Posted: 12/14/2008 6:01:34 AM EDT
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I just picked up a early 70's Fender Musicmaster bass amp. It's not a tweed Bassman but it is a 12 watt 1x12 hand wired point to point Fender tube amp. The amp is a little rough around the edges. It's filthy, the handle strap is missing, and the shaft on the volume pot is broken but it works.
This isn't exactly a collector piece to start with and given it's looks I'm sure it's not worth much. I only payed $40 for so I was thinking it might be a good candidate to hot rod. I plan to clean it up and replace the broken pot. Is there anything fairly simple I can do to get some drive out of this amp? I can crank it all the way and neither P-90s nor humbuckers will push it enough to any dirt. |
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I'd get the tubes checked first, and check the speaker. The tubes should be (2) 6V6's and (1) 12AX7. I'd make sure the speaker isn't blown, has the correct ohm rating, and that the tubes are good. If those things are correct I would get it looked at, because a 12 watt tube amp should break up pretty easily. I don't know how to hot rod stuff, but I've run enough tube amps to know that doesn't sound right, especially for a Fender. It could be something as simple as a bad connection somewhere. See what happens after replacing the volume pot, perhaps there was some other damage done when the shaft broke off.
ETA: they're listed on ebay from $450 - $599, so while you're right that it's not worth what a '59 Bassman would be, it does have some value. So if you get it running and still don't like it, with only $40 in it, you could always sell it and buy something else. |
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I'd get the tubes checked first, and check the speaker. The tubes should be (2) 6V6's and (1) 12AX7. I'd make sure the speaker isn't blown, has the correct ohm rating, and that the tubes are good. If those things are correct I would get it looked at, because a 12 watt tube amp should break up pretty easily. I don't know how to hot rod stuff, but I've run enough tube amps to know that doesn't sound right, especially for a Fender. It could be something as simple as a bad connection somewhere. See what happens after replacing the volume pot, perhaps there was some other damage done when the shaft broke off. ETA: they're listed on ebay from $450 - $599, so while you're right that it's not worth what a '59 Bassman would be, it does have some value. So if you get it running and still don't like it, with only $40 in it, you could always sell it and buy something else. It works now and is surprisingly loud for 12 watts. I can crank it to 10 and it's just really loud, full, and clean-like a twin on 3-4. The original speaker is pretty light duty so I plan to put a new Jensen or Celestion 12 in it for better bass response. |
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I'd run a Jensen P12R or P12Q, those would be probably closest to the original factory speaker. http://jensentone.com/vin-home.php
Jensen recommends: http://jensentone.com/replacement.php they don't list a Musicmaster Bass amp, but they list other 12 watt Fenders that run 12"s. ETA: did you replace the volume pot? |
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I'd run a Jensen P12R or P12Q, those would be probably closest to the original factory speaker. http://jensentone.com/vin-home.php Jensen recommends: http://jensentone.com/replacement.php they don't list a Musicmaster Bass amp, but they list other 12 watt Fenders that run 12"s. ETA: did you replace the volume pot? Not yet as I just picked it up yesterday. The shaft of the pot is broken off but I can adjust the volume through the full range with a pair of needle nose pliers. |
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I'd run a Jensen P12R or P12Q, those would be probably closest to the original factory speaker. http://jensentone.com/vin-home.php Jensen recommends: http://jensentone.com/replacement.php they don't list a Musicmaster Bass amp, but they list other 12 watt Fenders that run 12"s. ETA: did you replace the volume pot? Not yet as I just picked it up yesterday. The shaft of the pot is broken off but I can adjust the volume through the full range with a pair of needle nose pliers. Gotcha, I didn't know if you already had one to put in. So it works fine, it just won't break up? Someone probably put in a higher wattage speaker at some point, low watts break up easier/better. |
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Here's the schematic if it is an amp w/ (2) 6V6 tubes and one 12AX7 tube.
