Tube Biasing

[Salamander]

This is forward thinking, I'm not changing my KT88's just yet, but I have been concerned about biasing since my amp does not have those easy to use top plate access holes, so it's a case of taking the bottom plate off. But with all those volts lurking about, and me no electrician I feel I've got good reason to be concerned.

A bit of research and I saw a BiasMaster master used in a photo so Googled that, $230

, a bit rich for me so the search continued.
Finally came up with a similar item on AE, www.aliexpress.com/item/33021031440.html?spm=a2g0s.8937460.0.0.1f442e0e2TqmwO and thought I'd ask to see what the 'experts' say.
My thinking was you don't have to switch off and then on 4 times (4 x KT88), I know I will have to take the bottom plate off to get at the bias adjuster, but I will take great care in this, and also I thought it would make life easy for simply checking the bias, hopefully not having to adjust every time. I don't own a multi-meter (honest, I didn't know it was obligatory to be a member of this forum) so I'm guessing a saving of $20+ has also got to be taken into account since it comes with a meter. I look forward to your replies, there maybe more questions to come when I actually have to 'get me hands dirty' and change the valves.

 

[Salamander]

I've copied the above text from another forum where I asked the question also. I would like to stress, I'm new to this and would like to make biasing as easy and SAFE as possible.

 

[Ian Austen]

Prodding around on a circuit board with hundreds of volts DC isn't something I would recommend. Spend a few quid to get a local technician to do it - better than electrocuting yourself to death.

 

[Tony L]

What amp is it? Did it come with good instructions or are such available elsewhere? If so I’d advise learning how to do the biasing procedure in accordance with the manufacturer’s instructions. Don’t fear it. Assuming you have a good safe multimeter with nice shrouded probes you shouldn’t fear a valve amp. Just understand there are high voltage areas inside and capacitors can hold high voltage for quite some time after unplugging the amp from the mains. That learned it should not be hard to achieve.

Depending on the layout of the amp I’d expect the process was to connect one probe lead to ground (e.g. the negative speaker terminal or a casework screw), you can usually do this and leave it fixed in place so you only need to worry about waving one probe around. Have the probe’s plastic shroud in place so the actual metal bit is very short indeed and you have far less chance to knock against something you don’t want to. Once you have a reading you’ll need to adjust a trim pot somewhere. This will likely have a head like a flat-head screw and my advice would be to use a plastic screwdriver or spudger (e.g. these) to tweak it, again to remove the potential to hit something you don’t want and short anything out or electrocute yourself. Basically you never put yourself in a position where you are doing anything other than holding insulated plastic handles.

You will need to leave the speakers or a similar load resistor connected to the amp whilst you do this.

PS I’d not cheap-out on the multimeter, the throwaway Chinese eBay landfill junk often isn’t safe or fails in odd and misleading ways. Worth buying a nice one as you’ll end up using it far more than you expect for a whole variety of tasks. Whilst I’ve got a Fluke 87V myself (painfully expensive, but I figure I’ll have it the rest of my life) I’d personally recommend Dave Jones’s EEV Blog Brymen BM235. I subscribe to his YouTube channel and his multimeter reviews are obsessively critical and leave absolutely no room to hide bad features or function, especially in regard to safety design. He put a lot of work into this and it will be thoroughly safe and highly accurate. Fluke quality at half the price.

 

[Salamander]

Thanks everyone for their replies.
Ian, it is not easy for me to find a local techie who is knowledgeable on valve amps since I am in Cambodia at present, the amplifier was purchased to use here.

