The transformer was designed for this application. it will be happy withstanding the Dc current the circuit presents it.
The question I have is WHY do you feel the need to eliminate the parallel resistor? I doubt the designers were siting around and said HEY, lets throw a resistor in here to baffle people 50 years fro now.
The question I have is WHY do you feel the need to eliminate the parallel resistor? I doubt the designers were siting around and said HEY, lets throw a resistor in here to baffle people 50 years fro now.
I had a look in some of the old books to see why there would be a (15k) resistor in parallel with the transformer primary. I found this in Radiotron Designer's Handbook, 3rd edition, 1941, chapter 1, page 2:
'In some circumstances "loaded" transformers are employed. Either the primary or secondary is shunted by a resistance which tends to flatten out the response characteristic.'
(In other words reducing a resonant peak caused by the magnetizing inductance in parallel with the inter-turn capacitance.)
'In some circumstances "loaded" transformers are employed. Either the primary or secondary is shunted by a resistance which tends to flatten out the response characteristic.'
(In other words reducing a resonant peak caused by the magnetizing inductance in parallel with the inter-turn capacitance.)
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Yes, a lot of people online have eliminated the resistor and experienced a loss of treble. I suspect it's probably there for a multitude of reasons, damping any ringing resonances of the circuit, maybe also to carry some of the DC for the plate so the transformer core doesn't saturate, perhaps some accommodation of the available transformer.
Enzo, I'm not determined to get rid of that parallel resistor. I'm just trying to understand the circuit, and experiment a bit to learn something, and have some fun. This is not warranty repair, nor is it seme revered vintage treasure. This is a cheap toy to learn on, and hopefully enjoy afterward. This is a DIY site, and people like me will try stupid things you would not.
I'm really into the interstage transformer PI instead of more tube stages. And I've always liked single-ended outputs that are capacitor-coupled instead of the DC on the transformer, but at high power there are always major disadvantages. But I've stumbled onto an attractive topology, using capacitor coupling to an interstage transformer! Now suitable caps are more readily available or they probably would have done it back then! Even now, even with a transformer with a gap, adding 1 coupling cap and one plate resistor might improve the transformer performance. Admittedly a transformer design optimized for the redesign might work much better with capacitor coupling! I really would like to take one of these transformers apart to inspect the core; it's quite possible there is no air gap and the power is so low it just handles the DC. There actually are even some advantages to having that variable DC on the primary instead of just variable AC, because the typical single-ended transformer doesn't experience the nonlinearity of the magnetic hysteresis of the metal which occurs when the magnetism changes direction.
So to me it's an interesting experiment, and I appreciate the conversation. I doubt the transformer was designed to handle the DC or the bass well, the whole circuit was designed to be economical in a day when decent capacitors were a new emerging innovation.
And heck it's not even arrived yet! I'm just enthusiastic. I can't see how it could do anyharm to install the coupling cap and a reasonable plate resistor, and listen.
One big change this makes is that the transformer and its coupling cap would be between the plate and ground now, instead of between the B supply and plate. I'll have to digest how that affects the tube's ability to drive it. The phase will be different, which should be inconsequential when there's no feedback.
Enzo, I'm not determined to get rid of that parallel resistor. I'm just trying to understand the circuit, and experiment a bit to learn something, and have some fun. This is not warranty repair, nor is it seme revered vintage treasure. This is a cheap toy to learn on, and hopefully enjoy afterward. This is a DIY site, and people like me will try stupid things you would not.
I'm really into the interstage transformer PI instead of more tube stages. And I've always liked single-ended outputs that are capacitor-coupled instead of the DC on the transformer, but at high power there are always major disadvantages. But I've stumbled onto an attractive topology, using capacitor coupling to an interstage transformer! Now suitable caps are more readily available or they probably would have done it back then! Even now, even with a transformer with a gap, adding 1 coupling cap and one plate resistor might improve the transformer performance. Admittedly a transformer design optimized for the redesign might work much better with capacitor coupling! I really would like to take one of these transformers apart to inspect the core; it's quite possible there is no air gap and the power is so low it just handles the DC. There actually are even some advantages to having that variable DC on the primary instead of just variable AC, because the typical single-ended transformer doesn't experience the nonlinearity of the magnetic hysteresis of the metal which occurs when the magnetism changes direction.
