Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)

Post by Cdog »

That artificial centre tap using 2 resistors is pretty much how I’ve just wired my fender heaters. Lost my Tube Amp book in my pile of crap, can’t remember exactly why but from memory it is to reduce heater hum.

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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)

Post by Optical »

The heater winding must have some DC reference, amp will hum like crazy without one.
You can ground the winding, do an artificial CT with resistors and ground that, even give the winding or artificial CT a DC offset (eg from the power tube cathode). All are fine to do and reduce hum potential. You can pick whatever, as long as there is no noise then goal accomplished.

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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)

Post by MikeC »

Cdog wrote:That artificial centre tap using 2 resistors is pretty much how I’ve just wired my fender heaters. Lost my Tube Amp book in my pile of crap, can’t remember exactly why but from memory it is to reduce heater hum.
Use 1%, 100 ohm resistors (1W or 2W) & measure them to ensure they are as identical as possible. Connect each wire of the 6.3V AC heater via one of the 100 ohm resistors to the chassis ground. I usually do it at the 6.3V lamp.

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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)

Post by Jay »

Biased the EL84s with a 150 Ohm resistor which brought the plate amperage and valve dissipation within spec. Valves now at 98% of max dissipation at idle. I used a 2W 50 Ohm wound pot in series with the original 130 Ohm bias resistor and and tried 140, 150, 155 ohm. 150 still sounded good and brought EL84 dissipation within spec.

Today I tried the Trainwreck type 3 Master Volume (Rob Robinette web site) mod but it sounded shit and was only active over the last little part of a 1M pot. So I asked myself, do I need an MV?

Took the amp out into our living room and opened it full throttle with a Celestion Anni for speaker . Nice cleans for a good part of the volume pot and then glorious output valve distortion.

I don't think this amp suits a master volume but don't know why.

Anyway, time to put the chassis in the head cabinet and build the back and front panels. Front will have a small build in speaker, back will have power connection, external speaker jack and rotary switch for 4, 8 and 16 ohm.

I'll ask the wife to wrap it and put it under the Christmas tree for me :)
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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)

Post by Cdog »

Well done mate, that's great news! :D

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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by Jay »

Taken about a year to complete, but hey, well worth the wait. This is a very fine sounding amp indeed with a very low noise floor. Need to compare with the original to hear if there are major differences. This one will be brighter as I increased the treble pot from 350K to 500K and that was a noticeable difference. (The preamp valves are also different). You will note the little speaker I built in the head and it is actually quite good... makes it a real bedroom amp :)
'Jansen 4-Twelve' complete - 20191201_122153.jpg
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'Jansen 4-Twelve' complete - 20191201_222820.jpg
'Jansen 4-Twelve' complete - 20191201_222820.jpg (1.65 MiB) Viewed 1791 times
'Jansen 4-Twelve' complete - 20191204_113053.jpg
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by StrummersOfThunder »

Congratulations !
Great to hear that it sounds pleasing. Low noise is am achievment in itself.
It looks dreadful!

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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by JHorner »

Paging Dr BG for a tweed cab stat!

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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by Cdog »

Money shots! Pay no attention to the uncultivated... that blue tolex is where it's at :P ;)

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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by Jay »

I thought they cultivate bullkelp these days?
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by StrummersOfThunder »

You must have been smoking something cultivated when you chose that tolex

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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by Cdog »

StrummersOfThunder wrote:You must have been smoking something cultivated when you chose that tolex
I think you've made it quite clear you don't like the tolex. :thumbup:

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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by StrummersOfThunder »

Cdog wrote:
StrummersOfThunder wrote:You must have been smoking something cultivated when you chose that tolex
I think you've made it quite clear you don't like the tolex. :thumbup:
Don't worry, Jvpp and I have a special relationship. I let him borrow my denim shorts from time to time.

That's a good point. He could have covered it in ripped denim!!

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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by Bg »

I have some blue vinyl somewhere if you want me to give it a makeover, won't even cost you more than a donation to the forum :)

That SA is lovely too - especially the ears :P
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

Post by Jay »

StrummersOfThunder wrote:
Cdog wrote:
StrummersOfThunder wrote:You must have been smoking something cultivated when you chose that tolex
I think you've made it quite clear you don't like the tolex. :thumbup:
Don't worry, Jvpp and I have a special relationship. I let him borrow my denim shorts from time to time.

That's a good point. He could have covered it in ripped denim!!
If I remember correctly, yours were shorter than mine and definitely would be tighter than tight on me.
I didn't think ripped denim with fishy smell would be acceptable.
"Better a soiled dove in the hand than two in the bush"

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