http://www.tubefreak.com/1bass400.gif
http://www.tubefreak.com/2bass400.gif
I stumbled across a pair of schematic pages that outline a Mesa 400 bass head. The addresses of these pages are located above. It seems that it's not entirely complete, and I am hoping that someone on this forum may be able to enlighten me of a thing or six.
1) The 1st diagram leaves out a lot of the labels for the values of the capacitors. They show up as simply .1 with no indicator as to it's unit of measure; I don't know whether to interpret this as .1f, or if there is another standard value that is assumed when it is not marked. There are a few capacitors in the EQ section that are labeled as #/# such as 220/63 and I have no idea what that means.
2) There are also a number of resistor values that are not labeled in the schematics. I'm hoping these can be calculated in some way or another. In the preamp section, the only one not labeled seems to be the variable resistor for the high end volume pot. I would assume this is a 100k, but am not really sure. In the Buffer section, there are two resistors not labeled, one at the top and one at the bottom. I would assume that these are 33k ohm. The resistor just after the Slave output control, seems to be 4760 ohm, but that seems like such an odd number that I'm just not really sure. In the Power Supply section, there's on resistor that seems to be labeled BIAS that I have no idea what it should be. In the Equalizer section there is a little box in the upper right that says 220/63, I don't know what the R value is there, or the little + signs with x2 in the middle. There is another R not labeled just after Q3.
3) Neither of the transformers are labeled in a way that I can understand them.
4) I do not know what VR1 and VR2 are.
5) I do not know what the value for the Filter Choke should be.
I think that's about it for my first round of questions. If anyone can help me, or point me towards a more complete diagram of this head, I would be most appreciative. I'm itching for one of these heads, and I'm confident that if I have a complete set of schematics I'll be able to build one.
Thank you,
Tony
Mesa 400 bass head schematic woes.
Moderators: pompeiisneaks, Colossal
Re: Mesa 400 bass head schematic woes.
All caps should be read as uF (micro Farad) unless otherwise labeled. Minimum 400V, again unless otherwise. If you see 220/63 that typically means 200uF 63V.tonyhoff wrote:1) The 1st diagram leaves out a lot of the labels for the values of the capacitors. They show up as simply .1 with no indicator as to it's unit of measure; I don't know whether to interpret this as .1f, or if there is another standard value that is assumed when it is not marked. There are a few capacitors in the EQ section that are labeled as #/# such as 220/63 and I have no idea what that means.
The only one's I see not labeled are in the bias section and the bias voltage is labeled. I'd assume that you need to adjust that resistor until you have that voltage.2) There are also a number of resistor values that are not labeled in the schematics. I'm hoping these can be calculated in some way or another
Honestly I'd expect this to be more like 250k. Funny that it's not labeled.In the preamp section, the only one not labeled seems to be the variable resistor for the high end volume pot. I would assume this is a 100k, but am not really sure.
I don't see them, all resistors in that section are labeled.In the Buffer section, there are two resistors not labeled, one at the top and one at the bottom. I would assume that these are 33k ohm.
Maybe that's 470 ohm. It's kinda blurry so it's hard to tell.The resistor just after the Slave output control, seems to be 4760 ohm, but that seems like such an odd number that I'm just not really sure.
You'll need to measure the input voltage then use the voltage divider equation located at Aikenamps.com to calculate the correct value. Pretty poor not labeling that.In the Power Supply section, there's on resistor that seems to be labeled BIAS that I have no idea what it should be.
The voltages are listed so again I'd measure the voltage you have there and adjust that value until you have the correct voltage.In the Equalizer section there is a little box in the upper right that says 220/63, I don't know what the R value is there, or the little + signs with x2 in the middle. There is another R not labeled just after Q3.
That's common on manfactures schems. They custom wind these so you'll have to calculate the power draw, the voltages are labeled. IME Mesa will not give you these specs on current amps, only on obsolite ones.3) Neither of the transformers are labeled in a way that I can understand them.
Most likely metal oxide varistors.4) I do not know what VR1 and VR2 are.
Again Mesa only knows.5) I do not know what the value for the Filter Choke should be.
