Tube bass amp...

[Gingertube]

My approach for driving parallel output tubes is to use the "standard" phase splitter with a MOSFET source follower for EACH output tube, AC couple from the phase splitter.
Bias (for each tube) is then applied to the MOSFET gates.
The source follower load needs to go to a -ve supply of at least 3 x the normal bias voltage so that tubes can be driven to cut off.
Source Folower drain needs a supply of say +20 to +30V such that there is still say 20V left across the MOSFET on the most positive going signal swing at the source. This is to step around a nasty audible edge introduced by the MOSDFET Crss being modulated by the audio signal (Crss is low and unchanging when Vds is above about 20V but increases dramatically below that Vds and varies with Vds).
Output tube grids are direct coupled to the source follower (just a grid stop in between).
For HiFi I "spiff" that up by using a ring of two transistor current source as the source follower load instead of a resistor BUT that would most likely not be required for GIT Amps.

A 1M resistor (or even higher) can be used to tie the MOSFET gate to the bias pot so you can parallel 4 on each of the push pull sides and still have the "typical" 220K (250K in this case) load on the phase splitter.

Went looking for an example:
Here is an EL84 HiFi Amp design I did which shows the idea. Just add parallel MOSFET source followers and Output tubes each side.
What is teh main characteristic of that HiFi Amp? - TIGHT STRONG BASS.which is good going for a 10 Watt Amp

[bea]

Which is more or less the same approach as in the 400w Hiwatt power stage, just with transistors - except the latter uses only one source/cathode follower for each half instead of three.

Just summing this up: You could obtain even more power from those 6 KT88 (in that power stage, even 6550s could be used, couldn't it?) by increasing the anode voltage and of course using suitable transformers. I am aware that such an amp would be a real monster which should be too heavy for practical stage use. It might be more interesting with (up to) 4 KT88 or different tubes (GU50?).


Quite practically, the weight and transportability of an amplifier is an important criterion, and IMHO it would be preferable set a limit of about 20 kg which should not be exceeded. That would limit a design to about 200 W. Maybe 250.

[bea]

Back to the original question: KT90 and KT120 in bass amps.

I've been commissioned to design and build a tube bass amp. Initially, I though I would use 6550 power tubes but I'm thinking about the newer KT120 and/or KT150 tubes. Does anyone have any experience building using these new power tubes and if so, could you please elaborate on your findings?

Here are two threads on builds with these tubes - including schematics.
Quick summary: MOSFET source followers to drive the tubes and anode voltage as large as possible (with safety margin). The new tubes need quite a long time to fully heat up and deliver their power.

KT90:
Endstufe mit KT90

KT120:
Erste Erfahrungen mit der KT120

KT150:
PP Testendstufe mit KT150