Bad Tube ??

[playonit]

I have a 5881 tube in my 5e7 Bandmaster build that flashes bright white at the base when going off the stand by switch.... I am pretty sure the tube is cooked by the smell at it's base.. It all started when I was not getting any sound from the normal channel jacks. I was exploring with a chop stick looking for anything loose and nothing appeared out of place.... This amp has worked wonderful since I finished build 2 years ago and prior to the no output in the normal channel and the tube flash...

I switched the standby off and flash...... Any thoughts??

Thanks.....

[selloutrr]

does the top look like the frosting of a frosted flake?

[playonit]

does the top look like the frosting of a frosted flake?

Nope... clear. I may know what happened.. after the initial problem was discovered I un-hooked everything from the head to examine the jacks wiring.. When I plugged it back in I forgot to attach the speaker cable to the head... had a guitar plugged in and I think the rest is history.. I assume my OT is probably trashed to.. I need to read up and figure out how to test the OT?? and see if it survived...

[Phil_S]

Some basic tests on the OT are probably good enough to tell you if it's alive.

Disconnect it. Use your Ohm meter to check windings for continuity. Where there is a CT, it should be reasonably symmetrical, but not exactly 50/50 because the outer half of the winding has more length to make the same number of turns.

If it checks out on the Ohms, then hook up the secondary to a low voltage AC source. You can use your 6.3V filament supply for this. While under load, use your meter to check AC volts in and AC volts out. These numbers will give you the turns ratio.

I don't know what is appropriate for a 5E7, but I can give a rough example. If the primary is supposed to be 3.4K and the secondary 2 ohms, the turns ratio is SQRT(3400)/2 = 29:1. If input voltage on the secondary winding is 7VAC, then output on the primary will be about 204VAC. You see, 204/7 = ~29.

OK, go do the test!

[playonit]

Thanks for the quick response Phil...... I was wondering if you would talk me through the DMM checks for the OT?

Disconnect it: Remove from chassis? or just leads (what leads)?
Where exactly do I check? CT to ? I assume the CT is the 4k red center wire?

then hook up the secondary to a low voltage AC source. You can use your 6.3V filament supply for this. While under load, use your meter to check AC volts in and AC volts out. These numbers will give you the turns ratio. can you walk me through this ?

[/url]

http://www.classictone.net/40-18006.pdf

[Phil_S]

OK, here it is in detail. Disconnect means all wires should be free of connections. There some choices to make. Leave it bolted to the chassis for now.

The primary outer legs go to pin 3 of the power tube sockets. Pull the tubes and that's it for those two wires. They aren't connected to anything.

The primary CT is routed to pin 4 as a tie point. You can disconnect at the junction of the choke and the 10K at the filter caps or disconnect from pin 4. Do whatever is easiest. Pin 4 at V5 looks like the place to desolder it. The drawing shows this as a red wire, but I'd feel better telling you from a picture of the actual amp. It is fairly standard to have a red CT, with blue and brown as the outer legs.

On the secondary, lift (disconnect) the ground wire. This can be done at the chassis or at the jack, whichever is easier for you.

Now you are disconnected. If you are going to use the filament supply for the voltage test, pull all the tubes. That test is second.

First, set the meter to Ohms. Check the primary. Blue to red and brown to red (no don't use the chassis for any measurements described) should be about the same, not equal, and blue to brown should be the sum of the two halves.

Now, check the secondary. It will be very low ohms. Just look for whether it is open or not. put one meter probe on the "0" lug of the jack, and the other meter probe on the 8 and then the 16. In theory, the 8 should be 2x the ohms of the 16 (yeah, this isn't backwards), but your meter may not be able to give a reading. This is only a problem if the meter indicates Open. If it indicates open, you need go no further, sorry, that's bad news.

Be sure to get a good contact. Use clips, as they generally assure a better contact. I don't want you to see a false Open because you didn't make a a good contact.

Use two jumper wires with 'gator clips. You have a 5V supply at the rectifier. Use that. One jumper wire goes from V6 pin 2 to the "0" wire on the OT secondary. You can clip it right onto the output jack lug. The other wire goes from V6, pin 8 to the 8 Ohm OT wire, and you can clip this right to the jack, too. Make sure the clips don't make any stray connections.

