Endbell on ST70 Power Transformer

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Endbell on ST70 Power Transformer

Postby kt88pppamp » Mon Dec 29, 2008 10:21 pm

I have an old ST70 transformer with a rusty end bell. I have a replacement. Is there a proper procedure for transplanting end bells or do you simply just unscrew it and screw the new one on?
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Postby EWBrown » Mon Dec 29, 2008 10:38 pm

Basically just unscrew it and put on the new end bell and screw everything back together. If the screws are rusty, it is a good idea to replace them at the same time. Ditto for the fiber shoulder washers, if they are damages, it is a good idea to replace them. These can be found in the nuts 'n' bolts section of most hardware stores.

If the old end bell isn't too corroded / pitted, it can be sanded, smoothed and re-painted, and re-used.

If the lams are also rusted, they can be buffed clean and then rubbed with a litle light oil, or painted, as needed.

/ed B
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Postby Pyre » Tue Dec 30, 2008 9:29 am

You can get all the hardware parts for both the opt's and power transformer in the nut & bolt section at Home Depot. I replaced every nut bolt and washer on an ST-70 for about 12$. I just took one of each and matched them up.

My covers were really nasty. I tried for about an hour to sand off that black paint quite unsuccessfully. Got very sore black stained hands. I took them to a guy down the street who does shot blasting/Peening for the auto industry. Since he not busy he threw them in for me and the results were amazing. They came out a light frosty gray metallic. I clear coated them and left them like that. If you have anyone that does Peening in your area take them over.

I have since taken over the cage, a pas3 cover and some faceplates.

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Postby kt88pppamp » Tue Dec 30, 2008 4:53 pm

How much more voltage will a 5U4 drop in the stereo 70 as opposed to a 5AR4?
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Postby Ty_Bower » Tue Dec 30, 2008 5:46 pm

About 50 volts.
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Other things about a 5U4

Postby Bob01605 » Wed Dec 31, 2008 9:34 am

Ty_Bower wrote:About 50 volts.


The 5U4, as stated, will have a higher voltage drop and because of the lowered B+ you probably won't get the full 35 WPC out of the amp with a 5U4 as your rectifier. The other thing to remember is that a 5U4 is directly heated and will warm up quicker and be quicker to apply B+ high voltage. The 5AR4 is indirectly heated and is slower to apply high voltage which allows the output tubes to heat up a little more before applying high voltage to pins 3 (plate) and 4 (ultralinear screen tap) of each output tube. IMHO you should always use a GZ34/5AR4 in a stock ST-70 especially if it is older and running on mostly older parts and tubes. A 5U4 is great to have as a backup rectifier in the ST-70 to keep the music flowing should your GZ34 go bad.

Another thing > You will always have to rebias the amp if you go from a GZ34 to a 5U4. The use of a 5U4 will also drop the bias a little. No problem though - just turn the two bias pots up a little to compensate.

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