have used, teflon shielded wire for input signals. The circuit is almost ready and I just have connect the mains and O/P transformer to the valves.

    I think I’ll start testing tomm. I am thinking of making a 5W resistors bank to create a dummy 8 OHMs load and will analyse the distortion on an old Oscilloscope (20MHz).

    Yes, you are right about your 100W 6P45S monoblock tube amplifier, I’m following a similar construction approach, I have made a prototype design, uploading here, about how the complete assembly will look.
    This is my second tube amp, I have previsouly made an awesome sounding HIWATT DR504 which is my Guitar Amp and I love it ๐Ÿ™‚ (… Gilmour Fan ๐Ÿ™‚ )

  28. Gaurav Kumar says:

    Chassis Prototype.

  29. Mads Barnkob says:

    It is looking real good, I am looking forward to see the final result ๐Ÿ™‚

  30. Gaurav Kumar says:

    Hi,
    Yesterday night I powered it on and nothing abnormal happened, today morning I installed the valves and powered on, unfortunately the bias was not negative enough and i didn’t notice the red hot valves behind the base plate but sensed the heat after a minute, shut down immediately and corrected the bias to get about 400mA between Cathode and ground, the negative feedback was connected opposite, corrected that too.
    Tested the amp on my home made Transmission line speakers and played.
    Even without AC balance it sounds damn good. The Baxandall tone stack works flawlessly, i omitted the input cap and 1M dropdown resistor at the input stage.
    I used 100kA volume pot between tone stack and input to ef86.
    Amp sounds well balanced to my ears. I also tested the out put response of 1kHz signal which looks clean and undistorted, will do the rest adjustments and share with you.
    Need suggestions on following, my plate voltage supply is little on the higher side around 480V against the suggested 440 V, do u think its ok?
    Anything else you would like to recommend for testing… I have Pretty lean setup, just one old basic oscilloscope without FFT facility and DMMs.

    Above all, I extremely appreciate your help and guidance. Please let me know if you need anything from India.

  31. Mads Barnkob says:

    Hi Gaurav Kumar

    It sounds like you had a almost problem free first test, that you did get to do all these adjustments to fast and in one go is good.

    Looking at the plate current vs plate voltage curve with grid voltage plots, the EL34 is pretty linear at 440 to 480 VDC. The difference is in the range of 3-4 mA plate current.

    Just take account for the higher voltage when adjusting bias, so you set it a little lower to obtain the wanted Watt rating.

    About components from India, how are tube prices compared to Europe? Normally I would look at prices from Germany or Easteurope as I like to use the cheaper Russian tubes. I am not a fan of spending hundreds of Euro on something like 300B tubes ๐Ÿ™‚

    Kind regards
    Mads

  32. Gaurav Kumar says:

    Hi,
    Thanks for the suggestions.
    I took care of the power dissipation while setting up the bias.
    India is not actually a place for sourcing valves, I myself source from Germany.
    However, Over past few years I have evaluated some good transformer manufacturers and actually zeroed down to one and I am highly satisfied. The pricing are also not aggressive and they can custom manufacture in huge range.
    A 50W Output Transformer typically costs about 40 USD and for a 400 VA toroidal Power Transformer costs about 80-90 USD.

  33. Mads Barnkob says:

    Hey Gaurav Kumar

    Shipping iron across the planet is however not cheap ๐Ÿ™‚ I used dagnall electronics where I can get custom made transformers made in EU, so no import taxes, to prices in the range of 100 USD for a 250VA power transformer, shipping included.

    Kind regards
    Mads

  34. Gaurav Kumar says:

    You are right!
    It doesn’t make sense ๐Ÿ™‚
    Anyways, thanks for your help ๐Ÿ™‚

  35. Sunil J says:

    Hi Mads,

    In the PS, what is the 0-110v negative voltage at the bottom right side of the schematic ?

    Is a separate power supply required here ?

  36. Mads Barnkob says:

    Hi Sunil J

    The 0-110VAC winding is a separate winding on the same power transformer as the HV and filament voltages.

    Kind regards
    Mads

  37. Gaurav Kumar says:

    Hi,
    I’m making another unit and this time I am planning to add 2 layers of 470uF/500V pairs in the HV filter line to reduce the ripples even further.
    Any opinions on this?

  38. Mads Barnkob says:

    Hi Gaurav Kumar

    See my update in the main article from 9th February 2013.

    “….With only two 220uF capacitance on the 450VDC rail I would see 2,3% ripple. With the recommended four times 220uF there would be 1,05% ripple. As I had spare 1000uF capacitors I used those to bring it further down to a mere 0,4% ripple…..”

    I just copy pasted a small part of it out.

    While the result was measureable, it is still very hard to hear a reduction in hum. Even a few mV is enough to cause hearable hum in a tube amplifier.

    Kind regards
    Mads

  39. Sunil J says:

    Hi Mads, spent a lot of time relaying the components and this is sort of final. I’ve introduced three 20 tag strip boards. Two for the amplifier and one for the SS supply.

    What do you think ?

  40. Sunil J says:

    Front view, with top cover.

  41. Mads Barnkob says:

    Hi Sunil

    It looks good. You have the transformers oriented correctly in respect to each other, good sectionening of the pre-amplifier/splitter and output tubes and also furthest away from the mains transformer.

    If anything, would it be possible to move the amplifier strip board into the output tubes compartment?

    Kind regards
    Mads

  42. Sunil J says:

    Hi Mads

    Thanks for the quick reply. If that layout works I’m fine with it. It also helps with reducing the height, by introducing all the trannies on the lower plate. Please review the new layout.

  43. Mads Barnkob says:

    Hi Sunil

    It looks good. I am eager to see your results when it is done ๐Ÿ™‚

    Kind regards
    Mads

  44. Sunil J says:

    Thanks Mads.

    Your design and implementation is the inspiration after Claus’ initial work. I hope it turns out to sound good. Fingers crossed.

  45. Mads Barnkob says:

    Hi Sunil

    My best advice would be to pay very close attention to grounding so that you avoid ground loops, those are the reason for at least 90% of the hum I have removed from my amplifiers.

    I am sure you will get a good amplifier, you did a good research up front before building it ๐Ÿ™‚

    Kind regards
    Mads

  46. Does it have a phono stage ?

  47. Mads Barnkob says:

    Hi Edmond Jeganathan

    No, it has been modified to accept input levels to modern standards. You would have to build in a phono stage or use a external.

    Kind regards
    Mads

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