*** All-valve amp': KT88's / 6550's ***

[REDLAWMAN]

Imagine an all-valve head that's designed for four 12AX7's and four KT88's. Let's assume stock tubes are JJ.

1. Can you simply take out the KT88's and replace with 6550's (eg. Svetlana 'Winged-C's')?


2. I appreciate the evolution of both of these tubes on different sides of the Atlantic and I've read that there are different voltages, etc. in relation to certain parts of the tubes, but my current knowledge ends there.

What are the principal differences?


3. I see that Watford Valves sell 'Winged-C's' in KT88 (both regular and Cryo'-treated) and 6550 format:
Do the Svetlana KT88's sound largely similar and exhibit largely the same characteristics as the 6550's?


4. What's the real-world difference between the regular Svetlana KT88 and a Cryo'-treated one and is the latter really superior?


5. What changes am I likely to hear if I switch from the JJ's to the Svetlana?


Thanks everyone.

[Roland Rock]

I'm not an expert, but do have some experience with this. My amp came with 6550s (EHX) and I swapped these for KT88s (JJ). This worked fine, as at the time, my amp was fixed bias (it has since been changed to cathode bias)
They are interchangable, but you'll need an adjustment by a tech if your amp is not fixed bias.

Edited April 5, 2012 by Roland Rock

[REDLAWMAN]

This particular amp' has the facility to measure and adjust bias externally, for each individual tube independently, although I haven't got a clue quite how one goes about doing that!

[Bassman Steve]

My Boogie 400 uses 6550's and I switched to Winged C's and was really impressed with the sound. Can't help with the cryo bit though.

[Mr. Foxen]

Cryo is snake oil.

Careful with the term 'fixed bias' it is actually a whole way of operating the inside of an amp (as opposed to operating cathode bias, which is self biasing but less efficient), and is misused when applied to 'not having a bias pot', non-adjustable bias is probably better way of saying it.

Also, guess paste reply from other forum to same question here for benefit of others reading:

You can use 6550 instead of KT88 in most amps, but some might have too high plate voltage for the 6550 to take. Most amps, especially modern ones won't, the gap between the two types has closed up. There is a risk from putting KT88 into an amp designed for 6550 because the KT88 draw more heater current which can cook your transformer, but again, the gap isn't as large as it was so you'll probably get away with it. Basically, compare spec sheets for the valves, which vary between manufacturers, you'll see the heater current draw and safe maximum voltages across various parts on there.

The actual sound difference between valves is dependent on the amp, so you can't really generalise, some amps work in entirely different ways and the differences between valves will vary accordingly.