I don't have the answer for you, but the folks at AX84.com will be able to give you a few suggestions. Overall it should be fairly easy, just a few component value changes. You could probably model the preamp (12AX7 tube) on the P1 Extreme project. The P1 Extreme was designed to get the most drive out of one 12AX7 (preamp) and one EL34 (power amp). In this case your power amp would be served by the two 6V6 tubes instead of the EL34. You should be able to get preamp distortion from the two gain stages in the 12AX7 but I'm not sure if you can get enough drive to get distort the the 6V6 tubes when it uses a transformer as a phase inverter instead of another tube (12AX7) that provides more gain in the phase inverter. Post in "general amp discussion" and you should get a few ideas from real pros in amp design. |
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Here's the schematic if it is an amp w/ (2) 6V6 tubes and one 12AX7 tube. I don't have the answer for you, but the folks at AX84.com will be able to give you a few suggestions. Overall it should be fairly easy, just a few component value changes. You could probably model the preamp (12AX7 tube) on the P1 Extreme project. The P1 Extreme was designed to get the most drive out of one 12AX7 (preamp) and one EL34 (power amp). In this case your power amp would be served by the two 6V6 tubes instead of the EL34. You should be able to get preamp distortion from the two gain stages in the 12AX7 but I'm not sure if you can get enough drive to get distort the the 6V6 tubes when it uses a transformer as a phase inverter instead of another tube (12AX7) that provides more gain in the phase inverter. Post in "general amp discussion" and you should get a few ideas from real pros in amp design. Thanks for the info. I thought about switching the speaker from 8 ohms to 16 ohms to push the power tubes harder. |
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Here's the schematic if it is an amp w/ (2) 6V6 tubes and one 12AX7 tube. I don't have the answer for you, but the folks at AX84.com will be able to give you a few suggestions. Overall it should be fairly easy, just a few component value changes. You could probably model the preamp (12AX7 tube) on the P1 Extreme project. The P1 Extreme was designed to get the most drive out of one 12AX7 (preamp) and one EL34 (power amp). In this case your power amp would be served by the two 6V6 tubes instead of the EL34. You should be able to get preamp distortion from the two gain stages in the 12AX7 but I'm not sure if you can get enough drive to get distort the the 6V6 tubes when it uses a transformer as a phase inverter instead of another tube (12AX7) that provides more gain in the phase inverter. Post in "general amp discussion" and you should get a few ideas from real pros in amp design. Thanks for the info. I thought about switching the speaker from 8 ohms to 16 ohms to push the power tubes harder. I doubt that will do much for you, hardly worth the trouble compared to doing it right. |
| I was finally able to get a little crunch out of it by pushing the front end with an EQ pedal with the level boosted to the max. Its got sort of a Raconteurs-esque gritty light distortion that's clean on single notes and when strumming lightly but gets a little crunchy when you lay into power chords. Boosting the in level so much makes it insanely noisy. I need to pick up a Tubescreamer or somehting similar to try with it. |
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don't change speaker impedence, won't do jack shit except piss off the plates a bit.
here's my secret weapons for any fender amp front end - straight ahead tube swaps - nothing else needs to be changed; if you want more grind replace the 12ax7 with a 12bz7 if you want a really muscular clean tone replace the 12ax7 with a 5751 try it, fuckin' awesome, old fenders really respond to those tubes. these guys should have what you need... http://www.tubesandmore.com/ |
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don't change speaker impedence, won't do jack shit except piss off the plates a bit. here's my secret weapons for any fender amp front end - straight ahead tube swaps - nothing else needs to be changed; if you want more grind replace the 12ax7 with a 12bz7 if you want a really muscular clean tone replace the 12ax7 with a 5751 try it, fuckin' awesome, old fenders really respond to those tubes. these guys should have what you need... http://www.tubesandmore.com/ It looks like they only carry one kind of 12bz7. Any suggestions of which 6v6 I should get for more distortion? I might as well replace the power tubes as well and I would like to get some grind without having to boost the front end and crank the amp to 10. |
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12BZ7s aren't made anymore. For the best selection Ebay is probably your best bet. They'll all be pretty good as they are all NOS now. Pick a name you know GE, Mullard, etc. The 12BZ7 will drive the stock amp harder, but to really take advantage of the BZ tube internal components should be changed to optimize the operating point of the BZ.
