SPL Charisma Tube stage ideas.

Hi.

I have a SPL Charisma



Its a really dull and boring device. Really can't use is for anything.
A short description of the unit.

ICs for one channel
SSM 2141
TL072
SSM2142 for output
No transformers in audio path.

And a 12AX7 tube.

I would like to modd the Tube section abit to make it more useful. Add more dirt and more life.
Here is how the tube section of my unit looks like.
Tubes are strangely cold and not heating up like I am used. Probably some sort of tube saving method from under heating.



All the ideas are welcome!

Is this unit maybe a diode limiter followed by a tube stage.

I am always surprised that designers continue to use the 12AX7 for introducing 'dirt'. The 12AX7 is actually quite a linear tube so making it dirty is not exactly easy. The circuit you posted looks like a straightforward common cathode stage followed by a cathode follower with a 20dB pad at the output. With a 285V HT supply you are not even running it in starvation mode.

For more dirt, and some nice sounding second harmonic distortion, I would recommend the following:

1. Change the topology to an SRPP. THis places a relatively heavy load on the plate of the first triode stage which causes it to have relatively high distortion but, in tests I have found it to me mainly second and third harmonic which relatively little in the way of higher harmonics. It probably will not have the gain of the original but it has a much lower output impedance so changing the output pad should bring the level back.

2. An SRPP stage distortion depends on the load so varying the load is a simple way to vary the distortion.

3.  Triode distortion is pretty much directly proportional to output level which probably explains the drive ad output controls you have.

4. There are plenty of tubes that distort more than the 12AX7. One of my favourites is the 6992/E88CC  (industrial version of the 6DJ8/ECC88) which can produce several percent 2nd and 3rd harmonic distortion if driven hard enough. The only problem for you is that it needs a 6.3V heater and it looks like you only have 12V. An alternative would be the good old 12AU7. This is well known as a high distortion tube and it has the same heater connections as the 12AX7.

Cheers

Ian

Change R8 to zero (tack a jumper across it).

Change R9 to 100K or 300K.

With R8 there is significant NFB within the gain stage, reduces distortion.

At 285V supply you would need HUGE signal level to get any audible distortion. By making R9 very large the effective supply voltage is lowered, so you get "bentness" at more reasonable signal level.

Jumping R8 will increase gain. Lowering supply voltage has little effect on gain. However "there really should be" a B+ bypass cap at the bottom of R9. There isn't. And making R9 large will feed-back from V1B plate to V1A plate and reduce gain. This may make-up for the increased gain due to jumping R8.

FWIW---- I had a similar topology, did a bunch of jumpering inside, got it sounding "much better" (cleaner, more open). When I buttoned-up and turned-on, I noticed it came-up in a second, not 11 seconds it takes the tube to heat. My ears liked it better with-OUT the tube. (But I was going for a clear literal sound, not "color").

Hi.

Tried what PRR suggested.
Well This thing just sound so bad.

I also plugged 12AU7 in there. In the original setting, it sounds more clean. But it might be useful.

But over all there is something just missing. It just doesn't have that Wow effect.
I will run some analysis on the things I have done, lets see what changed.

Hi!
Did you have any luck moding your charisma Served?

Was thinking of picking one of the 8 channel ones up and modify it since they can be found quite cheap in Germany and other parts of Europe.

Have not used a Charisma but a friend, working for a Grammy winning producer, have one of the tube SPL Vitalizers and swears by it. Know they are quite different beast but perhaps it would be possible to modify it to something useful.