ST-70 / Variac Question

[fwphoto]

I've got an original-spec ST-70 that started blowing fuses almost 2 years ago & ended up in storage before I was ever able to troubleshoot & fix the problem. (See my old "ST-70 Blowing Fuses" post.) I'm now ready to get back to it & have a couple questions about the process.

I aparently didn't remember at the time that I had a spare set of EL-34 tubes available to put into the amp to see if it was, indeed, a power tube that had gone bad. (I did try pretty hard to find a someone with a tube tester at the time but never did.) So now I'm thinking that might be the first step.

So, should I invest in a Variac before I try to fire this puppy up with the old set of tubes? If so, will this one currently on eBay work:

http://www.ebay.com/itm/Variable-AC-Transformer-500VA-Variac-0-130V-TDGC2-0-5-/380681325334?pt=LH_DefaultDomain_0&hash=item58a2600b16

I'm open to suggestions but would rather not spend more than this if possible. I'm not looking for the "best," really just the cheapest useable one, IF I need it at all. Yes, I've read through a lot of old posts & am still a bit confused about variacs, but I really want to get my Dynaco gear going (& lasting!) so if I need one, I'm OK with the purchase. Hey, I'm going to spend more than that on new tubes anyway. :-)

TIA,

Frank

[mike stehr]

If you have a digital multimeter or a volt-ohm meter, why don't you check over the power supply transformer for possible shorts/continuity first. Then once you know if the PS xformer will pass this test, go ahead and disconnect the transformer's secondary and heater windings. Including that one wire that run the bias supply. Cover any exposed secondary wiring. Leave the primary connected to the line/power switch, and add another fuse.

This will run the PS xformer unloaded. It's kinda nice to have a Variac for this situation, but if it's shorted, it's shorted... Throw the power switch, and see if the fuse blows. If it does, the PS xformer is shorted.

If it doesn't, run the power supply transformer unloaded for a period of time. Say, a couple few hours or more, monitoring for overheating. (touching the endbells)

Unloaded, it shouldn't even get warm at all, barely at best. If it gets cooking hot to the touch unloaded over a period of time, then it's working on being shorted. If it stays cool, then it should be okay.

Now that you have the DMM or VOM out, you may as well disconnect the choke and check it for DCR/continuity, get that out of the way... Then go ahead and disconnect both output transformers, (you may be able to do it with just unhooking the primaries) and check them over for resistances/shorts. The resistance between each one won't be exact, but they should be within the ballpark. (pretty close)

You can do these sort of things before buying a Variac...but, you may as well get one...they are nice to have. I wish I had one with a current meter.

Edited November 16, 2013 by mike stehr

[Marvel]

I have a large variac that was used for lighting control in our theater dept. a long time ago. No current meter or volt meter, but it will handle a lot of current. Best of all, it was free. Once my wife and I get moved I'm going to try to get a small workbench set up where I can have my tools more at hand. Right now I have to unpack and pack when I want to do any work.

Bruce

[fwphoto]

Mike, thank you for the reply. I do have a VOM, but even though I understood most of your post, there's no way I have the skill to start poking around with the "guts." :-) I'm a little intimidated even with setting the bias with new tubes but I'm going to give it a shot with the next quad I put in anyway. I'm sure it will be an adventure.

I have a large variac that was used for lighting control in our theater dept. a long time ago. No current meter or volt meter, but it will handle a lot of current. Best of all, it was free. Once my wife and I get moved I'm going to try to get a small workbench set up where I can have my tools more at hand. Right now I have to unpack and pack when I want to do any work.

Bruce, we were trying to sell our house last year to get me a little closer to my work but it didn't happen. We do like our house & where we live, so when it didn't sell, we decided to make a few changes & "move back in." We had stored much of our "stuff" in one of those rental storage units in order to "de-clutter" for the house showings, so we did a bunch of modernizing and are just starting to move back in, so to speak. I'm setting myself up in a new room & am really ready to get my 2 channel system up & running! :-) My Marantz 1152DC is holding its own, but I'm ready for some tubes!

Take care guys,

Frank

[Guest " "]

as an arm chair tube tech, I can tell you there could be many reasons why a fuse blows in a tube amp. power tubes, driver tubes, rectifier tubes, if you have diodes...those too....

best bet....find someone who is not an arm chair tech.

If you can't do that...test all the tubes (you will need a transconductance tester) ......measure all capacitors (cap tester will run you 40 bucks for a cheap one)....test all transformers (volt meter for power suppy trannies...inductance meter for output trannies is better than DCR ing a tranny)....use a variac (but you need to bring them up super slow over hours.....theres a lot of articles on how to be an instant tube amp arm chair tech) ...cross your fingers.....then after the smoke clears...find a tube amp tech accept your lecture from them...hand over the cash...they will fix your amp.

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