The 6V6 that came with the amp is probably as good as you will get unless it's worn out. The differences between tube brands are so minimal I doubt a different tube will give you the magnitude of change you are looking for. I'm still a little suspicious that someone else says they get good distortion from their stock(?) amp and you get none. Makes me wonder if there is a deeper problem. |
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12BZ7s aren't made anymore. For the best selection Ebay is probably your best bet. They'll all be pretty good as they are all NOS now. Pick a name you know GE, Mullard, etc. The 12BZ7 will drive the stock amp harder, but to really take advantage of the BZ tube internal components should be changed to optimize the operating point of the BZ. The 6V6 that came with the amp is probably as good as you will get unless it's worn out. The differences between tube brands are so minimal I doubt a different tube will give you the magnitude of change you are looking for. I'm still a little suspicious that someone else says they get good distortion from their stock(?) amp and you get none. Makes me wonder if there is a deeper problem. Nah, I'm prety sure the amp works fine. I get a light overdrive with it cranked to 10 and some decent crunch with the EQ pedal. I plan to replace all the tubes as they are three likely the orginals and this amp looks like it's seen quite a bit of use. The 12AX7 is a Fender and the 6V6s are Sylvania. |
| I played around with it a bit tonight and it started sounding much better. All I really did was take the tubes out, clean the sockets, and put them back in. As soon as I cranked it back up and hit a chord I could tell a difference. It's still not a Boogie, obviously, but but it has a nice smooth classic sounding overdrive. The best way I can think of to describe it is Keith Richards tone in Honky Tonk Woman. |
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I played around with it a bit tonight and it started sounding much better. All I really did was take the tubes out, clean the sockets, and put them back in. As soon as I cranked it back up and hit a chord I could tell a difference. It's still not a Boogie, obviously, but but it has a nice smooth classic sounding overdrive. The best way I can think of to describe it is Keith Richards tone in Honky Tonk Woman. Great! I had a similar problem on my last build. Finished building it and double checked every connection. Carefully powered it up and nothing. All the voltage readings were good. Finally traced it to a dirty socket/pin on the first gain stage screen. Two hours of trouble shooting for a 30 second problem. 
Not being familiar with this specific amp at all, the Honky Tonk Woman tone is what I would expect looking at the schematic. Are you still getting new tubes or does this put you in the tone style you were expecting? |
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I played around with it a bit tonight and it started sounding much better. All I really did was take the tubes out, clean the sockets, and put them back in. As soon as I cranked it back up and hit a chord I could tell a difference. It's still not a Boogie, obviously, but but it has a nice smooth classic sounding overdrive. The best way I can think of to describe it is Keith Richards tone in Honky Tonk Woman. Great! I had a similar problem on my last build. Finished building it and double checked every connection. Carefully powered it up and nothing. All the voltage readings were good. Finally traced it to a dirty socket/pin on the first gain stage screen. Two hours of trouble shooting for a 30 second problem.
Not being familiar with this specific amp at all, the Honky Tonk Woman tone is what I would expect looking at the schematic. Are you still getting new tubes or does this put you in the tone style you were expecting? I would still like to get new tubes for a little more gain and less headroom. It would be nice to get some drive without having to crank it to ten. Right now, that's the only way I can get any dirt and it shakes the windows. Would pulling a power tube allow me to get the same tone at half the power? I know that works with some amps. |
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I have some nos tubes stashed down at my Mom's from the '60s that my Dad had left over from when he tried to make it repairing TVs. I'm not sure what they are, but some of the #s you guys are talking about sound familiar.
