Sweet video, Stuart. I did one a few years back on the HD150, which is the head version of this amp pretty much. I went into some detail about the design quirks.
It's a common grid configuration. Essentially it runs very close to class B, hence the low bias current. The very high HT is ok as the bias current is low. As a result, output valves last a long time. It a design that goes way back to the 1920s. It was used in a lot of RF power stages. The down side is that the input impedance is very low and needs high current to drive it, so that's what the power transistor is doing.
@@stuartukguitarampguy5830 I think it was a bit of a gimmick, although it would allow you to get lots of watts without overcooking the valves, as would have been the case with a standard AB1 push pull or needing more valves, plus a long life. It was a well known technique in RF world for boosting the output of a transmitter by simply strapping on a common grid stage to the output. The original transmitter output would be 75 ohms so low enough impedance to drive the common grid add-on but I think Music Man were the only people to use it in audio.
What sort of wattage do you get out of these amps? I’m getting 56Vpp at the output which is a bit low. I estimate that I should be seeing 76Vpp at the output to get 130 watts
Well, I have now watched every single Video over the last 2 months and I am now a fully qualified Amp Tech!! Ha Ha!! Very very informative videos and have loved every minute of them AND repaired my burnt out Trace Elliot Velocette 10, Very Happy. Thanks Stuart, Keep the Videos coming.
Great video Stuart. I had one of these back in the day. Here in the US these amps came with Sylvania 6CA7 valves, the American version of the EL34s. Crazy high plate voltages. The screens are at 350 volts. The 6CA7s had no problems operating under these conditions.
Thank you Stuart. I have the HD-130 Music Man head, I've never had to get into it but seeing your video has given me courage should the need ever arise to open it up. Great and informative video!
Stuart, thanks again for the clear explanations as you work through this amp. The diagrams are exceptionally helpful. I find myself understanding much more of what you are saying. Always entertaining as these amps are fascinating to see. I have played one of these before and it did sound good. I was looking for a clean amp at the time and decided on a Roland JC-120 instead. Keep up the good work!
I had an HD130 head in the mid 80s and it was a GREAT sounding loud amp. I also recall having a real printed schematic for it because I brought it to the local TV repair shop to fix a broken tube socket.
Old Leo sold Fender to CBS. Then he went and formed up MusicMan. It was good deal. Everybody was happy. Then later Leo formed G&L Instruments. I've never heard anyone complain about these MusicMan amps. Thanks Mr. Stuart.
After buy a Music Man 212 Sixty-Five that needs maintenance, I spent a lot of time reading and studying the schematic. I was able to successfully repair the amp and it sounds fantastic again. I replaced all e-caps, all dioes, all op-amps. A couple of resisistors that were out of spec. The 2 PNP transistors on the heat sinks too, these are on the phase inverter board. A couple of rotary control pots. These are wonderful amps. The other two small blue adjustment trim-pots that you mentioned on the pre-amp circuit board are for adjusting the tremolo.
Hey Stuart, back in the day I used to use a Music Man 112RD65, what an amp..! Just fab sound and powerful. Alas, it blew up eventually. Great video and nice to see one of those Music Man amps on your bench, getting some TLC, nice job...Ed..uk..😄
Somebody told me years ago that the output stage on these amps is run basically in class B which is why the plate current is so low. It would be interesting to know if pin 6 is used for anything; an American 6CA7 doesnt have a suppressor grid and doesn't use pin 6 (a 6L6 doesnt use it either), but a true EL34 brings the suppressor gris out via pin 6 and it usually gets tied to ground (or to the output tube cathode in a cathode-bias amp). Those 1.5k resistors are tied to pin 4 and are obviously screen resistors ( it would probably be better to have one for each tube rather than sharing a resistor per tube pair). The KT77 (kinkless tetrode) would probably be the closest European equivalent to an American 6CA7 ( beam power tube).
Whoops, a correction ---- the EL34 brings the suppressor grid out at pin 1, not pin 6, and in a fixed-bias amp it gets tied to ground (or to pin 8, the tube cathode, in a cathode-biased amp).
I have the exact amp in head cab form. The board with the dual op amp and the 2 3 leg transistors is the phase inverter/ driver board for the power tubes. The blue wheel on that board is what you use to set bias. My amp was factory wired to run either 6l6 or el34 tubes. And right now it's actually running both at same time. One of each per side. I'm just having a ongoing issue with the speed and intensity section now. At one point it was perfect, then a xener diod went out, after that I could swap the metal can op amps around and either I had reverb and no trem or I'd have trem but no reverb. New transistors for the trem circuit, the black half round 3 leggers, it's all there but if you don't hit the strings hard. Hit the strings hard then you get full sound and as it lowers the trem starts working again. Kinda stuck on this
I played one of these amps for 40 years on an average of 5 nights a week and it never went wrong on me . the only thing I did was change the tubes 3 times. best amp ever , when I bought this amp all my friend went for the cheaper Peavy amps , none of them lasted 10yrs on the road and had constaint problems
I really like Peavey amps, they were affordable but seem to be pretty rugged. Thousands still going strong years later and both the solid state and valve amps have a great tone. Possibly not rugged enough for 40+ years on the road, being slung into vans and constantly joggled about, or possibly not easy to repair, is that why you added the 'Ugh' comment Stuart?
