Fender Musicmaster Bass oddities

[Zina]

I've a Fender Musicmaster Bass amplifier. It seems to be working all right, but there are a few odd things that worry me, and after advice (thanks, Skipburz) I decided to leave it alone for now. Here's the SQ:

- 2 inputs, 1 volume, 1 tone, no on/off switch;
- volume knob also switches the amp on/off;
- 240V/50Hz 3-pronged european powerplug;
- pilotlight glows softly if plug inserted one way, not the other;
- tonecontrol only changes sound from 1-3;
- tonecontrol decreases volume after 7.

I would be sincerely grateful for answers to the following questions:

- are these minor quirks that do not affect the amp's functioning?
- do any of these signal possible circuit trouble, or even danger?
- could I remedy any of these myself (have soldering iron), if needed?

Here are front & rear pictures of the same model from reverb.com:

https://reverb-res.cloudinary.com/im...x5nuezv3tq.jpg

https://reverb-res.cloudinary.com/im...gdnfelmk3r.jpg

[Wildcat]

Find a good local amp tech is the way I would go. Get the circuit schematic for it would be a good start. Some one here probably could dig it up.

[Zina]

Thanks for replying; the schematic is on the Web, I saw. I can't bring it to a technician for various reasons, so for now I just want to know if there's any risk of damaging the amp, or of transforming myself into a flambée.

[Sam Sherry]

You have the earlier model with 6AQ5 power tubes. I had a pleasant experience with the later 6V6 version before swearing off store-bought amps.

volume knob also switches the amp on/off

If you're planning to keep the amp this is a mid-level safety concern. A more pressing safety concern is that the amp lacks a fuse !!!!

- pilotlight glows softly if plug inserted one way, not the other

Those fixtures and bulbs are notoriously wonky and that one is 50+ years on. Start with the easy fix: Shoot some DeOxIt into the fixture and onto the bottom of the bulb. If that doesn't help change the bulb. If that doesn't help it probably reflects a more serious problem requiring analysis and attention.

- tonecontrol only changes sound from 1-3;

Could be a faded capacitor. Could be that the speaker sucks so you can't hear the change. Could be that the pot is bad or crudded -- shoot some tuner cleaner in there; it can't hurt.

- tonecontrol decreases volume after 7.

Maybe a bad pot

- are these minor quirks that do not affect the amp's functioning?

No, these are not right.

- do any of these signal possible circuit trouble, or even danger?

They more likely reflect aged pots and maybe aged caps. HOWEVER, the danger of having the power switched by the volume pot is that if the pot fails there's live AC 'at hand.' Even more dangerous on an amp with no fuse!

- could I remedy any of these myself (have soldering iron), if needed?

Sure.

First decide if you want to put time and effort into a fun, quirky model -- there is no other Fender amp with 2x 6AQ5 tubes and a transformer phase inverter.

If you do, you can UNPLUG THE AMP, discharge the filter caps, replace the filter caps, install a power switch and fuse block and change out both pots, and see what you've got. While you're at it you might as well install a 1/4" jack for the speaker so you can try different cabinets.



With a little bit of attention these turn into decent, fun amps. Rest assured that they did not receive that attention at the factory in 1974 and most haven't since.

Take care -- please! -- and all good luck.

[pingu]

Important !!!

Do not mess with a Tube amp if you don't know what you're doing

you can get a nasty belt off one
even if unplugged from the wall

(the big electrolytic Caps store a lot of energy)

[Franz 1997]

If it has a Euro 3 pin plug, they are only supposed to plug in one ( safe) way into the socket - that's the whole point of that design. If you reverse it somehow, you may be disabling the ground wire and/ or wiring the neutral amp wire to the live socket wire. Where are you using it?

I' d disgree about the switch in the volume pot though. Evans amps have used those on/off pot switches for years, and they are expensive premium amps.

