'66 Ampeg B-15N In New Repair Shop, Getting Concerned

[PolkaHero]

Okay, at some point the guy who owned the '66 Ampeg B-15N that's currently in my possession modified the speaker output by installing a 1/4" speaker output cable. I took it in for repairs (bad tube socket, convert the two-prong power cable to three-prong), and the original tech couldn't fix it because whatever the previous owner did altered the schematic which threw the tech off. He did clean the contacts, tested the tubes and caps, and didn't charge me so that's cool.

I then took the amp to an authorized Ampeg repair center (where I take my SVT-VR which is still under warranty) last week and today the tech gives me a call. He said something on the external speaker jack was wired to ground incorrectly and that's what's causing problems. Then he told me the power output is 16 ohms, is that correct?

I thought the power output on these was 8 ohms. Isn't the speaker that's in the cabinet 8 ohms? Am I missing something or is the tech incorrect (he said he was reading the schematic). This amp is a classic and I want to make sure it's working correctly!

[Vintage-Blue]

It sounds to me like the tech is reading the schematic wrong. The B15N output transformer has an 8 and a 16 ohm tap. The hardwired speaker cable is normally connected to the 8 ohm tap. If an 8 ohm extension speaker is plugged in it is put in series with the internal speaker and the output is switched to the 16 ohm tap.

[PolkaHero]

Originally Posted by Vintage-Blue

It sounds to me like the tech is reading the schematic wrong. The B15N output transformer has an 8 and a 16 ohm tap. The hardwired speaker cable is normally connected to the 8 ohm tap. If an 8 ohm extension speaker is plugged in it is put in series with the internal speaker and the output is switched to the 16 ohm tap.

I think he's confused as well: he asked me if I wanted to keep the hard wired speaker cable thinking that wasn't original and instead offered to replace it with a 1/4" jack. This guy has been working on tube amps for years, wouldn't you think he would know more about these?

So what is the output impedance with both the internal and external speakers engaged, is it still 8 ohms?

[beans-on-toast]

There are three wires on the output transformer secondary: 8 ohm (yellow), 16 ohms (green), and a black ground wire.

You are correct, the speaker is 8 ohms.

When you plug the cable into the cab, the 8 ohm tap is connected to the speaker. When you plug an 8 ohm extension cab into the "ext spkr" jack on the back of the amp, the two speakers are connected internally in series (8 ohms + 8 ohms) and connected to the 16 ohm tap. As Mark said.

When removing the original cable with the 4-pin XLR and wiring in a jack, I install an isolated (shoulder washer on one side of the chassis, insulating washer on the other side -- part number S1029 and S1028) 1/4" switchcraft jack to avoid any currents in the chassis. Run a wire from the sleeve of the jack to the ground point of the phase inverter. Connect the black wire from the OPT secondary here as well. Point F on the schematic connected to the phase inverter 1K cathode resistor gets grounded at the PI as well. The tech should understand this....

[PolkaHero]

Originally Posted by beans-on-toast

There are three wires on the output transformer secondary: 8 ohm (yellow), 16 ohms (green), and a black ground wire.

You are correct, the speaker is 8 ohms.

When you plug the cable into the cab, the 8 ohm tap is connected to the speaker. When you plug an 8 ohm extension cab into the "ext spkr" jack on the back of the amp, the two speakers are connected internally in series (8 ohms + 8 ohms) and connected to the 16 ohm tap. As Mark said.

When removing the original cable with the 4-pin XLR and wiring in a jack, I install an isolated (shoulder washer on one side of the chassis, insulating washer on the other side -- part number S1029 and S1028) 1/4" switchcraft jack to avoid any currents in the chassis. Run a wire from the sleeve of the jack to the ground point of the phase inverter. Connect the black wire from the OPT secondary here as well. Point F on the schematic connected to the phase inverter 1K cathode resistor gets grounded at the PI as well. The tech should understand this....

Wow, that's some serious info there, thanks! I'll give the tech a call tomorrow and relay that info.

Now I probably know why I got shocked when I touched the chassis before. . .

[JimmyM]

No kidding? You learn something new every day...I always figured it would switch it to 4 ohms, not 16. Oh well, not like the output suffers when you use two cabs...it's still quite massive.

[beans-on-toast]

Originally Posted by JimmyM

No kidding? You learn something new every day...I always figured it would switch it to 4 ohms, not 16.