So to me it's an interesting experiment, and I appreciate the conversation. I doubt the transformer was designed to handle the DC or the bass well, the whole circuit was designed to be economical in a day when decent capacitors were a new emerging innovation.
And heck it's not even arrived yet! I'm just enthusiastic. I can't see how it could do anyharm to install the coupling cap and a reasonable plate resistor, and listen.
One big change this makes is that the transformer and its coupling cap would be between the plate and ground now, instead of between the B supply and plate. I'll have to digest how that affects the tube's ability to drive it. The phase will be different, which should be inconsequential when there's no feedback.
The more I read about this amp and listen to the YouTube clips, the less it seems to need. I wanted to capacitor-couple and move the interstage transformer to a typical load position, and add a plate resistor, like a parafeed without the choke. But...a lot of people like the transformer specifically because it eliminates the capacitor I'm contemplating adding...so anything I do will be reversible.
I found two direct replacements: Classic Tone 40-18093 $38.03 and MojoTone MOJO789 $39.14 one from . I found a Hammond interstage with a centertap secondary for PI duty which probably handles more power, and might be more appropriate if the transformer is changed to the typical parafeed position from plate to ground...perhaps with a change to a preamp tube with less gain and more current output.
I imagine it really comes down to how much bass is desired. With the larger 12" speaker cutout and a better speaker in place, everyone seems pleased with just byapssing the input cap, and I'll probably leave it in place and shunt across it with a wire. For more, an output transformer upgrade might be needed before the interstage transformer. Then you're just a power transformer away from a Deluxe and it's become a different amplifier.
So...thanks Enzo for giving me perspective to balance enthusiasm LOL. Tonight I have my guitar lesson, which will make more difference in how it sounds anyway! With a variety of interstage transformers available I might even build another just to experiment with.
I found two direct replacements: Classic Tone 40-18093 $38.03 and MojoTone MOJO789 $39.14 one from . I found a Hammond interstage with a centertap secondary for PI duty which probably handles more power, and might be more appropriate if the transformer is changed to the typical parafeed position from plate to ground...perhaps with a change to a preamp tube with less gain and more current output.
I imagine it really comes down to how much bass is desired. With the larger 12" speaker cutout and a better speaker in place, everyone seems pleased with just byapssing the input cap, and I'll probably leave it in place and shunt across it with a wire. For more, an output transformer upgrade might be needed before the interstage transformer. Then you're just a power transformer away from a Deluxe and it's become a different amplifier.
So...thanks Enzo for giving me perspective to balance enthusiasm LOL. Tonight I have my guitar lesson, which will make more difference in how it sounds anyway! With a variety of interstage transformers available I might even build another just to experiment with.
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Well it arrived. I'm just beginning to get to know it. The condition is much better than I expected, and the silver faceplate is in surprisingly good condition. Needs grille cloth, big deal!
For now I'm cleaning up the cosmetics; the unexpectedly good "unobtainable" faceplate changes everything. I got a new identical Fender original-type handle, I got two new chrome chassis straps and screws. It looked like someone spilled coca-cola on the old ones, badly rusted but only in the depressions. I need to order grille cloth ASAP, and I have to decide whether to replace the black tolex. It is pretty ratty, but glue and shoe polish can work wonders. This thing is made without any metal corners. I'm inclined to check out whether the cabinet corners are less rounded and too sharp to accept standard Fender corners; it wasn't made with any but it could use them and it would still look authentic enough as most Fender amps of the era did. This morning I just noticed the cabinet appears it might be just slightly pyramidal, with the front baffle pointing the speaker up just a tiny bit, barely noticeable.
Haven't had the house to myself to really crank it yet...
Haven't had the house to myself to really crank it yet...
FENDER MUSICMASTER BASS AMP | TalkBass.com
Some info on the amp and suggestions for improvement. Looks like the amp is only 15 watts, kinda low for bass, so other improvements might be a waste of time. On the other hand , might be a great guitar amp, if you can overdrive it enough.
