Modified PSU:
12.5mA is wrong value (calculation error?).
I= (367-342)/680 about 36.8mA.
If the 342V remains the same, the first two stage current also changeless, so 32mA don't growing to 36.8.
Please re-measure all voltages in PSU and amplifier. Make sure, that operating points doesn't change.
Adding a lowish value cap at the rectifier cathode converts the supply from choke input to cap input, and raises the DC voltage by a significant amount. The DC change will cause a bias change in the valves, which may be audible as the circuit appears to use no global NFB. The first cap may be too small to maintain low supply impedance at DC and low frequencies, so you have probably raised the supply impedance. This is generally a bad idea for audio and will affect bass more than treble.
Why did you make the change?
PS It would have helped if you had highlighted the change, to save us all from playing spot the difference. I was about to give up, but then saw that PRR had spotted the change and mentioned it.
Why did you make the change?
PS It would have helped if you had highlighted the change, to save us all from playing spot the difference. I was about to give up, but then saw that PRR had spotted the change and mentioned it.
hi DF96, thanks a lot. what value cap would you recommend at C1a?
the bass is precisely where the problem is most evident..
thanks everyone for your help.
I will update the voltage value of my psu after I deal with the probably too small value at C1a
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Unless you have something particular in mind, I recommend 0uF for C1a.
Alternatively, if you want to boost the supply rail by about 50% (and make any necessary changes to the circuit to cope with this) then use a slightly bigger cap for C1a. See the rectifier datasheet for the maximum recommended. Then calculate what you need for a suitably low supply impedance (DC/LF impedance is roughly 1/(4 f C) - where f is supply frequency and C is reservoir cap value - plus whatever resistance follows (e.g. the choke)).
Alternatively, if you want to boost the supply rail by about 50% (and make any necessary changes to the circuit to cope with this) then use a slightly bigger cap for C1a. See the rectifier datasheet for the maximum recommended. Then calculate what you need for a suitably low supply impedance (DC/LF impedance is roughly 1/(4 f C) - where f is supply frequency and C is reservoir cap value - plus whatever resistance follows (e.g. the choke)).
ok, Ive updated the psu voltage ratings!
Attachments
hi
in 2014, I upgraded two times my amp circuit with the help of member Rongon.
the last modification made to my amps were:
1- change the choke for hammond
2- put a serie resistor 150ohm 10W on the 5ar4 plates. Shown in the second attached file
3- replace R13 et R14 to 1500ohm 10 W
"The choke is there to increase the inductance (L) in the CLC "pi" filter that is the first power supply filter stage. (CLC means capacitor-inductor-capacitor. That's the reservoir capacitor from rectifiers to ground, then the series choke, then the filter capacitor to ground.) That will reduce hum (ripple current on the DC voltage) by raising the Fc(-3dB point) of the power supply filter (removes more of the 60Hz ripple current frequency). It will also provide more current delivery capacity for periods of peak demands on the power supply (lowers the power supply impedance). These are all very desirable things. "
the choke was also added to try to reduce the slight hum in the amp. the hum is not something I can hear at the listening position, but there non the less. I will ask my tech to rewire and reground the whole amp as well to try to remove the hum. my 6b4g runs on ac filaments so I know they can be perfectly silent and its not the biggest issue I have with my amps right now. the biggest and only problem I hear is in the bass where it sounds noticeably dull in the bass. but I find the mid and treble just incredible compared to my Sony ta 707
C1a was there before the new choke arrived since C1a was necessary with the previous choke.
So you are saying that with my new hammond choke, C1a could be safely removed?
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We would have to see rongon's calculations to know whether the changes were upgrades or not.
Anyway, at least the history of this circuit is beginning to emerge from the mist. Why do people always wait until we are halfway through the thread before telling us what really happened?
I don't know what you are aiming at. I don't know what rongon was aiming at. To get rid of hum you first have to determine where the hum is coming from. Changing from a choke input PSU to a cap input PSU is not an upgrade but a significant redesign with knock-on effects.
Anyway, at least the history of this circuit is beginning to emerge from the mist. Why do people always wait until we are halfway through the thread before telling us what really happened?
I don't know what you are aiming at. I don't know what rongon was aiming at. To get rid of hum you first have to determine where the hum is coming from. Changing from a choke input PSU to a cap input PSU is not an upgrade but a significant redesign with knock-on effects.
Im aiming to ameliorate the bass performance in my amps.
would removing C1a and returning to choke input psu like you suggested be worth a try to better bass performance?
The hum is not a big issue as at the LP I dont hear anything and I love the resolution of the amp but the bass is definitely slightly rolled off..
would removing C1a and returning to choke input psu like you suggested be worth a try to better bass performance?
The hum is not a big issue as at the LP I dont hear anything and I love the resolution of the amp but the bass is definitely slightly rolled off..
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