The Fender Musicmaster Amp Revisited

capnjuan

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This Fender Musicmaster from the 'Silverface' era is up for eBay auction and is the same model as our BBer Dane's; single 12AX7 and twin 6AQ5s (6005s).

Fenderebaymusicmasterbass.jpg



A friend of mine just bought one of these except that his is a twin 6V6 model, not 6AQ5. When the parts get here, I'm going to recap it for him. I went looking for the schematic and found the 6V6 version that includes an 'extra' transformer; Gibson GA35 shown on left for comparison:

transformers.jpg



According to Teagle and Sprung's Fender Amps, the Musicmaster was in production from 1970 to 1982. They go on to say that the function of the transformer is to couple the audio stage to the output and achieve phase inversion without the use of another tube and its necessary sidekick resistors and capacitors that, in most amps, act as the phase inverter. According to them, this is the only Fender design with a coupling/inverting transformer and explains why there is only one preamp tube, rare for a twin or push/pull amp. In this discussion of the differences between Black and Silverface Fender amps, they say the Musicmaster is a 'tone monster'! Additional Musicmaster specifications at Ampwares

In LTG's Voicing Amps thread, there's a lot of discussion about the effects of cathode bypass capacitors on tone and frequency response. The Musicmaster schematic below showing a large value 50uf cap on the left bypassing the preamp cathode resistor and a conventionally sized 25uf cap bypassing the output tube's cathode resistor on the right. The preamp cap acts like a low-pass filter; it prevents higher frequencies from passing while allowing the lower ones to pass. Every Fender Bassman has a 50uf cap in the same location.

cathodecaps.jpg



So what? Nearly every high-grade boutique or home-built 'Deluxe' has switchable preamp cathode bypass caps mounted; normally 25uf in one switch position and .68 'Marshall' style in the other because the low value cap acts like a high-pass filter; stopping low frequencies while allowing higher ones to pass. The switchable cap provides a fundemental change in the amp's frequency reponse and tone. When I get my friend's amp on the bench, I'll try to get him to listen to a 25uf there; whatever's in there is coming out anyway for maintenance reasons. If you get the time or inclination Dane, you might try the same thing; ought to make the amp sound like it's running a little hotter, more twang, and if you're interested, I still have that single, low-use 6AQ5/6005. CJ
 

danerectal

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Wow. That's exactly like mine 'cept for no cover on the 12AX7. I may just try your suggestion about swapping that cap. Oddly enough, I was able to snatch a pair of 6AQ5s for about $12, thanks for the offer.

CJ, do you know how the 6v6 version compares to a Princeton Reverb from about the same year? I don't need to know for anything. It just seems like my Princeton has a lot more headroom than the Musicmaster, and I wonder if it's due to the tubes.
 

Walter Broes

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I briefly had one of those, but I could sell it with a profit at a time that came in very handy. I wasn't that impressed by how it sounded, and it its unmodded state, wouldn't call it "a tone monster".
 

capnjuan

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danerectal said:
... Oddly enough, I was able to snatch a pair of 6AQ5s for about $12, thanks for the offer.
Hi Dane; good price for the tubes ... mine is like owning one really good sock that doesn't match any others.
danerectal said:
... It just seems like my Princeton has a lot more headroom than the Musicmaster, and I wonder if it's due to the tubes.
That 50uf cap in the preamp changes the gain characteristics of the first stage. I also don't think this is the kind of amp where they would go all-out with a high-grade output transformer the way they would with front-line amps like the Princeton. Generally, the smaller and cheesier, the quicker they are to get saturated and break up. If you haven't yet, you might try a 12AT7 in the preamp; lower gain factor might cost you a little volume but it ought to be a little cleaner. I don't know the 6AQ5 well enough to know whether it has headroom limitations but I think I'd look elsewhere first. CJ
 

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6aq5 is the 7 pin version of the 6v6. Electronically, they are identical.
"the purveyor of worthless knowledge"
 

capnjuan

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Default said:
6aq5 is the 7 pin version of the 6v6. Electronically, they are identical. "the purveyor of worthless knowledge"
We have a local plumbing company down here that shoots for the 'Island Trade' (jobs on Palm Beach). Painted on the side of their trucks: 'Purveyors of fine plumbing systems' Oh yeah ... Thankee for the 6aq5 factoid; had it been in wider use, I think more manufacturers would have messed with it and sooner or later, somebody would have made one that sounded better than somebody else's. It just so happens I have one in my box ... if you want, I could send it to you so you could put it in your box ...? CJ
 

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You're right cj. They were intended for tv's, not guitar amps. I would think that being a bass amp didn't help either.
I'll shoot you an email later.
 

capnjuan

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Another Musicmaster on eBay with chassis shots. Also a 6AQ5 model ... with something burfed up at one of the output sockets; doesn't match the other ... been replaced ... not a dainty job either; big wads of solder holding it in place ... that would the audio transformer/coupler between the tube sockets ...
 

danerectal

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capnjuan said:
something burfed up at one of the output sockets; doesn't match the other ... been replaced ... not a dainty job either; big wads of solder holding it in place ...

