tube filament in series for better DC filtering - Page 1
Forum section: electronics / projects

author: oz2cpu
author_rank: Frequent Contributor
timestamp: September 14, 2022, 04:51:04 pm
content: the classic anode resistors and capacitors in a long line towards the critical inputof old tube amplifier / instruments, is something we have seen many times before..but this one is a bit new to me :The filaments are also done in series and caps in a long line towards the inputcheck this out for all details, Danish Design from 1964Radiometer RV35 vacuum tube voltmeter 1964 with 9 tubeshttps://youtu.be/KJ_wFzMnahQ

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2022, 05:06:05 pm
content: The series connection is likely used to keep the current low and thus make it easier to stabilize the filament current. In the old days 24 V at 100 mA were much easier than 6 V at 400 mA.It depends on the tubes how well the filaments in series works. With indirect heating is usually should.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2022, 05:27:43 pm
content: In some Fisher tube amplifiers, the 12AX7/ECC83 heaters in the RIAA stages were wired in series with the output tube cathodes to provide DC current for low hum and bias voltage for the output tubes.The series connection of those heaters was nominal 12.6V at 150 mA, but the two sections could be wired in parallel for 6.3 V at 300 mA.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: September 14, 2022, 07:06:34 pm
content: in other words, they knew ohms law allready back then :-)less current, means you get more hum filtration using less uFso we are not impressed after all :-)I just love to see the basic fundamentals put into real world solutions.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: September 14, 2022, 11:29:43 pm
content: This is a little off on a tangent but tube manufacturers had parallel families of common tube models where instead of requiring a fixed voltage for the filament, e.g. 6.3V, they required a fixed current. For example the PCL84 was the same as the ECL84 but required 0.3A heater current at a higher voltage. A set of tubes from the P family with heaters in series would be driven from the mains avoiding a transformer winding for heaters. Often used in televisions which had enough tubes to take advantage of such a scheme.Yes I know the video is about using DC for heaters to reduce hum and also series connection to reduce current.

author: langwadt
author_rank: Super Contributor
timestamp: September 14, 2022, 11:31:39 pm
content: Quote from: oz2cpu on September 14, 2022, 07:06:34 pmbut need higher voltage capacitors ....

author: trobbins
author_rank: Frequent Contributor
timestamp: September 15, 2022, 12:05:31 am
content: All the tubes are the same type for that series heater connection - that is a key design requirement for balanced heater voltages and operating tolerance (which is not typically an available option for equipment which uses a variety of tube types). It looks like the filter caps in that chain are located close to their respective valves, so that residual ripple current through pcb traces and caps gets less as the powering requirement gets closer to the 3 input tubes. Distancing the input tubes from any nearby AC traces, as well as ac current through the heater, is the last course of action to avoid hum ingressing in to the input stages (from a variety of capacitive and magnetic coupling mechanisms). I've come across simpler series heater connections with intermediate filter cap that effectively forms a CRC filter for the final heater filaments, where the R is also a heater filament.The sad aspect of that video was the naïve awe by the commentator, and the lack of any subsequent performance or before/after testing.One could even suggest that instruments like this one spurred on the look/feel of the veritable HP3400A that came out 8 years later.