Hi Rob I can;t recall why I went ugh tbh. But there's a difference between how a musician views an amp and how a tech views one. E.g. people love their Mesas but they are a SOB to work on so techs hate them.
Stuart, glad to see, with your experience, that you were just as bewildered with this design as I was last year working on a 212RD65--it took me quite some time with the poor schematic pages.
Beautiful condition 1976/77? I wonder how many people (like me!) were screaming at you "the standby switch is on the back"!!! ha ha! I have the near identical amp 1976 but 65x 210!! Lovely clean amp and have NEVER had a problem with it!. Nice one Young Stuart!
My old guitar teacher's main amp was a Peavey Deuce which was solid state preamp / valve poweramp and he always got lovely tones out that (and with EMGs in his 60s/70s partscaster to boot!)
This amp most likely has the series filter cap. system where the mid point is attached to the secondary winding centre tap thus not requiring the voltage dividing resistors.
The springy tube clamps are known as "bear traps" here in the states. The Mullard EL34 outputs are almost certainly not original; in the US this amp would have been factory supplied with Sylvania STR "fat bottle" 6CA7 tubes which is the American "equivalent" to the EL34, the 6CA7 being a beam power tube and not a true pentode, but with the same pinout and somewhat similar characteristics to the EL34. "STR" stood for "special test requirement".
PS, If memory serves correctly the Bakelite base of the fat bottle 6CA7 would have been wider diameter than the base of an EL34 and that's probably why the bear clamps weren't touching the Mullard tubes.
Well done. I had wondered (complete guesswork on my part) whether the transistors in the cathode circuits were automatic bias (as on tubecad) but I may be completely wrong. Probably an illusion, what seems to be invisible soldering technique on the white-red wire (5:22).
Very strange output valve configuration. After a lifetime of involvement in electronic equipment maintenance, testing and design, I’ve only seen common base configuration used in weird impedance matching circuitry or in h.f. amplifiers as a method of reducing inter-electrode capacitance. I share your intrigue. Really interesting vlog, many thanks for posting it.
well im in my hd130rev. 2275 coming into a reverb issue having some distortion and effecting the trem system. take the reverb out of the loop and trem works perfect, verb in loop and normal playing on guitar the trem doesnt like it, easy on strings and trem likes that. swapped the ic cans around and tried nos so i know its not in them, looking for a better more eligible schem , and help be great
Very interesting Stuart.. I used one of these several years back ( loaned to me by a friend for a gig) and I was very impressed with it. Had I known what lay under the bonnet at the time, I'd have been very nervous 😅.. Music Man were eventually bought up by Ernie Ball . I can't help think that those FETS that substitute 12AX7's might be hard to find these days should that become necessary.
@@stuartukguitarampguy5830 Apparently Fender made something similar .. It was called The Showman (not to be confused with the 1960's version). Solid state pre-amp and valve output.
What type of amplifier class or biasing is it called when using a FET to modulate the cathode and keeping the grid at + 15vdc? How did you calculate the balancing resistor values? I'm not sure why you used 220K instead of 10K or 1K
Hi Wayne. I'm not sure pf the technical term. In transistor it would be common emitter or common base. I've never seen this configuration before. No you couldn;t use 1k! Do the power calculation for 350V across a 1K resistor! 220k is common. It dispates about half a watt so a 2W resistor is fine.
Hi Wayne. It's a bit of a catch-all loose term really. I'm not totally sure what causes it, possibly slight loss of vacuum. But it's when a high current flows from anode to cathode. It usually takes out the HT fuse etc. It's almost always a faulty valve.
As much as these amps can sound great, (think Brian Too Loud Macleod), I've heard some of these amps depending on year are "tube hungry" with the particular bias setup, or lack there of. Thanks to your video, it becomes quite clear why this can be the case. I know alot of amps have their design quirks though. One tech's dream can be another's nightmare. I've been looking for one as of late, and if I do get one, the resistor mod across the caps would seem crucial. Ive actually heard other techs mention it as well, so seems like a good idea.
