Hello everybody,
I've built a Darling Amp (my first build) and it's been working fine for the last few days - no obvious issues, no hum and it sounds good. However I've been scratching my head to find out why I'm getting a low B+ at 240V instead of the expected 265V.
the input tubes are the CV5311 which is a british military version of the 6J4 used by Bob Danielak.
I use a Hammond 369EX PT, 380VCT /75mA.
The diodes are 3x UF4007 (2 for full wave rectification and 1 for CLC decoupling), and there are 2x 180uF capacitors for filtering with a final bypass capacitor of 16uF.
the choke is 2H, 100mA, 175DCR.
the output transformers have a 10K primary impedance.
I've attached the power supply schematics below.
When I measure B+ I get 240V instead of 260V.
The power 1626 tube plate voltage is measured at 229V (compared to an expected 250V)
The power tube common cathode resistor (shared between the 2 tubes) voltage is 23.9V @500ohms which gives me 48mA - that falls close to the typical operating condition for a pair of 1626 tubes.
The input tube plate voltage is low also at 120V instead of the expected 165V for this tube.
the common cathode resistor for these tubes is 200 ohms and the voltage measured across it is only 1.26V. That gives me 8mA for the 2 tubes well below the 13.5mA times 2 specified as typical operation.
Any help to understand what's going on would be greatly appreciated !
Cheers,
Bastien
I've built a Darling Amp (my first build) and it's been working fine for the last few days - no obvious issues, no hum and it sounds good. However I've been scratching my head to find out why I'm getting a low B+ at 240V instead of the expected 265V.
the input tubes are the CV5311 which is a british military version of the 6J4 used by Bob Danielak.
I use a Hammond 369EX PT, 380VCT /75mA.
The diodes are 3x UF4007 (2 for full wave rectification and 1 for CLC decoupling), and there are 2x 180uF capacitors for filtering with a final bypass capacitor of 16uF.
the choke is 2H, 100mA, 175DCR.
the output transformers have a 10K primary impedance.
I've attached the power supply schematics below.
When I measure B+ I get 240V instead of 260V.
The power 1626 tube plate voltage is measured at 229V (compared to an expected 250V)
The power tube common cathode resistor (shared between the 2 tubes) voltage is 23.9V @500ohms which gives me 48mA - that falls close to the typical operating condition for a pair of 1626 tubes.
The input tube plate voltage is low also at 120V instead of the expected 165V for this tube.
the common cathode resistor for these tubes is 200 ohms and the voltage measured across it is only 1.26V. That gives me 8mA for the 2 tubes well below the 13.5mA times 2 specified as typical operation.
Any help to understand what's going on would be greatly appreciated !
Cheers,
Bastien
The single-ended 190V from your xformer gives a theoretical max of 262V peak at the 1st cap, minus some rectifier losses, so your 260 is pretty close.
That's peak voltage, and with the CLC you smooth that out and it gets lower, again 240 is not that bad. If you temporarily jumper the L you'll see you get close to 260.
So the values you get seem consistent with the design.
BTW that 3rd diode seems superfluous but taking that out would only give you less than a volt extra.
Jan
That's peak voltage, and with the CLC you smooth that out and it gets lower, again 240 is not that bad. If you temporarily jumper the L you'll see you get close to 260.
So the values you get seem consistent with the design.
BTW that 3rd diode seems superfluous but taking that out would only give you less than a volt extra.
Jan
Inductance seems low? Check out this discussion on critical inductance, post #6
https://www.diyaudio.com/community/threads/maximum-choke-size-in-lc-input-rectifier.365295/
https://www.diyaudio.com/community/threads/maximum-choke-size-in-lc-input-rectifier.365295/
thanks for your quick answers ! At least I'm glad it's not a wiring issue...
Regarding the 3rd diode, as I've been explained, it prevents the 1st capacitor to pull back current from the 2nd if it happens to be less charged ?
Is it correct in this case to use the formula 190Vx1.41 (capacitor input) = 266V and then 266Vx0.9 (choke input) = 239.4V ? It's matching my B+.
So if I want to correct and increase the input tubes cathode current closer to the 27mA (13.5x2) with common cathode resistor, should I just change the value of the 47K ohms plate resistor and/or cathode resistor ?
Again, thanks for the help, it's much appreciated
Regarding the 3rd diode, as I've been explained, it prevents the 1st capacitor to pull back current from the 2nd if it happens to be less charged ?
Is it correct in this case to use the formula 190Vx1.41 (capacitor input) = 266V and then 266Vx0.9 (choke input) = 239.4V ? It's matching my B+.
So if I want to correct and increase the input tubes cathode current closer to the 27mA (13.5x2) with common cathode resistor, should I just change the value of the 47K ohms plate resistor and/or cathode resistor ?
Again, thanks for the help, it's much appreciated
You set the current primarily by the Vgk. Since the grid voltage is fixed, you change the cathode resistor. Larger Rk means more voltage drop hence higher Vgk thus lower anode current. And vice versa.
Jan
All tube-work is +/-20%. Tubes vary more than that. Most parts vary almost that much.
20% drop of maximum output may be audible but sure is not "WOW! That sucks'. If you were really desperate to fill the stadium you would pick a bigger plan.
I say it is working fine. (Yes, I did once waste hundreds of bucks chasing losses to raise output from 9W to 17W.)
20% drop of maximum output may be audible but sure is not "WOW! That sucks'. If you were really desperate to fill the stadium you would pick a bigger plan.
I say it is working fine. (Yes, I did once waste hundreds of bucks chasing losses to raise output from 9W to 17W.)
I think you are right, I’m getting obsessed with this, but the amp is actually working really well and I’m enjoying it.
It has surprisingly deeper bass then my Jadis DA50S from the 90’ and is quite good sounding. I’m very pleased with it, specially for a first build ever !
It’s also just a few days old so I expect it could improve with sone burn in time.
I’m running it with altec 604Es in big 620 style cabinets, so I’ve got plenty of volume - it doesn’t feel like 3/4W !
Bastien
It has surprisingly deeper bass then my Jadis DA50S from the 90’ and is quite good sounding. I’m very pleased with it, specially for a first build ever !
It’s also just a few days old so I expect it could improve with sone burn in time.
I’m running it with altec 604Es in big 620 style cabinets, so I’ve got plenty of volume - it doesn’t feel like 3/4W !
Bastien