Metropoulos Forum

Hello all, my first post here, first I want to say what a great this place is. Many of my questions have been answered here just by using the search function and I really appreciate all who contribute to the forum.

Now for my question, is there anything i need to do (besides the correct bias) to install KT66's in my Metro 50 watter? Is it just a quick bias dial in or is there something else?

Thanks, BF

Bat Fastard wrote:Hello all, my first post here, first I want to say what a great this place is. Many of my questions have been answered here just by using the search function and I really appreciate all who contribute to the forum.

Now for my question, is there anything i need to do (besides the correct bias) to install KT66's in my Metro 50 watter? Is it just a quick bias dial in or is there something else?

Thanks, BF

1) Your bias voltage circuit may not be capable of the bias voltage necessary for KT66's, which is more negative than EL34's at any given plate voltage. You may have to change a resistor value to get the bias voltage range/window you need.

2) You may have to adjust your impedance selector switch one step lower
than with EL34's , i.e 16 ohm cab on 8 ohm impedance selection, etc.

I am aware that I may need to change the bias resistor, so no worries there. I read somewhere (on this forum) about changing the impedance. Could someone elaborate why I need to change from 16ohms to 8ohms? (my cab is rated at 16ohms)

BF

Because KT66's require a different load to EL34's. The load that the KT66's will see from the transformer, will be closer to the specs required for a 16ohm load when set to 8ohms.
This is why the KT66 amps have a different OT to the EL34 variety. It's all about the impedance of the secondary windings.

Thanks for the help guys, I changed the bias resistor and set it to 38mv (ive got 440v on the plates) and it sounds great. A lot fatter sounding than with the EL34's. Less gain also, I now have to turn vol 1 and 2 up to around 6 for the amount of gain I like, before they were around 4 or so. Next question is what ohm hot plate to use? I have 2 cabinets one 8ohm and the other 16ohm. Do I match the hot plate to the amps ohm setting or to the speaker ohm?

BF

The KT66's will run just fine without screwing with the impedance selector -- it's a different sound with the transformer in your amp compared to a higher-plate-impedance output tranny, and you can experiment with setting the selector differently, but it's not a necessity.

Flames1950 wrote:The KT66's will run just fine without screwing with the impedance selector -- it's a different sound with the transformer in your amp compared to a higher-plate-impedance output tranny, and you can experiment with setting the selector differently, but it's not a necessity.

Will it do any harm to my output tranny by running it on 8ohms into a 16ohm cab? The reason I ask is that I REALLY like the sound of it setup like that.

BF

I have an 8 ohm cab and running KT66's with the amp set at 4 ohms makes the mids really punchy, it almost makes the sound on the verge of harsh.

Running it on 8 ohms sounds a bit more compressed but not as compressed as running EL34's in that amp. I guess b/c I've been a Fender guy my whole life, I just don't get the darker compressed sound of an EL34. But KT66s just sound beautiful.