Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SleepLess on November 02, 2011, 10:15:37 am
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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:
(http://img257.imageshack.us/img257/4392/photo010xw.jpg)
(http://img534.imageshack.us/img534/7643/photo007vu.jpg)
(http://img573.imageshack.us/img573/6388/photo008d.jpg)
(http://img163.imageshack.us/img163/3205/photo009gp.jpg)
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Too much current, look for a short downstream.
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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
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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.
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Are you sure it's really 5.6K? That would be green/blue/red. Looks like green/blue/brown to me.
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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
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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...
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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
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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...
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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
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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
:BangHead: 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.
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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
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I swapped the new duet of EH 6V6GT for another new duet of EH and I have the same problem...
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Measure the cathode voltages of the tubes (i.e V1a+b, V2a, V2b and the power tubes)
/Leevi
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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?
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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!
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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.
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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
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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.
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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.
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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. :help:
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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
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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.
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What is the value of the cathode bias resistor for the power tubes?
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The red OT wire connected to the B+ IS the center tap.
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Why do you have the neutral AC wire connected to the rectifier socket ? A tie point ?
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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.
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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
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It's probably just the photo.. but are those 220K resistors soldered ,where they meet ?
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It's the photo, they are soldered at the eyelet and not before.
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I think you must have a broken wire somewhere..check continuity everywhere with a meter.
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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... :sad2:
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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.
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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.
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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
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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...
:BangHead:
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Preamp tubes in, power tubes out, check voltages to ground from these nodes.
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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
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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.
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To rule out failed capacitor, lift one end of the cathode bypass capacitor on the power tubes.
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Just done it. Same symptoms: 180V voltage drop at the 5.6K, turned off before the resistor started to burn more...
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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)?
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What is this resistor?
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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.
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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!)...
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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)
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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. :icon_biggrin:
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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. :icon_biggrin:
Gonna shoot some more pics for you. You are right about the zero readings, they are not, they are actually .1 (infinity)...
Thanks!
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(http://img403.imageshack.us/img403/382/photo017q.jpg)
(http://img440.imageshack.us/img440/7421/photo013n.jpg)
(http://img440.imageshack.us/img440/5052/photo014bt.jpg)
(http://img84.imageshack.us/img84/1969/photo015gy.jpg)
(http://img694.imageshack.us/img694/8372/photo016af.jpg)
:dontknow:
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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
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That's what I have... :dontknow:
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You said earlier that the resistor does not burn when the 6V6s are pulled. Leave the 6V6s out and measure voltages on every pin of both sockets. I'm particularly interested in the voltage on pin 5 of each tube.
Then turn the amp off and recheck resistance from pin 5 of each socket to chassis ground. We're looking for 220K so don't use your 2K range on your meter.
One meter lead should be directly on the socket pins for all the above readings. What are the numbers?
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OK, so here's what I have, every measure taken in AC:
V3
1: 0.6V 2:3.23V 3:0.56V 4:0V 5:0.62V 6:0.62V 7:3.20V 8:0V
V4
1: 0.55V 2:3.23V 3:0.67V 4:0V 5:0.46V 6:0.84V 7:3.21V 8:0V
Resistance on lug 5 V3: 255K and on lug 5 V4: 242K
If that leads somewhere...
Thanks!
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I think you will have to go back and take your measurements in D.C. With no signal you should only have A.C. on pins 2 and 7, D.C. on pins 3, 4 and 8. Pin 3 pin should be same as what the rectifier is at pin 8 (of the 5Y3 rectifier), pin 4 should be some 40 volts less than pin 3. and pin 8 should be 18 to 21 volt D.C. It seems as though you have a short around on of the power tubes. I would pull the pre amp tubes and one of the power tubes, power the amp up and take a voltage reading (D.C.) across the 5.6K resistor. Record this number. Power the amp down, Pull the power tube, Install power tube in other socket, power amp back up and take a voltage reading across the 5.6K resistor. Record this number..Compare the two numbers, if there is a big difference in the voltage drop then you know which power tube position has the problem.
