Troubleshooting an Ampeg B-15N

[sleeplessknight]

So, a theater I've been doing a lot of work for has an old Ampeg B-15N in their back room. I plugged it in, and while the tubes light up and flicker a bit when I play my bass into it, no sound comes from the speaker. I've spoken with the sound guy about the amp, and he's of the impression that the theater might be looking to get rid of it since A. it doesn't work and they have no money to repair it, and B. all the bassists that work there tend to bring their own amplification anyway. I'd like to make 'em an offer for it and see if I couldn't take it to my regular repair guy, but I have no idea what it's worth since it's non-working. Are there any 'quick troubleshooting tips' I could try to make sure the important bits are functional, or maybe even get the amp in a working state? What would be a fair offer to make to the theater for a 'non-working B15, with an unknown ceiling for repair bills'?

[BassmanPaul]

My first take on this is that if the output tubes glow blue and flicker when you play that the head is basically working. I suspect that either the speaker is blown or the speaker cable is broken.

[sleeplessknight]

Can I stick a multimeter on the cable and see what happens? If so, what could I set my 'meter to (what resistances, current, etc), and what pins would I put the positive/ground probes on?

[vin*tone]

The speaker should see about 40 Volts AC IIRC but a better way to test the speaker is to plug another amp into it. If there's a break there the amp would have been essentially running without a speaker load which isn't great news for the output tranny

Could also be worthwhile checking the 'amplifier' plug on the back which serves as both a power amp input and a pre amp out.

[coreyfyfe]

Originally Posted by sleeplessknight

Can I stick a multimeter on the cable and see what happens? If so, what could I set my 'meter to (what resistances, current, etc), and what pins would I put the positive/ground probes on?

You could try that. Use the lowest resistance setting that gives you a reading, the speaker should be 8 ohms so it will probably read around 6ish. If you don't have a 4-pin to 1/4" converter just lift off the head and touch where the leads off of the speaker are connected to the output on the side of the cab, that should work as well. If it reads as an open connection or a short you've found your problem.

I'm no where near as experienced as paul, but what he said sounds about right to me - if you're getting response from the tube as you play it could be as simple as a dead speaker.

Worst case, you're looking at a few hundred to fix it up if it needs a new output tranny. I got mine for $500 as a fixer, but it was actually in playable condition. If it's in that state of disrepair you'll probably need to do a cap job, but that should only run about $100. If you decide to pass but they're still looking to sell it for a reasonable price let me know, I might be interested (and I'm in your area ).

[sleeplessknight]

Originally Posted by vin*tone

The speaker should see about 40 Volts AC IIRC but a better way to test the speaker is to plug another amp into it. If there's a break there the amp would have been essentially running without a speaker load which isn't great news for the output tranny

Could also be worthwhile checking the 'amplifier' plug on the back which serves as both a power amp input and a pre amp out.

Well, the speaker input on the B15 is this weird 4-pin plug I've never seen before. I could plug my EA amp into the B15, but I'd need to find a Speakon-to-Ampeg adapter (or wire one up). I think it might be easier to go the multimeter route, but what's the pinout on that 4-pin Ampeg plug?

Edit: d'oh, I didn't read all of Coreyfyfe's post. I'll do exactly that tomorrow, thanks for the idea!

[JimmyM]

btw, the pinout is 1 and 4 are your speaker wires and 2 and 3 are wired together to somehow prevent you from taking it out of standby when not plugged into a speaker.

[jimbilly]

Don't try to play it anymore, if the wires to the speaker are simply disconnected you'll be killing the output transformer. I'll bet you can get it cheap enough to fix, since it's busted. I'd offer them $200 if it's blue, $100 if it's black.

[sleeplessknight]

Oh it's blue black, baby. Dusty, a little beat looking, with a hole in the grill, but very, very blue black :-)

[beans-on-toast]

There are some simple tests that you can do.

