I need guidance from experienced colleagues:
After a number of years I have decided to go back to the hobby and try to fix an old issue: I build my first tube 2A3 SET amp 13 years ago, due to inexprerience, I copied a circuit which probably had some error. As a result, I ended up with audible hum which was mainly due to PSU originated 2Vpp 100Hz ripple. The original PSU design was calling for a GZ37 with a 2uF input cap, 8H chocke and 47uF cap, with a 300VCT PT and was expected to deliver 322V B+ and aprox 100mA to 2A3 power stage and 6SL7 SRPP. I used a 5U4G instead of the GZ37, and with the original design I ended up having around 280V B+ and 2Vpp ripple. I upgraded the second cap to 100uF which helped to reduce hum to some extent. At the time, with the help of some of you and by using the Duncan PSUII simulatorI understood that my PSU needed improvement; definitely the input cap is too small. The problem is that modifying the input cap to a value that would take the PSU ripple to an acceptable level (200mVpp?), B+ goes higher than needed. I do have an additional 3H spare choke available, and a few 47uF&10uF caps, I was planning at the time to add a second LC stage to the filter with this choke but the choke is not easy to tuck into the chassis at this point, and I observed some damped transient damped oscillations in the simulations which are a cause of concern (are they?). I would apreciate some guidance from experienced folks on which route to take. Second question to the forum, the amp has been sitting unused for the last 6 years or so, should I be worried about the big 100uF cap condition?. I am attaching the schematics I started from for reference; I used 5U4G instead of GZ37 and upgraded the second cap to 100uF. Thanks a lot.
After a number of years I have decided to go back to the hobby and try to fix an old issue: I build my first tube 2A3 SET amp 13 years ago, due to inexprerience, I copied a circuit which probably had some error. As a result, I ended up with audible hum which was mainly due to PSU originated 2Vpp 100Hz ripple. The original PSU design was calling for a GZ37 with a 2uF input cap, 8H chocke and 47uF cap, with a 300VCT PT and was expected to deliver 322V B+ and aprox 100mA to 2A3 power stage and 6SL7 SRPP. I used a 5U4G instead of the GZ37, and with the original design I ended up having around 280V B+ and 2Vpp ripple. I upgraded the second cap to 100uF which helped to reduce hum to some extent. At the time, with the help of some of you and by using the Duncan PSUII simulatorI understood that my PSU needed improvement; definitely the input cap is too small. The problem is that modifying the input cap to a value that would take the PSU ripple to an acceptable level (200mVpp?), B+ goes higher than needed. I do have an additional 3H spare choke available, and a few 47uF&10uF caps, I was planning at the time to add a second LC stage to the filter with this choke but the choke is not easy to tuck into the chassis at this point, and I observed some damped transient damped oscillations in the simulations which are a cause of concern (are they?). I would apreciate some guidance from experienced folks on which route to take. Second question to the forum, the amp has been sitting unused for the last 6 years or so, should I be worried about the big 100uF cap condition?. I am attaching the schematics I started from for reference; I used 5U4G instead of GZ37 and upgraded the second cap to 100uF. Thanks a lot.
Attachments
The hum can be injected into the signal by the power supply of the heaters. The two wires supplying heaters should be twisted and .should not be routed among the circuit components. This minimises parasitic coupling. Hum is also reduced by using a virtual earth in the heaters power supply. This is achieved by connecting two series equal resistors across the heaters power supply. The midpoint between the resistors is grounded to create a virtual earth. You can watch some youtube videos by Uncle Doug to learn how to eliminate the hum.
Thank you for the reply, edbarx. I have twisted the heater wires already, and back in 2012 I had purchased the kit to upgrade to DC heating from a gentleman called Rod Coleman (I will eventually use those some day). I will watch the videos to learn more, thanks. Nevertheless, the noise I´ve measured with the scope seems to be 100Hz (Main freq is 50hz here) so I am assuming that the main contributor to hum is the PSU at this point. Cheers.
If your B+ goes too high when increasing input capacitance (of which I'd think you could easily add another 4.7µ/450V without distressing the rectifier), have you considered adding some resistance in series with your 8H? Or the odd zener diode? Getting rid of voltage tends to be much easier than finding more of it...
Thank you. I have started to consider a Mosfet amplified Zener, yes, although I have no practical experience. As for the resistance, which would be the obvious fix, I am unsure whether the variable voltage drop introduced by the resistance would be audible in a Class A (I do not consider myself a true audiophile) .