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Measured in DC as well now. I have 0VDC everywhere with both output tubes pulled out except on lugs 4 of V3 and V4 where I have 414VDC. I have 445VDC on pin 8 of the rectifier...
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I would pull the pre amp tubes and one of the power tubes, power the amp up and take a voltage reading (D.C.) across the 5.6K resistor. Record this number. Power the amp down, Pull the power tube, Install power tube in other socket, power amp back up and take a voltage reading across the 5.6K resistor. Record this number..Compare the two numbers, if there is a big difference in the voltage drop then you know which power tube position has the problem.
So I just did that. With the power tube in V4 I read 479VDC at the junction of the 5.6K and the OT B+ and 266VDC at the junction of the 5.6K and 22K. With the power tube in V3 I read 413VDC and 258DC.
If that helps...
I think that V3 is the problem. Do you think that it could be a bad socket?
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and 266VDC at the junction of the 5.6K and 22K.
How much is this voltage with preamp tubes in and power tubes out?
What power transformer are you using? And does it have a heater center-tap?
Your earlier resistance measurements should read the same for both tubes.
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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...
I'm using a Classictone PT. It does have a CT, that goes to the first filter cap, across the board from the 5.6K resistor.
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Ok, one more reading that I forgot to have you take. Alternating the power tubes as before take the D.C. voltage reading at pin 8 of the tube you have installed.
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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...
I'm using a Classictone PT. It does have a CT, that goes to the first filter cap, across the board from the 5.6K resistor.
You have the POWER TRANSFORMER center tap connected to the B+ ?????
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Is this your Power Transformer:
http://www.classictone.net/40-18078.html (http://www.classictone.net/40-18078.html)
If it is, then lift the GRN/YEL wire from ground.
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No, this is the one:
http://www.classictone.net/40-18017.html (http://www.classictone.net/40-18017.html)
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Ok, one more reading that I forgot to have you take. Alternating the power tubes as before take the D.C. voltage reading at pin 8 of the tube you have installed.
HI. I've just done it. I get the same measures either with the tube in V3 or V4 on lug 8. I have 7.3VDC...
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Take a real close look at your wiring around the Power tubes pins 4 and 5.
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Take a real close look at your wiring around the Power tubes pins 4 and 5.
Just have... Lugs 4 of V3 and V4 are linked together and go to 5.6K/22K. Lugs 5 just have a leg of the 1K5 resistor, not crowded there...
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Here are more resistance readings:
All measures were taken amp OFF with MM set on 200K range, black lead shoved into PT ground lug (along with power cord ground wire)
V3
1: infinity 2: 00.3 3: inf 4: inf. 5: inf 6:inf 7: 00.3 8: 00.2
V4
1: infinity 2: 00.3 3: inf 4: inf. 5: inf 6:inf 7: 00.3 8: 00.2
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When doing resistance measurements you don't keep the meter in one range. You click through the whole range and stop to the point that gives you most accurate result.
Here is some info on meter use:
http://www.tubelab.com/MeterUse.htm (http://www.tubelab.com/MeterUse.htm)
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When doing resistance measurements you don't keep the meter in one range. You click through the whole range and stop to the point that gives you most accurate result.
Yeah, I knew that but Bruce at Mission Amps told me to use one single medium low setting for all measurements. I just followed... SO here's what I got with varying the MM knob. The ohms in parenthesis are the set range on my MM.
V3:
1: overload (200M) 2: 0.321 (2K) 3: overload (200M) 4: 03.2 (200M) 5: 0.243 (2M) 6: 0.242 (2M) 7: 0.321 (2K) 8: 0.270(2K)
V4:
1: overload (200M) 2: 0.321 (2K) 3: overload (200M) 4: 03.2 (200M) 5: 0.241 (2M) 6: 0.240 (2M) 7: 0.321 (2K) 8: 0.270(2K)
Just note that lug 4 starts at 02.6 under 200M range and keeps going up slowly the longer I put the red MM lead on the lug. The MM charges it up with the small voltage it puts out...