[neatomic]

there are two versions of that musicmaster amp...different power tubes

sounds like a preamp tube issue...the tone stack issue

try changing the 12ax7...smaller one closest to the input jack


the 240 power wiring issue is a bit scarier...you want to make sure everything is properly grounded..dangerous if not!


cheers

[cosmic gumbo]

Yeah, European electrical house current is killer.

[Zina]

Thanks (et merci) for all your replies; I really appreciate it, and will heed all advice re. capacitors/condensateurs. Just now I realised I should have used illustrations re. the powerplug and sockets. There are 2 types:

https://upload.wikimedia.org/wikiped...wer-socket.jpg

https://www.nexxia.co.uk/wp-content/...NX-WPM-092.jpg

and the amplifier has this type of plug:

http://image.made-in-china.com/2f0j0...rd-UE-312-.jpg

When I plug it into a socket without the ground prong, depending on which direction it's in, the pilotlight will glow softly or not at all - NB: this is before I even switch on the amplifier!

I had a pleasant experience with the later 6V6 version before swearing off store-bought amps.

If you're planning to keep the amp this is a mid-level safety concern. A more pressing safety concern is that the amp lacks a fuse !!!!

Thank you for the detailed advice; I read your post, which gives me hope, and I will do as much as I can without taking risks.

Btw, what is that "death cap" that I saw mentioned, and is the name figurative or literal (capacitor kills tone or people..?)

Important !!! Do not mess with a Tube amp if you don't know what you're doing/ you can get a nasty belt off one
even if unplugged from the wall/ (the big electrolytic Caps store a lot of energy)

Thanks - I've heavy rubber cleaning gloves; I'll wear those.

If it has a Euro 3 pin plug, they are only supposed to plug in one ( safe) way into the socket - that's the whole point of that design. If you reverse it somehow, you may be disabling the ground wire and/ or wiring the neutral amp wire to the live socket wire. Where are you using it?

In Switzerland for now. Some sockets have the prong, like in France (am close to border), some 2 ground clips - I added pics.

try changing the 12ax7...smaller one closest to the input jack

the 240 power wiring issue is a bit scarier...you want to make sure everything is properly grounded..dangerous if not!

I will change the valve, thanks for the tip. When I bought the amp they gave me replacements; and the original vinyl cover (nice when travelling by bus/train). As for grounding... ...how do I know..?

Yeah, European electrical house current is killer.

Oh yes; we had some oaf come fix old electrical wiring in a washroom, and he was thrown off his stepladder after touching both wires. Had the hiccoughs for an hour, and his arm was trembling.

attention aux condensateurs, chère Zina, ils conservent le courant même quand l'ampli est débranché

Merci beaucoup; j'ai des gros gants en caoutchouc, et des bottes Wellington; je porterai ça, et déchargerai les condensateurs - on met un grand tournevis solide sur les deux pôles, n'est-pas?


EDIT: I want to keep this amplifier, because it's the only one I have, plus it's lighter than a suitcase; can carry it in one hand.

[Franz 1997]

Zina, I'd say it's too dangerous to use the amp without a solid ground connection. If anything lights up without the amp being even switched on, that suggests there is something wrong with the mains wiring in the amp and it will have to be fixed by a pro tech. Sorry, but I don't think this is about changing tubes, from what you have said it's about basic safety. Particularly, don't touch the chassis if plugged in but not grounded; 240v can kill.

On the positive side, this should be a simple, quick and inexpensive repair. Mains wiring is critical, but pretty simple. It's safe, or it's not.

[Patlotch]

the powerplug and sockets. There are 2 types

ce n'est pas normal. Dans un "État national" qui a 4 langues officielles, il devrait y avoir 4 types de prises mâles et femelles
this is not normal. In a "National State" that has 4 official languages, there should be four types of powerplug and socket

mais vous avez de la chance. Quand j'avais un ampli à lampes je captais la radio de la police. Cela donnait aux standards des paroles étranges
but you're lucky. When I had a tube amp I was picking up the police radio. This gave the standards strange words

essayez celles-ci, vous deviendrez peut-être une vedette internationale...
try these, you may become an international star ...

