valve bias?

[falcoron]

Hi guys,

need a little help again on valves (laymans terms please)

This is regarding the 6550 output valves in my ARC VSi55 amp

I know that a valve amp can and should be bias set for optimum sound, but if i was buying NOS valves in matched pairs, What does that mean "matched"

And say i bought 2 different pairs, say from 2 different sellers and both are advertising as matched pairs, what are they matched to ( the curtains, only joking)

I know they are matched so they are close numbered, but that just loses me im afraid.

If i bought these 2 different pairs ( same make and model) does that mean they won't work as a quad in my amp? i need 4

Is this where biasing comes in?

I buy any quad of the right valve no matter what the matched number im given ( dont know anything about the numbers either) and i set them into my amp and set the bias to the recommendations, does that mean the quad are now matched and will perform well?

sorry guys but its only easy if you know! i don't!

Can anyone help?

cheers

Ron

[i_should_coco]

Valve bias sets the operating point for the tube, by adjusting the grid voltage, we change the current through the tube. In a push-pull amplifier, we have two tubes which are connected to the output transformer. The transformer has two coils, one for each tube, wound in opposite directions. What we want is for the current through each coil to match so that the magnetic fields cancel leaving no net field. If they do not match, this can cause the output transformer to saturate, increasing distortion.

Generally, there are two parameters we measure for in tubes, plate current (Ip) and Transconductance (Gm). Tube can be matched for either, or both.

If the plate current is matched, then that means the tubes draw the same current for a specific grid voltage. However, this does NOT guarantee that they will match for any other grid voltage.

Matching for transconductance is more like matching the gain of the tubes. But, again, the transconductance of a tube varies with the plate current Ip, complicating things further.

For power tubes, the main thing is to match for plate current to avoid saturating the output transformer. Unfortunately, tubes that start matched often end up un-matched after some hours use. Some amps often allow you to adjust the bias on the tubes to compensate for slight mis-matches as tubes age. Ironically, this system is normally called "fixed" bias. Even is using matched pairs, you should check the bias ot make sure, as they're only guaranteed to match at the operating point they were tested at. The operating point in the amp may be different!

If you have two matched pairs, that is normally fine, as tubes only have to match each other in the push-pull pair.

BTW, matching is normally done to 10% or so.

[falcoron]

Thanks,

So if i got 2 pairs of say nos tubes and as long as the 2 are matched on the pull side and another pair matched on the push side all would be ok, even if they are different matches? eg if one pair was say rated at 70 and 72 and the other pair was 60 and 61 that would still be fine. Then set the bias to the manufacturers recommendations.

I am looking at 2 different pairs of tung sol 6550 valves they have differing sets of matching numbers but are being sold as matched pairs.

Thanks again for the info

Ron

[beckypumps]

Tung sol are good but also consider valve art 6550 or kt88. I have used them in ARC stuff and have 16 valve art kt88 now fitted to my monos.
You will have to put them on a valve tester to see if they match forget the numbers.

[falcoron]

Never heard of these are they nos or new modern valves? any links

where does one get the valves tested? can this be done with a shop bought tester or is it a specialist job?

Thanks

[Tel]

Ron.

What do you mean by matching numbers?

Are they NOS Tung Sol?

What markings are on them?

The matching valves should be fitted in the same channel one pair for the left channel one pair for the right channel

Edit : Just to add, if you have NOS Tung Sol you have the best 6550 ever made, better then NOS GE (and they are very good)

Be aware that there are new 'Tung Sol' being sold which have the same 'coke bottle' shape but the name and the shape are the only similarity to the original.

There is also a massive price difference

I think Valve Art are rebranded Shuguang modern production Chinese tubes

[i_should_coco]

falcoron wrote:

Thanks,

So if i got 2 pairs of say nos tubes and as long as the 2 are matched on the pull side and another pair matched on the push side all would be ok, even if they are different matches? eg if one pair was say rated at 70 and 72 and the other pair was 60 and 61 that would still be fine. Then set the bias to the manufacturers recommendations.

I am looking at 2 different pairs of tung sol 6550 valves they have differing sets of matching numbers but are being sold as matched pairs.

Thanks again for the info

Ron

Hi there, not on the "pull" or "push" side, but rather on each channel. So matched pair for left and matched pair for right will be fine.

[falcoron]

Cheers,

Sorry for the complete lack of knowledge. But i am enjoying the learning curve.

Is there a tube tester available at a resonable cost and easy to use?

Thanks

Ron

[i_should_coco]

Most testers are pretty expensive, sadly. They're only worthwhile if you have a lot of tubes.

If they're sold as a matched pair, they will have been matched on a tester anyway.

