How to bias the AVC (aka V.A.L.) EL34 integrated amplifier
First a quick summary of circuit types:
There are three main classes of biasing. In a tube circuit that uses FIXED-BIAS (whilst producing music, the bias is fixed), the grid-bias voltage is supplied from a power source external to the circuit. These normally use a separate power source to supply negative voltage to the grid (sometimes from a battery). In practice, it is even more convenient to provide the grid-bias DC voltage for the valve from the transformer itself, by insertion of a resistor, Rk, in the cathode lead (as opposed to using a separate battery cell). The polarity of the voltage developed across this cathode resistor is negative to the grid. So the grid itself is receiving its -ve bias from a transformer output (where the voltage delivered by the separate transformer source is altered via a trim-pot thereby altering the level of -ve DC voltage to the grid...the trim-pots are used to alter the bias when being "serviced"...the bias does not get continually altered whilst the amp is making music in normal use, so it is still a fixed-bias amp). The cathode is grounded so it is sometimes called a COMMON CATHODE circuit.
2 other types of amps use forms of SELF-BIAS (i.e. they bias themselves whilst operating): The first self-bias type is called CATHODE BIASING (such as used in a triode circuit for instance). In these circuits, grid voltage is kept at ground potential and the cathode is made to go positive relative to the grid. The effect of this is the same as making the grid negative relative to the cathode. Because the biasing resistor is in the cathode leg of the circuit, the method is called cathode biasing. Also, the grid is grounded in these circuits, so it is also called a COMMON GRID circuit. These circuits are used for triode valves and produce less power than fixed bias circuits.
The second self-bias type is called GRIDLEAK BIASING, which makes use of a portion of the input cycle to -ve bias the grid. As a result, this is also known as a cathode follower. With pentodes, the screen bypass g3 must be connected to the cathode and the anode is grounded. Sometimes called a COMMON ANODE circuit.
Without going into too much detail, basically the AVC amplifier has a -ve DC voltage (bias) applied to Grid 1, and it is this that is altered via the trim pots (i.e. it is an ajustable fixed bias (common cathode) Class A/B amplifier actually giving 30-35Wpc). It is NOT a non-adjustable fixed-bias amp, it is NOT Class A, it is NOT 40 Wpc.
Before I go any further VALVE AMPLIFIERS CAN KILL YOU. VERY VERY HIGH VOLTAGES ARE GENERATED INTERNALLY. I ACCEPT NO RESPONSIBILITY IF YOU DECIDE TO FIDDLE AND END UP FRYING YOURSELF. IF YOU DO NOT UNDERSTAND WHAT I HAVE MENTIONED SO FAR ANYWAY, THEN IT'S PROBABLY BEST YOU LEAVE BIASING TO A TECHNICIAN.
YOU HAVE BEEN WARNED !
N.B. This is version 2 of "how to bias a valve amp". My original v1 relied on you buying a quad of precisely matched tubes, but I have reliably been shown that even the best matching in the world isn't perfect. As a result here is v2. This takes into account the fact that you won't need to specifically match all 4 valves and is therefore a slightly more "robust" approach to correct biasing. Thanks goes out to Murray Johnson over at the 'wam for taking the time to help me.
BIASING YOUR VALVES v2.00
TOOLS REQUIRED
Digital multimeter
Long-handled slotted screwdriver
Philips screwdriver to remove the metal base of the amp
BASIC CALCULATIONS
V=IR These amps run correctly in class A/B push-pull configuration when the current draw of the valves is at 39mA (this ensures reliability and longevity). The resistors the current runs through in the circuit are 10Ω. Typing these into the calculation means that we want a meter reading of 10x39 = 390mV DC.
PROCEDURES
• Making sure the power switch is in the OFF position and THE VOLUME IS TURNED ALL THE WAY DOWN.
• Install all the tubes to the corresponding sockets using a gentle rocking motion.
• Connect your speaker leads, input cable and power cord.
• Turn the Amp upside down and remove the base. Turn the amplifier ON.
• LEAVE TO WARM UP FOR 45 MINUTES.
• Turn on the multimeter. Select the section for measuring up to DC 2000mV.
• Attach and secure the negative test lead to the chassis or to a -ve binding post.
• Now insert the positive test lead onto the EL34 base IN THE V1 TUBE SOCKET (pin1 is the Grid 3 point that is soldered to the 10Ω resistor see the red arrow in the picture below).
• Carefully insert a long-handled slotted screwdriver into the bias adjustment pot corresponding to the EL34 tube socket you're on (see the purple arrow in the picture below).
• Slowly rotate the screwdriver to adjust the bias voltage until it reads '390' on the multimeter (which means 0.39V on the setting we have it on).
• Repeat for all remaining EL34 tubes.
• You will need to return a few times until all four tubes are properly biased (there is some interaction, so re-adjustments back and forth may be needed before the voltages are all the same).
• Once you have finished biasing all four EL34 tubes, wait a few minutes and recheck the settings again.
• At this point, if a valve(s) continues to drift, it probably needs replacing - large amounts of drift is the sign of a bad tube, small amounts the sign of mains variations.
