View Full Version : Valve amp bias
Anyone near Brighton that can do this please ?
Firebottle
13-02-2019, 12:09
Might be helpful to say what amplifier.
You might be able to learn how to do it yourself, are there accessible controls and test points?
Hi Alan,it’s a kt88 as in my sig, John wood told me the bias is set at 30 me but I can’t for the life of me remember where he showed me to put the meter probes, I know roughly, I’ve emailed him to ask but thought I’d see here as well. I will post a close up pic later if that helps.
I’d like to check the bias because I seem to have a slight drop in gain and after he adjusted the bias last time it sounded great, the power valves according to him were about 63% but I wonder if some new ones might help?
I should add it’s a fixed bias and it’s adjusted by a small screwdriver .
Firebottle
13-02-2019, 14:15
The little blue 'trimpot' is where the adjustment is made, as you say.
I am assuming there is just one adjustment as you have said it is fixed bias, as opposed to individual bias adjustments for each valve.
The only drawback with fixed bias is that as the valves age they can become unbalanced, that is have different conduction.
Each valve has the blue adjustment pot, it’s knowing where to put the red and black bolt meter probs that I’m unsure about.i will post a better pic when I get home, I’m aware of the huge voltages hence why I’d like some advice first:)
here's a better photo,
https://i.postimg.cc/CKgHvbWF/IMG-3028.jpg
and close up.
https://i.postimg.cc/9Q7m1L5H/IMG-3026.jpg
Typically you measure the millivolts across the resistor, this gives the current in the circuit.
thanks,you mean the brown one or the blue one just behind the capacitor?
Firebottle
13-02-2019, 17:08
Second photo shows the sense resistor. It is 1 ohm, Brown, Black, Gold, Gold.
Connect the meter across each of these in turn and adjust the respective trimpot.
For 30mA you will measure 30mV (millivolt) on the most sensitive voltage range on the meter.
Job done ;)
My guess... would be the brown one.
Its in a very easy to get to position, something a good designer would do.
Second photo shows the sense resistor. It is 1 ohm, Brown, Black, Gold, Gold.
Connect the meter across each of these in turn and adjust the respective trimpot.
For 30mA you will measure 30mV (millivolt) on the most sensitive voltage range on the meter.
Job done ;)
My guess... would be the brown one.
Its in a very easy to get to position, something a good designer would do.
thanks guys oh and obviously dont use a metal screwdriver :) it actually sounds fine now so it might have been my ears but i will check to be sure.
AD Audio
13-02-2019, 23:42
30ma sounds very low for an amp with claimed 100watts per channel. It won't do any harm at that level but I'd expect 60ma to get the claimed output
Sent from my ONEPLUS A5000 using Tapatalk
I’m only going by what the maker told me, he also confirmed what has already been stated that it is indeed the brown resistor that the bias is measured from.
Could you tell me what doubling the bias would do in terms of what I’d hear, from the little I understand it would shorten the valve life, but if it brings a great improvement I could live with that.
AD Audio
14-02-2019, 09:05
The difference you'd hear depends on the amp design, your speakers and other factors. In my own amps I can hear the sound decrease in quality as bias is lowered from the optimum 'hot' spot for example. As for valve life, the Radford STA100 and TT100 run their valves at 60ma and valve life is decent.
But I'd check with John and go with his recommendation and /or ask him what he thinks about backing off the bias a bit. If the setting is in fact 60ma, backing off to 30ma is a huge reduction to around 17 watts dissipation per tube.
It’s the other way round,30 is what he told me to set the bias at. So I guess it would be upping it if anything.at 30 there is absolutely nothing I’m unhappy about, the sound is lovely,as always though there is that “what if” thinking lol
Bigman80
14-02-2019, 09:41
Do what JW said. You'll just end up spending large sums on replacement valves more often.
AD Audio
14-02-2019, 10:52
It’s the other way round,30 is what he told me to set the bias at. So I guess it would be upping it if anything.at 30 there is absolutely nothing I’m unhappy about, the sound is lovely,as always though there is that “what if” thinking lolWhat I'm saying is maybe 30ma bias is wrong and you misunderstood as 30ma bias will not get near the claimed 100watts a channel.
So I'm simply suggesting you double check with John and follow what he says.
Sent from my ONEPLUS A5000 using Tapatalk
I'm struggling to understand why a valve amp that is not self biasing has been set up so it is (seemingly) not easy for an owner to bias it.
I'm struggling to understand why a valve amp that is not self biasing has been set up so it is (seemingly) not easy for an owner to bias it.
