Linear Conchord 30 Watts valve guitar amplifier

Danger Danger! HIGH VOLTAGE!

The pieces of information present on this page are provided as it is and without warranty of success. I wouldn’t take any responsibility for any damage or mistake that could affect your equipment or yourself.

Introduction:

Rusty Linear Conchord Amplifier
Early in 2007 I was the lucky winner of a Linear Conchord amplifier auction on the auction website. Before I started bidding, I googled for “Linear Conchord” since Linear was an entirely unknown brand to me at the time – and found John Chambers’ site which has a few pages dedicated to the Linear amplifiers.

The amp I recieved was in a terrible cosmetic condition: it was rusted and covered with a thick layer of hair and dust, as if the amp served many years as a home for mice!

Original wiring and components on the linear conchord After I opened the amp I realised that not only the Conchord was entitled to a proper paintjob but also that its internal components needed to be changed since a few resistors and capacitors were heavily damaged.

After a careful examination of the Conchord’s schematic I succombed the temptation of powering-on the amp. To my surprise, all the valves started to glow! Good sign! At least the heating circuit is intact and the valves in a good state! (A terrible smell of burning dust quite rapidily spread out of the amp…).
Linear Conchord schematicExtremely carefully (one hand in my pocket, the other holding the multimeter probe) I checked the tensions at diffrent points in the circuit or obtained the following results:

  • 240VAC at the mains (UK)
  • 750VAC just before the rectifier tube
  • 500VDC after rectification
  • 420VDC at one of the EL34’s pin 3 (cathode)
  • 200VDC at the end of the HT line

Linear conchord front

As Johhny Tube once said: “High voltage stuff is scary“. Working inside a valve amplifier is hazardous indeed! Not only when the beast is connected to the mains and powered on, but ALSO when it is powered off (some poor designs always connect the transformer to the mains!) or not connected to anything at all and powered off. If something wrong with the amp, the discharge resistor may have been damaged and in that leave the power supply capacitors charged (+500VDC in this amp!)… Please always make sure the big capacitors are discharged before working the amp!

At this point I had to stop working inside the amp because I knew I wouldn’t get far without any speakers to connect the Linear Conchord to! I didn’t go the easy way here since I decided to build my own speakers.

Rewiring/rebuilding:

Later in 2007 after I built a 100W/16 Ohms/4×8″ spearker cabinet I resumed working on the valve amp.

One of the hardest thing was to source all the components I needed! After hours of search I ordered the resistors (metal film) and jack sockets from Maplin (UK based), and the various capacitors and potetiometers from Banzai Effects (Germany based).

Pior to desoldering all the components inside the Linear Conchord, I made a drawing of how things are wired inside the in order to ease the rebuilding. After having spend a few hours in front of the computer I ended with the wiring diagram bellow. What I ended up with was drawn with WYSIWYG sate of mind so please it a this ;).


Linear Conchord internal wiring Linear conchord internal wiring drawing

 

Post operation debriefing:

The only major problem I encountered after completing the rebuilding was that the 10K Ohms resistor – located in between the to + poles of the 32uF powersuppy capacitor – kept burning instantly after I switched on the amp. After a having spend hours checking and re-checking every single connection in the amp, I contacted John Chambers and he generously spend a few minutes on the phone with me and quickly identified the problem: I missread the PDF version of the Linear Conchord diagram available on his website and connected the output transformer’s organge cable after the 10K resistor where it had to be connected just before! (The correct circuit diagram you should use for the Linear Conchord is the one re-redrawn by Giulio Maiocco).

Tweaking/improving the amp:

As I was on the phone with John, he advised me to simply short the 10K resitor on the HT line so that the double 32uF can is tranformed into a 64uF (you shouldn’t go any higher in capacitance for the reservoir capactior with a GZ34 rectifier by the way). This easy tweak reduces resudual ripples in the HT line, hence reduces hum and also increases dynamic range of the preamp section! If you do this, please check that you are not pushing the other triple 16uF capactior can too hard (since you increase the tension on the rest of the HT line!). You could always add a 10K metal film resistor in series with the 33k resistor right after the 32uF can in order to restore the amp to its original specs tension-wise.

 

Linear Conchord - Back, upside-down view

Additionally, I took the liberty to add the following features to the Amp:

  • Two 1/4 jack socket inputs instead of the original coaxial sockets!
  • Two speakon outputs to double the existing 3 and 15 Ohms speaker outputs.
  • A female kettle socket
  • A power on switch (with a red neon glow)
  • A bypass switch

Please refer to the interal wiring PDF to see how the tweaks above were implemented.