Now, with the jumpers in place, turn on the power. (Set the meter to measure VAC.) This will send 5VAC into the secondary. I don't know what the primary is supposed to be, but you'll see roughly 200VAC on the primary if the OT is good, probably a bit less than that. First probe the source voltage at V6, pins 2 and 8 (no chassis contacts!!!) to know the actual voltage under load. Let's say it is 5.5V so I can give an example later, but you'll use whatever number you get on the meter.

Then meter the primary, using clips, one on V5 pin 3, the other on V4 pin 3. Let's say you see 220VAC.

Do the math: 220/5.5 = 40. That's the turns ratio 40:1. So, you know the reading was taken from an 8 ohm output. The square of the turns ratio is the impedance ratio. In this example, the primary is 1.6K (40 * 40). Your turns ratio should be closer to 20:1.

Can you try this? If you are unsure, post another question. Report results.

Edit: I didn't see your link to the OT schematic. Yes, you have a 4K primary. Red is the primary CT. I see there is a 4 ohm secondary (yellow). The 4 ohm will have the highest turns ratio of about 31.5:1. You should test with that if you can. This is a 40W OT. Input of 5 or 6v on the secondary shouldn't be a problem at all.

[playonit]

OK, here it is in detail. Disconnect means all wires should be free of connections. There some choices to make. Leave it bolted to the chassis for now.

The primary outer legs go to pin 3 of the power tube sockets. Pull the tubes and that's it for those two wires. They aren't connected to anything.

The primary CT is routed to pin 4 as a tie point. You can disconnect at the junction of the choke and the 10K at the filter caps or disconnect from pin 4. Do whatever is easiest. Pin 4 at V5 looks like the place to desolder it. The drawing shows this as a red wire, but I'd feel better telling you from a picture of the actual amp. It is fairly standard to have a red CT, with blue and brown as the outer legs.

On the secondary, lift (disconnect) the ground wire. This can be done at the chassis or at the jack, whichever is easier for you.

Now you are disconnected. If you are going to use the filament supply for the voltage test, pull all the tubes. That test is second.

First, set the meter to Ohms. Check the primary. Blue to red and brown to red (no don't use the chassis for any measurements described) should be about the same, not equal, and blue to brown should be the sum of the two halves.

Now, check the secondary. It will be very low ohms. Just look for whether it is open or not. put one meter probe on the "0" lug of the jack, and the other meter probe on the 8 and then the 16. In theory, the 8 should be 2x the ohms of the 16 (yeah, this isn't backwards), but your meter may not be able to give a reading. This is only a problem if the meter indicates Open. If it indicates open, you need go no further, sorry, that's bad news.

Be sure to get a good contact. Use clips, as they generally assure a better contact. I don't want you to see a false Open because you didn't make a a good contact.

Use two jumper wires with 'gator clips. You have a 5V supply at the rectifier. Use that. One jumper wire goes from V6 pin 2 to the "0" wire on the OT secondary. You can clip it right onto the output jack lug. The other wire goes from V6, pin 8 to the 8 Ohm OT wire, and you can clip this right to the jack, too. Make sure the clips don't make any stray connections.

Now, with the jumpers in place, turn on the power. (Set the meter to measure VAC.) This will send 5VAC into the secondary. I don't know what the primary is supposed to be, but you'll see roughly 200VAC on the primary if the OT is good, probably a bit less than that. First probe the source voltage at V6, pins 2 and 8 (no chassis contacts!!!) to know the actual voltage under load. Let's say it is 5.5V so I can give an example later, but you'll use whatever number you get on the meter.

Then meter the primary, using clips, one on V5 pin 3, the other on V4 pin 3. Let's say you see 220VAC.

Do the math: 220/5.5 = 40. That's the turns ratio 40:1. So, you know the reading was taken from an 8 ohm output. The square of the turns ratio is the impedance ratio. In this example, the primary is 1.6K (40 * 40). Your turns ratio should be closer to 20:1.

Can you try this? If you are unsure, post another question. Report results.