It will be easy once you know how - calculate the bias current you want at whichever pin it is, tweak pots and measure the current represented as a voltage at each valve base. That's roughly how mine goes. I'm sure biasing instructions will be available for a John Wood KT88 online somewhere.
Audio Al
14-02-2019, 13:29
Hi Alan,it’s a kt88 as in my sig, John wood told me the bias is set at 30 me but I can’t for the life of me remember where he showed me to put the meter probes, I know roughly, I’ve emailed him to ask but thought I’d see here as well. I will post a close up pic later if that helps.
I’d like to check the bias because I seem to have a slight drop in gain and after he adjusted the bias last time it sounded great, the power valves according to him were about 63% but I wonder if some new ones might help?
Jamie should that read 30 mv ? and not 30 me ? :scratch:
Audio Al
14-02-2019, 15:23
I thought the JW KT88 amp was 70 WPC :scratch:
would that not explain the low MV bias?
if maker says 30ma, then leave it at that. he ought to know the best setting
anthonyTD
14-02-2019, 16:58
30 ma per valve is just the idle current ie; with no signal, the output stage is not class A, as you drive the amp with signal, the output valves will draw more current!
Hope this helps.
A...
well ive just done it,i set it with the amp on hooked up to a reciever and speakers connected,should i have done it without anything connected then?
1 thing i did notice is the 1 valve (no 3 from left to right) was out compared to the others,this was the same valve that was out when john adjusted it,does this point to a worn valve?
For future reference,
I just heard from JW, it’s 35ma for each tube.
Good on you; doing this kind of thing for yourself pays dividends, literally.
anthonyTD
15-02-2019, 09:15
For future refrence;
You need to adjust the bias with no signal, if your amp has a volume control, then turn it all the way off, and make sure what ever is plugged into the input ie; CD, Streamer, etc is switched off.
And just be careful where your sticking your meter probes!
Hope this helps.
A...
well ive just done it,i set it with the amp on hooked up to a reciever and speakers connected,should i have done it without anything connected then?
1 thing i did notice is the 1 valve (no 3 from left to right) was out compared to the others,this was the same valve that was out when john adjusted it,does this point to a worn valve?
For future refrence;
You need to adjust the bias with no signal, if your amp has a volume control, then turn it all the way off, and make sure what ever is plugged into the input ie; CD, Streamer, etc is switched off.
And just be careful where your sticking your meter probes!
Hope this helps.
A...
Good shout; the amp needs to be in a quiescent state. I'm not sure what the result of pumping a signal in while biasing - should the OP revisit and reset the bias do you reckon? I probably would.
For future refrence;
You need to adjust the bias with no signal, if your amp has a volume control, then turn it all the way off, and make sure what ever is plugged into the input ie; CD, Streamer, etc is switched off.
And just be careful where your sticking your meter probes!
Hope this helps.
A...
Thanks mate, yep I’m using crocodile clips and only using 1 hand etc.
Thanks mate, yep I’m using crocodile clips and only using 1 hand etc.
Mine was on its side, not particularly well lit and I was laying on my side! Also, I was having fun adjusting a pot while monitoring the bias current of a totally different valve!
As long as you don't get a bolt it continues to be fun.
You right it is fun, I also noticed when you set 1 valve it changes the bias of anther one slightly so you end up going back and forth.
You right it is fun, I also noticed when you set 1 valve it changes the bias of anther one slightly so you end up going back and forth.
Tweak, tweak, return to V1, tweak - V1 & V2 happy. Tweak V3, twek V4, return to V3, happy. Quick measurement - V1 is out, and it starts again!
anthonyTD
15-02-2019, 10:26
If the amp is being modulated with signal, then the steady idle current will not be correctly set, hence The reason for biasing with no signal present!
Again, hope this helps.
A...
Good shout; the amp needs to be in a quiescent state. I'm not sure what the result of pumping a signal in while biasing - should the OP revisit and reset the bias do you reckon? I probably would.
anthonyTD
15-02-2019, 10:29
No worries, just stay safe, there is very high DC voltage in there, especialy around pin three and four of the output valves, in excess of 400v !!!
The bias of each valve in the channel you are adjusting will change slightly, as said, its a balancing act, dont beat yourself up though for a fraction of a ma, its not that critical!
Thanks mate, yep I’m using crocodile clips and only using 1 hand etc.
Audio Al
15-02-2019, 12:35
I will have a go at doing my amp as its a loooooong way from London to Nottingham and back :eek:
al,
having done mine properly last night i can say yes do it if its not been done for a while.
very much worth the effort imo.
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