• 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.

Help tubelab sse beginner problem

This is my first amplifier project and it's one of the most cool thing I've ever built. Unfortunately there is no sound from the left channel. All tubes lights up except the 12at7 seems not very bright.

Component list:
PT: Hammond 374BX
OT: Hammond 1628SEA
Choke: Hammond 193H
Auxilliary cap: Temco 100 uf
Tubes: 12AT7 Electro Harmonix, Sovtek 5AR4, EL34 Electro Harmonix

I took some voltage measures:
B+: 450 vdc
R17: 32
R27: 33
R10: 0
R20 0

I thought the problem was my potentiometer so I removed it: no change. I've redone all soldering on both side. I took basic triode wiring for my amplifier. The problem seems in the preamp section... I'm facing a wall, any advice ?
 
Thanks Steve Morley and kobabmx for the quick replies.

R17 and R27 are the voltage at the cathode.

I've tried to swap the output tubes and another 12AT7: no change. I've let the amplifier running longer, 12AT7 looks ok in the dark. I think those tubes are just less bright in general, it's the same thing in my phono preamp Yaqin ms-23b. I will do further tests later and going to post this in the tubelab forum as well.
 
R13 is reading 234 vdc and R23 228 vdc.

Those resistors connect to the 12AT7 plates, and those are the correct voltage readings. The 27 volts has to be a bad reading, perhaps you measured the heater DC voltage (pin 4, 5 or 9 should be around 30 volts). This implies that all of the tubes are operating correctly. I don't see any obvious mistakes in the pictures, but I can't make out all the colors on the resistors around the 12AT7.

I think that the problem may be in wiring to the input of the board, or the wiring from the OPT's to the speakers. Sometimes the speaker connectors or input connectors can get shorted where they pass through the chassis, or the hot and cold connections to the input jacks can get switched.
 
Common mistakes beginners and even the experienced make are putting in the wrong resistor, EG a 100r for a 10k, getting an electrolytic cap the wrong way round and connecting wires to the wrong places. also check valves are in the right bases. Might sound daft, but we have all made daft mistakes, even Einstein.

So with that in mind when your fresh out of bed and had a few coffee's, re-check your work, then do it again, and again if you have to, with a magnifying glass. Also check for shorts, leads touching, wires not properly soldered etc.

Next, you have a working channel, compare voltage readings between the two channels.

Andy.
 
Yes, I've made a mistake while measuring, here's the correct voltage measure at 12AT7 pins with other tubes connected:

1: 458
2: 0
3: 0
4: 27
5: 27
6: 462
7: 0
8: 0
9: 27

Something is defintely wrong. 12AT7 is probably dead with this voltage...I will look at my wiring too.

At least, I'm learning a lot with this project, I'm also reading books at the same time. I will take my time and precautions to get this amplifier working.
 
There should be a volt or two on pins 3 and 8. Measure across R14 and R24 if there is no voltage drop there is no current flowing. It could be a bad 12AT7 or both 10M45 are defective or wired incorrectly. Go to the 10M45 data sheet and compare the pin callouts with their connections on your board, tracing the board traces if you have to.

S.
 

kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
You should have a 12V differential between 4 and 5. By the looks of it, you have no heater voltage. Even if the tubes heater was open, you should still have a 12V difference on 4 and 5.

EDIT: If you are using a 6V heater supply, you should have 6V between pins 4,5 and pin 9.
 
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kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
You should have a 12V differential between 4 and 5. ...
EDIT: If you are using a 6V heater supply, you should have 6V between pins 4,5 and pin 9.

As another newb, I can say this is probably a bit confusing for the OP.

You could have quoted the whole post. If they can read it should be pretty straight forward to understand I think :D
 
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There are 2 ways of connecting the filament in a 12AT7 (AU7, AX7 etc.). To run the tubes on 12 volts, the fil. supply is connected to 4 and 5, 9 isn't connect. The filaments in the 2 sections of the tube (each section is 6 volts) are in series, = 12volts. The other way with 4 and 5 connected together and one side of the fil supply connected to 4,5 and the other side 9 allows running the tube on 6 volts. Two six in parallel= 6 volts. Please excuse if this offends. Not trying to talk down to anyone. Everyone is a noob sometime.


See my Tubelab EL84 build series at Wallofsound.ca 3 parts posted so far, 3 more to come.


DIY EL84 Amp Update! | Wall of Sound | Audio and Music Reviews
 
... here's the correct voltage measure at 12AT7 pins with other tubes connected:

1: 458

3: 0

6: 462

8: 0

Looks like both 10M45 are dead shorts. The cathode voltage on each triode should be 1.7 - 1.8 VDC ish. The tube may be deceased.

The 27 volts, if DC, looks about right. The heaters have an elevating DC voltage applied to them. You have to be careful about making sure your voltmeter is set for ac or dc when you are taking measurements, as many pins have both ac and dc voltages on them. For example, the heater pins ...