2. amps-preamps
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What are those extra tube inputs for?

Hi folks,
I have an amp built on a Dynaco st 70 chassie. It is only 8wpc and uses el34's. The guy who built it said the he used all platinum connections in it's guts and chrome plated it. Saying that the only Dynaco thing about it was the chassie. It has these extra tube input holes in the front coming out horizontally. I havent the slightest idea what they are for. Should I stick some extra tubes in there?
irishwolf
swampwalker's avatar
swampwalker
5,138 posts
 
bias adjustment
eldartford
4,262 posts
 
Those connectors on the side of the chassis are for a plug with wire from the original Dyna Kit preamp that had no power supply. It drew the power it needed (not much) from the power amp.
mlsstl
1,436 posts
 
Eldartford is correct. The original ST70 amp contained front panel sockets that you could plug the preamp into. It also had one of the socket holes marked on each side to show where to insert a voltage meter probe to check the bias of the output tubes.

Here is a link to a picture where you can see the original configuration: http://www.tnt-audio.com/ampli/dynaco2e.html
eldartford
4,262 posts
 
The picture reminds me that there are two of these sockets. That's because although this power amp is stereo, most preamps, including Dyna Kit's were monophonic, so you had to use two. Having two volume controls was a pain. I modified two Heathkit preamps to a stacked configuration and installed a dual pot in one with the signal from the other brought up to it and back.
ckoffend
684 posts
 
Generally my rule of thumb is that whenever I find a socket and I am unsure of its purpose, I find it most logical to stick my finger in it. If I get a shock, I figure it is probably important. I try to teach my kids this practical philosophy as well. Do be careful though, with certain sized sockets it is important not to stick your finger in too far, as it might get stuck - this would be a bad thing!
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