5E3 issue: SOLVED!

[SleepLess]

Hi there!
I'm having an isse with my latest 5E3 build. It's the third one I'm building so it should have been easy, but I had a problem when I turned it on. The 5K1 2W resistor started burning. It is the one that connects to the OT's red B+ wire. The other end goes to V4.

So far I've checked these things but to no avail:
- Tubes (except power tubes)
- Underboard wiring (yes, took it out... oh well...)
- All wire routes.
- Solder joints.
- Resistor values (all within specs)

I got a 342V B+ at the standby switch and at the rectifier, a NOS GE.
If you have any tips or any idea as to where I should look, please shoot, I'm lost here!
Thanks!

Some pics:

[FYL]

Too much current, look for a short downstream.

[Leevi]

Have you measured the dc resistance from the 5K1 resistor to the ground?
If that's ok remove the tubes one by one and check.
/Leevi

[SleepLess]

I didn't have a 5.1K resistor at hand so I put a 5.6K instead. I have  0.4Vdc at the resistance...
Thanks for your help already.

[sluckey]

Are you sure it's really 5.6K? That would be green/blue/red. Looks like green/blue/brown to me.

[Leevi]

I have  0.4Vdc at the resistance...

i.e. Voltage over the 5.6K resistor?

Measure resistance from 2nd end of the 5.6K resistor towards the chassis.

/Leevi

[SleepLess]

Are you sure it's really 5.6K? That would be green/blue/red. Looks like green/blue/brown to me.

The last stripe looks brown because it has started burning. It still tests as 5.6K though. The 0.4dc I have are one end to ground...

[Leevi]

The 0.4dc I have are one end to ground...

It means there is a short-cut somewhere.

Turn the amp off and measure the resistance (Ohms) from different
connection points towards chassis after the 5.6K resistor. If you get low values try to
find where the leakage is.

/Leevi

[SleepLess]

I'll try that. I thought 0.4VDC (MM set on 2V range) would be OK, but I really need to see 0.0V there?

I have tested the 3 Sprague 16uF caps by replacing each of them at a time with a TAD 22uF. Same conclusions: the 5.6K keeps burning and the amp emits no sound at all...

I am really dubious about a short, or then it would be a broken wire with the break hidden by the cloth... Mmm...

[Leevi]

I thought 0.4VDC (MM set on 2V range) would be OK, but I really need to see 0.0V there?

Maybe I have misunderstood something but I think you should have more than 300VDC there (from 5.6K resitor to ground)!
/Leevi

[SleepLess]

I thought 0.4VDC (MM set on 2V range) would be OK, but I really need to see 0.0V there?

Maybe I have misunderstood something but I think you should have more than 300VDC there (from 5.6K resitor to ground)!
/Leevi

  I measured that with the amp turned OFF... Damn!


So, with the amp turned ON.
B+ at standby switch 401VDC
B+ at 5.6K/OT B+ junction: 401VDC
Other end of the 5.6K/wire goinf to V4 pin 4: 212V

My amp is wired for 240V operation, I'm in the EU...

Thanks.

[Leevi]

Ok, no problem.

It seems that the voltage drop over the 5.6K resistor is 189V (401V-212V) which
means the current through the resistor is 33mA which is quite high. P = 6.2W which
means the resistor starts to burn.

There must be something wrong in the wiring or tubes.
If you remove the preamp tubes one by one is the problem still there?

If you remove the power tubes is the problem still there?

Double check the wiring.

/Leevi

[SleepLess]

I swapped the new duet of EH 6V6GT for another new duet of EH and I have the same problem...

[Leevi]

Measure the cathode voltages of the tubes (i.e V1a+b, V2a, V2b and the power tubes)
/Leevi

[sluckey]

Pull the 6V6s out.  Does the resistor still burn? If so...

Disconnect the red wire from the junction of the 5.6K and 22K. Does the resistor burn now?