Some info on the amp and suggestions for improvement. Looks like the amp is only 15 watts, kinda low for bass, so other improvements might be a waste of time. On the other hand , might be a great guitar amp, if you can overdrive it enough.
Yes, in those tube days a small "student" bass amp did not actually reproduce any bass, it was just "bass tolerant" in that it didn't blat, fart, etc. With only one preamp tube, no phase inverter, then the output tubes, it might really take well to a decent clean FET boost pedal for guitar.
These amps are great just as they are for guitar unless you simply have to get in there and "fix" something. Just make sure it's working as best as it can be.
If you take an output transformer, it must have a load or the impedance is wrong. The load can be either on the secondary, (loudspeaker) or primary (driver).
Hi Jon,
Yes, but this was a Phase Inverter transformer rather than an OT. So the loads on it are the grid leaks for the power stage valves.
Malcolm
Well, I started with some non-electronic issues. I'm favoring function and consistent style more than being exactly authentic. This Fender cabinet is awful MDF, not some nice finger-jointed Birch ply or solid pine, but I'm going to try to retain it. The bottom sounds rattly when I rap it with my knuckles. The previous owner obviously didn't know the grille was removable, as the flat heads of the speaker bolts were sticking out from the actual baffle board. The chrome chassis straps on top and their bolts were pretty rusted, as were the handle brackets.
Stripping the tolex was a pain, the old-style high-quality backing was glued down stronger than the MDF or the vinyl, and the fuzz clogged up a new belt sander. So I scraped the fuzz off with a hacksaw blade first. The MDF bottom has minor cracks right at the glue joints in the corners, which explain the rattle. At least they are in a decent location. The speaker is retained by only 4 bolts.
I got a new genuine fender handle with the exact original 1-bolt chrome ends, but I'm going to retain the original black rubber strap which is a bit thinner and authentic. I got new genuine Fender chassis straps and bolts, which are still available because they are used on other more popular models like the champ. The amp didn't originally have any metal corners. I got genuine fender corners, and they must be genuine for some crappy model, as they had no recess "dents" to act as counterbore to make dome-head (nearly flat-head) counterbore screw heads flush...the last thing I wanted was some round-head screws protruding from some cheap thin stamped corners! So I got some more havier-duty authentic corners. I got lots of tolex, in case I screw up, and twice as much grillecloth for the same reason LOL. The grillecloth isn't quite genuine, each little rectangle in the pattern is just a bit larger, but it's very close.
I had some discussion with others about gluing the MDF cracks in the corner joints. There are lots of options. Surprisingly frequently, very experienced people told me to get very thin superglue and clamp it first, and apply liberally, and it will wick in and penetrate deep, rather than the more traditional method of titebond pushed into the crack with the crack spread then clamp and wipe. I'm also going to cut some wood strips into a right-triangle cross-section and glue them in as corner braces: bottom to sides, bottom to baffle, baffle to sides. With the removable grille the baffle can be permanent.
Adding 4 more speaker bolts is really easy and seems worthwhile.
Now I need to get some better feet. There are several options. It was built with some crummy teensie 3-point buttons pounded in like staples. Some steel cookies with rubber mount to the tolex would be a good upgrade, or I might just use some rubber stopper-shaped feet.
The amp is higher than a champ because of the 12" speaker, so I'm contemplating adding some of the shorter Fender tilt-back legs. Most tilt-back legs never get used. In some ways they're perfect for this amp. The amp is not very top-heavy. The tilt-back legs are often used with heavy amps that have removable wheels, and it's a pain to remove the wheels before tilting it back, so often the tilt-back isn't used. This one can be plopped down, legs pulled out, and leaned back! This little amp could probably benefit from tilting up and pointing at the player a bit too! The downside is that the legs are heavy, and one virtue of this amp is its extremely light weight.
