Fender Silverface Musicmaster Bass w/no Mains Fuse?? Install one!

[SoulFetish]

We had a 70's Musicmaster Bass amp that came in for general cleaning/service (dates to '79, it looks like).
The molded plug end had been replaced by an old 3-prong replacement with screw terminals. We see this a lot, and I always check the condition and wiring inside. More often than not, it's a disaster and it gets replaced–as this one did. One of the other routine checks I make, particularly in vintage amps, is confirming the fuse value and rating is the one specified.
So, after replacing the mains power cord with a new molded plug, I went to check the fuse and.. there was no fuse. Not like, the fuse was missing from the fuse holder. I mean, there was no mains fuse at all, and there never was in this amp!!. I double checked on the schematic and confirmed that it was designed and built without one, which is crazy to me. It has a UL sticker on the back panel as well. Apparently, fender installed what appears to be a thermal fuse in some other/later models.
Inside, there was the typical 70's fender tag strip with one of the legs of the incoming mains coupled to the chassis with the blue radial cap. The cap was removed, and installed an inline cap from the live to the power switch.
Before choosing a value, I ran the amp into clipping at full power, while monitoring the mains current draw. The mains current didn't exceed much more than ≈500mA at full output, so I chose a value of 1A/slow.
Have any of you run into this on this model?

[nosaj]

We had a 70's Musicmaster Bass amp that came in for general cleaning/service (dates to '79, it looks like).
The molded plug end had been replaced by an old 3-prong replacement with screw terminals. We see this a lot, and I always check the condition and wiring inside. More often than not, it's a disaster and it gets replaced–as this one did. One of the other routine checks I make, particularly in vintage amps, is confirming the fuse value and rating is the one specified.
So, after replacing the mains power cord with a new molded plug, I went to check the fuse and.. there was no fuse. Not like, the fuse was missing from the fuse holder. I mean, there was no mains fuse at all, and there never was in this amp!!. I double checked on the schematic and confirmed that it was designed and built without one, which is crazy to me. It has a UL sticker on the back panel as well. Apparently, fender installed what appears to be a thermal fuse in some other/later models.
Inside, there was the typical 70's fender tag strip with one of the legs of the incoming mains coupled to the chassis with the blue radial cap. The cap was removed, and installed an inline cap from the live to the power switch.
Before choosing a value, I ran the amp into clipping at full power, while monitoring the mains current draw. The mains current didn't exceed much more than ≈500mA at full output, so I chose a value of 1A/slow.
Have any of you run into this on this model?
[ATTACH=CONFIG]54144[/ATTACH]

Now that's cheap and dangerous of the bean counter.
nosaj

[Leo_Gnardo]

after replacing the mains power cord with a new molded plug, I went to check the fuse and.. there was no fuse. Not like, the fuse was missing from the fuse holder. I mean, there was no mains fuse at all, and there never was in this amp!!. I double checked on the schematic and confirmed that it was designed and built without one, which is crazy to me. It has a UL sticker on the back panel as well.

Good ol' Undertaker's Laboratories, making sure everything is safe & sound. Shows you what a load of bollocks their seal of approval is. It's the American Way!

[SoulFetish]

Yeah, this is a head scratcher. I mean, what if there was a shorted diode, or fault in the transformer? Where's just the minimum amount of protection to prevent a fire or worse?

[The Dude]

Not sure if yours has it- some didn't. Some of those amps had a sort of primitive thermal fuse. It basically looks like a thin piece of lead wire off the mains input.

[SoulFetish]

Not sure if yours has it- some didn't. Some of those amps had a sort of primitive thermal fuse. It basically looks like a thin piece of lead wire off the mains input.

Hmmm. Thanks Dude for the heads up. I’ll do another check when I get back to the shop.
I don’t remember seeing anything like that, but it may not have looked familiar to me. lemme try and find an image so I know what im looking for. I removed the original power cord, so Im near positive I didn’t see it directly connected to one of those leads.
I do remember that there were a bunch of solid wire leads that used as groundS, so I may have mistaken it for one of those.

[The Dude]

FWIW, I wasn't saying you should depend on that. You did a good thing by actually putting a fuse holder and fuse in there. More often than not, that "thermal fuse" (piece of wire) burns up and somebody solders a piece of regular lead wire in there.