[Zina]

Zina, I'd say it's too dangerous to use the amp without a solid ground connection. If anything lights up without the amp being even switched on, that suggests there is something wrong with the mains wiring in the amp and it will have to be fixed by a pro tech. Sorry, but I don't think this is about changing tubes, from what you have said it's about basic safety. Particularly, don't touch the chassis if plugged in but not grounded; 240v can kill.

On the positive side, this should be a simple, quick and inexpensive repair. Mains wiring is critical, but pretty simple. It's safe, or it's not.

Thank you; that clinches it then, and I won't touch it for now. My guitar has a big acoustic sound anyway (miraculously, for only a thinline), so it's not the end of the world. Thanks for the warning.

but you're lucky. When I had a tube amp I was picking up the police radio. This gave the standards strange words

"Smooth road, clear day..."
<bzzt huge pile-up on m'way due to avalanche bzzt>
"You go to my head..."
<krrk terrible drunk spotted near jazzbar krrk>
"I get misty..."
<tcch what did I say about that pile-up, huh? tcch>

[EllenGtrGrl]

A couple of thoughts:

1. If you want squeaky clean sound, a Musicmaster Bass amp isn't for you. They can get pretty dirty sounding, when turned up. In a way, they're almost like a poor man's Tweed Bassman. Oh yeah, and don't expect super loud - the amps put out 12 watts of power.

2 As was mentioned, the earlier versions of the Musicmaster Bass amp use 6AQ5 tubes. Those are sweep tubes. Sweep tubes were originally developed to help run the sweep circuits for TV picture tubes, but were discovered to work fine as power tubes (with certain restrictions needing to be kept in mind - they don't tolerate being out of resonance/tune in radio application [I have a ham radio license, and tune my vintage ham radios with sweep tubes, very carefully]), and due to their low cost (you used be able to pick up replacements cheeep at your neighborhood drugstore), were used in some amplifier circuits. Unfortunately, ALL sweep tubes are extinct nowadays - they haven't been made in decades. So replacements are more difficult to find than 6L6s, 6V6s, EL34/6CA7s, etc., and can be pretty expensive (a few years ago, I spent over $90 for a pair of USED [not NOS] 6KD6s, that looked, to be in OK shape, so I'd have a set up backup final/power tubes for a 46 year old Yaesu FTdx-401b ham radio I have, that puts out about 300 watts of power). Be prepared to do some searching for replacements, and if you want NOS, spending even more than what used sweep tubes normally go for.

3. I myself, am not wild about on and off switches for high voltage circuits being from the volume knob. As was mentioned earlier, this is a bit of a safety concern.

[Franz 1997]

3. I myself, am not wild about on and off switches for high voltage circuits being from the volume knob. As was mentioned earlier, this is a bit of a safety concern.

I'm not crazy about them myself- but I think it's intuitive, rather than rational. They should be wired only to the neutral side of the mains, which theoretically is at zero volts, or near that - what the switch does is just complete the 'on' circuit. They are made as separate units, which then get attached mechanically to the volume pot, and which will have thick plastic sides, thicker than any high voltage wire in the amp. And finally, they have to pass stringent official tests to be able to be used in this application in a production amp like a Fender.

So I think in practice, the only significant risk is that they break eventually, if wired according to regulations. But I get it, it feels a bit unsafe...

In some older Fender amps, the presence pot actually can have a low dc voltage on it - I mean the pot itself. But the amps still got UL certified...

[sgosnell]

Back in the old days when everything ran on vacuum tubes, most of it used the volume as the on/off switch. Very few devices had a separate on/off switch, you just turned the volume all the way to the left until the switch clicked. It works just like a toggle switch, except that it's rotary. Safe as any other switch, other than the slight difficulty of being able to see if it's really off, and one learned to check that carefully before attempting any repairs.

