• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

KT88 or 6550?

I am going to try building this VTV amp:http://www.plitron.com/pages/Products/Audio/vtvkt88.htm
and was wondering whether i should use kt88s or 6550s. I see that they are regarded as being interchangeable and was interested in opinions as regards to how they sound and suitability as to this circuit. Also i would love to get some opinions regarding this amp. This will be my first push pull project and i`m trying to keep it relatively simple.


Paid Member
2003-06-12 7:04 pm
Maine USA
mclink.it shows essentially identical curves:



I'd even say they are the SAME in triode and UL. Slight difference in Pentode is similar to the difference between two tubes of the same type. Note that these are curve-traces, not data-book sales-charts, and the pictures show what appear to be old-stock tubes.

And while 1950s-1960s KT88 and 6550 came from different factories, current-production 6550 and KT88 are very likely to be the SAME guts in different bottles and pin-outs (6550's G3 is loose from the cathode).

Anyway: between ultra-linear, cathode resistors, and significant overall NFB, if you hear any difference between different big-Beam tubes in that VTV plan then you probably have (a) defective tube(s).