I got new output tubes, just some JJ's. The B+ is usually very low on this model, so I don't have to be very selective. I could not save the original paper tube chart, it was just too deteriorated from ozone oxidation but I do wish I'd noted the code with build date. Oh well, too late to be a responsible conservator now, it had to come off with the tolex and crumbled to dust. It was all original except someone fitted a 3-prong cord and replaced the "death cap". It doesn't hum but I'm inclined to replace the cardboard-tube main filters anyway, they're about 42 years old now. I haven't measured anything yet.
I want to get a speaker jack, and a genuine 1/4" angle-plug with the "F" Fender logo to put on the speaker leads.
I'm inclined to add a brace to the bottom, and add screws from the small low cabinet back board into it, so the back is held on by 6 screws instead of 4 and actually braces the cabinet against racking.
In the past I have painted MDF cabinets with epoxy paint, and it made the MDF much stronger and less vulnerable to water damage, which is is very susceptible to. Even just dew in the grass can damage cheap MDF that's made without marine-grade binder glue. On the other hand, I've used some thick black paint on industrial-style speaker cabinets with great success and a lot less bother. Anything to make this cheap cabinet stronger is prudent, and a good paint layer would make the tolex come off next time without taking MDF with it! Good contact cement will stick to the paint fine.
Stripping the tolex was a pain, the old-style high-quality backing was glued down stronger than the MDF or the vinyl, and the fuzz clogged up a new belt sander. So I scraped the fuzz off with a hacksaw blade first. The MDF bottom has minor cracks right at the glue joints in the corners, which explain the rattle. At least they are in a decent location. The speaker is retained by only 4 bolts.
I got a new genuine fender handle with the exact original 1-bolt chrome ends, but I'm going to retain the original black rubber strap which is a bit thinner and authentic. I got new genuine Fender chassis straps and bolts, which are still available because they are used on other more popular models like the champ. The amp didn't originally have any metal corners. I got genuine fender corners, and they must be genuine for some crappy model, as they had no recess "dents" to act as counterbore to make dome-head (nearly flat-head) counterbore screw heads flush...the last thing I wanted was some round-head screws protruding from some cheap thin stamped corners! So I got some more havier-duty authentic corners. I got lots of tolex, in case I screw up, and twice as much grillecloth for the same reason LOL. The grillecloth isn't quite genuine, each little rectangle in the pattern is just a bit larger, but it's very close.
I had some discussion with others about gluing the MDF cracks in the corner joints. There are lots of options. Surprisingly frequently, very experienced people told me to get very thin superglue and clamp it first, and apply liberally, and it will wick in and penetrate deep, rather than the more traditional method of titebond pushed into the crack with the crack spread then clamp and wipe. I'm also going to cut some wood strips into a right-triangle cross-section and glue them in as corner braces: bottom to sides, bottom to baffle, baffle to sides. With the removable grille the baffle can be permanent.
Adding 4 more speaker bolts is really easy and seems worthwhile.
Now I need to get some better feet. There are several options. It was built with some crummy teensie 3-point buttons pounded in like staples. Some steel cookies with rubber mount to the tolex would be a good upgrade, or I might just use some rubber stopper-shaped feet.
The amp is higher than a champ because of the 12" speaker, so I'm contemplating adding some of the shorter Fender tilt-back legs. Most tilt-back legs never get used. In some ways they're perfect for this amp. The amp is not very top-heavy. The tilt-back legs are often used with heavy amps that have removable wheels, and it's a pain to remove the wheels before tilting it back, so often the tilt-back isn't used. This one can be plopped down, legs pulled out, and leaned back! This little amp could probably benefit from tilting up and pointing at the player a bit too! The downside is that the legs are heavy, and one virtue of this amp is its extremely light weight.
I got new output tubes, just some JJ's. The B+ is usually very low on this model, so I don't have to be very selective. I could not save the original paper tube chart, it was just too deteriorated from ozone oxidation but I do wish I'd noted the code with build date. Oh well, too late to be a responsible conservator now, it had to come off with the tolex and crumbled to dust. It was all original except someone fitted a 3-prong cord and replaced the "death cap". It doesn't hum but I'm inclined to replace the cardboard-tube main filters anyway, they're about 42 years old now. I haven't measured anything yet.
I want to get a speaker jack, and a genuine 1/4" angle-plug with the "F" Fender logo to put on the speaker leads.