It does seem strange, 4 ohms is more intuitive. You don't see 16 ohms on a lot of amps.

[PolkaHero]

Originally Posted by JimmyM

No kidding? You learn something new every day...I always figured it would switch it to 4 ohms, not 16. Oh well, not like the output suffers when you use two cabs...it's still quite massive.

Correct me if I'm wrong, but I don't believe the output power changes on a tube amp with an impedance change.

[beans-on-toast]

I deleted my last post, I wanted to add that the tip of the new jack goes to the tip of the ext spkr jack. The 8 ohm tap of the OPT goes to the shunt (switch) on the ext spkr jack. As shown on the schematic.

[beans-on-toast]

Originally Posted by PolkaHero

Correct me if I'm wrong, but I don't believe the output power changes on a tube amp with an impedance change.

True if the cab is matched to the impedance of the amp. A 16 ohm cab (or two 8 ohm cabs connected in series as in this case) connected to a 16 ohm transformer tap will put out the same power as one 8 ohm cab connected to the 8 ohm tap.

It will be louder because you've added a second cab.

[JimmyM]

Originally Posted by PolkaHero

Correct me if I'm wrong, but I don't believe the output power changes on a tube amp with an impedance change.

Right. That was why I said that

[PolkaHero]

Amp is fine and ready to be picked up. He said something wasn't wired correctly in the output transformer and he also replaced the speaker cable as the wires were starting to fray inside. He also replaced the two-prong power cord with a three-prong, and replaced the internal light bulb which I planned on replacing anyway with a different colored one. He said it "sounds really good", no kidding it's the recording industry standard for a reason!

It's not in the best shape, here's a picture again of the head:



I think some metal polish (I have Simichrome) will take some of the oxidation off the chrome. I may also paint the transformers with a flat black, any recommendations on brand/type of paint to use? Should I take the transformers out and paint or mask everything and paint them mounted to the chassis? Any idea where I can get a replacement colored bulb for the inside?

[JimmyM]

Be careful that your chrome polish doesn't remove the lettering. I forget who makes it, but there's a certain kind of crinkly finish flat black that Ampeg used. I would definitely paint them mounted to the chassis unless you can wire them back on yourself. Someone else will have to help you with the bulb, though...I know you can get bulbs at Welcome to Fliptops -- Parts for Ampeg and other Vintage Amp Lovers but I've seen dudes buying LED bulbs with colors that are pretty cool and I don't think Fliptops sells them.

[PolkaHero]

Originally Posted by JimmyM

Be careful that your chrome polish doesn't remove the lettering. I forget who makes it, but there's a certain kind of crinkly finish flat black that Ampeg used. I would definitely paint them mounted to the chassis unless you can wire them back on yourself. Someone else will have to help you with the bulb, though...I know you can get bulbs at Welcome to Fliptops -- Parts for Ampeg and other Vintage Amp Lovers but I've seen dudes buying LED bulbs with colors that are pretty cool and I don't think Fliptops sells them.

Thanks for that info, I don't think I'll touch the front plate with the Simichrome, just the top portion. I thought I read on here that someone painted the bottom of the lucite logo with fingernail polish and achieved similar results. Maybe I'll go that route, I'm thinking a neon blue!

[JimmyM]

If you do that, I'd recommend getting a new one and not messing up the original.

[Ben Clarke]

ext speaker jacks in series were pretty typical back then. Pete-era Traynors are the same.

[teleharmonium]

Painting the transformers will definitely hurt the value of the amp.

I like the 4 pin cannon jack as originally used by Ampeg; wired correctly it does protect you from a no-load situation.

[JimmyM]

Originally Posted by teleharmonium

Painting the transformers will definitely hurt the value of the amp.

I see your point, but I don't know how that plays out on the market. I've seen a lot of B-15's with replaced transformers getting top dollar, so I believe as long as it's done well, it probably won't hurt the value. However, there are folks out there who would feel differently. My advice is don't sell to them if you ever want to sell it

[PolkaHero]

Originally Posted by teleharmonium

Painting the transformers will definitely hurt the value of the amp.

I like the 4 pin cannon jack as originally used by Ampeg; wired correctly it does protect you from a no-load situation.

Do I detect a purist?

[PolkaHero]

Originally Posted by JimmyM

If you do that, I'd recommend getting a new one and not messing up the original.

Where does one find new Ampeg lucite logos?