I think I broke my Ampeg B-15N

[Landy67]

I was messing around tonight and cranked up the 1960's era amp, and I suddenly started getting loud static out of it over the sound of the bass. I tried moving the cable to the 2nd channel and even removing the instrument cable entirely, and I'm still getting the static with nothing connected.

The volume control doesn't affect the sound, so does this mean I messed up the output tube?

Is there something I can do to troubleshoot the problem, or should I seek professional help?

 

[David Jayne]

Could be the tube (s? Don't know how many they have). Or it could be a capacitor. My advice is to bring it to a tech. If you don't really know your stuff, you could make it worse or even get hurt (very high voltage inside even unplugged!).

 

[Sid Fang]

If you're not used to working on tube gear, I hesitate to encourage you to do too much in the way of your own troubleshooting - even in a little amp like the B15 can hold a dangerous charge in its caps long after it's been unplugged. NEVER TRY TO REPAIR A TUBE AMP IF YOU ARE NOT FAMILIAR AND COMFORTABLE WITH HIGH VOLTAGE HAZARDS AND PRECAUTIONS. Tubes can generally be replaced without having to risk contact with high voltage wiring, but those are the only user-serviceable parts.

If you know a good amp tech, and can afford it, you should take it to the shop. If not, here are some things that you can try without too much trouble/danger.

if you've got another amp that will take pre-amp output (a power amp, or even the effects loop return on some other amp), you should then take the Ext. Amp output from the Ampeg and see if the noise is there. If it isn't, your problem is in the power stage. If it is, it's the pre-amp section.

Next, the laying-on-of-hands. With the amp cold and unplugged, pull the tubes one at a time, checking the pins for corrosion and crud, and carefully re-insert them. If your previous step isolated the problem to the pre-amp, you can just do the V1 and V2 pre-amp tubes. This actually works enough of the time to be worth trying, but don't expect a miracle.

If it looks like the problem is in the pre-amp, try running the amp without first the V1 tube, then without the V2, with the amp of course completely shut down during removals and insertions. These are the pre-amp tubes for channel 1 and channel 2 respectively. The affected channel won't work, of course, but if the noise goes away, you know it was a tube and you know which one, and can replace it. If there's noise in the Ext. Amp output regardless of pre-amp tubes, take it to the shop.

If it's in the power stage, and your B15N is one of the type where the input buffer to the power stage (V3) is also a 6SL7, you can try replacing V3 with the presumed-good V2 (or V1, if you prefer to play through channel 2). If that makes the noise from the speaker go away, you've got a working single channel amp until you get a replacement tube for the pre-amp.

The design of the output stage doesn't lend itself to debugging by tube swapping, so if that's where the noise is coming from, back to plan A - find a tech.

I was messing around tonight and cranked up the 1960's era amp, and I suddenly started getting loud static out of it over the sound of the bass. I tried moving the cable to the 2nd channel and even removing the instrument cable entirely, and I'm still getting the static with nothing connected.

The volume control doesn't affect the sound, so does this mean I messed up the output tube?

Is there something I can do to troubleshoot the problem, or should I seek professional help?

 

[Landy67]

Thank you! That is exactly what I was looking for. I'm going to print this out and keep it.

I used to fix TV's and radio's in the mid 70's, so I remember those residual charges you're talking about. It's also been long enough that I'm a little fuzzy about where the sparks used to come from, so I'll take it to a tech if it's anything more than a tube.

 

[Landy67]

The problem is a tube, and it's one of the pre-amp tubes. Its a Sylvania tube and it has a piece of masking tape on the base and it says 6SL7. It's the tube for channel 1, without the tube, channel 2 works just fine.

Any recommendations on a replacement? Should I buy a modern replacement or a vintage tube? Should I buy a matched set, or a mix of brands?

Here's a funny note... The last time I bought tubes was in 1976. Sylvania had a promotion at the time, where if you bought 12 tubes you got an indian head penny. I still have 8 of those penny's from the promotion. Looking at the price of tubes today, I wish I had saved some of the tubes!

 

[B-string]

I personally would go with old production while you can Invalid Link Removed buy at least 2.

 

[JimmyM]

NOS. Word I'm getting from the 6SL7 crowd is that none of the currently made ones are too hot, and I wasn't a fan of the Sovteks my tech used to replace a couple bum preamp Sylvanias I had in it. I sort of was OK with the Sovteks at first, but the Sylvanias had more output and more overall balls. And I think that's why Ampeg went with NOS JAN Philips 6188's (same as Sylvania's 6SL7's) for the Heritage B-15, and 12AX7's for the cheaper Heritage B-15N, as it's much easier to find good 12AX7's currently being made than good 6SL7's.

