tonal differences between kt66s and kt88s?

BowynMadley

Evening all.

My amp currently runs on a pair of kt66s (cathode bias) and is punching out about 28 watts. If in the future I feel I need more headroom and uprate the valves to kt88s which Rob who built my amp assures me is very easily done what tonal differences might I find?

Cheers chaps

Bo

ICBM

Bigger and fatter, like the bottles .

ecc83

Hmm? Don't see where you will get any extra power from, HT is the same and the valves have a similar a-a load value. Even the cathode resistor is of similar value but then much depends on the actual HT in the amp.

If you HAVE a pair to try by all means do so but a bit pricey if no result.

BTW, avoid a certain "curried" 88. I had 4 pop off one after tother in just a few hours of a transformer test!

Dave.

BowynMadley

The amp has a 50 watt transformer and can accommodate kt66s, el34s, 6l6s, 5881s and kt88s. I was under the impression that I would get another say 12 watts? making it about 40 watts with a pair of kt88s

ecc83

BowynMadley said:

The amp has a 50 watt transformer and can accommodate kt66s, el34s, 6l6s, 5881s and kt88s. I was under the impression that I would get another say 12 watts? making it about 40 watts with a pair of kt88s

The only way to get more power before clipping is to increase the output voltage swing into a given load or run the valves at a higher anode current and drop the load impedance. (or a bit of both!)

Then again I know nothing of the amplifiers design details. Can you link to a user manual?

Dave.

BowynMadley

It's a custom built head Dave @ecc83 basically a dumble preamp, fender tonestack and a bluesbreaker output section. Obviously I'll chat to Rob who built for actually making the change. I'm just after rough tone comparisons between tubes.
Cheers

ICBM

Different valve types can make a difference to the final power output if the amp is capable of the higher power - it's not that the higher spec valves give more, it's that the lower spec ones give less, if that makes sense… you can't gain more power from nowhere, but you can restrict it. If he designed it to give the full power with KT88s, it wouldn't be surprising if 66s gave less.

BowynMadley

I'm almost certain he designed it to run at full power using 6l6/5881/kt88s (all of which produce around 45-50watts as a pair) with the option to run it using a pair of el34s/kt66s (which produce between 26-32 watts)

ecc83

ICBM said:

Different valve types can make a difference to the final power output if the amp is capable of the higher power - it's not that the higher spec valves give more, it's that the lower spec ones give less, if that makes sense… you can't gain more power from nowhere, but you can restrict it. If he designed it to give the full power with KT88s, it wouldn't be surprising if 66s gave less.Ah

Ah So! If the HT etc is set for 50W and KT88s yes I see that would work. Is there a switch for bias then? If I were to plug 4 66s in an S1 200 I and do nothing else I think they would melt or more likely blow a fuse with ~700V on them!

Dave.

ICBM

If it's cathode biased it should work without switching, run as conservatively as anything which will give only 50W for a pair of KT88s. They could be way hotter than the usual 70% rule without getting anywhere near max dissipation, there's a huge margin for error.

I'd expect it will be designed with voltages in the 6L6/KT66 range and loading in the KT88 range, so it will be fine with any EL34/6L6/KT family valve.

If I remember correctly the THD Univalve did that sort of thing but single-ended, and might even have been able to take a 6V6.

I know this is not "proper" design! This gtamp design, diffrent .

BowynMadley

I've always fancied trying a univalve.
I think I'll make a decision early next year on whether to upgrade the tubes