[g1]

Another possibility is a transformer with an internal fuse. Such a fuse is not always shown on the schematic, but might satisfy UL.
Which schematic matches this unit?

[SoulFetish]

Another possibility is a transformer with an internal fuse. Such a fuse is not always shown on the schematic, but might satisfy UL.
Which schematic matches this unit?

I couldn't find the exact one online I was looking at earlier. I will take a pic of the one we have in our folder, if there isn't a digital copy on our server. But, if someone happens to have the different versions of this amp, it's the version with 6V6s, and does not show any fuse in the mains wiring (thermal or otherwise).

[Justin Thomas]

I did a quick search...

Edit: seems all of the "homenade" schematics show 6AQ5s... But they also look a good bit different from any official Fender schem I've seen, compared to others of thecsame period...

I found two official Fender schematics; one of which showed a "CSA Thermal Protector" & the other which showed a regular fuse. But a third "unofficial" schematic seemed common that was NOT official Fender, that did not show anything. The single Fender layout diagram I found did not show anything either.

Is it possible you're working drom the "unofficial" schematic? And thatcthe creator's example had one of those batshit bare wire fuses but it was mistaken for a piece of wire?

Justin

[SoulFetish]

I did a quick search...

Edit: seems all of the "homenade" schematics show 6AQ5s... But they also look a good bit different from any official Fender schem I've seen, compared to others of thecsame period...

I found two official Fender schematics; one of which showed a "CSA Thermal Protector" & the other which showed a regular fuse. But a third "unofficial" schematic seemed common that was NOT official Fender, that did not show anything. The single Fender layout diagram I found did not show anything either.

Is it possible you're working drom the "unofficial" schematic? And thatcthe creator's example had one of those batshit bare wire fuses but it was mistaken for a piece of wire?

Justin

It’s possible. But Im pretty sure I remember it looking like original Fender. The only thing i really not sure of (thinking about it now) is whether or not the the schematic had 6V6s or if it was the 6BQ5(EL84) version.
All the ones I found online that with the mains drawn the way Im describing have 6BQ5s, and the amp I was working on had 6V6s.

[Justin Thomas]

You mean 6AQ5? There was no 6BQ5 version. The 6AQ5 is touted as a 6V6 in a 7-pin bottle. Can you post the schem you're using? The "unofficial" ones I was seeing look kinda official but didn't have the tube outlines drawn around the tubes; reminded me more of old Gibson schematics. By the time the MMB came around I thought Fender was pretty consistent in their drawing style...

Eirher way, yeah, fuse that sucker! What an inconvenience to not have a "normal" fuse in it.

Justin

[SoulFetish]

You mean 6AQ5? There was no 6BQ5 version. The 6AQ5 is touted as a 6V6 in a 7-pin bottle. Can you post the schem you're using? The "unofficial" ones I was seeing look kinda official but didn't have the tube outlines drawn around the tubes; reminded me more of old Gibson schematics. By the time the MMB came around I thought Fender was pretty consistent in their drawing style...

Eirher way, yeah, fuse that sucker! What an inconvenience to not have a "normal" fuse in it.

Justin

I’m sorry, you’re correct. I misread it. I mean 6AQ5 (I like those tube too, I just never really see them in commercial designs.)
In any case, Ill try and get the schematic uploaded today if I get some time

[Leo_Gnardo]

I mean 6AQ5 (I like those tube too, I just never really see them in commercial designs.)

Just worked on an early 60's Gibson GA-5, pair of 6AQ5. A whopping 5.5 watts with 275V B+. Very clear, direct sounding amp. I doubt you can get full 6V6 power out of an AQ5, OTOH as long as you work within their limits, very good tubes. Treat 'em as mini 6V6, you'll do all right. I've had a back burner project to someday build a junior power JTM45 style head with 6AQ5's.

Now back to the program already in progress.

[SoulFetish]

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So here’s the schematic. It is the 6AQ5 version I was looking at. I looked inside the chassis again and it was either wired like the one show here, the thermal fuse is internal to the transformer, or it was removed at some point (Although that seems unlikely I don’t think anyone’s been in this amp yet)