[selloutrr]

[EDIT] Nevermind

[Phil_S]

OK, I see the picture now. To get to the secondary, I would clip the black lead (secondary 0) and leave it in the air. Just be careful with it. This will allow you to leave the green secondary ground alone. You can get to the yellow 4 ohm lead at the selector switch.

When you reconnect the black wire, do this. Run a bit of solid wire through all 4 of the sleeve lugs on the output jacks. It looks as if you may have already done this, but it is not in the picture. Connect the black wire at the end closer to where it enters the chassis. It doesn't matter where you connect it on that sleeve buss - either end is the same.

What are you doing with the buss wire on the output jack tips? I can't really see. It looks like you have to output jacks in parallel on the same tap. I hope you are clear that two 8 ohm speaker loads plugged in at the same time is a 4 ohm load. It also means, you shouldn't use a 4 ohm and an 8 ohm load at the same time, as that's 6 ohms or 4+16, which is 10 ohms.

Also, I hope you didn't ground the yellow wire. Are you using that nut on the tube socket as the OT ground? That ground goes to the same ground as the power CT, though you might get away with it.

I see, for your chassis ground, you have not followed the "best practice." You are using a transformer bolt. You have plenty of room. Drill a hole in the chassis and use a stud dedicated to making the ground. If you chassis is aluminum, use a bit of NoAlOx or similar compound to prevent oxidation. All grounds should be made with wires soldered to ring lugs. Then, use a toothed washer and double bolt to make sure it stays tight.

Where are you making the grounds for the parts on the board? Wire under the board? That's OK. I've learned not to do that because I can't see it and I can't service it. I don't like it, but that's personal and there is nothing wrong with the practice of placing a wire under the board.

[playonit]

OK, I see the picture now. To get to the secondary, I would clip the black lead (secondary 0) and leave it in the air. Just be careful with it. This will allow you to leave the green secondary ground alone. You can get to the yellow 4 ohm lead at the selector switch.

When you reconnect the black wire, do this. Run a bit of solid wire through all 4 of the sleeve lugs on the output jacks. It looks as if you may have already done this, but it is not in the picture. Connect the black wire at the end closer to where it enters the chassis. It doesn't matter where you connect it on that sleeve buss - either end is the same.

What are you doing with the buss wire on the output jack tips? I can't really see. It looks like you have to output jacks in parallel on the same tap. I hope you are clear that two 8 ohm speaker loads plugged in at the same time is a 4 ohm load. It also means, you shouldn't use a 4 ohm and an 8 ohm load at the same time, as that's 6 ohms or 4+16, which is 10 ohms.

Also, I hope you didn't ground the yellow wire. Are you using that nut on the tube socket as the OT ground? That ground goes to the same ground as the power CT, though you might get away with it.

I see, for your chassis ground, you have not followed the "best practice." You are using a transformer bolt. You have plenty of room. Drill a hole in the chassis and use a stud dedicated to making the ground. If you chassis is aluminum, use a bit of NoAlOx or similar compound to prevent oxidation. All grounds should be made with wires soldered to ring lugs. Then, use a toothed washer and double bolt to make sure it stays tight.

Where are you making the grounds for the parts on the board? Wire under the board? That's OK. I've learned not to do that because I can't see it and I can't service it. I don't like it, but that's personal and there is nothing wrong with the practice of placing a wire under the board.

Phil.. thanks for the tips on the grounds I will fix them up. I took a lot of well deserved flack for not using the correct wire color codes when I posted pics when the build was first finished some time back..

Here's where I am at:

primary's

blue to red - 116Ω

brown to red - 116Ω

brown to blue - 233Ω


Secondary's

8ohm - 0.2 Ω

16ohm - 1.0 Ω

also when metering both 8 and 16 I get my meter's continuity beeps

I just made the 2 jumpers and will attach at the 5v pins
tomorrow.. and let you know what I find there..

Thanks for the help... I will update later.....

Cheers Greg

[martin manning]

...put one meter probe on the "0" lug of the jack, and the other meter probe on the 8 and then the 16. In theory, the 8 should be 2x the ohms of the 16 (yeah, this isn't backwards), but your meter may not be able to give a reading.