Conclusion:

It was fascinating to work on this little amp! Not to mention an excellent opportunity to extend my electronic skills on step further with regards to electronic valves.

Feature-wise the I bet this amp could be compared to an early VOX AC30 guitar amp, without the extra channels and the speaker. Rebuilding the Linear Conchord from scratch wasn’t too expensive: £150 including a new set of valves!, definitely cheaper that a buying a brand new valve amplifier.

Sound-wise the amp sounds very good (sweet! I wish I had no neigbours!) but lacks of brillance and there is a lot of hiss (and only a very little hum (heaters on the powertubes are out of phase, I bet this helps reducing the hum to its minimum actually, if you wire the heaters of the power tubes out of phase it will the amp will produce a lot of hum. Because the power-stage is a pull-push design, you must wire the heaters in phase so that each power-tube cancels the hum of the other).

I will now work on reducing the hum to virtually nothing by introducting regulated DC heaters on the preamp tubes and I will also try to alter the amp’s preamp section to reduce the hiss. Maybe reduce the amp to just one input and use the first volume as a gain and the second as a volume……

Thank you for your interest and many thanks to John Chambers and Johnny Tube for their help.

Postscript 1:

I managed to drastically improve the amp by:

  • Swapping (and adequately rewiring!!!) the first EF86 tube in the circuit for an ECC83.
  • Carefully rewiring the heating circuit (espetially the ECC83’s).

These two actions reduced the hum and hiss drastically.

Postscript 2:

Richard kindly send me pictures of his Linear Conchord. Nice paint job and very neat wiring indeed! Thanks!

Links:

  • CHAMP ELECTRONICS: Visit the vintage amp hospital. John Chambers is a master-chief electronic engineer who fixes and restores tube amps in his Nottingham workshop. It’s on John’s website that I found the Linear Conchord schematics. John also helped me a lot as on this project and I would like thank him again.
  • audiocircuit.com: Discution thread on the Linear Conchord.
  • Maplin Electronics: A good source if you need to order electronic components in the UK.
  • Banzai Effects: Excellent supplier: they stock hard to find capacitors, potentiometers and other guitar effects and amplifiers related components.
  • Watford Valves: Recommended online shop. They sell exellent valves at a reasonable price.
  • The Valve Wizard: All you need to know about valve amp designs!

13 thoughts on “Linear Conchord 30 Watts valve guitar amplifier”

  1. Hi bounav – ive just rebuilt a conchord using your and john chambers schematics and diagrams and it works great – even the original mullards that came with it works 😉 im starting to tweak the amp and even though there is hardly any hum, there still is quite a lot of hiss (heheheh you know whats coming 🙂 ) what did you follow or a better question is what did you do to convert the gain stage of the amp from ef86 to ecc83?

    Cheers – your diagram helped the build massively!!

    Richard

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    1. Hello Richard,

      Glad to know the diagram helped. I know exactly where you are hiss-wise indeed ;).

      I’m a bit ashamed because I experimented so much with this amp that i’ve lost track of the changes I’ve made and I will have to reverse engineer them and post about it!

      Now that you are familiar with the amp I asume you can divide it’s design into logical units: preamplifier & eq (1st EF86), eq (eq potentiometers + 2nd EF86), phase splitter (ECC83) and power amp (EL34 x 2). The conchord is very close in spirit to a Vox AC30, so what I did is found a few service manuals for AC30 (google help, there are loads if different flavours of AC30), then removed the EF86 from the conchord and built AC30 preamplifier sections that feature ECC83 valves into the conchord.

      I had to improvise a bit with the EQ and gain of each valve (the valvle wizard website will help you a lot in that area).

      I’ll have a quick look at the amp this weekend an see if I missed something ;).

      Ben

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  2. Oh don’t reverse engineer the amp!! I thought it might have been the ac30 you were modelling the changes after 🙂 Im tempted to change valves around a bit to try and find the one which hisses the least. Ive heard the ef806s are good (just a shame i cant afford the telefunken ones at 240 odd pounds for two!!) but ive seen the jj and svetlana winged c ones about for 30 pounds for two and I’d be tempted to try them first but if not sadly the efs may become eccs. Before then though, after I posted the first message, I found that I can turn down the gain stage to see if any hiss goes (also no matter what the level is, whether it be 0 or 10 the hiss is at the same constant level so its definately the ef86s). Im very happy theres next to no hum though 🙂 Ill have to have a good play first with that but its nice to know there was once someone in the same position as me heheheh. Also I think I’m going to move round the start of the heater wiring to the closest place to the transformer ie the el34s instead of the light where it is currently set up as a ring main and that should completely destroy any hum.