Good luck. Meas doesn't want you copying this so you're pretty much on your own. Tube manuals might help you do the calculations.
Re: Mesa 400 bass head schematic woes.
What Bob-I said. But you have a lot of questions so here's more.
First: I have seen booby-traps on M/B's plans. Things that clearly won't work. Hidden in obscure corners where the average Service Tech never needs to look, but will confound people who steal blindly. I don't see anything at a glance, but you better understand everything if you are putting your cash on the line.
> I don't know whether to interpret this as .1f
Go price a 0.1F cap. Today you can buy one, in 5V rating. 0.1uFd in 600V rating can't be shipped UPS; anyway you can't pay for one. That's hi-energy physics-lab or atom-bomb stuff.
Compare with other amps. Fender throws 0.1uFd or 0.047uFd in a lot of the same places. Don't make sense that M/B would use 1,000 times bigger caps in an amp that's only 4X-10X the size of most Fenders.
> Equalizer section there is a little box in the upper right that says 220/63, I don't know what the R value is there, or the little + signs with x2 in the middle.
This seems to be a 2-section ("X2") 220uFd 63V cap. You will buy two caps. (Actually, it probably was two caps, but the draftsman got cramped.)
The R value computes from the stage current. Mostly two "3300" resistors showing "20.7"V, or 12.54mA. Round-up to 13mA for stray current we are too lazy to compute. (The 22K has 2.02V across, which is roughly 0.1mA; there's a smaller leak-path through Q1 bias... say 13mA.) The unknown resistor drops 53V to 32V, or 21V. Ohm's Law says 21V/13mA is 1,615 ohms. Betcha it is really a 1.5K resistor. Heat is 21V*13mA or 273mW or 0.27 Watts, use at least a 1/2W part.
Note that this whole contraption runs on NEGative voltage, since the only handy source for stuff transistors can eat was the negative bias supply. That's why the "little + signs" go "the wrong way".
> There is another R not labeled just after Q3
And the same in front of Q2.
They should be equal. They are probably somewhat larger than the "3300" resistors, but not a heap larger. If you knew the +/-dB of the EQ, you could estimate from the series resistors on the EQ L-C networks. +/-20dB is possible. We see 470 on the big coils and 680 on the last smallest coil. The big coils have internal R, the small one has less.... assume the ideal resistance is maybe 700 ohms. Then for 20dB the unknown resistors are almost 10 times bigger or 7,000.... use 6K8. This is indeed larger than 3300, but not so much so as to make the 3300 wasteful.
3K3 or 3K9 also make sense in light of the transistors sitting 2/3 of the way up with 3K3 resistors. Either value will "worK". WTH. Put those two resistors out where you can monkey with them.
> Neither of the transformers are labeled in a way that I can understand them.
As Bob-I sez: Boogie Special. "Obviously" the output is well over 150 Watts and maybe down to 40Hz, impedance suitable for 3 pair of very over-worked 6L6. The RCA manual shows 6L6GC working at lower B+ and 5K6pp to make 56 Watts... this seems to be working 25% higher voltage on plate and screen. The apparent assumption is they used book-spec (which is good if safe) and jacked the voltage until they could not special-order a 6L6GC to stand it (do NOT use 6L6, use 6550). Over 200 Watts, assuming the same 5K6 load per pair. (That's at low THD; the "400" may be gross-distortion watts, which may be "valid" on older geetar amps). So look-up the RCA spec for plate current, do 1.25 times higher for hot-rodding, and triple for 3 pair. That's most of your B+ drain and thus your PT rating. Likewise you figure heater current from book specs and tube count. The PT HV rating is right there, though you need to read it "200-0-200" or "400VCT".
Hammond has a monster hi-fi OT iron and IIRC in 2KCTpp spec, this is ballpark enough for 5K6/3.
You may not find that PT as any stock item.
EsPECially with that whacko fan winding.
Have you hunted Heyboer? They list replacements for many odd old amps.
> what the value for the Filter Choke should be.