Thanks! :BangHead:
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:BangHead:
Spent three hours on it today. I unsoldered every wire going to V3 and took the tube socket out to inspect it closely. I didn't notice any burnt or black/brown spot. This socket seems to be fine. I also changed every wire on V3, even the jumper wires going to the board and and the wires connecting V3 t V4. It's not in the wires... It's not in the tubes... What in the hell can it be...? :w2:
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In the first picture on page 1 it seems that the 220K power amp resistors
are not connected to ground!
/Leevi
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Thanks for the idea but they actually are. A ground wire goes underneath the board and connects the 220K resistors, the three filter caps and the 270 ohms/22uf 100V cap. The ground wire then goes from that spot to the brass plate under the pots which is the amp's ground.
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and 266VDC at the junction of the 5.6K and 22K.
What happens to the voltages if you lift one end of the 22k resistor?
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What happens to the voltages if you lift one end of the 22k resistor?
Same stuff. 413VDC at the 5.6K/OT B+ and 275VDC at the 5.6K/22K junction...
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The problem is somewhere in the area of OT/power tubes. You mentioned in one reply
that when you remove the power tubes the resistor does not burn anymore.
The screen grids (pin 4) take probably too much current. Can you measure the cathode voltage of the
power tubes. That can be done by measuring the the DC voltage from the 270 Ohm resistor/cap towards the ground.
/Leevi
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The problem is somewhere in the area of OT/power tubes. You mentioned in one reply
that when you remove the power tubes the resistor does not burn anymore.
The screen grids (pin 4) take probably too much current. Can you measure the cathode voltage of the
power tubes. That can be done by measuring the the DC voltage from the 270 Ohm resistor/cap towards the ground.
/Leevi
Sure, took three measures:
Lug 4 of V3: 224VDC (1000V range)
Lug 4 of V4: 224VDC (1000V range)
270 Ohm resistor/cap towards the ground: 8.2VDC (20V range)
Both measures taken with all tubes in and power ON obviously. Remember I'm wired for 240V operation.
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The power tube biasing sounds quite cold 8.2V/270 = 30mA which means 15mA/tube.
Are you sure that both tubes are operating normally? Are they glowing equally, are they
both equal hot?
/Leevi
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The power tube biasing sounds quite cold 8.2V/270 = 30mA which means 15mA/tube.
Are you sure that both tubes are operating normally? Are they glowing equally, are they
both equal hot?
/Leevi
Well, I never got to have them stay powered up too long because of the burning 5.6K but they look pretty equal to me. I have tried two other duets of 6V6s as well, so I ruled the tubes out...
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Then I propose the following measurement (proposed that already earlier):
- turn off the amp
- set the meter to Ohms
- start with 200 Ohm area
- measure resistance:
1) from V3-pin 3 to 5.6K/first filter cap
2) from V4-pin 3 to 5.6K/first filter cap
/Leevi
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Thought I'd already done it, but there you go: overload for both measures! I even went up to 200M range, and got down to 200ohms range... The meter shows 1. which is overload. Did it with black lead of the MM on lug 3 of power tube and red lead of the MM on 5.6K/OT B+ and the opposite as well...
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Measure from brown wire (lug 3) to 5.6K and Blue wire (lug 3) to 5.6K.
The results should be about 100-200 Ohms.
/Leevi
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Sorry, I must have been unclear in my previous message. That's what I did. Lug 3 of power tube (blue wire) to 5.6K and then lug 3 of V4 (brown wire) to 5.6K.
I don't get 100 to 200 ohms but infinity or overload, it's the same: meter shows 1.
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even with 2K range? or 2M range?
/Leevi
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Yes, even with 200M range...
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Then the B+ is not connected to the CT of the OT or the OT is broken!
/Leevi
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Then the B+ is not connected to the CT of the OT or the OT is broken!
/Leevi
:sad2: I was about to unsolder the two wires that go to the standby switch to check if they are good but then I realized that they must be good or I wouldn't have any readings would I? So I know that the B+ is connected to the CT of the OT. Crap, it's a broken OT then? I guess that I have to try the 5E3 head's OT that I have with this build to see if I do have the same results. Gonna take some time, I'll do it tomorrow. If you guys have other ideas... Just shoot because I'm not really into unsoldering components from a working amp (I'm talking about my 5E3 head)...