    Ill have to send some photos of the finished piece once ive tinkered a bit more 🙂

    Thank you very much for your help Ben

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  3. Hi Richard,

    I tried swapping the Mullard EF86s with a new pair EF86 (can’t remember the make) and it was just a noisy. Decreasing the gain should reduce the noise in theory. I’m curious to hear if it works for you.

    I remeber having a chat about EF86 noise with John Chambers and he told be that EF86 valves built with a mesh (as opposed to plate) sheild tend to be noisier…

    Please let us know if moving the heater wires closer to the EL34 reduces hum.

    I’d love to see pictures of your amp, don’t hesitate sending some and I’ll put them here.

    Cheers!

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  4. Hi Ben.
    Firstly, thanks for this excellent information. I’ve got 3 (yes, three) of these amps and I’ve just started a refurb job on 2 of them. Given their age, they aren’t actually in too bad condition (the third is a basket case, and will need a ground up rebuild) I’m getting hiss and hum from both, obviously, and starting out by replacing the 500v dual 32uf cap.
    I’m also trying to source a couple of 350v duals, to replace those as well, but can’t find any. I notice that you’ve used 500v for the capacitor on top, not 350. Is that ok to do? (I’m not an electronics engineer, and while I know the basics and can work on these things safely, I’m nowhere near smart enough to know when different value components are appropriate / safe to use)
    Any help would be appreciated
    Iain

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    1. Hello Iain,

      Thank you for your message.

      The tension goes up to 500 Volts DC just after the GZ34 rectifier valve so you must use 500VDC caps here (or use a network of caps and tension dividing resistors as explained here at the bottom of the page). It’s always OK to use capacitors with a higher voltage rating, but not the other way around (if you do they will blow).

      The last smoothing capacitors can be rated at 350 VDC since the supply tensions is lower (around 200-300 VDC) in this section.

      Banzai Music has a wide selection of large electrolytic capacitors, it’s probably worth having a look at their online shop if you can’t find what you are after anywhere local.

      Hope this helps!

      Ben

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  5. Great advice again. Thanks. As an update – I’ve replaced the cap just after the rectifier, and the hum has immediately down to acceptable levels – you have to stick your ear right inside the speaker to hear it! There’s still a bit of noise from the preamp side, but again its acceptable – I think it’s more of a grounding problem with the guitar / lead than the amp. Having said that, I think that replacing the other electrolytic caps, all of which are probably 30 years old, , and a valve replace exercise on the ef86’s and 12ax7 will be as far as I need to go.
    Thanks again!

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    1. Replacing all electrolytic capacitors is a good idea indeed and will help reducing the hum and general performance of the amp. You could also replace all the other capacitors (especially the greasy aluminium foil ones). Replacing the resistor with metal film one should also help reducing noise and clarity a bit.

      As far as I recall the original design sets the EF86s in very high gain pentode mode, which has the drawback of generating a lot of white noise and making the EF86s very prone to produce microphonics.

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  6. Hi all, I am in need of the linear diatonic circuit. If anyone out there know where I can get one… Want to rebuild mine as its over heating now and I suspect it needs loads of new components, in particular the 96 ohm 4 watt bleed resistor across the large capacitor. Thanks.

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  7. Hi there
    Great site of information you got going here, it’s just what i need. Now im no pro sparkie but i know my way around an amp!. I’v had this concord for about 12 years, i was lucky enough to pull it out of a skip would you belive, it also had a cab with two 12″ goodmans speakers inside perfect!! happy days they were hahaha. Anyhow i’v been looking around the net now and again and i can only see concords that are built using two small circut boards! but mine is not like them and i can’t seem to find any info about it. Basicly the diference is that mine has no boards at all and is totaly hand wired. Now im sure the schematic will be the same but i was just wondering if you knew of any others wired this way. I was trying to contact John chambers but he’s not taking emails at the moment. If you would like to see a picture i can email it to you.
    Thanks.
    Mark

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