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Old 2nd November 2010, 03:37 PM   #1
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Default st70 problem

i have the triode electronics st70 kit. i have had some problems with it. i have gone through several rectifier tubes and now use ss rectification with a current limiter (CL90). then i had to replace a 10ohm grounding resistor from one channel and now i have lost an output tube.

the voltages on the input board are wrong i think.

on pin 1 of the driver tubes, i get 360volts from the pin 1 to ground. the guide says that the voltage should be 55-80v on pin 1 but this voltage is too high. maybe i misunderstand? pin 6 on the ef86 has about 400 volts on it and these readings are on both tubes. this is also too high

pin 1 screen grid should be 55-80 volts on the ef86 but i get 360v
pin 2 should be 0 and i read 0
pin 3 should be 1.2 - 1.4 and i read .9 on both tubes
pin 4 and 5 are filament
pin 6 anode should be 80-110 and i read 410 v
the remaining pins are 0 volts

i took these measurements with the SS rectifier unit installed and no tubes

i have checked the values all the components and the wiring all seems to be fine.

http://www.flickr.com/photos/55195510@N02/5140112930/

thanks




Last edited by chopchip; 2nd November 2010 at 03:45 PM.
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Old 2nd November 2010, 04:14 PM   #2
llwhtt is offline llwhtt  United States
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Measuring the voltages without tubes installed is meaningless as there is no current draw in the circuit. All of the B+ voltages will be higher than normal due to no current draw. All of the plate voltages will be the same as the B+ voltages. All of the cathode voltages will be zero as there are no tubes installed. This is a good time to use a Variac in your troubleshooting.

Craig
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Old 2nd November 2010, 04:45 PM   #3
DF96 is offline DF96  England
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I am not familiar with this kit, but it sounds like you have wiring errors in the power supply. Re-check what you have done. Test it with a variac or lamp limiter. There is no point in measuring valve voltages with the valve removed!
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Old 2nd November 2010, 05:45 PM   #4
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Quote:
Originally Posted by llwhtt View Post
Measuring the voltages without tubes installed is meaningless as there is no current draw in the circuit. This is a good time to use a Variac in your troubleshooting.

Craig

the instructions say that i should check these voltages with only the rectifier plugged in.

i have had the amp for a while now and have used it for probably 50hrs or so. but losing this output tube and now these voltage readings makes me think that there is something amiss.

maybe ill buy a variac.

thanks
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Old 2nd November 2010, 06:02 PM   #5
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take the measurements with the tubes in and report those - with no current draw, voltage readings are going to be high.

I would also be interested in your B+ voltage after the choke with all the tubes plugged in. Modern rectifiers usually aren't the best, though the Sovtek 5AR4 seems to handle abuse fairly well. Chinese rectifiers ... meh...

Output tube failure could also be from too high voltage/current or just a poorly made tube.
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Old 2nd November 2010, 06:03 PM   #6
DF96 is offline DF96  England
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Either you have misunderstood the instructions, or the person writing them was confused. You can't measure valve voltages with the valve not present.

Losing rectifiers usually means that something is drawing far too much current, or the rectifiers were very poor quality. With everything in place, check the output valve currents by checking the voltage across the cathode resistors. The output valves are about the only thing in a typical amplifier which can draw significantly too much current without themselves failing fairly quickly (they fail slowly instead!).
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Old 2nd November 2010, 06:46 PM   #7
llwhtt is offline llwhtt  United States
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The only voltage to check and be correct without tubes would be the bias voltage. That is probably OK as you would have more problems than you do now. The rectifier tube is going to be hit and miss unless you spring for a Mullard. The blown output tube was probably just a bad tube. What brand of tubes are you using. If you are using JJs, in short don't, they have been very problematic the couple of years.

Craig
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Old 2nd November 2010, 07:14 PM   #8
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Quote:
Originally Posted by kstagger View Post
take the measurements with the tubes in and report those - with no current draw, voltage readings are going to be high.

I

the B+ voltage after the choke is about 500volts and the voltage at pin 1 of the input tube is 65v. one output tube is fried though, i dont think that makes a diff.

thanks
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Old 2nd November 2010, 07:15 PM   #9
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Originally Posted by llwhtt View Post
If you are using JJs, in short don't, they have been very problematic the couple of years.

Craig
yeah, these are all JJs. are electroharmonix a better tube?

thanks
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Old 2nd November 2010, 08:42 PM   #10
llwhtt is offline llwhtt  United States
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I don't have much experience with EH tubes, I do know they sound fairly bright in guitar amplifiers. I build guitar amps for a living and went thru many JJ E34L and GZ34 tubes and that's how I came to the conclusion that JJ octal tubes have a problem. This has been noted by several other posters. We have a bunch of Sovtek tubes on the way to try out, don't know exactly what we're getting tho. I have a set of Triode Electronics Dynaco MKIIIs that I am putting together now and will be trying to use JJ KT88s as I bought them long before the problems started, if they don't work out I'm going to try the new Sovtek Gold Lions. A couple of years ago I repaired an ST70 for a friend and used JJ tubes, it wasn't long before one the E34Ls crapped out. Don't get discouraged it's only bad tubes you are dealing.

Craig
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