Here we go again that boring old subject of biasing. Anybody with half a brain can bung in a new set of valves, can't they ? Pull out the old ones, plug the new ones in. What could be simpler ? They are all the same make, all the same age, they all look the same so must be matched, right? They all light up, the amp works now, it didn't before. Don’t know anything about biasing, but if the amp works ok it can't be far out, can it?
Wrong. Wrong. Wrong. on all counts!!!!!
(Well almost ! See article at the end of this feature):-(
If your car's idle speed is set too low you will stall your engine every time you come to a halt.
Dangerous and a nuisance!! On the other hand if it’s set too high, it will run away and be difficult to stop. Also dangerous and a nuisance. At the extreme, running flat out very difficult to stop. If you do, with the engine running at that speed, somethins’ a soona’ gonna’ goa’ BANG!!! Oil and bits all over the road. That’s why the idle speed is set at about 600 to 800 RPM so the engine is running happily at idle.
The bias of your amp acts in much the same way as the throttle on your car.
Correct setting of the bias keeps your amp running happily, but you need to fit properly electronically matched output valves for it to run in balance and smoothly. Just like in your car’s engine, the cylinder compressions need to be equal. If you have one compression lower than the rest, the engine will misfire run lumpy and lack power and performance. If the compressions are all over the place, forget it fit a new engine. But new the engine will still need to be set up properly to give its best performance. But then everybody knows that, don't they?
Fitting matched valves ensures among other things, that they all draw the same amount of electrical current from the (the fuel system) power supply. Assuming the amp is otherwise ok, if the amp’s bias is set correctly or not, the matched valves will each draw an equal amount of current. This may be insufficient, the amp will sound horrible and run cold, may not even work at all. It may be to excessive, amp running very hot you can smell it is not right, probably hums, not tight. Its soon gonna’ go BANG!
Adjusting the bias to achieve the correct idling current is hugely important for good sound and longevity of your amplifier and its expensive new glassware.
Your amp tech, (if you can find one) will check the anode currents of your new valves, and set the amp to draw about 30 ma per valve, for EL34’s. It may be a little more 35 ma or a little less 25 ma. or anywhere in between, depending on the make of amp, and the anode voltages present. The bias is a NEGATIVE voltage applied to the control grid of each valve. Example. -44 volts. Increasing or decreasing that voltage by a small amount will affect the amount of current drawn by the anode. Just 1 volt difference in the bias voltage can make a difference of 10 or even more ma, of anode current. Just checking the bias voltage alone is not good enough, unless your new valves are of the exactly same electrical characteristics as the originals, (most unlikely) you will need to check the anode current, and adjust the bias voltage to achieve the correct setting. Just like adjusting that throttle on your car so you don't stall or over rev the motor.
If you read a Mesa Boogie owners manual, it will tell you that it is not necessary to re bias a Boogie amp when fitting new replacement valves. This is complete and utter bollocks, Boogies are no different to any other amp when it comes to replacing valves. What they actually mean is you must buy only Boogie valves with the same colour code as your originals and you will never need to bias your Boogie. If you fit vales of other manufacture (they don't make their own by the way) and the amp gives trouble, they will blame the valves not the amp. But you can bias it to fit valves of your choice. But as always, only fit high quality matched sets. I personally have found many Boogie badged valves to be of very poor match. And have you seen the prices? Not just my "BIASED" opinion try the following link for some interesting reading on the Boogie bias subject. 50 - Music Electronics Forum
Boogies have a fixed resistor that sets the bias voltage. It is not adjustable, it would need to be replaced with one of a different value to alter the bias, or better still convert to adjustable bias.
All valves of all manufacture vary enormously in their electrical characteristics. They are rarely matched by the factories, but are sold in large batches just as they come, off of the assembly lines, to dealers throughout the world. If you buy your valves from a reputable dealer, that dealer will test the valves on highly a sophisticated test rig, and record the details. In this way he can select pairs, quads, sextets, or more that have the same electrical characteristics. These are then sold to the likes of me and other engineers and shops as matched sets. Numbers or colour codes will be put on the boxes to identify the match. If you buy 4 valves all boxes should be marked with the same number, i.e. 68 ma or perhaps 65/10 or RED or perhaps BLUE. Whatever, they should all be the same. If not or there are no numbers or colour codes, then you ain't’ got a yourself matched set of valves. Put elm back and buy some elsewhere.
Out of a batch of a hundred valves, the tester would be lucky to find any more than about 12 / 15 matched pairs, even more difficult with quads and sextets.
The whole point of this article is as follows.
A young lad struggled into my workshop with a large Laney VH 100R amp head.