I'll check them out tomorrow when I'm down there & if any of them match, I'll grab 'em. |
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I played around with it a bit tonight and it started sounding much better. All I really did was take the tubes out, clean the sockets, and put them back in. As soon as I cranked it back up and hit a chord I could tell a difference. It's still not a Boogie, obviously, but but it has a nice smooth classic sounding overdrive. The best way I can think of to describe it is Keith Richards tone in Honky Tonk Woman. Great! I had a similar problem on my last build. Finished building it and double checked every connection. Carefully powered it up and nothing. All the voltage readings were good. Finally traced it to a dirty socket/pin on the first gain stage screen. Two hours of trouble shooting for a 30 second problem.
Not being familiar with this specific amp at all, the Honky Tonk Woman tone is what I would expect looking at the schematic. Are you still getting new tubes or does this put you in the tone style you were expecting? I would still like to get new tubes for a little more gain and less headroom. It would be nice to get some drive without having to crank it to ten. Right now, that's the only way I can get any dirt and it shakes the windows. Would pulling a power tube allow me to get the same tone at half the power? I know that works with some amps. No, unfortunately, that only works when you have pairs of output tubes as in a 100 watt Marshall where there are four el34s. By pulling a pair, one from each side of the output transformer, and adjusting the impedance for the load on the tranny you can turn the 100 watter into a 50 watter. This works because in a push pull power amp such as yours and the Marshall, one tube amplifies the "positive" portion of the sine wave and one tube amplifies the "negative" portion of the sine wave. In amps with two tubes amplifying each portion of the sine wave you can pull one pair of tubes, one from the positive side and one from the negative. Then because there is now a different input to the power tranny you change the speaker impedance to make it look the same to the tubes. |
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Well, I just brought the whole tube stash home with me. I have one RCA 12AX7A/ECC83 & one Sylvania 6V6GTA (will this one work???). I couldn't find any of the alternate tubes mentioned though. Pics: http://i218.photobucket.com/albums/cc317/GoneShootin/DSCF0806.jpg http://i218.photobucket.com/albums/cc317/GoneShootin/DSCF0804.jpg http://i218.photobucket.com/albums/cc317/GoneShootin/DSCF0803.jpg http://i218.photobucket.com/albums/cc317/GoneShootin/DSCF0802.jpg http://i218.photobucket.com/albums/cc317/GoneShootin/DSCF0801.jpg Let me know if any on these tubes will help. Unfortunately, other than the ones you mentioned, most of those tubes are radio and TV tubes. The ones that caught my eye as an amp homebuilder are the 6SN7s, 6K6, and 5Y3GT. The 6SN7 is a comon tube for stereo preamps and is used in the guitar amp bulding hobby as a very low power power tube for a 2 watt push pull amp. The 6K6 is also good for the homebuilder as it allows the building a push pull amp of about 10 watts. The 5Y3 is a common tube rectifier used in older tube amps. |
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Thanks for looking , GoneShootin. So does anyone know if I can pull a power tube on this thing to run it at half power? Yeah, no problem.  There will be a problem if it's the amp for which I posted a schematic in this thread on 12/15/08. Sorry, brain fart. I forgot to highlight the part I was talking about: "Thanks for looking , GoneShootin." |
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Thanks for looking , GoneShootin. So does anyone know if I can pull a power tube on this thing to run it at half power? Yeah, no problem. There will be a problem if it's the amp for which I posted a schematic in this thread on 12/15/08. Sorry, brain fart. I forgot to highlight the part I was talking about: "Thanks for looking , GoneShootin." Ah... Gotcha. 
Makes perfect sense the way you explain it. Amazing how complicated this business of converting thoughts into text can be sometimes. 
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<snip> Ah... Gotcha. 
Makes perfect sense the way you explain it. Amazing how complicated this business of converting thoughts into text can be sometimes.  Yeah, tell me about it. I've discovered that it's very easy to get misunderstood when posting on a message board.
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