I'm inclined to add a brace to the bottom, and add screws from the small low cabinet back board into it, so the back is held on by 6 screws instead of 4 and actually braces the cabinet against racking.
In the past I have painted MDF cabinets with epoxy paint, and it made the MDF much stronger and less vulnerable to water damage, which is is very susceptible to. Even just dew in the grass can damage cheap MDF that's made without marine-grade binder glue. On the other hand, I've used some thick black paint on industrial-style speaker cabinets with great success and a lot less bother. Anything to make this cheap cabinet stronger is prudent, and a good paint layer would make the tolex come off next time without taking MDF with it! Good contact cement will stick to the paint fine.
MDF cabinet has more problems than I first thought, I should have just had a new higher-quality cabinet made, but I stripped this one and have been salvaging it.
Cute trick I learned from a guy on facebook: big ounce-size bottles of very thin anaerobic cyanoacrylate (crazy glue) can be bought at hobby shops. It wicks into the "end grain" of MDF quickly and makes it much stronger, setting quickly and producing considerable heat. I just flood it and it soaks in fast and deep. From now on this is the treatment I will employ around the driver cutout and in every screwhole whenever I make a speaker baffle out of MDF.
Cute trick I learned from a guy on facebook: big ounce-size bottles of very thin anaerobic cyanoacrylate (crazy glue) can be bought at hobby shops. It wicks into the "end grain" of MDF quickly and makes it much stronger, setting quickly and producing considerable heat. I just flood it and it soaks in fast and deep. From now on this is the treatment I will employ around the driver cutout and in every screwhole whenever I make a speaker baffle out of MDF.
Oh Come on Enzo,
....of course they did.
They were sitting around
a table smokin' weed and
said to one another laughing
uncontrollably while eating
Frito's before they banned
the Frito Bandito from ads.
They said, c'mon dude, let's
screw with peoples minds
and in 50 years the'll look
at the amp and be baffled
at the resistor in here.
Then one of the guy's said,
"f" this, who ran off with the
Laura Scudders, man.
Just tracing through those resistor values, I'm thinking the difference in jacks is NOT inconsequential. For the two 68k resistors, the "paralel" connection goes through 34k, the coupling cap, and the 470k grid-to-ground resistor, for a total of about 500k load. This is pretty good, though changing the 470k to 1M might help make it brighter.
When you plug into the other, you've got the two 68ks in series going to ground, for a total of 136k (in parallel with that 470k, which makes it even a little lower), and that's going to put a substantial load on the pickups, and cut the highs. If you somehow WANT your tone cut like that, you can plug into this, but I'd rather use the other input to get "full response" and cut the tone on the guitar if I wanted it cut.
I think we are not looking at it close enough. Plug into the high level jack and the two 68k are in parallel for 34k in series with the signal. Plug into the low level jack, and the two 68k are now in series. One end of the two goes to the input tip, the other end to ground. The signal into the first stage grid comes from the junction of the two resistors. So they form a 2/1 voltage divider, cutting the signal level in half. A 6b pad.
If you plug a signal into both jacks, then each has a 68k series resistor, and the ground shunt is open, so no voltage division occurs. The amp was not designed with that in mind though.
If you plug a signal into both jacks, then each has a 68k series resistor, and the ground shunt is open, so no voltage division occurs. The amp was not designed with that in mind though.
Great Thread!
I too have one incoming in the mail. I'll be watching this thread closely for ideas from you more knowledgable than I on this topic!
Alot of folks say to jumper that .0047 input cap as its not necassary for a guitar signal.What about using an axial errite bead for the jumper, wouldnt that help with the RF protection?
I too have one incoming in the mail. I'll be watching this thread closely for ideas from you more knowledgable than I on this topic!
Alot of folks say to jumper that .0047 input cap as its not necassary for a guitar signal.What about using an axial errite bead for the jumper, wouldnt that help with the RF protection?
Attachments
RF protection?
The design calls for specific valves that work best at audio frequencies, the transformer cuts off at 20kHZ so RF is not an issue here. Most solid state amplifiers and consumer devices use RF blockers.
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