MAC Autoformers?

The thing that concerns me about 6AS7s, it that they emit cathode sparks in my Tektronix 530 curver tracer at zero grid.That is consistant with your statement of cathode coating failure as it appears the sparks are bits of cathode coating. They actually look like sparks off of a 4th of July sparkler.

The 6AS7G begins to develop grid current at about -15V or so; 0 volts on the grid is pushing them pretty hard (a bank of 12 power tubes at 0v on the grid can pop a 15 amp fuse rather quickly BTW). Despite that our driver circuit can push them to +15 volts on the grid while maintaining linearity.
Here are some tips for reducing arcing:
Precondition the tubes by putting filament current on them for at least 72 hours if you have the Russian variant (6H13C). If you have the Chinese (6N13) or American (6AS7G) tubes you will want to precondition for 96 hours. If B+ is applied before preconditioning or at any time during the process, you may consider the process ended. We built a jig for just this purpose. Preconditioning reduces premature arcing and can double the life of any power tube.
The Russian and Chinese tubes hold up much better in this regard. The 6AS7GA and its 6080 variant to the GA should be avoided for use in our amps- the grid heatsink is much smaller and tends to warp with grid current.

@atmasphere - Hi Ralph, I know we have had conversations about the 6AS7GA tubes and your comment above echoes the advice you gave me. However, Roger has a nice stock of those tubes and he asked me if I wanted to try them in the M-60s. So I took it over to the shop and he put in matched quads (2 matched per monoblock). Now I have no idea if the grid heat sink is warping and I wouldn't even begin to know where to look to check, but I have to say these have been the best tubes that I have had in the amp. The Chinese were not very reliable, even after preconditioning, the Russian were much better, and the RCA on par with the Russian. So far I think it has been a good 6 months since those tubes have been in the amp and no issues with arcing or lint shorts (Roger's term) or other type of failure. In addition, Roger didn't precondition the tubes. Now as to lasting 10,000 hours I guess we will see. Obviously still too early to tell.

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