• You have now biased your amp. Switch it off and CAREFULLY screw the base plate back on and turn over (it's HOT). Relax and Enjoy!
The advantage of measuring the grid 3 voltage directly in this way is that you are directly calculating the current of that particular tube, and hence the current being supplied by the transformer. Also, in this amp, remember that grid 3 is pin 1 that is directly connected to pin 8, which is the cathode. The other leg of the resistor is soldered onto pin 6, which is grounded - hence the black lead coming off it (I have labelled the pin numbers on the valve base for you to make it easier to read).
A more precise method would be to measure the exact resistances of each of the resistors beforehand and calculating the individual voltages required for each of them to give 39mA....however, this is probably wasted here, where the resistors are only accurate to within tolerances of up to 10% anyway (if you do want to do this, measure the resistances beforehand when the amp is still switched off - the procedure is otherwise the same).
Valves are EL34 power tubes (6CA7 are worse quality), pre-amp section is GE JAN 5670 (which are basically 2C51 valves. 6N3 are worse quality and 396A are best quality). I think the Svetlana EL-34 valves are a very good make to try. Pre-amp valves do not need to be biased.
If you follow the above procedure, you do not NEED to go back to specific/unreliable manufacturers for tubes but can order 4 pre-amp tubes and 4 EL34's from the net and bias the amp correctly after replacing. I repeat, there is NO NEED to rely on any specific company's customer service in order to maintain your amplifier once it's out of warranty (over 2 years old). Feel free to try other makes of EL34/5670 and experience the different sounds on offer from different makers. Just make sure you order similar type tubes specifying they'll be used in a common cathode Class A/B, -ve fixed-bias amp and "micro-adjust" the amp as necessary following the above....DON'T ELECTROCUTE YOURSELF.
P.S. On his train wreck of a website, the owner of AVC seems to have an issue with other better valve amps. Just for the record, I own both PrimaLuna and AVC amps, and at 20Hz, the AVC is pretty gutless c.f. a PrimaLuna (and plenty of others agree with me).
There are a number of advantages to the hand-built PrimaLuna over the cheaply mass produced AVC. Firstly, the PrimaLuna uses ONLY point to point hardwiring in the signal path, unlike the AVC which has a large, cheap PCB holding most of the circuitry. Hardwiring in the PrimaLuna allows for more mechanically robust contacts, and a better signal propagation, which all means a vastly improved sound. Learn more about these advantages by taking a look inside the PrimaLuna.
There actually is a circuit board inside the PrimaLuna, but this is not in the actual signal path. It is a circuit that allows for continued adjustment of the bias "on-the-fly" (AutoBias) so that the tubes are each running at their own individual optimums. It's far superior to standard fixed-biasing, which is what the AVC uses.
In fact, as Lim hasn't even tried a PrimaLuna before commenting on it (Oh dear Oh dear!) I thought I'd show you an idea of his...erm...professionalism, shall we say, from his own site (with his own emphases). My comments are in brown:
"Unfortunately, when the hurricane hit America, the American computer stopped working"what, all of it? How can you tell who is 100% honest, and who is "bent" ?
"WARNING = BEWARE OF COUNTERFEIT ITEMS FROM CHINA"what, like the AVC amp?...oh, sorry, that's (ahem) 'yours' isn't it? (see below) "This is not a con to get money out of you"
"Since the amplifier is two and a half years old (out of guarantee), and it was his own stupid fault, we told him what we thought of him, and we do not like to be blackmailed. Do you think it would be fair to call him a liar and a blackmailer ?"referring to one of his own customers "It is strange that some people will listen to what some idiot say on some Internet forum"
"You MUST buy replacement valves from us"why? you've seen how to adjust bias for any EL34 above "We do NOT support second hand amplifier, that you bought from somebody else."there goes your 2nd hand value "Just compare the size of OUR transformers"
"This is not a con to get money out of you"
"I thought about this point, looked at all the safety aspect of how can I design an amplifier"He didn't design it. Looking inside there's nothing special/different about what he sells (despite what he claims...notice the same (massive massIVE MASSIVE) transformers used in the mass produced V.A.L. VP-100). V.A.L. supplies his amps as is. "The problem is that most "fashionable" gurus do not understand electronics, AND THEY ARE TOO LAZY OR TOO PROUD TO SIT DOWN AND READ A TEXTBOOK"
"Fashionable" false guru cannot explain any technical details, BECAUSE THEY DO NOT HAVE A BASIC UNDERSTANDING OF VALVE AMPLIFIERS"now now, play nice "We are fed up of idiots who tamper with our amplifiers. WHY THE BLOODY HELL DO YOU WANT TO TAMPER WITH OUR AMPLIFIERS !"see above "I would like to the valuable America eBay customer for his questions"eh? "HUGE HUGE HUGE POTTED OUTPUT TRANSFORMERS"
"Please stop messing about with DIY (Do It Yourself) speakers + buy our GBP £650 speakers"
"This is not a con to get money out of you"
etc etc......CAVEAT EMPTOR
Svetlana EL-34 valves are a very good make to try.