Re: Replacing Tubes in Class A
Guys - Mincer is asking about a Mesa Blue Angel - it's
cathode biased unless Schematic Heaven has it dead wrong:
http://www.schematicheaven.com/boogieamps/boogie_blueangel.pdf
DISCLAIMER: please stop reading if this schematic doesn't apply to Mincer's amp.
Another way to think of "cathode biased" is as "self-biased". It's not adjustable because it generally doesn't need to be, unless you are switching the actual type of tube (like a 6L6 in place of a 6V6). That's why this amp uses a different resistor/bypass capacitor combination for the 6V6 tubes versus the EL-84s. Look at the power amp section on the second page of the schematic. These components and the "progressive linkage" switch are just about dead center.
Even if the amp were fixed bias (i.e. bias provided by a negative voltage applied to the grid of the power tubes instead of by raising the voltage at the cathode with a resistor), the amp is not going to blow up and you weiner is not going to fall off if you use another brand of tubes or, heaven forbid, unmatched tubes. Unless I'm completely mistaken (which has happened before) this is particularly true in a cathode biased amp. It will sound different with different tubes or even with the same tubes in different positions - maybe better, maybe worse.
Bob at Eurotubes makes lots of people happy with JJ tubes. However, the idea that you must have a special set of 6 tubes handpicked to make
your amp sound its best seems just silly to me.
(now I see the set includes a "balanced phase inverter" and all the preamp tubes too... )
Three sets of matched pairs - that makes some sense. Placing the 6V6s is no problem. Don't know how to be sure which EL-84 sockets are in parallel on opposite sides of the phase inverter, but you could just move them around until noise is at a minimum and it sounds good.
The idea that Mesa is doing a better job of making and matching tubes than anyone else is absurd. The idea that your tubes must meet especially stringent specs in a cathode biased amp just doesn't make sense. Running a super-hot quad of EL-84s together with a super-cool pair of 6V6s may not sound as sweet as the amp is capable of, but I'm guessing that you would have to go to the extremes to notice a huge difference in tone.
Eurotubes is very popular. I've also had good experiences with
Doug's Tubes - got me to try Ei EL-84s in my Blues Jr. and they're my favorite set out of JJ, EH, and Tung Sol. (BTW: do NOT get Ei 12AX7s due to microphonics and reliability problems) Doug seems to know what will work well in specific amps. Bob does too.
Hope this helps,
Chip
That pretty much spells it out, But like what is said in this artice... There Still is a lot of smoke & mirrors as to what can be marketed as a class A amp.:eyecrazy:
k: But Mesa has another system. I think 1 thru 10 or something like that? I never bought a Mesa power tube, But i remember them doing something like that.