That does look pretty sickly... I need to fix my pilot light (hard to say if I'll find one like that). I have a jewel-style one that may fit, but I'd rather not change anything like that.

That guy has an odd intensity to me... :shock:
 

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Guy also doesn't know jack shinola about tubes either. The only connection between 6aq5's and 6bq5's is that they both plug into something. :roll:


The video does not help, either.
 

capnjuan

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danerectal said:
... I need to fix my pilot light (hard to say if I'll find one like that). I have a jewel-style one that may fit, but I'd rather not change anything like that. That guy has an odd intensity to me... :shock:
Can't mess with the magic ... is yours the type with the lamp in a socket behind the jewel as opposed to the typical Fender assembly with the faceted jewel?
 

capnjuan

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Walter Broes said:
Wow, I thought my guitar faces were bad! :lol:
Amazing ... you'd think somebody would at least look at these before posting them ... this guy looks dazed and confused At least Walter you look engaged in what you're doing ... this guy looks like Kramer on Seinfeld after he cut one ....
 

capnjuan

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Default said:
Guy also doesn't know jack shinola about tubes either. The only connection between 6aq5's and 6bq5's is that they both plug into something...
Transparent effort to drag the reputation of the 6BQ5/EL84 onto the 6AQ5 ... "...gotta be a good tube if the tube ID is so similar to the 6BQ5..."
 

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capnjuan said:
Can't mess with the magic ... is yours the type with the lamp in a socket behind the jewel as opposed to the typical Fender assembly with the faceted jewel?
The one in the amp I have is the stationary lense. I have an extra screw on jewel-type that I've contemplated, but it looks too large for the hole. It would also be pretty out of place.
 

capnjuan

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My friend Paul brought his 2X6V6 Musicmaster by this morning: he has a 12" Eminence Red Coat The Tonker in it (so named because it tonks), ceramic magnet, 50 watts or so power handling:

frontback.jpg



Underside of chassis: Paul wanted fresh power supply and cathode bypass caps. Object in red box is the audio- or coupling / inverting- transformer that is the hallmark of this model taking the place of a conventional tube phase inverter / driver:

autotransformer.jpg


In the image below; the upper left and center red boxes are the cathode resistors and bypass caps on the 12AX7 preamp. The red box on the right is the output tube bypass cap. The blue boxes are the three power supply filtering caps. The brown cigar in the middle of the lower pic is a two-section X 20uf X 300V cap. The small green circles are the tone cap in the schematic and the amp:

chassis06.jpg



The upper panel is the original parts, the lower is the finished work. The three black FT caps are singletons; the two on the right had to be mounted between two small wiring strips. To secure the wiring strips, the self-tapping machine screws that fasten the output transformer to the deck were removed, the holes drilled out slightly, and bolts put it to keep the wiring strips in place. The preamp cathode resistors were replaced and are under the preamp bypass caps.


chassis07.jpg



Finished job including the little yellow Mallory .002 tone cap at the lower right of the pic:

circuit02.jpg



Paul is currently running this pair of late 1940s Sylvania JAN 6V6s ... Dig the US Navy ensign on the side:

tubes01.jpg



Finally Dane, this is the as-found light assembly; Fender-style socket and combination screw-in bezel and jewel. I did not think to check the pot codes for date of manufacture; still don't know which of the 6V6 and 6AQ5 models preceded the other.


lamp01.jpg
 

capnjuan

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Hi Walter; Plenty of signal, good enough sound-stage as the stereo geeks would say ... but I thought it a little limited or pinched sounding. Gotta remember other than turning up the volume, I'm pretty much a doofless toe-tapper of a musician ... I turn it to 4 and I either like it or turn it off; it's designed for loud, clean, and later breakup but I like dirty early breakup. Paul has a couple of pedals that Marshallize the amp. He's a strong musician and adept recorder so the application is different.

He sticks a pedal in front of it, and he's Ted Nugent, in part because he can play like Ted Nugent ... I play like Ted Nougat ... Anyway, it isn't for me, it has plenty of volume for his needs, and he has pedals that convert it to what he wants it to do. I tried to get him to audition my Jensen P12N in it - that alnico and 6V6s were alchemy ... nothing doing. It's also a bass guitar and I don't have a bass to put through it; unlike the amps that I enjoy most that are a musical end in themselves, this one isn't. Strong enough, loud enough, but more of a vehicle for pedals than something with enough character to carry its own.
 

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Thanks John, I'm a little suprised at that, as I remember the one I had (not the 6V6 variety, btw) as not very loud, and having fairly early breakup. I do think the stock speaker in it was as wimpy as they come, and not efficient at all compared to that Eminence, so that probably limited the headroom of the amp severely.

I understand the pedal bit though, as I remember it as fairly bland and not very interesting sounding.
 
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