Replace 4xEL34 with 2xKT88

[MWJB]

Because you do not bias fixed bias, push pull amps to 100% dissipation...unless you own a tube factory that is! :-)

That 120-140W is not clean W RMS, as specified in the amp's schematic/service notes. A 100W amp is an amp that makes 100W clean, as specified by the manufacturer's prescribed test procedure. Sure, no amps seem to make 21W, or 87W...they always get rounded to the nearest 5W, usually rounded UP at that. An amp with 4XEL34 might make 100W, might make a shade less, perhaps a teeny bit more with more powerful tubes (nominal rating is 25W per tube...note use of the word "nominal"). I have seen 4x6L6 amps that make <40W because someone was shooting for a low voltage, high current, sweet toned amp, rather than shooting for max W RMS. I can switch my 50W 2x6L6 amp to 6W if I want.

Number of tubes is the number of tubes, wattage is wattage, the 2 criteria are not directly related. Amps can make less than the potential max...many do because the builder might prefer the tone at a specific plate voltage, but only amps that actually are efficient, make close to max potential efficiency.

[SlashLP97]

When I said I was "biasing to 50W" I didn't mean idle plate power. I meant I would bias it like a regular 50W amp with two tubes (35-40mA/tube). My original idea was to bias it super hot and drive it super hard to try to make 100W with the two tubes.

[defaced]

Step back for a second. Doubling power is only a 3dB increase in gain. Given the effort to make 100w out of two KT-88s, I'd seriously evaluate if the the 3dB increase is worth the work. My approach would be to see if two KT-88s biased normally and with a 3.8k load (16 ohm cab, 8 ohm tap) meets what you're after. If it does not, then start jumping through hoops to make 100w out of two KT-88s.

[SlashLP97]

That is what I am going to do.

[MWJB]

KT88 will give you tighter dynamics & should be good for 70W-ish output...it will sound different to 4xEL34...SlashLP97 may like it, he may not...either way +/- 3dB is not a realistic way of evaluating the difference inthe 2 set ups. KT88 are nominally rated for 42W each, 2 of them won't make 100W clean.

SlashLP97 - "When I said I was "biasing to 50W" I didn't mean idle plate power. I meant I would bias it like a regular 50W amp with two tubes (35-40mA/tube). My original idea was to bias it super hot and drive it super hard to try to make 100W with the two tubes." 40mA x is 18.4W idle dissipation, x2 tubes is 36W plate dissipation...that's not "biased to 50W". Biasing "super hot" will normally cause power to drop, not rise....you have to forget the notion that there is some kind of direct correlation between idle plate current and Watts AC RMS power out, there isn't. Bias to where it sounds best and nothing burns up...no one's going to want to play the amp if you bias it specifically for max power, if this is not where it sounds best.

[defaced]

KT88 will give you tighter dynamics & should be good for 70W-ish output...it will sound different to 4xEL34...SlashLP97 may like it, he may not...either way +/- 3dB is not a realistic way of evaluating the difference inthe 2 set ups. KT88 are nominally rated for 42W each, 2 of them won't make 100W clean.

The details of +/- 3dB is kinda splitting hairs. Exacts don't make a whole lot of difference here. I know KT-88s sound different than EL-34s, I use them for that very reason. The heart of my post was more to consider the potential goal with the amount of work and don't kill yoruself for something that may not be all that important.

[Wilder Amplification]

OK SlashLP...power does not come from the valves. Valves do not "make power" like people have been led to believe (I can tell this is how you've been led to think based on your posts).

The power is already there...the power supply transforms a portion of the power that is available at the wall socket into what the amp can use, and it is this power that gets transformed by the output transformer and dissipated by the speaker. It's already sitting there ready to go. All the valves do is control the current that the power supply can draw through the OT at any given point in the input signal swing. But the power is already there.

That being said, you don't "bias to 50 watts". You CAN set the idle bias so that the valves are dissipating no more than 70% of their maximum rating but depending on the design that is not always necessary...ESPECIALLY when you're running a high current valve like a KT88 in a (roughly) 450V design.

Now if you were running the KT88 at max potential such as in a dual rail amp with a 700V B+ and a 350V screen voltage, then you may need to bias them to 70% of their rated dissipation for satisfactory results tone-wise because they exhibit a bit lower transconductance figure in this type of design. But certainly not in a 450V single rail amp where they exhibit a much higher transconductance figure. I would bias them the same as I'd bias EL34s in that scenario.

For the record you don't bias a 50W any different than you'd bias a 100 watt amp. If you're reading plate current across each 1/2 of the OT primary, sure you will see double the plate current than what you'd see in a 50W. But the valves by themselves are drawing exactly the same as what they'd draw in a 50 watt...there's just 2 more valves present, hence two paths for current to flow, hence double the overall idle current through the OT primary. But the valves themselves are only seeing 1/2 of what the OT is seeing through each 1/2 of its primary.

[Amp Kat]

I wouldn't even worry about power,trust me they will be as loud as you need. I use KT-88's in almosty all my hundred watters and absolutely love their tone and big punch sound and plenty of power. I just bias them up to about 65 to 70% and they do fine and last a long time. Much different tone than EL-34's and not as bright.
Every now and then I have to change the range on the bias pot to accomodate the extra ma's but not a big deal. Really like them.