Phil, I think that is backwards, and in theory the resistance from the 16-ohm tap to the common should be 1.4x the resistance of the 8-ohm to the common.

It also means, you shouldn't use a 4 ohm and an 8 ohm load at the same time, as that's 6 ohms or 4+16, which is 10 ohms.

This is not making sense to me. If you mean a 4-ohm speaker connected to the 4-ohm tap and an 8-ohm speaker connected to the 8-ohm tap at the same time, the reflected primary impedance will be half of what it should be, same as if two 8-ohm speakers in parallel were connected to the 8-ohm tap.

[Phil_S]

Hello Martin. Thank you for taking me to task. I know that sometimes I write things that I shouldn't. In this case, I think I was simply not clear. Let's see if I can do better.

...put one meter probe on the "0" lug of the jack, and the other meter probe on the 8 and then the 16. In theory, the 8 should be 2x the ohms of the 16 (yeah, this isn't backwards), but your meter may not be able to give a reading.

Phil, I think that is backwards, and in theory the resistance from the 16-ohm tap to the common should be 1.4x the resistance of the 8-ohm to the common.

No disagreement here. I didn't say what I really meant. The 4 ohm tap, having the fewest turns will have the biggest turns ratio and also produce the largest voltage differential. I reasoned, the larger spread between the two voltage readings improves the ratio calculation; this is an accuracy/tolerance thing, assuming a cheap meter is in use. Thinking about it again, I suppose the proper test is the check all three taps on the OT in case there is a short on one of the outer (8, 16) windings.

It also means, you shouldn't use a 4 ohm and an 8 ohm load at the same time, as that's 6 ohms or 4+16, which is 10 ohms.

This is not making sense to me. If you mean a 4-ohm speaker connected to the 4-ohm tap and an 8-ohm speaker connected to the 8-ohm tap at the same time, the reflected primary impedance will be half of what it should be, same as if two 8-ohm speakers in parallel were connected to the 8-ohm tap.

Martin, maybe I didn't interpret the photo correctly? I see an impedance selector switch to which all three taps are wired. There is one jumper from the switch to the output jacks going to a buss across all four tip lugs on the output jacks. That means there are two jacks parallel to whatever single OT secondary has been selected at the switch. Using two loads of different impedance means the total load is the sum of the loads divided by 2.

Of course, if the jacks were wired directly to the OT without the selector switch, then, well....if you've got three jacks 4-8-16, and you want to plug in 8 and 16 ohm loads at the same time, you plug the 8 into the 4 and the 16 into the 8. The setup I see in the picture, it looks to me like this isn't applicable. Maybe it's just me, but I'd only use one jack with a switch because, as a practical matter, two jacks allows potential mismatch. Then again, maybe he's got two 16 ohm loads, which will work great with the 8 ohm secondary selected; or two 8 ohm loads with the 4 ohm secondary selected.

[Phil_S]

primary's
blue to red - 116Ω
brown to red - 116Ω
brown to blue - 233Ω

Secondary's
8ohm - 0.2 Ω
16ohm - 1.0 Ω
also when metering both 8 and 16 I get my meter's continuity beeps

These readings look very reasonable to me. Just based on those readings, I'm going out on a limb and saying your OT is OK. If you are able to contact the manufacturer, they will tell you if those readings are as they should be. I imagine that it should be fairly easy to get someone at Magnetic Components to confirm the numbers.

If it was mine, I'd skip the voltage test at this point and look for some other problem. If you are all set, there is no harm doing the test. For 5VAC input, you should see about 79-112-158 VAC on the primary side depending on which tap you select. Likely you will see numbers a bit higher, as I expect your 5V supply will be closer to 6V and there isn't much of a load provided by the OT as compared to the rectifier heater that's meant for that 5V supply. Since you have a selector switch, and you have those nice buss wires on the jacks, you can just clip one jumper to each buss (tip, sleeve). Then rotate the switch through all three selections and you'll have your readings very quickly.

[playonit]

OK, here it is in detail. Disconnect means all wires should be free of connections. There some choices to make. Leave it bolted to the chassis for now.

The primary outer legs go to pin 3 of the power tube sockets. Pull the tubes and that's it for those two wires. They aren't connected to anything.

