Do power tubes need to be matched if...

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boogiemon
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Do power tubes need to be matched if...

Post by boogiemon » Sun Mar 18, 2012 2:59 am

...they're going into a cathode-biased, class A amp?

My understanding is that class A/B is push-pull & so the tubes need to be matched, but how does this relate to class A?

Also, if the biasing is automatic (cathode) and the amp supports either 6L6's or EL34's will it also allow
mixing of the two, say one 6L6 & one EL34?



thnx
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stokes
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Re: Do power tubes need to be matched if...

Post by stokes » Sun Mar 18, 2012 3:58 pm

In a true class A "single ended"cathode biased amp you can safely run 2 mismatched power tubes.It must be SE,PP can also be biasewd into class A.But one will run hotter than the other,which could sound better or worse than a matched pair,you also have to be sure one is not drawing too much current with a given cathode resistor value.In the other example,mixing a 6L6 and an EL34,as long as one of them is not drawing too much current you can safely do it,again,the same theory applies,it may be better or worse sounding.I built a small SE amp using a 6V6 and EL84 in parallel,I ended up using a seperate cathode resistor on each to get it sounding its best.Also had to use 2 different screen resistor values to keep the EL84 screen in check.

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boogiemon
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Re: Do power tubes need to be matched if...

Post by boogiemon » Sun Mar 18, 2012 4:31 pm

Thnx! good to know. got another question then. Forgive my ignorance:

If a tube is not broken/defective, is it even possible for it to operate outside the paramaters of a given circuit given that the cathode resistor is a fixed value? That is, doesn't the cathode resistor limit the bias somehow? Given one 6L6 & one EL34 in the aforementioned SE, Class A circuit and w/the same cathode resistor values, could one of them be affected by the bias point of the other to the point of causing a problem?

thnx again,
j


stokes wrote:In a true class A "single ended"cathode biased amp you can safely run 2 mismatched power tubes.It must be SE,PP can also be biasewd into class A.But one will run hotter than the other,which could sound better or worse than a matched pair,you also have to be sure one is not drawing too much current with a given cathode resistor value.In the other example,mixing a 6L6 and an EL34,as long as one of them is not drawing too much current you can safely do it,again,the same theory applies,it may be better or worse sounding.I built a small SE amp using a 6V6 and EL84 in parallel,I ended up using a seperate cathode resistor on each to get it sounding its best.Also had to use 2 different screen resistor values to keep the EL84 screen in check.
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stokes
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Re: Do power tubes need to be matched if...

Post by stokes » Sun Mar 18, 2012 6:41 pm

A cathode resistor does limit a tubes operating point,but only within its own limits.In other words,simply having a cathode resistor doesnt insure that the tube wont operate outside its own optimum range.If the resistors value is too low the tube will run too hot.When I did the 6V6/EL84 output,the value that had both tubes idling at a safe current draw caused the 6V6 to run too cold,when I chose a value that got that tube cooking the other one was way too hot.So although the cathode bias design will limit current flow,its only when a signal is introduced that this limiting takes place.If it starts out too high at idle it can get too high when a signal is added to the equation.

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