current matching in cathode bias amps...

Messages
1,471
I have a chance to buy some nice NOS el84's for my cathode biased amps. The problem is that while they test very good for transconductance, they were never rated for the current draw, and the seller doesn't have a way to get that done.

How important are matched current tubes in cathode biased amps?
What type of problems can it cause (I've been told about humming)
and is it harmfull for the amp?

thanks
 

adamzappa

Member
Messages
91
Its not harmful to the amp. Matched tubes will sound better. As long as none of your tubes glow red you can go wild.
 

riffmeister

Member
Messages
16,615
You might be right about the hum. I know that mismatched power tubes in fixed bias amps will add hum to the signal.
 
Messages
1,471
Thanks for the reply.
Any of you amp guru's know of any other potential problems ?
Thanks
Just a final bump for this... Any of you amp builders want to chime in ?
I would have to guess that it's very important because all tube vendors recommend matching the power tubes, but I'd like some details.
If not, I guess I'll take the chance on those tubes..
 

59Vampire

Platinum Supporting Member
Messages
4,444
very simple and many refuse to believe, just have tech bias it properly. you can put un matched 84 in a a cathode biased amp. BUt, there is always the chance of red plating to occur. Simple fix. Buy your tubes put them in go to your tech and have him bias it. I have a bias probe and have matched a zillion, really, a zillion el84s in my day. The only time un matched tubes redplated were ina fixed bias amp. and well, thats an easy fix. If you are dropping major coin on unmolested mullards at 250 a pair bring it to a tech. If they ar are anos try it out and watch the back of your amp
 

SatelliteAmps

Member
Messages
6,170
Tube matching is not as important as some people make it out to be (for fixed or cathode biased amps). If the tubes are extremely off, there is a chance of red plating. A lot of people actually prefer slightly mismatched tubes rather than perfectly matched.

If it really came down to it, on a cathode biased amp it is extremely simple to just add a a second cathode resistor & cap and have two different biases for the tubes.
 
Messages
1,471
Tube matching is not as important as some people make it out to be (for fixed or cathode biased amps). If the tubes are extremely off, there is a chance of red plating. A lot of people actually prefer slightly mismatched tubes rather than perfectly matched.

If it really came down to it, on a cathode biased amp it is extremely simple to just add a a second cathode resistor & cap and have two different biases for the tubes.
Thank you. I don't want to mod the amp though, it's still under warranty.
 
Messages
1,471
and the tube thats drawing more current can burn up faster.
fwiw.
Thanks !

I'm familiar with biasing my EL34 amp, But I'm new to the cathode bias thing. Does it pay me to throw a meter like a biasrite on those tubes to see what the draw is?
I'm confused because it's cathode biased, and in effect "self adjusting". Is there anything to gain by metering it ?

I don't want to mod the amp, but I don't want to cook it either.... A blown tube I can live with. It's a 30 watt amp, so a blown tube will give me a pair and a spare for the 12 watt amp ;-)
 




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