He said it just faded away and stopped working. Now alarm bells started ringing when he told me he had taken it to a highly respected Devon music shop. There they replaced his output valves with a brand new set of Electro Harmonics EL34’s. Nothing wrong with Electro Harmonics, but I asked, was it a matched set? Eh, was the reply. Did they bias the amp to suit, I asked? Eh, yea it has been working fine ever since was the reply. Until now I said. Better have a look then. As suspected the HT fuse was blown. What blows fuses? Answer, too much current. Why too much current? Answer amp not biased, valves not matched. The valves of only a few months old, had obviously been running too hot the bases were cracked and the graphics were discoloured.
I checked the anode current of each valve as the pictures below show. Remember a matched set should all read the same whether set correctly or not. These readings were 76.3 ma. 63.9 ma. 76.4 ma. and a whopping 90.4 ma. 26.5 ma difference between the lowest and highest. Was this a matched quad? Don’t think so do you? looks to me as though a pair were, but the other 2, well! I guess someone else might have bought a pair just like them!!!
This adds up to a total of 307 milliamps at idle. Should be around 120 ma. When the amp is being driven hard the current will go significantly higher, more throttle, more fuel. Finally the weedy little 1 amp fuse could stand no more, and just expired with sheer exhaustion!
The real solution here would have been to fit a properly matched quad of valves and to adjust the bias to obtain that magic 30 ma per valve setting, the anode or plate voltage being 475 volts.
What I actually did was to adjust the bias in order to bring the unmatched set of valves, within safer operating parameters. 24 ma on the lowest one with about 38ma on the highest one, the other two around the 30 ma area. Not the best scenario but kinder on the owners pocket, and he might just get a few more months out of this set of valves.
The pictures show the uneven Anode/Plate currents drawn by the so called matched quad of EL34's
Clearly from left to right V 1 and V 3 are an excellent match and were most likely in stock as a matched pair. Fine, but what about V 2 and V 4? Nowhere near a match. This guy was charged big money by this well known music shop for the supply and biasing of this set of valves.
The devices used to check the valves are adapters I made myself. They are plugged into the valve bases, and the valves are then plugged into them. The red leads are connected to the test meters where the anode currents can then be read.
This should be done every time you replace your outputclass valves/tubes. Adjustments are made inside the amp. A dangerous place to be poking about!
SCREEN GRID RESISTORS
Nothing to do with biasing, but I thought due to my reference to Boogie valves in the previous article, this was worth listing here.
Funny, but within a few days of me writing the above article, I was presented with a Mesa Boogie F 50. You may have gathered from the previous article, that I am not a lover of anything Boogie.
They are over rated, over weight, over priced and over here. There have been so many different models over the years, with all the different Marks, 1/2/3/4/5 A , B C & even C+ versions Symul-class Nomads, Rockets, Rectoverbs Tremoverbs Dual Rectifiers, Stiletto, Deuce, Trident, Road Kings, Lone Star, Blue Angel, The Maverick. The list goes on and on, all too complicated too many knobs and pull switches. The best of the lot was the Mk1. If they had a successful model wouldn't they stick with it? Look at the Vox AC30 for example. By the way, I use a Tweed Peavey Classic 30, the best 1 X 12 combo I have ever had, and believe me I have tried a few. It cost me all of £170 on ebay. I have even had a MK3 Boogie and a Nomad 45, I’ll say no more, someone is sure to get upset.
Now this F-50 only a short while ago, and at great expense was fitted with a new pair of Mesa Boogie 6L6GC STR430 valves with a yellow match colour. I didn't supply the valves, but was asked to check the bias. True to what Boogie say it was in spec.
(STR. Special Tube Request. I will look into that subject in another article!)
Now only a few weeks later the owner was complaining of low power and poor quality strangled sound. On removal of the chassis I heard something loose inside. As I tipped it on its side, it slid along the chassis. After a little shake, a resistor fell out on to the bench. It was one of the 470 ohm screen grid resistors. The resistor had overheated to the extent that it had melted its own solder and just fell out of the board while the amp was in use. I tested the resistor which was a wire wound 3 watt device, it was still in spec, I refitted it to the amp switched on to find out what was going on in there. I measured the voltage drop across the resistor which was soon getting hot. The volt drop was 2 volts. I then checked the volt drop across the other screen resistor, which was only 0.42 of a volt, just over 1.5 volts difference. WHY? Swapping the valves over transferred the 2 volt drop to the other resistor. Obviously we have a faulty valve drawing an excessive screen grid current. This would get worse under playing conditions.
I fitted a new pair of G-T 6L6 valves, straight out of the box the bias was spot on, both the screen grid resistors then had an equal 0.46 volt drop across them. Problem solved.