[rpjazzguitar]

I'll add my voice to the chorus.

I don't want to see anything lit up when the amp is supposed to be off.

It indicates that electricity is getting somewhere it shouldn't be and that sounds dangerous.

For the money it costs to fix this, to the point where it's safe and you're convinced that it's safe, I imagine you could buy a better amp.

[sgosnell]

It's always possible to rewire anything so that the power switch is bypassed. It's also possible that the switch is faulty and doesn't actually break contact. Almost anything is possible with an old amp. I agree that if things are lighting up with the switch really off, then you really, really need to get a competent technician to fix it. If you can't afford to pay one, save up your money until you can, or else sell the amp. Electricity plays no favorites, it always goes to the easiest place it can find, and all too often that place is a human body, which it leaves the worse for wear.

[Patlotch]

in my youth in a small French town, the 50s, when we heard about "electric guitar" on the radio, it was because a guitarist had died electrocuted on stage. It was very impressive for a child!

Guitarists Who Were Electrocuted While Playing

[neatomic]

the great keith relf!! perfect!

funny..

but oh so dark!

cheers

[Zina]

1. clean sound
2. 6AQ5 tubes

[+ much helpful information]

Thanks a lot; I'd been wondering about several points you mention.

Re. sound, that's a disappointment; I hoped it would go far into the clean range with new parts and tubes, perhaps another speaker as suggested earlier - I found a scrapped old ElectroVoice 15" 300W bass speaker that I could make a box for (if the amp can even move that).

As for the tubes, that's bad news too. I can't sell it in this state of course, that would be a low move, but perhaps some hobbyist wants it. I'd better cut my losses and save up for a simple sturdy amp head.

I don't want to see anything lit up when the amp is supposed to be off.

Put like that it's clear, yes. I won't connect it anymore and mothball it.

If you can't afford to pay one, save up your money until you can, or else sell the amp.

Yep, that's the problem, plus finding a reliable tube technician. I'll give it to a hobbyist some day, and save for better in the meantime.

....................................

Sincere thanks for all tips and warnings. This has been most helpful,

Z.

[sgosnell]

I don't think selling it would be wrong, as long as everything is disclosed. Someone might be willing to pay good money for it as it is, with the ability to renovate it. Hiding potentially lethal defects is undoubtedly wrong, but as long as the buyer is informed of the issues, I would have no second thoughts about selling as is. But your morals are not mine, so do as your conscience dictates.

[Zina]

Oh I see, yes. If someone wants to pay for it in spite of the defects, it's all right I suppose. I'll print out the entire thread (will blank out usernames, don't worry) for reference, and should anyone local want it, I'll pass it on.

[Gitfiddler]

Here's another suggestion...

Since your MusicMaster Bass Amp has a few issues that may not be cost effective to repair, maybe consider converting it to a much better amp.

How? Glad you asked. :-)

1. Gut the MM innards and drop in a Silverface Vibro Champ chassis. They have identical widths (17").

VChamp Tubes: 2x6V6/1x5y3GT rectifier/1x12ax7

2. Next, install an efficient 12" speaker of your choosing into the tall 20 3/4" box.

Used Silverface Vibro Champs range from $500 and up, but be patient and check for well maintained clean examples.

You might want to have this conversion done by an amp tech, but it could be a fun DYI project if you are up to it. However it shakes out, you'd end up with a very useable/giggable, lightweight amp with great tone, better eq and vibrato! I'm not sure if the newer Super Champ X2 would fit, but if so, that would be another consideration.

[Zina]

That is an attractive idea, and I would indeed try that (while keeping the MMB chassis for a rainy day), but I haven't the money. I seldom need or even want any, but that attitude is now biting me in the behind! I've a job on the sly[*] though, so I'll save up and look at options, incl. yours, in a bit.

EDIT:[*]not undeclared/black; just circumventing an arbitrary restriction