Tony thanks for the extra info, I only went on the internet yesterday and came across a fairly detailed, with photos, how to bias an amplifier, in all fairness it was a guitar amp but never the less the procedure would be the same. The gentleman who wrote the article, which I did find helpful, pretty much gave the same info as yourself, although he did use a BiasMaster BIASMASTER System (tubeampdoctor.com) (this is the best photo of it, although it now retails at $230 which is a bit much for me) which simplified things in my opinion. Like you say Tony the negative lead was attached to the case and the the positive probe had to go to the appropriate valve, again only one hand going in and the other kept well out of the way, he suggested a pocket. My feeling was you could switch the amplifier off, yes, and leave it to discharge, remove valves and place those attachments in, followed by the valve. Then simply turn the amplifier over, remove the base plate, switch on and leave to warm up. It would save switching on and off 3 more times to complete the four valves. Reviews on the BiasMaster seem very good, once used, it becomes irreplaceable, and the reviews are also good on AliExpress. It is not easy to get suppliers to post to Cambodia, I don't know why, I purchased a pair of speakers on here, the gentleman was kind enough to pack them well, DHL pick them up from your own home and they were delivered safely. I have less trouble ordering from AE, odd items can go astray, but then you get a refund. I know some of these Chinese products are cheap and don't last 5 minutes, but the amplifier I purchased cost £515 delivered to my door, it's solidly built, sounds excellent and the headphone amplifier alone is worth the £500, it is that good. The problem for me is this biasing and of course no instructions. I will be having a look inside before replacing the valves, making sure I have easy access and that I can safely do the job. I have still to look on YouTube, I know there are some good videos on there, which should be more than useful.
Just seen the BiasMaster for 169 euros at Thomann.

 

[Tony L]

Personally if I was going to drop £150-200 I’d far prefer to buy a really nice safe multi-meter (e.g. the EEV linked upthread) which will have all manner of uses rather than a single use tool which to be honest doesn’t make things that much easier.

What is the amp? It will be designed to be biased so will likely have defined meter points, if not hitting the right pin on an octal base really isn’t hard, they are pretty big and the tags splay outwards.

Look at the valve bases on the far left and far right of this picture. This is the same type of octal base a KT88 plugs into. You only need to touch one of those tags with a shrouded probe lead to get a reading, it really isn’t hard. You could even use an oscilloscope-style clip-probe if you didn’t want to hold it. It is safe as all you will be holding is insulated plastic (definitely use a plastic screwdriver or spudger to adjust any trim pots, nothing with a metal bit).

PS I managed to probe every pin on every tube-base without issue, and the little 9 pin ones are way, way harder and many quite awkward to get to due to overhanging wires. It is possible, and I’m absolutely no expert.

 

[Ian Austen]

Thanks everyone for their replies.
Ian, it is not easy for me to find a local techie who is knowledgeable on valve amps since I am in Cambodia at present, the amplifier was purchased to use here.

Ah. Are there any musician shops around that sells/repairs electric guitar amps and the like? These are biased exactly the same way.

If you don't have an option, practice taking readings on lower voltage equipment until you're comfortable with the process. Avoid working with both hands in the amp. Stay safe

 

[Tony L]

Here’s an example of some ‘hands off’ measurement using clip probes. Worth getting a set if you are lacking in confidence or afraid of shorting anything. This way means you only need to concentrate on the meter reading and trim-pot.

 

[Salamander]

Thanks very much again Tony for some lovely clear photos and procedures. Since I last posted I've been giving this some more thought, and would like to verify something, that is can you have all the tubes in place before switching off and taking the base plate off? That would mean when you switch on again all you have to do is touch the pin on the octal plate, take a reading, adjust the bias accordingly with a screw driver, then move on to the second, third and forth tubes doing exactly the same. I've had it in my head that when you do the first one, the amp has to be switched off, the old tube changed for the new one and repeat, for the next three tubes. If my thinking is incorrect, my previous post would make no sense and confuse everyone who knows how to bias tubes.

 

[Tony L]

Depending on the amp (what is it you actually have?) it should be fine. You would only take the baseplate off once, go through and bias all valves, then put the baseplate back on. The amp would be either upside down or on it’s side depending on what it is and how it wants to stand. When I was measuring the little Leaks I had them upside down with a suitable ‘prop’ (actually a spice jar) holding the front up so they weren’t resting on the hot valve tops. With these particular amps the transformers are taller than the valves, so easy to flip over to work on. It depends on what you have. If it has a tube cage it couldn’t be easier!

 

[Arkless Electronics]

Just get a decent DMM! If the amp has instructions on biasing then it will likely have small cathode resistors to allow measurement of bias. The biasmaster and cheaper version merely replicate this.... at a cost... and are not required so long as there is a means of measuring bias provided. I actually built a DIY adaptor to allow adjustment of some Marshall guitar amps which don't have cathode resistors... it's just an octal plug and socket with a 1R resistor inserted in the cathode line and suitable lead out wires for the DMM.
With cheapo Chinese KT88's you'll be lucky if they last long enough to need rebiasing and they will probably drift like crazy anyway...
You don't need to remove the valves to do it the way I suggested. Just get it in a position where you can easily see and gain access to the relevant parts/test points. Allow at least 15 mins warm up before doing it... and don't slip with the meter probe!

