Choke or No Choke ?

[TomMcNally]

I'm working on my ST-70 with 7591's and Heathkit AA-100 transformers clone. Any opinions on the power supply ? Heathkit had no choke - CT of the output transformers and a 60 mfd/500 volt cap right off the GZ34 rectifier, then the screens after 1500 ohms with 25 mfd. I bought a couple of Hammond 156R chokes, but they are only rated at 400 volts and I don't have a lot of room, so I'm thinking of leaving them out. My caps and the parts for the driver board will be here Friday, so I'll be ready to fire it up pretty fast.

[EWBrown]

The usual mounting location for the C354 choke (functionally identical to the Hammond choke thaqt you have) is under the chassis, below one of the output trannies, the two screws are installed and the choke mounted, before the OPT is mounted.

If you have the room under the chassis, you might want to install, but not yet connnect it into the B+ at first. (This saves a lot of prep work after tha fact, if it is needed) If the 7591 / AA100 modified amp sounds to your liking, then no additional filtering is needed. If the choke is needed, then it is a fairly simple matter to break the connection between the GZ34 cathode and 60 uF cap, then wire the choke in series, and add a 20 to 40 uF, 500V cap between GZ34 cathode and the common ground point, and at the input to the choke. The choke's DC resistance is between 50 and 60 ohms, so the DC voltage drop will be about 10-12VDC, presuming 50 mA current per 7591 (200 mA total).

Whoops, I wasn't fully awake, re the OPTs being the thru the chassis type.. So mounting chokes under the chassis is out...
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/ed

/ed B in NH

[TomMcNally]

The AA-100 transformers are the "through the chassis" type - so I can't mount underneath. But I may have a little room to squeeze a choke in - maybe even use two, one for each channel. I agree with your thinking - try it and see how it sounds. What do you think about the 400 Volt rating of those little chokes ?

[erichayes]

Hi All,

I'm educated-guessing, and I haven't talked with my winder, but I suspect that the 400 volt (or whatever) rating is a dielectric specification referenced to power supply drain (ground). The voltage drop from one point of a pi filter to the next will rarely exceed 100 volts, so the choke is obviously within spec. in that regard. If, however, you're running 600 volts to the plates of a couple of 6550s and have one of these 400V chokes physically mounted to the metal chassis. . . well, I'd say all bets are off regarding the choke's life expectancy.

[TomMcNally]

My day job is as a broadcast engineer, and in the big tube transmitters (usually 10,00 watts and up) they typically mount the choke on some huge ceramic insulators. That way if it shorts out, you just get a lot of hum and not a lot of fireworks. I assume you can jack up the voltage to ground rating also.