You can estymate the current. What does it feed? Not the Plates, thank goodness. All the 6L6 screens, and everything else. We can actually spot the G2 current: 2K7 resistors have 535V one end and 526V(?) the other end. 3mA per screen, 18mA. The next big suck is probably driver V4... ah, more unknown resistors. The answer is "47K"; you should prove it with your own brain. 4mA there. Similar napkin-calcs show about 1mA in 6 more tubes. 28mA, you need a 30mA choke. The Henries can't be known, but other amps use 1H-8H in this position, and ANY choke is better than no choke. And though it usually is not critical, we can nail the DCR: it drops 5V when passing 28mA, it is 178 ohms. More or less.... none of these pencil-numbers is exact. Yet another clue would be to eyeball a real MB 400 choke's physical size: if it is a 10-pound lump or a 9-ounce joke.
> variable resistor for the high end volume pot
You mean the Treble control? Oddly, that may not be critical. Depends on the 250/1000pFd caps and how you like your treble boost. Looks like most other Fender-steal tone stacks, should have similar pots. Duncan TSC could show how far off a wrong-guess could be. For guitar, Bob-I's 250K seems likely. For Bass, they may have gone higher.
Oh. They's gotta be a missing resistor at V2B grid. Booby-trap? or just an oversight? Cute. The missing part will never burn, so repair-dudes don't need that detail. A blind-stealer could build it just this way, no smoke to point to the problem, even get a wisp of hum/hiss in the speaker, yet it just would NOT work.
That Blend pot can't possibly work with all FX boxes.
V4 grids look wrong but may actually be right. It should be possible to save 24 cents there... I'm sure it came about by try it and like it, not research or analysis, and in context the "excess resistors" are moot.
Ah. V1 grids are connected impossibly. Again, the factory amps had no flaw here, but copycats get no smoke to give clues to the clueless.
You got a clue on the transistor types?
> I'm confident that if I have a complete set of schematics I'll be able to build one.
A Good Guitar (or Bass) Amp is NOT just a "set of schematics"!!!
Schematics never tell all, even if they are nominally complete and accurate.
Layout is VITAL. Both big-picture and small-details.
And after a lot of messing with audio, I think the kharma of the builder matters. I know some builders who use ordinary parts and plans and build extra-musical amps; others who can build an amp that "works" but doesn't "sing". And the early M/B years had some obsessed and enthusiastic workers in a good home.
First: I have seen booby-traps on M/B's plans. Things that clearly won't work. Hidden in obscure corners where the average Service Tech never needs to look, but will confound people who steal blindly. I don't see anything at a glance, but you better understand everything if you are putting your cash on the line.
> I don't know whether to interpret this as .1f
Go price a 0.1F cap. Today you can buy one, in 5V rating. 0.1uFd in 600V rating can't be shipped UPS; anyway you can't pay for one. That's hi-energy physics-lab or atom-bomb stuff.
Compare with other amps. Fender throws 0.1uFd or 0.047uFd in a lot of the same places. Don't make sense that M/B would use 1,000 times bigger caps in an amp that's only 4X-10X the size of most Fenders.
> Equalizer section there is a little box in the upper right that says 220/63, I don't know what the R value is there, or the little + signs with x2 in the middle.
This seems to be a 2-section ("X2") 220uFd 63V cap. You will buy two caps. (Actually, it probably was two caps, but the draftsman got cramped.)
The R value computes from the stage current. Mostly two "3300" resistors showing "20.7"V, or 12.54mA. Round-up to 13mA for stray current we are too lazy to compute. (The 22K has 2.02V across, which is roughly 0.1mA; there's a smaller leak-path through Q1 bias... say 13mA.) The unknown resistor drops 53V to 32V, or 21V. Ohm's Law says 21V/13mA is 1,615 ohms. Betcha it is really a 1.5K resistor. Heat is 21V*13mA or 273mW or 0.27 Watts, use at least a 1/2W part.
Note that this whole contraption runs on NEGative voltage, since the only handy source for stuff transistors can eat was the negative bias supply. That's why the "little + signs" go "the wrong way".
> There is another R not labeled just after Q3
And the same in front of Q2.