 

[Tony L]

Just for clarity: this shows what I’m on about with shrouded probes. With them in place, as on the red lead, you are way less likely to hit other stuff or bridge anything. Chances are you won’t need to worry about the black lead as you can just connect it to the negative speaker terminal, use a crocodile clip on a case screw or whatever. It depends on the amp, which I would hope came with full biasing instructions.

 

[Salamander]

Many, many thanks again Tony, and to you Jez, I have said already, it never came with any instructions, I did message and even talk to a Mr Lee, helpful up to a point, I guess he cannot make the manufacturer supply instructions although he could do that himself and photo copy, putting them in the box before sending. He does test stuff before sending, this is the amplifier by the way, not from this seller though, but the photo is better: RFTLYS A2 KT88 Push Pull Tube Amplifier HIFI EXQUIS Integrated 12AU7 Lamp AMP With Bluetooth|Amplifier| - AliExpress
I do remember Mr Lee saying attach the negative to the casework, and I will buy an alligator clip for sure.
Earlier today I did some checking on multi-meter's, it would cost approx £50-60 to send to Cambodia, my brother could do this and also put in a few pre-amp tubes I've purchased , but before I do that, I know you will say, especially after all the help you've given me but here goes. Two I've picked out from AE: Free Shipping New Fluke 101/101kit/106/107 Handheld Digital Auto Range Digital Multimeter AC/DC Easily Carried mini DMM meter|digital multimeter auto range|multimeter auto rangemultimeter auto - AliExpress this would cost me $49 delivered and the second one: UNI T UT89X UT89XD Digital Multimeter True RMS AC/DC Voltmete LED NCV Professional Automatic Backlight Anti fall Tester|Multimeters| - AliExpress and this one $28 delivered. Just have a look, I know you have said, buying the more expensive one, it will come in for many little jobs, but I can assure you it won't, I rent a room here, and I don't really have to do any maintenance, although I do bits, but I don't have the tools to do most jobs
I'll finish by telling you, I think this amplifier is excellent sounding, the tubes, although Chinese manufactured sound great, it will only be when I swop the pre amp tubes I will really find out. I brought my NVA headphone amp out here, this cost £300 when new and is no slouch in the sq dept. but the headphone output on this bettered it by a long way, I was really surprised. I have messaged a couple of other people who have also purchased one of these so called cheap Chinese amps as well, both are delighted with them, just do your checking before you buy, not always that easy but there are a few reviews on YouTube now, which helps. I look at it this way, if the amplifier only lasted 3yrs, it will have cost me just over £3 per week, I hope it lasts me longer.
Should have added, I will purchase a insulated screw driver, after I have had a look inside and know what size to buy.

 

[karma67]

here's what i do,your amp may be different.
firstly the amp must be turned on and the speakers connected.let it warm up for 30 mins.
you can see 4 blue boxes in a row ( circled in red) ,these are the variable resistors,there is a small screw on top that is turned to adjust the tube bias either up or down.
just behind each one is a brown resistor,( see red arrows) (in front of the yellow cap) this is used to measure the bias, clip or touch the 2 legs of each resistor with the multimeter probes.the meter should be set to dc current and read in mv.if you get a minus number on you meter just swap the leads around on the resistor legs.
as i've already told you please be very careful and only use 1 hand,mine for example has 670v floating around inside!

 

[Tony L]

Two I've picked out from AE: Free Shipping New Fluke 101/101kit/106/107 Handheld Digital Auto Range Digital Multimeter AC/DC Easily Carried mini DMM meter|digital multimeter auto range|multimeter auto rangemultimeter auto - AliExpress this would cost me $49 delivered and the second one: UNI T UT89X UT89XD Digital Multimeter True RMS AC/DC Voltmete LED NCV Professional Automatic Backlight Anti fall Tester|Multimeters| - AliExpress and this one $28 delivered.

