I have just pre-assembled the amplifier to test and setup the valve bias. All test voltages were OK. I fitted the 2 x 8 ohm load resistors to the output terminals. With all bias switches off the extreme right-hand LED12 is permanently on. Upon switching bias switch 1 down (on) to set the bias voltage for V1 LED12 went off and I slowly turned RV1 clockwise. I ‘overshot’ the desired LED 2-3 ‘on’ position and was about to turn the RV1 anti-clockwise to start again when R99 (10 ohm 1w) resistor blew (burnt-out and went open circuit). Could over adjusting the bias voltage cause this resistor to fail? I subsequently turned the amplifier back on again and left it for 10 minutes and tried to adjust other valve bias settings but the whole bias-adjustment function seems to have gone into a strange state with most of the LEDs glimmering/flicking. The supplier of the kit is sending me a replacement resistor but can you advise on this as I don’t want to risk blowing another resistor. Is it likely that IC1 has been damaged?
After replacing the resistor, please remove all EL34 valves and measure the negative bias voltage for each valve.
Make sure it is at approx -50V when the bias is set to minimum. Next, adjust bias and check if voltage can be adjusted smoothly, without interruptions.
CAREFUL: THE VOLTAGES IN THIS AMP CAN KILL. PLEASE BE EXTREMELY CAREFUL WHILE MEASURING ON A LIVE AMP.
I will do, before receiving your reply I fitted 5W 10 ohm resistors to all the bias circuits (rather than the 1/2w you supplied). About 60 seconds after switch on the bias resistor for V1 went white hot and blew! V1 EL34 also glowed brightly thus I suspect that this is also damaged!
I’m not a happy chap as this kit was circa £1000.00!
Sorry to read that you experience problems.
We can understand that you are frustrated, however, please don’t blame the kit.
A stunning 95% of all returned kits have assembly problems.
Manufacturing problems are very rare, and if they occur, we fix them for free.
Please check your complete circuit very carefully. If you lack knowledge or experience, please seek local help.
We do offer inspection/repair, but that would require that you return the unit.
I built a kit recently and are very pleased. Sound like you have a problem with the negative supply or a dry joint to V1 gate. Don’t upgrade the resistors they are designed to go O/C in event of a fault and limit damage. You may have had a faulty valve but check the pots do adjust the grid voltages for about -25V to -50V before fitting any valves. If you have 0V on the grid the EL34 will go into full conduction. In any case that valve is broken now. Don’t understand why you see the bar graph like that unless one of the supply electrolytics is faulty/wrong way round producing hum.
I have found that the output transformer primary wire colours shown in the assembly book are reversed for both channels. VI/V2 and V5/V6 are physically wired to the green and violet wires of the primary windings and and V3/V4 and V7/V8 are wired to the orange and brown wires i.e. the labeling on the PCB needs revising. I don’t think this will make any difference to the operation of the amplifier but it did confuse me when fault-finding.
That explains why the spice simulation of the amp showed positive rather than negative feedback. I just assumed the transformer windings were reversed. Thanks.
Hello, just to say that the problem is fixed and to thank you for your help. The problem was a bad joint on the bias pot for V1. After re-soldering I setup the whole amplifier (bias etc.) When I connected speakers I was amazed how quiet the amp is (hum etc.)
I have epoxy-glued the 8 x bias pots to the circuit board as I think when exerting pressure with a screwdriver during adjustment could cause future joint/connection problems.
Thanks again for your help.
Super to here that did the EL34 survive! I think you should replace it anyway (get the same manufacturer). I would suggest you change the screen grid resistors to around 500R 2W from 180R to prevent any parasitic oscillations around the EL34 output valves and check the heater voltage. Mine was out of spec (7.1V) of the valves with correct mains voltage and voltage selection set. Both mods will protect the output valves from long term damage. I added a 0.1R 6W resistor in one leg of each of the heater leads from the transformer. Its also worth writing down the grid voltages for each valve and then swapping them around to get the best matched pair in each leg (although this does not balance mu) to reduce second harmonic distortion. If you do this remember to reset the pots. The valves supplied are fine for the amp. It will also work with KT77’s but you will need more negative bias which will require a couple of resistor changes.
Hello - no the EL34 did not survive (I’m not surprised - it glowed very brightly as it failed!). The next part of the project is to re-build my Mullard 5/20 stereo amp. The unit I built comprised two 5/20 power amps with the Mullard stereo pre-amp as the ‘front-end’ all built one chassis. The pre-amp comprised a pair of EF86’s per channel. It sounded great but ran very hot. I propose to rebuild just the pre-amp part of the system and use that as the ‘front-end’ for the K4040. As part of the rebuild I’m incorporating a Bluetooth receiver and (possibly) an MP3 dock. The pre-amp will have its own power supply incorporating DC valve heaters.
Thanks for your help and interest.
[quote=“baudouin”]Super to here that did the EL34 survive! I think you should replace it anyway (get the same manufacturer). I would suggest you change the screen grid resistors to around 500R 2W from 180R to prevent any parasitic oscillations around the EL34 output valves and check the heater voltage. Mine was out of spec (7.1V) of the valves with correct mains voltage and voltage selection set. Both mods will protect the output valves from long term damage. I added a 0.1R 6W resistor in one leg of each of the heater leads from the transformer. Its also worth writing down the grid voltages for each valve and then swapping them around to get the best matched pair in each leg (although this does not balance mu) to reduce second harmonic distortion. If you do this remember to reset the pots. The valves supplied are fine for the amp. It will also work with KT77’s but you will need more negative bias which will require a couple of resistor changes.
Enjoy.[/quote]
Hello …YES YES…Just wanted to point out I discovered .1ohm resistor fix a LONG time ago…seems all of the kits trannies put out way too much (as high as 7.7VAC). with the resistor inline for each side, I run at about 6.59VAC …well within spec …If I added another, it would be too low…sooo
This is what I think BURNED a lot of EL34s. Personally, I have 8 6L6GC’s (GE) from 1961 that are incredible…