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I need some help biasing my Soldano...


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So let's start this story from the begining. A few weeks ago I blew a fuse on my Soldano Hot Rod 50 +. I was pretty sure it was the power tubes blowing, they were pretty old and nothing else would cause that.

 

So finally got around to installing them yesterday. So I bias the amp up and noticed that the tubes were running pretty hot, about 50mA per tube on the lowest setting on the bias pot. Also, I wasn't getting any sound out of it.

 

So I took another look at it today and I noticed a lead running from the board to the power choke snapped off. So I re-soldered the lead back to the board. Now I'm getting sound out of it and sounds like it should. But now I'm getting a weird reading on my bias probe. I have it set to xx.x mA on my multimeter and I'm getting a reading of around 01.1 mA on the lowest setting. I tried bumping up the bias, but I was starting to get some blue glow and red plating in the tubes towards the end of the pot. It hits around 09.0 when this happens.

 

Any idea what could be going on??? The amp sounds fine and works great, but my bias reading is WAY off. I'm stumped.

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Hey Ryan, sorry to hijack, but are you going to Austin this Saturday for the ampfest?


If you bring it to ampfest, somebody will be able to help you. :poke:

 

 

Nah, Jason. I couldn't afford the trip or time off to make it out there. I really wish I could go.

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So let's start this story from the begining. A few weeks ago I blew a fuse on my Soldano Hot Rod 50 +. I was pretty sure it was the power tubes blowing, they were pretty old and nothing else would cause that.


So finally got around to installing them yesterday. So I bias the amp up and noticed that the tubes were running pretty hot, about 50mA per tube on the lowest setting on the bias pot. Also, I wasn't getting any sound out of it.


So I took another look at it today and I noticed a lead running from the board to the power choke snapped off. So I re-soldered the lead back to the board. Now I'm getting sound out of it and sounds like it should. But now I'm getting a weird reading on my bias probe. I have it set to xx.x mA on my multimeter and I'm getting a reading of around 01.1 mA on the lowest setting. I tried bumping up the bias, but I was starting to get some blue glow and red plating in the tubes towards the end of the pot. It hits around 09.0 when this happens.


Any idea what could be going on??? The amp sounds fine and works great, but my bias reading is WAY off. I'm stumped.

 

 

What kind of probe are you using? You should be reading voltage when using a bias probe, not current. Are you sure you hooked the choke wire back to the correct spot on the board? Something is fishy, you should have had no bias reading on the tubes if the choke was disconnected.

Jerry

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What kind of probe are you using? You should be reading voltage when using a bias probe, not current. Are you sure you hooked the choke wire back to the correct spot on the board? Something is fishy, you should have had no bias reading on the tubes if the choke was disconnected.

Jerry

 

Well, my bias probe hooks up between the power tube and tube socket. I usually read amperage on it. I do read voltage when I measure the plate voltage. This is the first time it has not worked right.

 

What range should I be measuring voltage in? I was reading around 1265 mV per tube on my bias probe.

 

I'm pretty sure I hooked the lead up to the right spot. It's the only place I could have put it, plus I could see where the wire snapped.

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Any pics of the choke wiring and the board where you connected it? The typical bias probe has a 1 ohm resistor in the cathode circuit of the tube and you measure voltage drop across the resistor which works out to equal the ma current reading due to ohms law. When measuring bias you'll be reading a mv reading and when measuring plate voltage it should be up around 450-480V if I remember right.

If you call Soldano they can probably help you out too. Those guys are awesome when it comes to customer service.

Jerry

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it seems like you're off by a factor of 10...like, 11mA at the lowest setting, and redplating at 90mA. that's just an assumption on my end, mind you. there could be something else wrong.

 

Actually, that's what's going on. I started taking other measurements to see if my multimeter is screwed up. Sure enough, I measured my plate voltage and it was off by a power of 10. So I borrowed my buddy's good multimeter and low and behold, that one was dead on.

 

So I guess something screwed up in my multimeter. It's kind of a cheap one. I think I bought it on eBay for like $5. I should probably go out and buy a good one now.

 

Thanks for your help, guys:love:.

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