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Re: Jansen 4/TWELVE SPECIAL

Posted: Tue Nov 27, 2018 3:27 pm
by sizzlingbadger
I can't see that single stage input overdriving the PI much (especially as it has to drive the tone stack first) so the master volume probably isn't going to be much use in this amp. I would say most of the drive tones will come from the interaction of the PI with the output tubes, the master volume will screw with that too.

Re: Jansen 4/TWELVE SPECIAL

Posted: Tue Nov 27, 2018 4:12 pm
by Slowy
sizzlingbadger wrote:I can't see that single stage input overdriving the PI much (especially as it has to drive the tone stack first) so the master volume probably isn't going to be much use in this amp. I would say most of the drive tones will come from the interaction of the PI with the output tubes, the master volume will screw with that too.
As I have said, without pedals, this amp only produces output stage distortion. And a glorious thing it is.

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 7:36 am
by Optical
How do you know it's output stage distortion? And do you mean the power tubes, or just everything after the MV?
I'm just curious.. the MV sits after the first gain stage and tone stack, there is one more gain stage and the cathodyne PI (which is also sort of a gain stage) after that which will both start to clip at higher MV settings.
The PPIMV might be interesting as it will allow the second gain stage and PI to clip before the power tubes. But you could go full retard and install an L-pad into it which would be cool - basically a built in attenuation. Potential problem with those things is they are configured to match a particular speaker impedance.

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 7:44 am
by AiRdAd
I'm getting the feeling that by the time this amp is finished, it's going to have 'strymon' levels of complexity and sophistication!!! :-)

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 8:23 am
by RectifiedAmps
Why not just build the amp as per the original, see how it sounds, then make any adjustments to personal taste? Provided you allow for space considerations for pots for PPIMVs, L-pads, VVRs, etc, etc, you can always add things in later and you may just decide you don't need them. Slowy's amp has survived ca.50 years without being excessively modded or butchered, so the circuit must be doing something right as it is!

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 9:06 am
by jeremyb
AiRdAd wrote:I'm getting the feeling that by the time this amp is finished, it's going to have 'strymon' levels of complexity and sophistication!!! :-)
And sound nothing like the original :lol:

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 9:35 am
by Conway
RectifiedAmps wrote:Slowy has survived ca.60 years without being excessively modded or butchered, so he must be doing something right as is!
Pure luck. :wink:

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 4:38 pm
by Jay
Optical wrote:How do you know it's output stage distortion? And do you mean the power tubes, or just everything after the MV?
I'm just curious.. the MV sits after the first gain stage and tone stack, there is one more gain stage and the cathodyne PI (which is also sort of a gain stage) after that which will both start to clip at higher MV settings.
The PPIMV might be interesting as it will allow the second gain stage and PI to clip before the power tubes. But you could go full retard and install an L-pad into it which would be cool - basically a built in attenuation. Potential problem with those things is they are configured to match a particular speaker impedance.
Just noting that the original amp does not have a Master Volume, only a Volume pot...

Not sure if PPIMV stands for Post or Pre phase Master Volume in your reference?

I could just leave the gang pot in and remove the two 1.8 MOhm resistors. When the gang pot is at full 250KOhm it basically represents the original circuit. If the MV sounds shit between less than full and zero, then just remove the pot and put the two resistors back in.

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 4:45 pm
by Jay
RectifiedAmps wrote:Why not just build the amp as per the original, see how it sounds, then make any adjustments to personal taste? Provided you allow for space considerations for pots for PPIMVs, L-pads, VVRs, etc, etc, you can always add things in later and you may just decide you don't need them. Slowy's amp has survived ca.50 years without being excessively modded or butchered, so the circuit must be doing something right as it is!
You are far too sensible! And yes, that is probably the best way since this is my first valve amp build.

Re: Jansen 4/TWELVE SPECIAL

Posted: Wed Nov 28, 2018 6:00 pm
by Slowy
Conway wrote:
RectifiedAmps wrote:Slowy has survived ca.60 years without being excessively modded or butchered, so he must be doing something right as is!
Pure luck. :wink:
Absolutely. I'm a lucky guy. :D

Re: Jansen 4/TWELVE SPECIAL

Posted: Tue Jan 08, 2019 5:57 pm
by Jehu
Wow, what a cool project... I can't wait to see how this turns out. I haven't heard one of these amps, but it sounds like they're pretty awesome.

Re: Jansen 4/TWELVE SPECIAL

Posted: Tue Jan 08, 2019 7:15 pm
by Jay
Jehu wrote:Wow, what a cool project... I can't wait to see how this turns out. I haven't heard one of these amps, but it sounds like they're pretty awesome.
It sounded great at the Auckland gearfest, but probably a fair bit of nostalgia involved too :wink:

Here is the build thread, and you probably have already seen that and thought WTF :lol:
viewtopic.php?f=41&t=35767