MAC Autoformers?

Paul was awesome. I had the pleasure at hanging out with him at RMAF several years ago. We met in Bobby P.’s room as Paul was a Merlin owner. It was there that I mentioned to him the Lightspeed and he bought it soon thereafter. In addition to the 9SE, Paul also loved his RM-10 and used the Lightspeed with that amp as well. I can't be certain but I have owned my Lightspeed for around 9 or 10 years.

@georgehifi - Yes let's go with the MkIII. I'm sure while ruffling some feathers on the active preamp side, you could ruffle a few more as there have been a few vendors venturing into the passive LDR space at prices quite a bit above what I paid for the Lightspeed. I still consider it one of the best bangs for the buck in this business.

@bifwynne - I work with Roger and handle the online tube store for him (tubeaudiostore.com). I know we have a quad of KT-150s lying around, but I imagine we can get more if necessary. You may want to read some of Roger's articles on tubes and tube matching. You can find them here:

http://tubeaudiostore.com/tubin1.html

Also, Ken Stevens of CAT has trusted Roger to match tubes for his customers CAT amps and preamps who he sends to us on a regular basis. Roger does the matching to Ken's stringent specifications. The cost is not cheap as there is a lot of work involved. As you have experienced, you may be able to find tubes at a lower cost, but you subtract quality from that as well as it relates to proper matching. I have purchased tubes from some very reputable vendors, but IMO no one tests and matches them as well as Roger. In fact, Roger has been working on developing a tube tracer that can be used by the layman to test tubes. It's been a slow process, but it hopefully it will see the light of day soon.

In addition to Brooks, John Ruttan at Audio Connection used to sell Vandersteen and MR combos and IIRC correctly it was the RM-10 and 2s. Given the new cost of the RM-200 a used one for under $3k is quite a bargain. It should also be noted that the RM-200, unlike most vacuum tube amplifiers which lose power while the speaker load impedance drops, increases output power as the speaker load becomes more challenging. It is stable down to 1 ohm and includes a 1 ohm tap.

While light loading sounds better to me, Roger would also be the first to say use the tap that sounds best to you. There is no reason to not experiment. It's a cheap and easy tweak.

@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.

The link Roger posted did not seem to work so here it is again:

https://www.stereophile.com/content/devore-fidelity-orangutan-o96-loudspeaker-measurements

To clarify a bit I made reference to Class A3 operation as implemented by Jack Elliano and used in the design of his A3-500 300B/6A3 amplifier. As Jack does not design amplifiers with multiple taps, this Class A3 operation achieves similar results as light loading. Jack achieves this with a combination of the transformer winding, as well as changes in the plate dissipation, load requirement, and grid drive of the circuit.