Damn... :BangHead:
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OK, it's late here and I have to go to sleep
good night
/Leevi
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I didn't know that you have a stand-by switch there. Where is it located, hopefully before the first filter cap?
If it's located between B+ and OT CT you wont get any results before you set the stand-by off (i.e. the amp is like in operation, but the power switch is still off).
Do the corresponding measurements with your working 5E3 head and compare the results with each other.
/Leevi
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Yeah, I'll try that tonight if I manage to find a couple of hours to delve into both amps. Meanwhile if you guys ahve any ideas, don't hesitate!
Thanks!
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I didn't know that you have a stand-by switch there. Where is it located, hopefully before the first filter cap?
If it's located between B+ and OT CT you wont get any results before you set the stand-by off
All my measurements were done with the amp fully ON (Power ON and Standby OFF) as if it were ready to play. It is wired this way: left lug of standby switch goes to pin 8 of rectifier. Right lug of standby switch goes to the eyelet board has a connection with the first filter cap and hence the 5.6K resistor.
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So, here is my comparison chart between my working 5E3 head and my non-working 5E3 combo:
All the readings I gave you yesterday are extremely identical except the readings on lugs 3 of the power tubes:
Head: 1.3 (200M range) // combo: overload.
So do I have to deduce that I either have a bad cable going to these lugs or a bad OT?
If someone can tell me how I can check I have a bad OT without making me unsolder the head's oT and using it on the combo that would be cool. If it's the only solution then I'll do it but...
Meanwhile I'm gonna change the wires going to the standby switch just in case they'd be responsible for that. I thought I had a bad socket but that is unlikely or I would have only one of the lugs 3 that would give me an incorrect figure, right?
Thanks!
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I think you've already done that, but here it is again.
Measure the resistance of the OT primary.
1. Turn the amp off and let the filter caps discharge.
2. Set your meter to read ohms on the 2K scale. (you'll be looking for a reading of a few hundred ohms)
3. Locate the OT primary plate wires. There is a brown on one tube socket pin 3 and there is a blue on another tube socket pin 3.
4. Measure the resistance between the blue and brown wires. (no need to disconnect from tube sockets)
5. Locate the OT primary center tap. It should be red and should be connected to the junction of the first filter cap and the 5K resistor.
6. Measure the resistance between the red wire and blue wire. Then measure the resistance between the red wire and the brown wire.
Any open circuit (infinity, overload, etc.) reading in step 4 and/or step 6 means bad OT, or broken lead, or red center tap isn't connected where we think it is, or you're not measuring correctly.
For a good OT, the reading in step 4 will be twice the readings in step 6. Or another way to say this is the sum of the two readings in step 6 will equal the reading in step 4.
If the OT primary is open, the screen may be drawing excessive current and burning that 5.1K resistor. This is just an educated guess based on the fact that there is 8.2 volts on the cathodes, indicating about 30ma of current flowing somewhere. That somewhere must be the screen since the plate has zero volts on it. That 30ma must also flow thru the 5.1K and that calculates to 4.7 watts being dissipated by that 2 or 3 watt resistor.
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I think you've already done that, but here it is again.
Measure the resistance of the OT primary.
1. Turn the amp off and let the filter caps discharge.
2. Set your meter to read ohms on the 2K scale. (you'll be looking for a reading of a few hundred ohms)
3. Locate the OT primary plate wires. There is a brown on one tube socket pin 3 and there is a blue on another tube socket pin 3.
4. Measure the resistance between the blue and brown wires. (no need to disconnect from tube sockets)
5. Locate the OT primary center tap. It should be red and should be connected to the junction of the first filter cap and the 5K resistor.
6. Measure the resistance between the red wire and blue wire. Then measure the resistance between the red wire and the brown wire.
Any open circuit (infinity, overload, etc.) reading in step 4 and/or step 6 means bad OT, or you're not measuring correctly.
The reading in step 4 will be twice the readings in step 6. Or another way to say this is the sum of the two readings in step 6 will equal the reading in step 4.