Remove the tube cage and mark the tubes so that you know which tube is from which socket. The first 6SL7 is for channel 1, the second is for channel 2, the third is the phase inverter, the next two tubes are the 6L6 power tubes, the last tube is the 5AR4 rectifier tube.

Remove the two 6L6 power tubes. Connect the speaker cable. Connect an instrument cable from the ext amp jack at back of the amp to the input of another instrument amp. This will allow you to test the pre-amp. Test each channel through the range of volume and tone settings. The treble and bass controls at noon is flat. If a channel doesn't work, try swapping the phase inverter tube with the 6SL7 for that channel. If the pre-amp works, still try swapping one of the channel 6SL7's with the phase inverter tube. This allows you to test the phase inverter tube. Replace all tubes as they were originally.

If it worked so far, you have tested the pre and the first three tubes in the amp.

Do a battery test on the speaker. Get a small battery, any voltage will do. I like to use a 9V. Disconnect the speaker cable. Attach a wire between the the (-) battery terminal and one of the speaker terminals. Connect another wire to the other speaker terminal. While looking at the back of the speaker cone, MOMENTARILY (do not hold the wire on the terminal, just quickly tap the connection on and off) make contact with the other end of wire and the (+) terminal. If the speaker cone moves in or out, the speaker coil is not burnt out. Reverse the battery terminals and repeat the test, the speaker cone should move in the opposite direction than it did before.

If this works, the problem is in the power amp, output transformer, or the speaker cable. Replace the 6L6 power tubes and test the amp. If something is blowing the tubes, your test set would be at risk with this test.

Otherwise you are going to have to open the amp to perform further tests. They are best performed by a tech.The speaker cable should be tested. The power amp bias circuit and voltage as well as the power supply components and voltages should be tested. Then check the output transformer.

[lbwdog]

Just ask 'em "how much fer it?". They may give it to you. They probably don't know its vintage value. (whatever that is)

[stiles72]

Originally Posted by lbwdog

Just ask 'em "how much fer it?". They may give it to you. They probably don't know its vintage value. (whatever that is)

+1 That's pretty much how I got my BT-15C

[aborgman]

Originally Posted by beans-on-toast

Remove the two 6L6 power tubes. Disconnect the speaker cable. Connect an instrument cable from the ext amp jack at back of the amp to the input of another amp. This will allow you to test the pre-amp.

Note - you'd better be driving the instrument input of another instrument amp, not the line level input of a power amp. The input impedance of a standard power amp is too low for the "ext. amp" jack of a B-15 to drive.

[BassmanPaul]

Originally Posted by JimmyM

btw, the pinout is 1 and 4 are your speaker wires and 2 and 3 are wired together to somehow prevent you from taking it out of standby when not plugged into a speaker.

Those two pins complete the ground from the power supply. It is in essence a secondary standby switch.

[BassmanPaul]

Originally Posted by beans-on-toast

There are some simple tests that you can do.

Disconnect the speaker cable. Connect an instrument cable from the ext amp jack at back of the amp to the input of another amp. This will allow you to test the pre-amp.

You've forgotten that removing the speaker cable disconnects the power. This test will work if the cable is left connected.

[beans-on-toast]

Originally Posted by BassmanPaul

You've forgotten that removing the speaker cable disconnects the power. This test will work if the cable is left connected.

It was a hard day's night and I was more than half asleep. Thanks Paul and Aborgman. I'll fix my post above.

Yes, the speaker cable needs to be connected so that pins 2 and 3 are shorted.

[sleeplessknight]

Sounds good guys, I'll hit up the amp with a 9-volt battery and my multimeter tonight, and post what I find! Thanks so much!

--Lee

[jimbilly]

I'd buy it before fixing it. If you fix it then try to buy it, you'll have to pay a lot more.

[vin*tone]

Originally Posted by beans-on-toast

There are some simple tests that you can do.

Remove the tube cage and mark the tubes so that you know which tube is from which socket. The first 6SL7 is for channel 1, the second is for channel 2, the third is the phase inverter, the next two tubes are the 6L6 power tubes, the last tube is the 5AR4 rectifier tube.

