Received the amps today in perfect condition. I hooked them up and played them after a warm-up. All seems to be OK but each KT150 have a round black spot on one side about the size if a silver dollar. The opposite side has the same spot but in chrome. Should I worry about this? Do the KT150 need replacement?

I believe you are referring to the getter flash, as long as its not white you should be alright.

Funny, I am waiting on my Zu free lunch to arrive in the mail.... Submitted a picture a while ago when we first moved into the house, setup then was considerably different.

Seems normal to me. How do the tubes sound?

What Simonatsea said.......

I've got that gray spot on my KT150's as well as on some other tubes. Sometimes it's just the dome, sometimes one side or both. If you are interested, check the quote and link I put in a post about a week ago in the discussion about Pope 6V6's and the full gray glass.

I take back some of my previous comments regarding the Mcintosh MA230. Fresh from the shop with a preamp bypass mod (removable jumpers on rear using Tape output and Tape mon input). The dyeing volume pot and the "Beta" version solid state preamp were what was holding this amp back. It is not silent like the Inspire, but there is something to be said about close to 40 lbs of output transformers. The sound stage is huge with the right recording, lots of tone and texture and power and it runs cool..... Will it be my everyday driver?... probably not. It lacks the tube rolling ability and shear fun of the Inspire, which has so many different personalities with right tubes. I do think, much to my wifes chagrin, that the Old 63' Mac will have a permanent place in my system, even if only for those sunday drives.... When you've got 16ohm speakers its not a bad idea to have a 5-10w Single ended 16ohm amp and a 30w push pull 16ohm amp around.... :D

My Pope 6V6GT's should be here tomorrow. As an aside, Dennis indicated that the Popes can handle a higher voltage, so he said a 5U4 rectifier might work also with those. Standard 6V6GT's might still need the 5Y3 with the larger voltage drop.

Any recommendations on new production and NOS 274B and 5U4G tubes? Dennis suggested I should try these. This is my first foray into SET.

Based on 4 hours of listening with the KT150 the mid-range and low-end is not as refined as compared to the Quicksilver Triode 6c33c mono amps, the sound-stage is similar (wide). I'm thinking that at least a 100 hours of break in is rewired on the tubes.

My preamp is an Audio Research Ref 5 and speakers are Kharma 3.2. I had to set the volume between 80 and 90, typically I never go beyond 60 with the Quicksilver's.

My suggestion is if you're running the KT-150's is to find a 60's era Mullard 5AR4 with the f32 or f31 date codes made in Blackburn England. Yeah, they cost $100 for a good one - but it will save you spending $35 on this one, $30 on that one $50 on that one...trying different ones out. The 274B and 5U4G have a larger voltage drop, so the total volume output at a given setting will be less - if I understand the circuit right. They last forever and won't ever need to be replaced.

However, if you're running 6V6GTs or a lower output tube, then yes, you might need a 5Y3, 5U4, etc.

Holy moly. I just popped in the Pope 6V6GTs and was letting them warm up / break in with some Eva Cassidy "Live at Blues Alley" - and these things get LOUD. Really surprised me. I'll let them break-in even more before drawing conclusions, but so far I'm impressed.

Fwiw, this is a KT-150 amp that I've "converted" to the 6V6GT by using the following tube complement:

Rectifier: Bendix 6106 (uber-5Y3GT)
Driver: Telefunken E88CC
Preamp: LP-2 w/ vintage RCA 5Y3G rectifier

Gotta love the flexibility you get from 97 dB/W efficient speakers!