- Post operation debriefing
- Tweaking/improving the amp
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.
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!
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…).
Extremely 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
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.
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 ;).
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).
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.
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.
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.
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.
Richard kindly send me pictures of his Linear Conchord. Nice paint job and very neat wiring indeed! Thanks!
- 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!