[SleepLess]

Pull the 6V6s out.  Does the resistor still burn? If so...

I've just done that. It seems the resistor is not burning anymore. I have 447VDC at the 5.6K resistor/B+ junction and 417VDC at 5.6K/22K junction...

Don't tell me that both EH duets are bad... What's the next step? Sorry for being such a noob and thanks!

[sluckey]

Don't blame the tubes quite yet. See the red wire at the junction of the 5.6K and 22K? Follow it to a 6V6 tube. Make sure it is on pin 4 and no other pin. There should be another wire on that 6V6 that goes to the other 6V6. Make sure it is on pin 4 and no other pin.

[Leevi]

Check that the OT primary is correctly wired i.e. the B+ goes to the center tap.
That can be tested by using the Ohm meter.

Check the screen grids as Sluckey pointed.
/Leevi

[SleepLess]

Don't blame the tubes quite yet. See the red wire at the junction of the 5.6K and 22K? Follow it to a 6V6 tube. Make sure it is on pin 4 and no other pin. There should be another wire on that 6V6 that goes to the other 6V6. Make sure it is on pin 4 and no other pin.

Done! It is on pin 4 of V3 and a wire goes from there to pin 4 of V4...

The PT CT wire is connected at the first filter cap which has the OT B+ at the other end.

[sluckey]

You have 4 6V6s? Put only one in either V3 or V4 socket. If it burns the resistor pull it out and label it bad and lay aside. Repeat for the remaining 6V6s. Maybe you'll get lucky and have two that are not labeled bad. If so, put the two good tubes in the amp and see if the resistor holds up.

Otherwise, time to try another set of tubes.

[SleepLess]

I even have 8... But The 4 remaining ones are RCAs and Sylvanias, I'd rather not harm them for the sake of testing... I'll try the RCAs though...

Test done. I'm absolutely now sure that the tubes are not the culprits. I have tried the two RCAs and the resistor started burning again. No sound. The RCAs were tested in a 5E3 head last week and sounded glorious.

Something is bad, but it's not the tubes. I guess I'm back to pulling the board out again and triple checking what I've done. 

[Leevi]

Check that the OT primary is correctly wired

And that the OT is in a good shape.

Take an Ohm meter and measure DC resistance from the OT center tap to both ends.
The values should be close to each other.

/Leevi

[SleepLess]

Check that the OT primary is correctly wired

And that the OT is in a good shape.

Take an Ohm meter and measure DC resistance from the OT center tap to both ends.
The values should be close to each other.

/Leevi

Excuse my lack of knowledge here but the OT doesn't have any Center Tap, the PT does... The OT primary wires are Brown, Blue (both go to lugs 3 of either V3 or V4) and Red (B+) that goes to the 5.6K resistor. The seconday wires are Yellow (8 ohms) Green (4 ohms) and Black (Ground).

It's a ClassicTone OT made for Mission Amps.
Thanks.

[VMS]

What is the value of the cathode bias resistor for the power tubes?

[worth]

The red OT wire connected to the B+ IS the center tap.

[worth]

Why do you have the neutral AC wire connected to the rectifier socket ? A tie point ?

[SleepLess]

Why do you have the neutral AC wire connected to the rectifier ? A tie point ?

Because it is connected to the power jack common wire and both go to lug 3 of the rectifier, which is unused. That's what Bruce at Mission Amps recommends. It's a tie point absolutely.

[SleepLess]

Check that the OT primary is correctly wired

And that the OT is in a good shape.

Take an Ohm meter and measure DC resistance from the OT center tap to both ends.
The values should be close to each other.

/Leevi

I'm not sure I did what you asked. I put the MM black lead to ground and measured DC at the 5.6K/OT B+ junction and I had 402V. Then I put the red lead across the same filter cap that goes to ground and PT CT and had 0.0VDC

The cathode bias resistor (if it's the right one I'm giving you, the ones that connects to lugs 8 of the power tubes) is 270 ohms at 5 watts, with a 22uF@100V cap

[worth]

It's probably just the photo.. but are those 220K resistors soldered ,where they meet ?