The primary CT is routed to pin 4 as a tie point. You can disconnect at the junction of the choke and the 10K at the filter caps or disconnect from pin 4. Do whatever is easiest. Pin 4 at V5 looks like the place to desolder it. The drawing shows this as a red wire, but I'd feel better telling you from a picture of the actual amp. It is fairly standard to have a red CT, with blue and brown as the outer legs.

On the secondary, lift (disconnect) the ground wire. This can be done at the chassis or at the jack, whichever is easier for you.

Now you are disconnected. If you are going to use the filament supply for the voltage test, pull all the tubes. That test is second.

First, set the meter to Ohms. Check the primary. Blue to red and brown to red (no don't use the chassis for any measurements described) should be about the same, not equal, and blue to brown should be the sum of the two halves.

Now, check the secondary. It will be very low ohms. Just look for whether it is open or not. put one meter probe on the "0" lug of the jack, and the other meter probe on the 8 and then the 16. In theory, the 8 should be 2x the ohms of the 16 (yeah, this isn't backwards), but your meter may not be able to give a reading. This is only a problem if the meter indicates Open. If it indicates open, you need go no further, sorry, that's bad news.

Be sure to get a good contact. Use clips, as they generally assure a better contact. I don't want you to see a false Open because you didn't make a a good contact.

Use two jumper wires with 'gator clips. You have a 5V supply at the rectifier. Use that. One jumper wire goes from V6 pin 2 to the "0" wire on the OT secondary. You can clip it right onto the output jack lug. The other wire goes from V6, pin 8 to the 8 Ohm OT wire, and you can clip this right to the jack, too. Make sure the clips don't make any stray connections.

Now, with the jumpers in place, turn on the power. (Set the meter to measure VAC.) This will send 5VAC into the secondary. I don't know what the primary is supposed to be, but you'll see roughly 200VAC on the primary if the OT is good, probably a bit less than that. First probe the source voltage at V6, pins 2 and 8 (no chassis contacts!!!) to know the actual voltage under load. Let's say it is 5.5V so I can give an example later, but you'll use whatever number you get on the meter.

Then meter the primary, using clips, one on V5 pin 3, the other on V4 pin 3. Let's say you see 220VAC.

Do the math: 220/5.5 = 40. That's the turns ratio 40:1. So, you know the reading was taken from an 8 ohm output. The square of the turns ratio is the impedance ratio. In this example, the primary is 1.6K (40 * 40). Your turns ratio should be closer to 20:1.

Can you try this? If you are unsure, post another question. Report results.

Edit: I didn't see your link to the OT schematic. Yes, you have a 4K primary. Red is the primary CT. I see there is a 4 ohm secondary (yellow). The 4 ohm will have the highest turns ratio of about 31.5:1. You should test with that if you can. This is a 40W OT. Input of 5 or 6v on the secondary shouldn't be a problem at all.

First probe the source voltage at V6, pins 2 and 8 (no chassis contacts!!!)

Are you saying one meter probe on V6/2 the other on V6/8 at the same time??

Then meter the primary, using clips, one on V5 pin 3, the other on V4 pin 3. [/b]

Clip one meter lead to V5/3 and the other lead to V4/3 at the same time??

[Phil_S]

First probe the source voltage at V6, pins 2 and 8 (no chassis contacts!!!)
Are you saying one meter probe on V6/2 the other on V6/8 at the same time??

Yes, one probe on each pin. This is generally described as "across 2 and 8." This tells you the input voltage, which you will need as the divisor/denominator to calculate the turns ratio. This measurement needs to be under the load of the transformer primary to be accurate.

This is likely to change a little bit depending on which secondary is connected, but probably not enough to matter since the DCR is so low on the secondary.

Point to chassis is for measure DCV.

Then meter the primary, using clips, one on V5 pin 3, the other on V4 pin 3.
Clip one meter lead to V5/3 and the other lead to V4/3 at the same time??

Yes, measure across V5 and V4, pin 3 on both. This is AC, no chassis ground here. This number is your numerator in the turns ratio calc. As I said before, this is probably in the 100V range, give or take, see earlier post for more precise estimates.

I don't think you will find a problem here based on the DCR you reported.