Marked by red arrow - the overheating resistor
Voltage drop across problem resistor
Voltage drop across normal resistor
Above - the test results. Below - the Boogie valves.
You can see the match code yellow printed on the bases.
All that glitters is not gold
During recent tests, I have found the Chinese Shuguang EL34B and the reddish brown based 6L6GC very durable, withstands high plate voltage and current ratings, sounds fantastic, in Boogies, Matchless & Marshalls. The Shuguang 6L6GC is now the only tube I will fit in the old Fender Silver faced Twin Reverb and the Peavey Deuce, Mace and VT series amps.
Any of these valves / tubes are of a decent quality and won't cost you an arm and a leg. They will withstand high plate voltages, and give good service at a fraction of Boogie and GT prices.
The Svetlana 6L6GC/STR387
The Tung-Sol 6L6GC STR
The Electro Harmonics 6L6EH
Valves manufactured by SOVTEK
The Russian Sovtek company
manufacture a range of quality valves, and sell them in their thousands to the
likes of Mesa Engineering (Boogie) and Fender, Groove Tubes, Electro Harmonics,
etc. With the individual company logo's printed on them, and at hugely
discounted prices. They are then marketed by those companies and sold on
to the likes of you an me in some cases at greatly over inflated prices.
To be fair the exception would be Electro Harmonics. These do retail at a fair
market price. There is a lot of hype about valves which makes it a
difficult choice for the end user. You can buy from any reputable
valve supplier, such as here in the U.K. Hot- Rox and
Watford Valves, Sovtek or Svetlana valves
at a fraction of the price of branded ones. They are of the same high quality
and spec as Boogie, GT, or Fender.
No amplifier manufacturer makes their own valves, they all buy from the major factories.
Above you see a group of valves. From left to right 1/ 5881/6L6WGC Sovtek Small base. 2/ Electro Harmonics 6L6EH. 3/ 5881WXT Sovtek normal base 4/ 5881/6L6WGC Fender. And on the right, 6L6WXT+ Sovtek. They are all the same manufacture. The cheapest and best buy, the SOVTEK brand
A Fender badged Sovtek valve. You can see the original Sovtek markings on the valve with the Fender logo on the other side.
Question: What is the difference between these two valves?
Answer: Just the price. Left about £22 a pair, right about £55 a pair
Next question ?
Most Boogie owners will insist on
Boogie badged valves purely because of all the marketing hype.
Maybe I should get some made up with Amp-Fix printed on them and do the same!
Another example below, this time we are looking at four identical Russian made EL84's
Four identical valves all made in the same factory
Same four valves with logos revealed.
Two Boogie, two Reflector
The two Reflector are NOS Russian Military 6n14n the n is the Russian symbol for P. The valves are therefore 6P14P and are amongst the best available. Often branded with someone else's logo, as above TAD is another and sold at greatly inflated prices as the EL84-M. Often 1980's stock all unused, sold off by the Russian Military as surplus stock to valve dealers in large quantities. If you have an AC30 or class A amplifier, if you can find them fit a set of these and it will sound like an AC30 should, the valves will safely handle the Class A load and the high vibration valves are subjected to in a combo cabinet, without the high price tag of branded valves. I have a customer running 6P14P in a Matchless DC30, he says it has never sounded better.
Specification information from Svetlana:
The Svetlana SV6L6GC is a glass envelope beam-power tetrode intended for high-power audio amplifier service. Close manufacturing specification tolerances and improved processing provide improved reliability and superior sonic performance. The Svetlana SV6L6GC is manufactured in the Svetlana factory in St. Petersburg, Russia, and is designed to be a direct replacement for any 6L6 type.
The Svetlana SV6L6GC features:
Design and construction based on the Sylvania 6L6GC/STR387. Extra-rugged construction for use in music amplifiers - thick mica spacers and extra bracing reduce microphonic effects and resist mechanical and thermal shocks. Increased peak cathode emission from new cathode materials. Gold-plated grid and extended processing and aging for stability and reliability. Tri-plate anode for superior dissipation. Precise grid/screen alignment. Comprehensive static and audio amplifier testing before and after aging. May be operated in inverted position - base fits into socket clamps in Fender guitar amplifiers
MARSHALL use EL34 valves manufactured by Svetlana and market them under the Marshall brand name. You can again buy these from valve suppliers, with only the Svetlana name on the valve. But just to confuse you, there are two types of Svetlana EL34. The cheaper version, I understand is made by Sovtek, who acquired the rights to the brand name. Nothing wrong with these, I use them all the time.
