by Bob Latino 9th January 2011, 2:33 pm
Hi,
The reason for using two chokes instead of one is that two chokes can handle (together) more current than a single choke. The original Dynaco C-354 choke on the ST-70 had a rated continuous current handling capacity of 200 milliamps. When you bias an ST-70 at 1.56 VDC per channel, each tube is biased at 50 milliamps. This eats up the entire 200 milliamp rating of the choke without even figuring the current used by the two 7199 tubes and the rectifier tube. Since an original ST-70 choke rarely goes bad, I suspect that the current rating on these chokes was conservative. In an original ST-70 though these chokes do get pretty hot after hours of continuous use of the amp. Many an original ST-70's C-354 choke will melt out a "brown goo" after years of heat abuse. Since the choke is in the upside down position in the amp, the "brown goo" usually drips down onto the amp's bottom cover under the choke. If you have an original choke and it is leaking you should replace it with a new C-354 choke from Dynakitparts. The C-354 chokes from Dynakitparts are better made than the original Dynaco C-354 chokes and won't leak. To test a choke in an ST-70, by the way, just measure across the two quad cap terminals that the choke is connected to with the amp OFF. If you get somewhere between 45 and 65 ohms, the choke is probably OK. If it is leaking, though, I would replace it with a new C-354 choke from Dynakitparts.
The ST-120 and the M-125 amps each use TWO C24-X chokes which are the same physical size as the C-354 in the ST-70. The C24-X chokes, however, are rated at 240 milliamps each. When two are used together, they give a current handling rating of 480 milliamps. In the M-125 the four output tubes are biased at 60 milliamps each (This has now been lowered to 50 milliamps each) and in conjunction with the two driver tubes and rectifier give a current flow well over 240 milliamps. I could have used a single larger choke but it wouldn't fit inside the 1 3/4 inch depth of the M-125 chassis so you would, if you used a single larger choke, have to find a spot on the TOP side of the chassis to mount the choke. The topside choke also has the issue of two very visible wires protruding through a hole in the chassis AND each wire is carrying 500 - 515 VDC. Keeping the choke inside the amp is (IMHO) a safer bet.
Bob
Last edited by Bob Latino on 9th February 2018, 7:10 pm; edited 1 time in total