If the OT primary is open, the screen may be drawing excessive current and burning that 5.1K resistor. This is just an educated guess based on the fact that there is 8.2 volts on the cathodes, indicating about 30ma of current flowing somewhere. That somewhere must be the screen since the plate has zero volts on it. That 30ma must also flow thru the 5.1K and that calculates to 4.7 watts being dissipated by that 2 or 3 watt resistor.
WOWOWO bingo Sluckey. Measured the head then the combo.
I do have 628 ohms between brown and blue. Then 328 ohms between OT CT and blue and 300 ohms between OT CT and brown.
On the combo's OT I have overload... So, am I ordering a new OT here? :think1:
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yes, you need another OT.
To be absolutely sure you can disconnect only the primary red CT wire in the good amp. Then use a gator clip lead to connect this dangling red lead into the new amp. Use two more clip leads to connect both pins 3 of the good amp to pins 3 of the new amp. Pull the 6V6s from the good amp and don't power it on.
You should have your new amp 6V6s connected to the OT (and speaker) of the good amp. Give it a test.
If you don't understand some of this, just pull the OT from the good amp and temporarily connect it to the new amp. Give it a test.
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Congrats SleepLess, you have probably found the root cause.
One learning from this is: Never give up!
/Leevi
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YEEEEEEEEEAAAAAAAAAAAAAAAAAHHHHHHHHHH!
Diagnosis confirmed! Tried the head's OT with the alligator clips and now the combo works! I had a faint sound but I think that is because of the alligator clips. Readings across the 5.6K are good.
I really want to thank ALL this community and ewery single soul that helped. I couldn't have done it without you! :worthy1:
THANK YOU!!!!!!!!!!!! Now on my way to ClassicTone!
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We sure ran down some wrong rabbit holes chasing this one! Most of the time problems with a new build will be a wiring error or wrong component value. Rarely do you see an open OT primary. The smoking caterpillar (er, resistor) sure threw us off the trail. But looking back, it makes sense. With the plate circuit open, the screen was acting as the plate, drawing electrons off the cathode. Drawing too many electrons for the 5.1K resistor to handle. A good set of voltage readings with the tubes out of circuit early on would probably have found this problem fairly quick.
Still, a good thought exercise. :think1:
Now, let me ask, was that a brand new OT?
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Yes brand new one, shipped along the 5E3 kit from Mission Amps... :dontknow:
Thanks a lot Sluckey!
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I bought a OT that was bad from Kendrick amps once years ago. I called him and talked with him and he sent me a new no charge.
Might be worth a try? :dontknow:
Glad to hear you got it fixed.
Brad :icon_biggrin:
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Yeah, I have just sent an email to Bruce at Mission Amps, let's see what he answers back...
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Glad you got it fixed, and thanks for sticking with it and talking everything out. I had nothing to contribute, but was following the thread with a lot of interest. :smiley:
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Glad you got it fixed, and thanks for sticking with it and talking everything out. I had nothing to contribute, but was following the thread with a lot of interest. :smiley:
Thank you John! We are both still noobs compared to guys like Sluckey. I am truly amazed at how knowledgeable some on this board are. They know tube amps like they all invented them. Amazing board to learn!
Can you believe that this topic has made me turn into a level 2 on its own? I have learnt a lot on this one. Really a lot. I have learnt more through this single amp and this "simple issue" than through the 15 previous amps I built all combined...
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Heh. I'm a noob compared to everyone!
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I may buy a Hoffman Tweed Deluxe OT instead of the Classictone if Bruce doesn't send me one new for free... What do you think?
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Many guys here have used them in a lot builds reporting very good results. I have a few laying around here that I haven't used yet, but I bought them because of what others have reported after using them. Than again there's quit a few places out there to buy a good OT.
Price for shipping might be a - big factor - depending on what country your in.
Brad :icon_biggrin:
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Glad you found the problem. This is the exact reason I bench test all new power transformers for proper voltages and do turns ratio tests on output transformers before installation. It only takes a few minutes and you know what you got. QC is not what it used to be. If I take in a non working amp that looks to be hacked, I always test the transformers and check the mains fuse for proper size. You never know what a hack has put in there.