The other more expensive and original Svetlana as used by Marshall, is known as the WINGED LOGO version, and is arguably a better sounding valve. A matter of choice either are ok by my ears.
The Sovtek manufactured
The Original Svetlana
Above copied from
The Tung - Sol 6L6GC STR
I am sorry, but I drifted well off the original subject of biasing as one thing lead to another. But whilst on the subject of valves, I just felt these things were worth a mention. I hope you found the information useful, but remember it is just my opinion.
As you probably know, there several methods that can be used to bias an amp. Which ever way it is done, the purpose of it is to insure the output valves are adjusted to operate within the anode / plate current range they are designed for. This will ensure long life of the valves and a good tonal response.
Some like them hot some cold. Without getting too heavy with maths and algebra, a tech from a market leader amp maker (no names mentioned) who tends to bias on the hot side, told me this.
The maximum output from an EL34 is about 25 watts. So measure the anode voltage of the amp, for this exercise we will assume that to be 445 volts. Divide the 25 by the 445 then multiply by 0.7 the answer in ma. is the 70% plate load bias setting required for that amplifier.
The 0.7 means 70% Hot anode / plate dissipation. Therefore you can use 0.5 (50%) as a Cool AB setting, 0.6 (60%) as a Warmer AB setting the one I would prefer. Or go for the 0.7 or even 0.75 (70 - 75%) if you like it hot.
Example: 25 / 445 X 0.7 = 39.3 ma. Bias
The bias figure will depend on the anode voltage which will vary according to the current drawn by the valve. More current less volts, less current more volts. So you may need to do the calculation several times if large adjustments are made.
Example 2. 25 / 480 X 0.7 = 36.4 ma. Bias
Example 3. 25 / 410 X 0.7 = 42.6 ma. Bias
If looking for the Class A requirement, that's 100% plate dissipation. The figure to multiply by would be 1 but check the valve is operating within recommended plate voltages. Here is an EL34 example with 300 volts on the plate / anode.
25 watts max o/p of the valve divide by plate volts 300 multiply by 1 = bias current requirement for class A operation.
Example. 25 / 300 X 1 = 0.083 = 83ma. or 25 / 350 X 1 = 0.071 = 71ma.
No rocket science - dead easy elementary stuff.
If you have a cathode biased amp, you can measure the bias current drawn by dividing the voltage drop across the cathode resistor, by its resistance. Example Cathode resistor 470 ohms voltage measured across it 25 volts.
25 / 470 = 0.053 = 53ma
53 ma flowing through the cathode resistor is the sum of the anode and the screen grid currents together. Deduct about 5% screen current 2.65 subtracted = 50.35ma anode current.
I will talk more about cathode bias later, but as a general rule a bigger value resistor the valve will draw less current, but the plate voltage will rise. Lower value resistor the valve will draw more current, the plate voltage will drop.
Update 26 March 2017
I did mention in a previous final paragraph a new device
called a TubeSync "Bias Engine", a very clever "intelligent device" invented and
marketed by an enterprising BRITISH company, KBO Dynamics Ltd as an aftermarket
accessory. It was basically an amp tech sitting inside your amp constantly
monitoring the bias current of each of your output valves / tubes. Should
any valve drift out of spec the onboard tech would readjust it to the
correct bias current automatically. Under normal circumstances if a valve
developed a fault condition the anode / plate current might rise to the extent
that at worst, might damage your amplifier or at best blow a fuse. Either way your amp
died on you and you would need to get it fixed, which might well cost a couple of
hundred quid or more. The on board tech, the "Bias Engine", would have
detected such a fault and shut down the faulty valve before it caused any
further trouble. An LED would illuminate indicating which valve was at fault.
You could then simply replace it with another of the same type of any make and
providing it was a good one, the Bias Engine would automatically set the bias to
match the remaining valves and the show would go on.
Bung in a new pair or a quad no biasing needed, the onboard tech will do that for you at no extra cost, possibly saving you 30 quid or more any time you replace the power valves.
An amp with 4 output vales would just keep working because the faulty valve would automatically switch off and the amp would keep running on the other three, the faulty one indicated by the illuminated LED. Other than a slight drop in volume, you might not even notice that anything had gone wrong!
The down side was that these things didn't come cheap and it might take an experienced tech the best part of a day to install it. Maybe a couple of days for the less experienced, possibly up to a week for the DIYer. The majority of guitarists work for peanuts and couldn't justify investing hard earned money in such a device, and sadly because of the lack of that demand it now means the TubeSync Bias Engine is no longer available as an aftermarket accessory. IMO a great pity because, in my experience as a registered installer, the backup from KBO Dynamics was first class.
Copyright © John Beer March 2017 All rights reserved