Building a Jansen 4/TWELVE SPECIAL copy (COMPLETE)

[Optical]

Your node voltages will come down once you have tubes in - there's no current drawn without tubes in, so you get no voltage drop. i.e. it'll read high now and lower with the tubes in.
Also if you want to match the original node voltages, it's likely you'll need to play with your psu dropper resistor values. That doesnt matter for a first test though.

[Jay]

Your node voltages will come down once you have tubes in - there's no current drawn without tubes in, so you get no voltage drop. i.e. it'll read high now and lower with the tubes in.
Also if you want to match the original node voltages, it's likely you'll need to play with your psu dropper resistor values. That doesnt matter for a first test though.

Thanks, powered it up with valves in. So far so good. Proud Mary coming through loud and clear. Couple of issues I note though... The EL84 valves are getting really hot, can't touch them, not sure if that is okay or not. Voltages are 358V on OT, 338V on next stage (EL84 V3&V4), 317V (V2) on next and 305V (V1) on last one. Are these too high? Is the EL84 biasing causing them to run too hot?

Also have a bothersome hum but that can wait until I sorted out the hot EL84s.

Any advice welcome.

[Cdog]

Congrats on getting this far... it's looking rather grand.
RE: EL84 heat - Check your heater voltages?

[Jay]

Congrats on getting this far... it's looking rather grand.
RE: EL84 heat - Check your heater voltages?

Yes, straight from the transformer, 6.7vac

[Anvil Amps Alan]

If your voltages are higher you will need to up the value of the shared cathode resistor on the EL84s otherwise the dreaded red plates will visit!
What voltage do you have on the resistor when the amp is warmed up? 6 volts?

You need to get the EL84 current right before you tweak the node resistors.

[Anvil Amps Alan]

Any plan for the unused triode on V1?

[Optical]

EL84s do run very very hot, hotter than you might expect. You could increase the cathode resistor by 10R if you are concerned about the bias, but your bias resistor looked in spec for 340V on the plates. Measure the bias current to be sure though!
Q: what is the voltage across your cathode resistor?

6.7V is also a bit high - likely because your transformer is spec'd to power several more tubes. You could load up the heater rail with an appropriate resistor to drop it down a bit, or convert to a regulated supply.

[Jay]

EL84s do run very very hot, hotter than you might expect. You could increase the cathode resistor by 10R if you are concerned about the bias, but your bias resistor looked in spec for 340V on the plates. Measure the bias current to be sure though!
Q: what is the voltage across your cathode resistor?

6.7V is also a bit high - likely because your transformer is spec'd to power several more tubes. You could load up the heater rail with an appropriate resistor to drop it down a bit, or convert to a regulated supply.

Thanks chaps,
EL84 voltage across the 130ohm/ 5W resistor is 11.6 VDC.
Re heater voltage, yes i will work out a resistor value to force a voltage drop of about 0.4volt.

[Jay]

Any plan for the unused triode on V1?

As per previous response, 11.6 volt. I am unsure what i can achieve with a single triode? Tremolo?

[Optical]

EL84s do run very very hot, hotter than you might expect. You could increase the cathode resistor by 10R if you are concerned about the bias, but your bias resistor looked in spec for 340V on the plates. Measure the bias current to be sure though!
Q: what is the voltage across your cathode resistor?

6.7V is also a bit high - likely because your transformer is spec'd to power several more tubes. You could load up the heater rail with an appropriate resistor to drop it down a bit, or convert to a regulated supply.

Thanks chaps,
EL84 voltage across the 130ohm/ 5W resistor is 11.6 VDC.
Re heater voltage, yes i will work out a resistor value to force a voltage drop of about 0.4volt.

Ok, you're at max plate dissipation, i.e.running the tubes as hot as you'd ever want to. I run my 84s here, but you could try dropping the bias a bit and see if you still like how it sounds. The tubes will last longer if you do as well. Try a 140R and 150R, or just insert one/some 10R in series with the 130R and see

[Optical]

Spare triode = switchable boost option

[Jay]

EL84s do run very very hot, hotter than you might expect. You could increase the cathode resistor by 10R if you are concerned about the bias, but your bias resistor looked in spec for 340V on the plates. Measure the bias current to be sure though!
Q: what is the voltage across your cathode resistor?

6.7V is also a bit high - likely because your transformer is spec'd to power several more tubes. You could load up the heater rail with an appropriate resistor to drop it down a bit, or convert to a regulated supply.

Thanks chaps,
EL84 voltage across the 130ohm/ 5W resistor is 11.6 VDC.
Re heater voltage, yes i will work out a resistor value to force a voltage drop of about 0.4volt.

Ok, you're at max plate dissipation, i.e.running the tubes as hot as you'd ever want to. I run my 84s here, but you could try dropping the bias a bit and see if you still like how it sounds. The tubes will last longer if you do as well. Try a 140R and 150R, or just insert one/some 10R in series with the 130R and see

Thanks Optical
I just checked Slowy's amp photo again and note that R20 (first resistor after first power cap) is 1800ohm 10watt... I have no idea why i went down to 1000ohm.... My gut feel is that i should change that one first back to 1800 as it will also drop all the other voltages a bit. Hopefully the bias current will come down too then...

[jeremyb]

Pretty excited by how close this is to completion!!

[Optical]

Thanks Optical
I just checked Slowy's amp photo again and note that R20 (first resistor after first power cap) is 1800ohm 10watt... I have no idea why i went down to 1000ohm.... My gut feel is that i should change that one first back to 1800 as it will also drop all the other voltages a bit. Hopefully the bias current will come down too then...

Generally - aim to match the node voltages rather than the dropper resistor values. Then set your plate dissipation/bias current to desired spec.
Yes the larger dropper will lower all the node voltages and the bias current, but whether that's desirable I don't know... Your ears are the best judge ultimately. For example I have a 20k high wattage pot that i use for varying node voltages (usually only bother with the preamp) and figuring out whatever sounds the best to me, then swap that out for a fixed value resistor.

Anyway good work!

[Bg]

Personally I think its attempting suicide because 'Proud Mary'.

If you did 'April Sun in Cuba' it would definitely finish it off. And a dose of 'Reminiscing' would make resurrection out of the question.