Pulling Tubes

[Sustain]

so i got called back from mesa, and they said that it is not a half for half conversion on impedance when you pull 2 power tubes, but they said that if I dont run it as if it IS then I will expect lesser tube life.

What they basically said was that it was a partial mismatch if I run 4ohm jack to 4ohm cab with only 2 tubes. They suggested that I could run in spongy to help prolong tube life with how im rolling now, or i could go half which would be a mismatch again, but safer than the overage.

So I guess noone was right. Bottom line is it is not a half tube = half impedance ratio, its less than that but its still safer to run under than over.

I was not really satisfied with this answer and at my earliest convenience will test my taps with 2 tubes and also with 4 tubes. Whenever I get around to it. When I do I will post results....dont hold your breath.

 

[nomad100hd]

I'm an amp builder/tech i realize only nerds know these things.

1. Think of tubes as resistors 4 in parallel will give you a lower resistance than 2 of the same size in parallel. The impedance they want to see works in exactly the same way so when you have two tubes the primary of the OT needs to have an higher impedance than with 4.

2. Tubes prefer a short to an open so a lower impedance is always better than a higher one.

3. If the amp was designed with an ideal load on the tubes than a miss match of 2:1 and 1:2 is okay. Which mean if your cab is 8Ω and your amp does 4-16Ω you can plug into which ever gives you the best tone! With tubes there is no perfect match, with different voltages and bias it changes. Every tube biases a little different so...

4. My mesa nomad manual says to change which tap an 8Ω cab goes into when running in half power.

5. You only need to pull out a recto if you want more sag than you get with both running only 2 tubes.

6. The statement of the amp not sounding like the 50W version with only 2 tubes is absolutely correct! If your looking for the sound of transformer saturation. Over rated transformers generally give more bass response. Which can equal to mud depending on the amp.

7. 50W vs 100W is more about feel than volume. 3db isn't that big of a volumes boost or cut.

 

[MikeK]

se3k4life101: excellent thread ... I appreciate the input from everyone, thank you!

nomad100hd: your post cleared up any questions I had, especially about the rectifier tubes, thanks.

Sustain: at least now we know for sure we're running as best as possible without any harmful side effects. Thanks for sharing that conversation you had with Mesa.

 

[nomad100hd]

just trying to put my nerd'dum to use!