Remove the two 6L6 power tubes. Connect the speaker cable. Connect an instrument cable from the ext amp jack at back of the amp to the input of another instrument amp. This will allow you to test the pre-amp. Test each channel through the range of volume and tone settings. The treble and bass controls at noon is flat. If a channel doesn't work, try swapping the phase inverter tube with the 6SL7 for that channel. If the pre-amp works, still try swapping one of the channel 6SL7's with the phase inverter tube. This allows you to test the phase inverter tube. Replace all tubes as they were originally.

If it worked so far, you have tested the pre and the first three tubes in the amp.

Do a battery test on the speaker. Get a small battery, any voltage will do. I like to use a 9V. Disconnect the speaker cable. Attach a wire between the the (-) battery terminal and one of the speaker terminals. Connect another wire to the other speaker terminal. While looking at the back of the speaker cone, MOMENTARILY (do not hold the wire on the terminal, just quickly tap the connection on and off) make contact with the other end of wire and the (+) terminal. If the speaker cone moves in or out, the speaker coil is not burnt out. Reverse the battery terminals and repeat the test, the speaker cone should move in the opposite direction than it did before.

If this works, the problem is in the power amp, output transformer, or the speaker cable. Replace the 6L6 power tubes and test the amp. If something is blowing the tubes, your test set would be at risk with this test.

Otherwise you are going to have to open the amp to perform further tests. They are best performed by a tech.The speaker cable should be tested. The power amp bias circuit and voltage as well as the power supply components and voltages should be tested. Then check the output transformer.

Some good tips there.

Slight derail to ask your expertise.....Channel 1 is out on my '65 B15N and swapping the tubes gives no change (Channel 2 still works). Any suggestions on what the problem might be? I've been running it fine through Channel 2 for over a year now.

[beans-on-toast]

Originally Posted by vin*tone

Channel 1 is out on my '65 B15N and swapping the tubes gives no change (Channel 2 still works). Any suggestions on what the problem might be?

If the tube is fine, the next step would be to open the amp and take a look at the circuits related to channel 1. This part of the amp is relatively simple. Most of the time, in a case of a channel going out, the problem is a broken wire or a bad solder joint. It should be an inexpensive fix if you take it to a tech.

It can be very dangerous to poke around the circuits unless the amp is unplugged and the capacitors are fully discharged. You have to know what you are doing to undertake this.

I would do the following:

Make sure that the input jacks are clean and the nuts are tight.

Visually inspect the circuits related to channel 1, look for any obvious broken wires, bad solder connections, or stray wire strands that could be causing a short circuit. Sometimes a connection can be broken but it looks fine. You need to physically check by carefully pulling the leads wherever there is a solder joint. This has to be done carefully. Every time you move an old wire, you risk breaking it.

Check the continuity of each wire with a resistance meter. The gray shielded wires from the input jack and tone controls are very delicate and prone to breaking.

If you are lucky, you would have found a bad connection. This is a very common problem and an easy fix by re-soldering the joint.

The tube socket contacts need to be inspected, cleaned and pinched closed a bit (re-tensioned) if necessary.

Test the volume and tone pots with an ohm meter. There are three solder lugs on each pot. Check the resistance across the two outside lugs and that the resistance changes between the middle lug and either outside lug as you rotate the pot. You are checking that you have some resistance, not looking if the values are correct. Proper readings can not always be taken when a component is in the circuit. The other components in the circuit affect the readings.

Now that the visual, physical, and continuity tests are done, the next step is to power up the amp and check that the power is getting to the circuits. The DC voltages are indicated on some of the B-15 schematics. If a schematic isn't available, readings can be taken from the working channel 2 and used as a reference.

If the problem still isn't found, you will need to use a signal tracer and/or scope to follow the signal through the circuits and find the problem. It might be necessary to unsolder components to test them.

Hope that this helps.