[falcoron]

Tel

Unfortunatly not tung sol but NOS GE 6550

This is the info i got when asked for test results;

These tubes were checked on our Amplitrex AT-1000 Digital Tester against G.E.'s specifications. 250 volts on the plate and 6.3 volts applied to the heater. A regulated power supply ensures accuracy on all tests. The results are as follows:

Tube 1: iP 140.0 (100%) Gm 11090 (100%)

Tube 2: iP 134.5 (96%) Gm 10800 (98%)


Again this means feck all to me but they are from a reliable tube seller, so i think im ok. garenteed original nos GE's

[Tel]

Do they look like the one on the left or the middle?

Should have 6550A printed in white as you can see on the middle one

'V' getter etc?

[falcoron]

no green JAN writting

this is them



I have these also,

Used but tested good



Cheers

Ron

[Tel]

I wouldn't worry about the JAN green writing, they are NOS GE's

Excellent 6550's

[i_should_coco]

"JAN" stands for "Joint-Army-Navy" and doesn't really mean anything. It was just a labelling exercise for the military. There's normally no difference between JAN and regular consumer tubes.

[falcoron]

What do you think of this little tool?
http://cgi.ebay.co.uk/Digital-Tube-B...d=p3286.c0.m14
would make bias setting a breeze i would think.
any thoughts.

if using a multi meter to set bias how do you do it?
my amp has little bias holes on the back and holes beside each valve to insert a special plastic screwdriver tool.

[Tel]

What does it say in the Operating Instructions?

My CJ has a similar plastic screwdriver (although a long terminal screwdriver is quite adequate).

There is an LED associated with each valve, I turn the screw to the right until the Led comes on then turn left until it just goes out.
Even after one is set, adjusting the other valve of the pair can upset the 'balance' of the first so a small amount of fiddling is required, but once set that is it for 6 months or so!

I didn't need any meters.

I'm not saying the ARC is the same but the two firms do a lot of similar things so it might be!

Check the manual.

[i_should_coco]

It looks like a neat idea, but only useful if your amp has no provision for checking the bias either with a built in meter (or some other system, such as the LEDs on CJ amps) or test points for an external multi-meter (which is available for 4.99 from Maplin and good enough for this purpose.)

[mrwippy]

falcoron wrote:

What do you think of this little tool?
http://cgi.ebay.co.uk/Digital-Tube-B...d=p3286.c0.m14
would make bias setting a breeze i would think.
any thoughts.

if using a multi meter to set bias how do you do it?
my amp has little bias holes on the back and holes beside each valve to insert a special plastic screwdriver tool.

These devices are actually a bit of a pain to use as when you bias one tube it (marginally) changes the setting of the other so unplug valve plug in socket, plug in valve, adjust, unplug valve unplug socket, plug in valve, unplug next valve, plug in socket.....well you get the idea, and then by rights you need to go through it again to fine tune (unless you were intending to buy 4 of course)! - also completely unnecessary as you have test points built into your amp.



I don't know the year of yours, and the only thing I could find on the database for bias setting was the note that

"..In 2007, VSi55 had their tubes changed from 6550EH (Electro Harmonix) to 6550C (Svetlana) at the production line. The output tube bias was changed to 55mV, providing better sonics and improved reliability..."

[mrwippy]

Sorry the above didn't answer the "how to" bit.

but essentially (after you read the manual, warm the amp up and have no signal going through it?) you plug the meter into the test point for the first valvewith it set to the correct scale (yellow = +, black = - at a guess) if it reads 55mV (based on the above post)move onto the next valve, if not insert plastic screwdriver into hole next to that specific valve and slowly turn and try to set it to 55mV, when done move onto the next valve and repeat - when all four are done, go back and recheck/reset/fine tune- how often you repeat this is up to you or as instructed in the manual.

All the above is just based on the fact that I've only ever had valve amps that required user bias (so called fixed bias)and I must stress I don't consider myself to be a technical person.

Did you get the manual with it? If not you should try and source one as I consider, especially with valve amps, the manual to be an essential part, more so than say the mains lead!

[falcoron]

yes i have the manual but if not a techy its a bit confusing. What you have told me is what i had thought, just wanted to be sure. It seems pretty straight forward.

red plug from multi meter into the yellow hole and black plug into black hole.

see result and adjust.

spoke to arc yesterday and they said i could adjust from 55mv to 65 mv according to the sound i want. New svetlana valves are set at 55mv

55mv will be warmer and will make the valves last a long time

65mv will be a more forward sound and shortens the valve life. My amp is a tad too farward so will be interesting to see what its set at.

So next, (and hopfully last) questions

which multi meter should i use?

what do i set it at to do the readings?

wish i had listened at school now!

Thanks a million for all you help its been really good,

keep it coming its a wet old day today im in work til 11.30 am then home. so im going to fiddle about then listen.

Will let you all know the results

Thanks again

Ron