diytube.com

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?

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

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.

How much more voltage will a 5U4 drop in the stereo 70 as opposed to a 5AR4?

About 50 volts.

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.

Bob