MAC Autoformers?

I neglected to mention that the 6AS7GA tubes I am using are General Electric military spec. In addition, as Roger had performed measurements and had data on the tubes he was able to match them so they were drawing an equal amount of current. Matching the tubes also allowed the position of the DC offset adjustment screw to be set at the middle (near 12 o'clock) of its range to achieve the appropriate reading on the meter. My previous experience with other tubes was such that while you could still set the offset, you may not have had as much flexibility for future adjustments. In some cases I also had to swap tubes around a number of times to achieve the proper DC offset. Matching the tubes made the process a lot easier. So regardless of the type of tubes used, and even though Ralph doesn't require the tubes to be matched, based on my experience I would suggest matching them if you had a means to do so.

If you have the tubes tested in a rigorous manner as Roger does, it makes sense that they would hold up better :)  It *also* makes a difference that they are mil spec- that might even be the bigger difference!

That does not seem like the sort of tube of which ample supplies exist. We stay away from the collector market as a general rule but if you mess with NOS tubes of course you can get better results.

We've had such poor luck with the garden variety of GA-style tubes that we don't mess with them. Usually what happens is someone buys a mess of them and installs them in the amps without any sort of testing. Then we get a call about a malfunctioning amplifier, but then it turns out to just be tubes. With the Russian tubes (which is what we've mostly been shipping recently) we have few enough problems that doing a warranty for a year is easy.

After the MkIIIs went into production about 14 years ago, we found we were shipping about 10% of replacement tubes that we were doing before that. Its good that you have a set of American tubes working for you- IMO and from all the feedback we have, they sound the best (and the amp makes more power too, an extra 20 watts).

@ramtubes 


Roger, you wrote that "a typical 100 watt amp puts out 100 watts into any matched tap and somewhere around 75 watts into a tap mismatched by one step. In this case the tube are loafing along, distortion is reduced and damping increased. But this requires that the load does not go significantly below the tap impedance.

However going in the other direction where the load is lower than the tap impedance bad things happen. In that case the amplifier puts out less power, works harder and the tubes get overly hot to the point or radically shortening their life."

Let me understand better what you are saying.  If my speakers have a roller coaster impedance curve (4 ohm to 20+ ohms), is my tube amp "happier" if I use the 4 ohm taps.  I think you are saying that the amp may produce less power, but it will produced better sound with less distortion and less tube wear and tear.

I assume this is so because the back impedance presented to the output tubes off the primary winding of the output tranny will be equal to or be higher than the output tubes' output impedance.  

Is that correct?

BIF

BIF,

The tubes will be happiest if you use the tap that is equal to or lower than the lowest impedance of the speaker. So if your lowest is 4 ohms use the 4 ohm tap.

Although David Manley (rest his soul) believed that we match the primary of the tranformer to the tube’s impedance this is not true. Once again its all about the voltage and current that the tube is comfortable with. With that in mind, a reduced impedance will require more current from the tube. If the tube is already at max current minimum voltage (the correct place to be at full power) the reduced load impedance will demand too much current and cause a great increase in voltage drop across the tube and overheat it.

A 10% overcurrent may double or triple the voltage drop aross the tube increasing the dissipation by many times. Its a horrible situation. In fact the RM-9 Special addressed this problem by allowing the output tubes to go into AB2 mode thus saving the tubes. The speaker that prompted this was a Theil whose impedance dropped below 4 ohms in the treble region where loud trumpet music would just bake the tubes in a regular RM-9. The AB2 mode is also used in the RM-200 as i liked what it did.

For normal tube amplifiers it would be best to connect the amplifier to the tap that matches the lowest impedance of the speaker in the region where there is a lot of music.

Speaker manufacturers are not so honest about their impedance range and a curve is the only way to know. If they didn’t do so many tricks in the crossover we would not have this problem. The drivers are not the problem, the crossover is.

In my experience most speaker designers do not know much about electronics or care about what the amplifier may have to do.