In previous posts schematic and assembly steps of transformer coupled mono blocks with the 6GE5 as output tube have been shown. As promised a cost optimised version will also be presented in form of a stereo amplifier.
Going stereo instead of two mono blocks is already a considerable cost saving measure with minimal impact on sound quality. To reduce cost further we need to let go of the interstage transformer and use RC coupling in the driver stage instead. Next is the reduction of the number of chokes which requires to go for a cap input filter instead of choke input. And lastly the exchange of oil caps with electrolytics. Here is the schematic:
I wanted to keep the ultra path connection in the output stage, hence two separate chokes for each channel to isolate them. All other capacitors are electrolytic. The first cap after the rectifier sees the highest voltage peaks, hence two 22uF/350V caps in series. This results in 11uF capacitance which is still reasonably low and does not stress the rectifiers too much. The two 150uF caps after the chokes can be implemented with 3 47uF in parallel as I did. These caps can also be increased but the 150uF proved sufficient for quiet operation. The bypass caps at the cathode resistors of the 6GE5 are needed to suppress any residual ripple which would otherwise be coupled from the high voltage to the cathode. The driver stage is a standard RC coupled stage.
A rather simple circuit which is easy to build. I will show the completed amp in the second part and will also offer this version as a kit. Stay tuned!
Best regards
Thomas
Hallo Thomas!
ReplyDeleteWhy is the cathode bypass cap needed on the 6GE5? Did you find out it hummed after building the circuit or is there a principle that already made this obvious when designing the circuit in theory?
All the best for the new year,
Daniel
Hi Daniel,
DeleteThis is a cost optimised circuit, I use just one choke per channel which results in more residual ripple on the B+. To suppress the hum either the B+ filtering needs to be improved or the cathode bypass cap. Since cost was a concern in this design I chose the latter.
Best regards
Thomas
I have read that one shouldn't operate sweep tubes in triode mode with a plate voltage that exceeds their max screen voltage. Have you had any longer-term issues with running the 6GE5 at 350 Vp? Thanks.
ReplyDeleteno issues
Delete