KT66s in place of KT88 tubes

@dogberry

if you look at the quad website, quad ii/40’s run kt88’s and rating is 40 wpc (as mono blocks) ... whereas quad ii classic run kt66’s and are 15 wpc as a stereo amp, so here you see apples to apples the kt88 application has about 25% more power than kt66 (5 wpc more)

old school quad ii’s always ran kt66’s iirc (there are many refurbed old ones for sale, as can be seen via a simple google search), so it is evident in the new ii/40’s they uprated the internals to run more powerful kt88s to gain more output power

i think you could easily ping quad in the uk, or a usa quad dealer or quad importer, and ask if they advise users with ii/40’s it is ok to run kt66’s as a downgrade

rule of thumb i use is never ever run a power tube in a tube amp that was not designed for it... only bad things can happen (though some amps, like primaluna’s do have variable or auto-biasing that do allow a choice of output tubes... but this is usually not the case)

all this said, if they tell you that it won’t hurt anything to run kt66’s in the ii/40’s (other than having less power), then i suspect you will get a more ’tubey’ sound with the kt66 tube ... for the reasons i described in my earlier comments

Oh, yes, I'm au fait with all that, and the 'more tubey' sound was what I had in mind. I know there will be a lot of internal changes to make, now, after talking to the guy I use to keep my Quads in repair (for example, he has already replaced the caps and resistors in them with better quality ones after some failed). I know a 15W amp sounds OK through these speakers as I used to use them with a pair of AES SE-811 amps. But rather than buy old Quad IIs and upgrade them, I was rather looking for a shortcut. To do the former would probably cost as much as buying a new pair of SET amps if I look around carefully.

All the same, I'd better wait till I have my proper speakers back and decide then if there is any improvement to be made. Thank you for your helpful thoughts.

@weebeesdad what jjss49 said is essentially correct though I'd say the blurring and loss of transient speed is overstated. To me EL34s, KT66s and 77s just sound more like tubes, higher powered KT variants might as well be solid state.