[SleepLess]

It's the photo, they are soldered at the eyelet and not before.

[worth]

I think you must have a broken wire somewhere..check continuity everywhere with a meter.

[SleepLess]

OK, I'll do that tomorrow as it's going to take some time and it's late here. I'll start with the 5.6K area.

A broken wire, the amp builder's nightmare... 

[worth]

A broken wire has been rare for me, but it has happened. Stranded wire, although not as easy to work with as pushback wire ,  has much less chance of breaking.

[FYL]

Excuse my lack of knowledge here but the OT doesn't have any Center Tap, the PT does... The OT primary wires are Brown, Blue (both go to lugs 3 of either V3 or V4) and Red (B+) that goes to the 5.6K resistor. The seconday wires are Yellow (8 ohms) Green (4 ohms) and Black (Ground).

It's a ClassicTone OT made for Mission Amps.

The OT B+ (Red) is a center tap. It should be connected *before* the 5K6 resistor. If it's connected *after* it, the 5K6 sees all of the current and overheats. Cf. the attached partial schemo.

[Leevi]

Excuse my lack of knowledge here but the OT doesn't have any Center Tap, the PT does... The OT primary wires are Brown, Blue (both go to lugs 3 of either V3 or V4) and Red (B+) that goes to the 5.6K resistor. The seconday wires are Yellow (8 ohms) Green (4 ohms) and Black (Ground).

There must be a Center Tap in the OT since 5E3 has a push-pull power amp or you are trying to use a wrong type of OT.
Doublecheck the wires of the OT.

/Leevi

[SleepLess]

I know I'm getting closer but it's still not working and I still have the resistor burning.

So I pulled the red wire from eyelet on theboard, right at the end of the 5K6 resistor... to the 6V6 socket. And not the resistor didn't burn ANYMORE! I had 440VDC at the B+/5.6K junction and 410VDC at the other end (5.6K/22K)

So I figured I had a broken wire there so I changed it for a new one. Same thing... Burning resistor, no sound.

Then I tried to measure DC resistance in ohms to ground at the 5.6K/22K junction and also at pins 4 of V3 and V4. My MM doesn't give me any reading under any rating (from 220 ohms to 200M ohms)... Should I put it in DC range or in ohms range? I figured ohms since you said so...

The OT I have is perfect for the deluxe. It says Deluxe OT on it, and I have already successfully installed two of those before... I know what I'm doing, but it's not working...

 

[VMS]

Preamp tubes in, power tubes out, check voltages to ground from these nodes.

[Leevi]

Should I put it in DC range or in ohms range?

Ohms range if you are measuring resistance. Note that the amp must be off when measuring Ohms.
The idea of the resistance measurement is to verify that there are not shortcuts in the circuit.
It sounds that there are not since you didn't get any value.

I recommended that you check all the wires around the OT, power tubes and filtering.
Compare your wiring to the circuit FYL has sent in this thread. The OT primary center tap (red)
must be connected to the first fitler cap/5.6K resistor. The brown and blue OT primary wires
must go to the power tube pins 3. The connection point after 5.6K resistor at the second filter cap must
be connected to the power tube pins 4.

/Leevi

[SleepLess]

Preamp tubes in, power tubes out, check voltages to ground from these nodes.

Done it, I have 0V at both nodes.

Should I put it in DC range or in ohms range?

Ohms range if you are measuring resistance. Note that the amp must be off when measuring Ohms.
The idea of the resistance measurement is to verify that there are not shortcuts in the circuit.
It sounds that there are not since you didn't get any value.

I recommended that you check all the wires around the OT, power tubes and filtering.
Compare your wiring to the circuit FYL has sent in this thread. The OT primary center tap (red)
must be connected to the first fitler cap/5.6K resistor. The brown and blue OT primary wires
must go to the power tube pins 3. The connection point after 5.6K resistor at the second filter cap must
be connected to the power tube pins 4.

