• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

st70 problem

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chopchip

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Joined 2010
#1
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



 
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#2
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
 
#4
llwhtt said:
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
Click to expand...


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
 
#5
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.
 
#6
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!).
 
#7
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
 
#10
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
 
#11
I will make some suggestions some of which may already have been stated by someone else. With the amplifier unplugged and no voltage present in the power supply I would suggest going thru the whole amplifier. Check all the resistor values on the board because your voltages aren't correct and no you cannot get proper voltages without the tubes in the sockets. Start with R1 and check the values with an ohm meter against the values listed on your input board parts list. Make a copy of this sheet and make your notations on this copy instead of the original. Example..R24 which is the plate resistor on the EF86 should read 221K if my copy is correct. Go thru the whole driver board and check ALL resistor values and write down your readings. Note* Pin one on the tube socket as viewed from the bottom is the square and you go around clockwise. Note this is the view from the bottom. When done there and have recorded the values and made changes if needed go to the power supply and re-check the resistor values and placement as well as choke placement and supplies coming off the board or capacitor bepending if you have the power supply upgrade. Do this first and report your findings.
 
#12
6BG6GA said:
I will make some suggestions some of which may already have been stated by someone else. With the amplifier unplugged and no voltage present in the power supply I would suggest going thru the whole amplifier. Check all the resistor values on the board because your voltages aren't correct and no you cannot get proper voltages without the tubes in the sockets. Start with R1 and check the values with an ohm meter against the values listed on your input board parts list.
Click to expand...

i have done all this and sent the results to triode electronics, i think the problem was my lack of experience. i wasnt reading the voltages correctly but now that i am reading them correctly i find them to be a bit high. the mains voltage here is 121v today and it the vac from the rectifier is 375v, the range it should be in is 230 to 260volts. can you recommend a way to deal with this? a resistor or a variac? what is the total current draw of this amp so i can get an idea of the kind of resistor i can use to bring the voltage down.

thanks
 
#13
Using a variac to bring the voltage down would just be a sticking plaster to hide the real problem. Are you confusing peak AC voltage with RMS AC voltage? You/we will not have much success at finding the problem if you are unclear how to measure voltages and currents. A wrong measurement could send us all off down the wrong path. Maybe you should get someone else to look at it for you.
 
#14
The HVAC from the stock power xfmr is 360-0-360 and that is with 117VAC from the wall. Since your wall has 121VAC your HVAC will be more like 370-0-370. This is causing your B+ to be higher than normal. I have the MKIII power xfmr and it too is designed for 117VAC, why Triode didn't adjust the primary is beyond me. Anyway as long as your filter capacitor has the voltage rating all you need to do is set the bias so the amp runs a little cooler. Normally the amp runs the output stage at 100ma with a B+ of 435VDC = 43.5 Watts per channel at idle. So take 43.5 and divide by your high B+ to get your new ma setting. To answer your question about how much current the amp draws that's most of it, the only miniscule current left is the front end. Which board are you using? If cathode voltages are listed divide that voltage by the cathode resistor and you will have the current draw for that tube. Do that for each tube in the channel then X2 + the output stage= total current draw. Your high B+ should NOT be a problem as long as you make an adjustment in the idle current. If you want to try it with the lower idle current you don't need to figure out the front end currents.

Craig
 
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#15
DF96 said:
Using a variac to bring the voltage down would just be a sticking plaster to hide the real problem. Are you confusing peak AC voltage with RMS AC voltage?
Click to expand...

i think that peak voltage in mains voltage is about 170volts. i built a voltage doubler to provide high voltage supply for that minilblok 2 amp i built. the B+ in that amp is well over 300v but drops to 220 or so when current is drawn by the tubes. the voltage drop is due to impedance in the output transformer and some resistors.

i measured the st70 voltage on the rectifier pins 4 and 6 with a DMM and it reads rms 370 or so when the sheet says 330-360 so i would like to bring it down a bit. im not sure if this is necessary but it feels right. im just wondering what others think about this. i have lost a few tubes and a resistor for some reason and i suspect it is this excessive voltage but im not sure.

i sent pictures of the amp into the Triodeelecs and all the wiring is good.

thanks
 
#16
llwhtt said:
The HVAC from the stock power xfmr is 360-0-360 and that is with 117VAC from the wall. Since your wall has 121VAC your HVAC will be more like 370-0-370. This is causing your B+ to be higher than normal. I have the MKIII power xfmr and it too is designed for 117VAC, why Triode didn't adjust the primary is beyond me. Anyway as long as your filter capacitor has the voltage rating all you need to do is set the bias so the amp runs a little cooler. Normally the amp runs the output stage at 100ma with a B+ of 435VDC = 43.5 Watts per channel at idle.

Craig
Click to expand...

interesting. thanks. i thought this amp put out about 35 watts per side. i have to think about what you posted and let it sink in a bit. the bias is generaly set to 1v. so you are saying that i should reduce this value? wouldnt that reduce the headroom some? i dont know, im still not absolutely clear on the operation of this amp.
 
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