I’d personally go for the entry-level Fluke 101. It is a very simple, easy to use meter and I absolutely trust Fluke’s Cat III rating (i.e. it won’t blow your hand off if you do something really stupid). I used a similar Chinese market (they are legit) Fluke 17 for years and only replaced it with the 87V as there were a couple of other functions I really wanted (peak hold, min/max etc). Anything with a Fluke badge is accurate, solid and trustworthy. I’m not saying the other isn’t, but personally I’d go with the Fluke.

PS You may think you’ll only use it for this task, but you’ll end up using it for all manner of things, e.g. checking batteries, checking cable joints, turntable ground continuity etc etc. I’m no electronics engineer but mine gets used several times a week for something or other. Everyone should have a good multimeter IMO.

 

[Arkless Electronics]

I hope this is a silly question... but does the OP know that this is a fixed bias amplifier and therefore actually needs the bias setting and has provision for it? Just checking...
Up to about 50WPC ish it could well be a cathode bias topology like the Leaks referred to up thread and doesn't need adjusting (nor can it be adjusted).

Ah now looked at the link above and it does mention "bias adjustable inside the amplifier"... but it's only 35WPC so bizarre that it's even fixed bias! I guess with such cheap and dodgy valves it's about as much power as they can get without a valve life of 2 days....

Pin 8 of the KT88's is the cathode. Looking at the valve sockets from the bottom there will be a location spigot and it's the pin to the right of this. Pins 3 and 4 have potentially (ha!) lethal voltages on them so get this right. With no power applied and after waiting a good while after UNPLUGGING it (do not merely switch it off at the front panel!) measure the resistance between pin 8 and ground. You should find 1 Ohm (most likely) as the resistor across which a voltage, derived from cathode current, can be measured. Switching the meter to Volts or millivolts, turn on the amp and allow to warm up for 15 mins or more and then measure the voltage between pin 8 and ground on all 4 output valves. For KT88's about 60mA would be a pretty typical bias current figure and across the typical 1 Ohm cathode resistor this gives 60mV (hence why 1 Ohm is usually chosen), which is 0.06V.

A higher voltage in this measurement means more class A working and potentially better sound at the expense of less life from the output valves and a lower voltage vice versa.... Mind you this thing is likely so dodgy that it's quite possible the makers intend it to be set to only say 35mV (mA current through valves) to prevent valve life being a day or 6!

Whatever current you go for the idea is to match all four of them. Again due to likely dodgyness you may well find it drifts all over the place and that it "needs doing" every month rather than staying pretty steady and needing a quick check a couple of times a year as you would expect with genuine GEC KT88's when fitted to a quality amplifier.

 

[Salamander]

Thank you Jamie, Tony, and Jez, your replies are very much appreciated. I will order a multi-meter asap, I can buy one in Phnom Penh, but they cost less than they do on AE so I suspect the quality is not all that good. Just waiting for a new bank card, the bank aren't rushing to get me one, but that's Cambodia for you, but it does have other attractions. I'll take a look inside over the next few days, so I could well be asking a couple more questions then.

 

[Salamander]

I've ordered a 15B Plus FLUKE 15BPlus/17B+ 15B+Digital Multimeters Auto Range Multimeter With Temperature Measurement Function|Multimeters| - AliExpress
It should take between 16 and 28 days to get here, from past experience it could take longer than that, but who knows.

 

[Tony L]

Excellent, I had a 17 (not +) for a long while and it is a very, very good multimeter. You will not be disappointed at all. It will also be bigger than you are expecting!

 

[Salamander]

Hello again, well today I have tried to bias the the tubes. Things have not gone too badly, except having the Fluke set to mV's rather than just V's, and also where do I put the positive probe don't answer that) to get a reading. I did read on the net to use pin number 3 and then looked on this thread again and Jez's post pointing at pin number 8, well pin number 8 worked. I have tried to set the bias to 370, unsure whether or not that is best, and I could easily be 3 or 4 mV's out since the slightest turn either way could result in a 30 - 50mV change and also the reading would never stay in one spot. My only problem was number 3 tube, the maximum I could get was 250, does this mean that tube is on the way out and also if one goes do I replace all 4? All your previous replies have helped me a lot and to be able to at least try and do this task.