 

[Sid Fang]

Thank the Bass Gods (and, I suppose, Vint Cerf and company) for the Internet. Back in the 1980s, I used to go crazy rummaging around the bricks-and-mortar electronics shops of the SF Bay area looking for preamp tubes for my old Ampegs - praise be unto Al Lasher's in Berkeley. Now I can put "NOS 6SL7" into Google and get 68,000 hits (not all of which are tubes for sale, but still...).

I personally would go with old production while you can Invalid Link Removed buy at least 2.

Glad we were able to save you a trip to the repair shop...

 

[Sid Fang]

The problem is a tube, and it's one of the pre-amp tubes. Its a Sylvania tube and it has a piece of masking tape on the base and it says 6SL7. ... Should I buy a matched set, or a mix of brands?

Matched sets are only important for push/pull pairs in output stages, to keep the biasing reasonable. For this pre-amp, you just want one that sounds good.

 

[Landy67]

I may still have to take it to the shop. I just noticed that there's a coil of some kind that's blocking one of the holes that's used to mount the head to the chassis tray, and I'm going to have to have that moved.

I personally would go with old production while you can Invalid Link Removed buy at least 2.

B-string, how do you know these are NOS? i don't see anything on the page that said old stock. When I did a Google search, I find the prices are everywhere. I've seen NOS 6SL7's for anywhere from $5 to $400.

Also, what's the difference when they add the GT at the end of the number? Are they interchangeable?

 

[B-string]

Antique radio lists new production tubes separately and used separately as well. Old production can be from various manufacturers unless specifically stated. GT is the glass tube. Without the G or GT at the end they could be the old metal cans and do not interchange.

 

[JimmyM]

B-string, how do you know these are NOS? i don't see anything on the page that said old stock. When I did a Google search, I find the prices are everywhere. I've seen NOS 6SL7's for anywhere from $5 to $400.

Also, what's the difference when they add the GT at the end of the number? Are they interchangeable?

Yes, they are interchangeable. As for those tubes being NOS, I don't think they are, either. But you don't have to spend $400 on one! Ampeg used Sylvanias mostly back in the day, so that's what I usually get, and you can get them for $20-25.

 

[B-string]

I have never had the metal and glass envelopes interchange with anything but poor results. YMMV, check the plate load curves before you spent the money and time.

 

[Landy67]

I found a NOS 6SL7 Sylvania tube on eBay for $15. I should have channel 1 back up and running in no time!

 

[DaveDeVille]

subscribed for pictures !!

 

[Landy67]

Matched sets are only important for push/pull pairs in output stages, to keep the biasing reasonable. For this pre-amp, you just want one that sounds good.

I tried to do my research on this before asking, but how important is it to BIAS the output tubes on the amp?
1 I found lots of conflicting info about when it's needed.
2 I don't know if this amp is fixed BIAS (I don't see anything that looks adjustable inside) or low enough wattage that it doesn't matter.

 

[JimmyM]

I've found it very unimportant to bias the power tubes. I just plop new ones in when I need them, and they always work and sound fine.

 

[coreyfyfe]

I tried to do my research on this before asking, but how important is it to BIAS the output tubes on the amp?
1 I found lots of conflicting info about when it's needed.
2 I don't know if this amp is fixed BIAS (I don't see anything that looks adjustable inside) or low enough wattage that it doesn't matter.

Depends on the era of amp. Pictures would help us ID it, specifically of the schematic under the amp chassis or the bottom of the amp chassis itself if you are willing to open it up. Be cautious as the caps can store lethal voltage.

With the 1960-early 1965 amps they were cathode biased and as long as the bias resistor and cap that are connected to the cathode good you can just plug in your matched pair of tubes and you're good. These are the B-15N, NA, NB, and NC revisions.

After early 1965 the amps were fixed biased (B-15NF revision) and the bias was set by resistors that should be checked. You* can install an adjustment to correct the bias to optimize for the pair of tubes you install, or just install some and be sure the range is correct. *Or your amp tech, I'm using "you" loosely here and not implying it's a task that should be undertaken without a thorough knowledge of what to do.

 

[Landy67]

There isn't a board with schematic...

[Invalid or Expired Link Removed]

 

[Landy67]

That Schematic seems to be the closest to what I have. This one shows a 7199 tube in the V3 position, where mine is another 6SL7, but the rest of it seems right.