/Leevi

Done that ten times at least. The wires go to where they should go. I have a working 5E3 head besides and I have triple-checked with that one along as well. Every wire is at the same position as in the 100% working head! I'm totally puzzled.

[VMS]

To rule out failed capacitor, lift one end of the cathode bypass capacitor on the power tubes.

[SleepLess]

Just done it. Same symptoms: 180V voltage drop at the 5.6K, turned off before the resistor started to burn more...

[FYL]

Done that ten times at least. The wires go to where they should go. I have a working 5E3 head besides and I have triple-checked with that one along as well. Every wire is at the same position as in the 100% working head! I'm totally puzzled.

Are you sure?

Compare the pic from your build with a layo. What about the red wire(s)?

[VMS]

What is this resistor?

[VMS]

Nevermind, it's probably a heater ct resistor.

Without power and power tubes, measure and report resistances from power tube pins 1,2,3,4,5,6,7 and 8 to ground. Also between points in the picture.

[SleepLess]

Done that ten times at least. The wires go to where they should go. I have a working 5E3 head besides and I have triple-checked with that one along as well. Every wire is at the same position as in the 100% working head! I'm totally puzzled.

Are you sure?

Compare the pic from your build with a layo. What about the red wire(s)?

I don't have this layout. I have Bruce's oer at Mission Amps. Your OT B+ goes to the rectifier. Mine goes to the first filter cap which then goes to the rectifier lug 8 through the standby switch. Again it's the third Mission Amps Tweed I'm building. The two previous ones work beautifully with Bruce's layout

What is this resistor?

It's a 100ohm resistor. Meant to reduce hum. It does work (when the amp works obviously!)...

[SleepLess]

Nevermind, it's probably a heater ct resistor.

Without power and power tubes, measure and report resistances from power tube pins 1,2,3,4,5,6,7 and 8 to ground. Also between points in the picture.

So with MM set on 2K range:
     V3: All pins 0 ohms except pin 8 270ohms.
     V4: 1:0 ohms 2: 321 ohms 3: 0 ohms 4: 0 ohms 5: 0 ohms 6: 0 ohms 7: 321 ohms 8: 270 ohms.

On the 2 220K resistors nodes I have a 0.240 ohms reading (2M range on MM)

[sluckey]

Look closely at the filament wiring. Pins 2 and pins 7 should read exactly the same on both tubes. Are you sure about all those zero readings. Maybe they are infinity or open or overload. Zero is what you get when you touch your leads together.

I'd love to see some closeup pics that are focused on V3 and V4 sockets. Show the notch key slot in the socket if you can. Maybe a couple different angles so I can figure out how they are wired.

This is gonna be a simple problem. We just ain't seen it yet. I'll bet you a beer this one ain't wired like your other two amps.   

[SleepLess]

Look closely at the filament wiring. Pins 2 and pins 7 should read exactly the same on both tubes. Are you sure about all those zero readings. Maybe they are infinity or open or overload. Zero is what you get when you touch your leads together.

I'd love to see some closeup pics that are focused on V3 and V4 sockets. Show the notch key slot in the socket if you can. Maybe a couple different angles so I can figure out how they are wired.

This is gonna be a simple problem. We just ain't seen it yet. I'll bet you a beer this one ain't wired like your other two amps.   

Gonna shoot some more pics for you. You are right about the zero readings, they are not, they are actually .1 (infinity)...
Thanks!

[SleepLess]

 

[FYL]

I don't have this layout. I have Bruce's oer at Mission Amps. Your OT B+ goes to the rectifier. Mine goes to the first filter cap which then goes to the rectifier lug 8 through the standby switch.

OK, you should have recto => C1 ^ OT CT and standby => 5K6 => C2 ^ screens