They should be equal. They are probably somewhat larger than the "3300" resistors, but not a heap larger. If you knew the +/-dB of the EQ, you could estimate from the series resistors on the EQ L-C networks. +/-20dB is possible. We see 470 on the big coils and 680 on the last smallest coil. The big coils have internal R, the small one has less.... assume the ideal resistance is maybe 700 ohms. Then for 20dB the unknown resistors are almost 10 times bigger or 7,000.... use 6K8. This is indeed larger than 3300, but not so much so as to make the 3300 wasteful.
3K3 or 3K9 also make sense in light of the transistors sitting 2/3 of the way up with 3K3 resistors. Either value will "worK". WTH. Put those two resistors out where you can monkey with them.
> Neither of the transformers are labeled in a way that I can understand them.
As Bob-I sez: Boogie Special. "Obviously" the output is well over 150 Watts and maybe down to 40Hz, impedance suitable for 3 pair of very over-worked 6L6. The RCA manual shows 6L6GC working at lower B+ and 5K6pp to make 56 Watts... this seems to be working 25% higher voltage on plate and screen. The apparent assumption is they used book-spec (which is good if safe) and jacked the voltage until they could not special-order a 6L6GC to stand it (do NOT use 6L6, use 6550). Over 200 Watts, assuming the same 5K6 load per pair. (That's at low THD; the "400" may be gross-distortion watts, which may be "valid" on older geetar amps). So look-up the RCA spec for plate current, do 1.25 times higher for hot-rodding, and triple for 3 pair. That's most of your B+ drain and thus your PT rating. Likewise you figure heater current from book specs and tube count. The PT HV rating is right there, though you need to read it "200-0-200" or "400VCT".
Hammond has a monster hi-fi OT iron and IIRC in 2KCTpp spec, this is ballpark enough for 5K6/3.
You may not find that PT as any stock item.
EsPECially with that whacko fan winding.
Have you hunted Heyboer? They list replacements for many odd old amps.
> what the value for the Filter Choke should be.
You can estymate the current. What does it feed? Not the Plates, thank goodness. All the 6L6 screens, and everything else. We can actually spot the G2 current: 2K7 resistors have 535V one end and 526V(?) the other end. 3mA per screen, 18mA. The next big suck is probably driver V4... ah, more unknown resistors. The answer is "47K"; you should prove it with your own brain. 4mA there. Similar napkin-calcs show about 1mA in 6 more tubes. 28mA, you need a 30mA choke. The Henries can't be known, but other amps use 1H-8H in this position, and ANY choke is better than no choke. And though it usually is not critical, we can nail the DCR: it drops 5V when passing 28mA, it is 178 ohms. More or less.... none of these pencil-numbers is exact. Yet another clue would be to eyeball a real MB 400 choke's physical size: if it is a 10-pound lump or a 9-ounce joke.
> variable resistor for the high end volume pot
You mean the Treble control? Oddly, that may not be critical. Depends on the 250/1000pFd caps and how you like your treble boost. Looks like most other Fender-steal tone stacks, should have similar pots. Duncan TSC could show how far off a wrong-guess could be. For guitar, Bob-I's 250K seems likely. For Bass, they may have gone higher.
Oh. They's gotta be a missing resistor at V2B grid. Booby-trap? or just an oversight? Cute. The missing part will never burn, so repair-dudes don't need that detail. A blind-stealer could build it just this way, no smoke to point to the problem, even get a wisp of hum/hiss in the speaker, yet it just would NOT work.
That Blend pot can't possibly work with all FX boxes.
V4 grids look wrong but may actually be right. It should be possible to save 24 cents there... I'm sure it came about by try it and like it, not research or analysis, and in context the "excess resistors" are moot.
Ah. V1 grids are connected impossibly. Again, the factory amps had no flaw here, but copycats get no smoke to give clues to the clueless.
You got a clue on the transistor types?
> I'm confident that if I have a complete set of schematics I'll be able to build one.
A Good Guitar (or Bass) Amp is NOT just a "set of schematics"!!!
Schematics never tell all, even if they are nominally complete and accurate.
Layout is VITAL. Both big-picture and small-details.
And after a lot of messing with audio, I think the kharma of the builder matters. I know some builders who use ordinary parts and plans and build extra-musical amps; others who can build an amp that "works" but doesn't "sing". And the early M/B years had some obsessed and enthusiastic workers in a good home.