I Have an a 300b amp that is driven by one D3a and one 7721. These tubes are supposed to be the same. When I checked voltages one was running at 182 and the other at 203 volts. I reversed their positions and the voltage readings moved with the tubes. Two questions. Will this effect imaging? Why the difference in two tubes that are supposed to be the same.
These tubes are about half century old ones.
If the bias is the same, one of them more worn out, than other.
The higher anode voltage decreases the maximum swing.
The biggest factor is that they are high-µ and high-gm which will never be the same unless you tightly match them. And even then, when they get older they will might start to stray again at different rates.
If you are comparing two brands then there might be even more difference between them.
If you are comparing two brands then there might be even more difference between them.
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I take it they are RC loaded by the different anode voltages? What values for the Ra and Rk are in the amp? It seems they are pulling different current, but how much difference depends on the values or the anode resistor. If there is a 10K resistor there then that would mean about 2.2mA difference, if the value of Ra is higher then the current difference will be lower. Not a whole lot when this tube is running at 20-30mA.
It's never a good idea to run a different brand type in one channel vs the other though. Mechanically they are not the same unless they come from the same manufacturer and production period, which means you are listening to two different tubes in the L and R channel.
New tubes that match might be an option, but I doubt anyone is selling matched pairs of the D3a or 7721 now? If you don't hear anything now, I'd not worry about it.
It's never a good idea to run a different brand type in one channel vs the other though. Mechanically they are not the same unless they come from the same manufacturer and production period, which means you are listening to two different tubes in the L and R channel.
New tubes that match might be an option, but I doubt anyone is selling matched pairs of the D3a or 7721 now? If you don't hear anything now, I'd not worry about it.
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The resistors for both tubes are 12.2k Here is a pic of the two tubes https://ci3.googleusercontent.com/p...yimg.com/images/g/r4wAAOSwlMFZJIWU/s-l275.jpg
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