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Do power tubes lose mA as they get old or was my amp just set cold?

sixstring531

Silver Supporting Member
Messages
3,221
Traditionally I'm an el84 guy and tube biasing never really enters my mind. I recently picked up a custom shop Bad Cat Fat Cat 50 with 6l6s and just threw in some new TAD 6l6GCs, rebiased...the whole bit. Before I did that though, I decided to check the bias on the existing TAD GCs and was surprised at how low it was set. The plate voltage (off pin 3) was 444 and the mA reading (after leaving it on a long time) settled on 27.2 mA. These are 30 watt tubes and by my calculation it looks like it was biased at around 40%, which seems pretty cold. I do know they are the original tubes, but it's only a little more than two years old. Of course I don't know the history of the amp or how many hours it had on it.

So - do you think it was just biased cold from BadCat or were the tubes wearing out?

With the new matched set in, I had 455 on pin three and biased it at 43.3, or around 65%. One of the new tubes did have a blueish glow, but I understand this is nothing to write home about. I won't be able to crank it until Wednesday, but I do expect a little different life in the tone.
 

Bruce Gerard

Member
Messages
136
Yes, power tubes can indeed draw less current as the cathode becomes depleted. Chances are, in the case of your amp that the tubes previously in it ran a little different than the ones in it now. It's my opinion that the bias / idle current should always be checked and set when tubes are replaced, or the condition of the amp and tubes is unknown to you. This includes any tube change, regardless of the claims of the reseller. It only take a moment to check, can prevent costly repairs, and prevent you from wondering if the amp is set up correctly.
 

sixstring531

Silver Supporting Member
Messages
3,221
Thanks, Bruce! At minimum I change the power section, PI and sometimes Rectifier on any new (used) amp I get. I don't know where its been and, as I stated, I am an el84 guy, so somewhat new to the whole "bias it yourself" game.
 

sixstring531

Silver Supporting Member
Messages
3,221
Yup -just takes some time. I don't have a full meter like the Bias Rite, but take the voltage off pin three and use a tube adapter for the current reading.
 

donnyjaguar

Member
Messages
4,194
Actually, your measurements are contradictory. The higher the bias current, the lower the plate voltage. That said, the plate voltage is effected by the line voltage to the amplifier, usually by an order of 4. So if your normally 120Vac plug runs 480V on the plates, if it were to drop to 110Vac you'd have 440V on the plates and lower bias current.
 

sixstring531

Silver Supporting Member
Messages
3,221
So, assuming I left the bias where it was on the existing tubes and threw a new set in (having different readings from the manufacturer), would the plate voltage not be different? It went from 444 to 455. I was waiting for someone to let me know where I was wrong. This is TGP and I welcome all manner of advices. (said with my best Arnold voice) :)
 
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sixstring531

Silver Supporting Member
Messages
3,221
So this lunch break was the first time I could head home and crank it up without the kiddos and she sounds beautiful! I think the new set was just what it needed.
 






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