• 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.

My first "from scratch" design, SE KT88..........

cadaverdog said:

How are you going to fix the bias?

cadaverdog said:

A quick look at the triode curves, that does not look like a particularly linear place to run this tube. I've never used one, though, so maybe people like it there.

cadaverdog said:

I'd increase the 56nF cap to 220nF.

I had to do a class presentation, so I showed the design as having fixed bias (when in reality, I'm probably going to cathode bias.) I didn't want to explain cathode biasing.

As far as the operating points go, I was using a simulation- and this was the combo that seemed to yield the least distortion.

Right now, I am looking for suggestions to add a volume control in order to make an integrated amp.

The input stage (according to the sim) has a gain of roughly 15. I could use higher gain tubes, but then I couldn't make use of my stock of some 200+ 6sn7's!!

While I'm on the input stage, do you think it would be wise to add chokes to the preamp power supply (one for each channel?)
Re: Answers...............

cadaverdog said:
I had to do a class presentation, so I showed the design as having fixed bias (when in reality, I'm probably going to cathode bias.) I didn't want to explain cathode biasing.

Huh? What kind of class can appreciate even the simplest of aspects concerning vacuum tube amplification (or any kind of audio amplifier for that matter), but can't "get" the idea behind cathode biasing?

You've got a resistor on the cathode. There is a current flowing though the tube that must flow through the cathode resistor. V equals I times R, and consequently there must be a voltage drop across the cathode resistor. Ergo, one end of the cathode resistor is at ground (0 volt potential) and the other end, i.e. the cathode itself, is some volts above ground. Since the cathode is some volts above ground, and the grid is at ground (while quiescent), the whole thing is rather conveniently biased, isn't it?

Sorry, I don't mean to sound argumentive. It just caught me off guard and surprised me when you implied there was some difficultly in explaining cathode bias.
I am an engineering student who's stuck in a general project-management class that's full of people who could care less about what I have to say.

Yes, this is a real project- and one I have to see through to completion, but it's also one I care to discuss only with people who are truly "in the know"; that's why I posted it here.
cadaverdog said:
I am doing this for a class project. My professors laugh at me when I say that I like using vacuum tubes!!

BTW, I'm sure that I left a few things out of the schematic.

Questions? Criticism? Comments?

I would use a choke in the power supply, a larger coupling cap and would set the current through the KT88 at about 75mA at least. How much power are you looking for out of this circuit? A convenient set of operating points is 300v on the plate, -30v on the grid and 75mA through the tube. That would give you roughly 4.5watts in triode mode. I would also make sure the gain stage is running at 7- 10 mA to deal with the highish input capacitance of the KT88. YMMV.

Coupling cap:
The reason I didn't use a larger coupling cap was because I thought I could trade a little bass for a bit less muddiness in the midrange. I have since then changed my mind and will probably go with a 1uF.

I was hoping to get away with not using chokes being that the preamp is ccs loaded (thus hopefully eliminating the crosstalk.) I'll listen to the prototype before I make a decision to add some more weight to this beast!:cannotbe:

Input stage current:
Upping the current is a good idea; I could easily manipulate the resistors in the input stage to get 10mA

I am hoping for approximately 5 Watts.