Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
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- Cdog
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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)
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.
Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)
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.
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.
- MikeC
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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)
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.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.
Whakanuia o mea kei a koe
- Jay
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Re: Building a Jansen 4/TWELVE SPECIAL copy (slowly)
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
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
When faced with quality, I recognise it every time.
- Cdog
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- Jay
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
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
When faced with quality, I recognise it every time.
- StrummersOfThunder
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
Congratulations !
Great to hear that it sounds pleasing. Low noise is am achievment in itself.
It looks dreadful!
Great to hear that it sounds pleasing. Low noise is am achievment in itself.
It looks dreadful!
- Cdog
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
Money shots! Pay no attention to the uncultivated... that blue tolex is where it's at
- Jay
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
I thought they cultivate bullkelp these days?
When faced with quality, I recognise it every time.
- StrummersOfThunder
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
You must have been smoking something cultivated when you chose that tolex
- Cdog
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
I think you've made it quite clear you don't like the tolex.StrummersOfThunder wrote:You must have been smoking something cultivated when you chose that tolex
- StrummersOfThunder
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
Don't worry, Jvpp and I have a special relationship. I let him borrow my denim shorts from time to time.Cdog wrote:I think you've made it quite clear you don't like the tolex.StrummersOfThunder wrote:You must have been smoking something cultivated when you chose that tolex
That's a good point. He could have covered it in ripped denim!!
- Bg
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
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
That SA is lovely too - especially the ears
So, is that low alcohol or no alcohol at all? mmmm, no alcohol, do you want to try it? Noooooooooo.
- Jay
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Re: Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)
If I remember correctly, yours were shorter than mine and definitely would be tighter than tight on me.StrummersOfThunder wrote:Don't worry, Jvpp and I have a special relationship. I let him borrow my denim shorts from time to time.Cdog wrote:I think you've made it quite clear you don't like the tolex.StrummersOfThunder wrote:You must have been smoking something cultivated when you chose that tolex
That's a good point. He could have covered it in ripped denim!!
I didn't think ripped denim with fishy smell would be acceptable.
When faced with quality, I recognise it every time.