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MARKET CENTRE Cash in your surplus gear. Advertise it here in SILICON CHIP FOR SALE tronixlabs.com - Australia’s best value for hobbyist and enthusiast electronics from adafruit, DFRobot, Freetronics, Raspberry Pi, Seeedstudio and more, with same-day shipping. PCB MANUFACTURE: single to multi­ layer. Bare board tested. One-offs to any quantity. 48 hour service. Artwork design. Excellent prices. Check out our specials: www.ldelectronics.com.au PCBs MADE, ONE OR MANY. Any format, hobbyists welcome. Sesame Electronics Phone 0434 781 191. sesame<at>sesame.com.au www.sesame.com.au LEDs, BRAND NAME and generic LEDs. Heatsinks, fans, LED drivers, power supplies, LED ribbon, kits, components, hardware, EL wire. www. ledsales.com.au PCBs & Micros: SILICON CHIP can supply PCBs and programmed microcontrollers and other specialist parts for recent projects and some not so recent projects: www.siliconchip.com.au or phone (02) 9939 3295. KIT ASSEMBLY & REPAIR VINTAGE RADIO REPAIRS: electrical mechanical fitter with 36 years ex­ perience and extensive knowledge of valve and transistor radios. Professional and reliable repairs. All workmanship guaranteed. $10 inspection fee plus charges for parts and labour as required. Labour fees $35 p/h. Pensioner discounts available on application. Contact Alan on 0425 122 415 or email bigal radioshack<at>gmail.com KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith 0409 662 794. keith.rippon<at>gmail.com DAVE THOMPSON (the Serviceman from SILICON CHIP) is available to help you with kit assembly, project troubleshooting, general electronics and custom design work. No job too small. Announcing Pioneer Hill Software SpectraPLUS 24bit DAQ ADC spectrogram, t.h.d. and i.m.d. analysis, f.f.t, acoustic tools, 3D surface plot, sig. gen. etc. Fully shielded SpecctraDAQ200 ADC/DAC 24bit/192kHz dual channel, Wolfson. AKM converters … USB3 interface to laptop/PC As 2ch. 24bit recorder t.h.d. = 0.002%max see : www.spectraplus.com Order direct, USA contact : John Pattee (pioneer<at>spectraplus.com) Local agent : DSCAPE Melbourne s/w , h/w package ca. USD $1500 Aus. Distributor : Julian Driscoll CEO jcdrisc<at>tpg.com.au for support Based in Christchurch, NZ but service available Australia/NZ wide. Phone NZ (+64 3) 366 6588 or email dave<at> davethompson.co.nz WANTED WANTED: EARLY HIFIs, AMPLIFIERS, Speakers, Turntables, Valves, Books, Quad, Leak, Pye, Lowther, Ortofon, SME, Western Electric, Altec, Marantz, McIntosh, Tannoy, Goodmans, Wharfe­ dale, radio and wireless. Collector/ Hobbyist will pay cash. (07) 5471 1062. johnmurt<at>highprofile.com.au ADVERTISING IN MARKET CENTRE Classified Ad Rates: $32.00 for up to 20 words plus 95 cents for each additional word. Display ads in Market Centre (minimum 2cm deep, maximum 10cm deep): $82.50 per column centimetre per insertion. All prices include GST. Closing date: 5 weeks prior to month of sale. To book, email the text to silicon<at>siliconchip.com.au and include your name, address & credit card details, or phone Glyn (02) 9939 3295 or 0431 792 293. Ask SILICON CHIP . . . continued from page 94 they are running very hot and the heat melts the solder. The HT voltage that feeds the valve plate was 282V and the cathode voltage was 47V. I double-checked all the components around the circuit and they are correct. Would you be able to advise me what the problem might be? (M. M., via email). • 47V sounds like a very high cathode voltage; it’s designed to run at about siliconchip.com.au 22V – see the circuit diagram on page 31 of the November 2014 issue. Assuming that they are 330Ω as specified, those resistors would be dissipating around 7W so it’s not surprising that they’re overheating. Could it be that the two small wire links next to LK4 and LK5 are missing from the PCB? These connect the 6L6s’ grid bias to GND. If the grid bias is floating, that could explain the high plate current. The only other explanation we can think is that you’ve used slightly different valves which have a different grid bias requirement. If you’re still not sure what’s going on, check the voltage on pin 5 of the 6L6 sockets (ie, the grid) which connects to one end of the 10kΩ bias resistor between the 6L6s and the 12AX7s. This should have a DC potential of 0V relative to a convenient ground point (eg, valve socket mounting screws). If everything seems correct, you could increase the 330Ω 5W resistors to say 680Ω 5W to reduce the operating current to a safe level however they will still be dissipating more than the circuit was designed for due to the SC higher cathode voltage. February 2016  95