2 questions: When you were "setting the bias" did you have the amp in high power mode? Did you measure the actual values of the 3.9 ohm emitter resistors before starting to see if that could be the cause of the imbalance between the 2 sides?
From memory it was in high power mode. No I didn;t actually measure the 3.9ohm res. Probably should have but I'd be surprised if they were much different from each other.
I had the 2x10 65 combo but it was earlier and had a valve rectifier. A decent enough amp that I swapped my silverface deluxe for. I don't miss the Music Man but I do miss that deluxe. Any way thanks for the video Stuart.
I have one of these I bought over 30 years ago from the original owner. Here in the US mine came with Sylvania 6ca7 fat bottle tubes! The earlier version was really like a bassman, no reverb ,no tremolo but had a 12ax7 phase inverter tube. you can see the block off plate where the tube would’ve been in this newer version that’s on the bench here . I have one of those two and it had 6ca7 Sylvania’sis in it too! I would say the earlier version sounds a bit better.
Hallo! I also have in my MM 130Hd US Sylvania. Can't say are they better or not because I had only 115RP with Siemens EL34 and 130hd head with EVM 12L, both of speakers have their benefits. Another story is Bob Luly... I've noticed "RL" on the back side. Maybe that's just another guy or woman. I had also "RL" but inside wood box. Thank you for video and sorry for my englisch.
Hello thanks tor that detail vid, I had many MM's and they came from the factory with Sylvania 6CA7 tubes, Blue and Yellow marks ;) please correct me if someone has some info about MM's 1974-78 first run tubes topic
It is very very strange indeed i have 1974 hd130 reverb head it to me is a weird cathode bias clearly in the states op amps and transistors were the “new thing” i have the gb-1 board 1/12ax7 phase inverter and four el34 and the bias pot is near the r45/ 46 your are required by music man per the listed schematic to read 0.5 VDC and yes the high power side is the benchmark setting of course no signal applied it took my a whole night to trouble shoot my red plating tubes and two 5 amp fuses blown before i could figure it out as well sheesh its rough!! But i got it stable and it functions rather loudly and consistently at full level its 130 dB!!! Just looking at my boards schematic it looks class a cathode bias for sure still confuses me terribly 😂😂 After looking at my schematic again, it appears to be a grid bias set up, but somehow you read point “y” which is tied to the cathode to read 1/2 vdc but the input to the boas trim is near 46vdc inverted like it should to the grid (g1) and trimmed to spec to me that weird also, I have done research on Music mean in the relationship to Leo Fender. From what I understand he just ran. The count me in was very upset that there was a solid state pre amp from what I understand. He wanted tube, preamps and solid state output but mini Fender isms appear to be found like the filter caps under the Doghouse on top of the chassis the overall look, and of course the loudness 😂😂 So with my issue, long story short, I’m sure I’m too late is that the tooth red bleeding because of the grit not being biased correctly with the right amount of negative voltage which makes sense Excellent work Stu!!!
I'm in the process of repairing another MusicMan amp from this time period, also 6CA7s, internet research says they changed to 6L6GC eventually, but 6CA7/EL34 is probably correct for when this was made.
You needed to get a circuit diagram before you start servicing this amp. There was a lot of stuff you got wrong. There are two 150K resistors on a board on the right side of the chassis. So the 220K resistors you added weren’t required. Not to be mean, just giving feedback.
Common base, common emitter? No that isn't the point at all. The output stages are a "cascode" hybrid design. It's a great improvement over the usual stages in many ways. (I expect the purists who "know amps" will disagree violently with that".) The transistor is much better at controlling currents, bipolars are current controlled devices by their nature, and as it sits under the valve's cathode it runs at a very low voltage allowing a higher frequency lower voltage transistor to be used. Heat dissipation is also pretty much not a big issue. The valve only serves to handle the high voltages of the power supply and transfer the BJT's current through to the transformer. You get much better bias stability, lower distortion which is what Music Man want, hugely improved upper frequency range. It's an absolutely brilliant "clean amp" solution to keeping the sound "valve" while letting the far superior transistor act as the amplifying element. If you don't know it you really should look up "cascode".
Che io ricordi gli ampli MUSIC MAN erano equipaggiati con 6L6 io ho avuto un RD12 65 e RD212. 100 entrambi con 6L6 acquistati nuovi nell' 80 e 82 - grazie della risposta
Hi Stuart, I enjoy your videos. You should defenitely spend a bit more time on that ugly schematic, there is a lot of usefull information to be found. I have spent hours.. The amp already has balancing resistors across the big capacitors, its the 150k resistors R16 and R67 on the schematic. The driver transistors are bipolar NPN transistors, not FET's. If there are problems with adjusting the bias or distorted output signal (and the tubes are ok), I have had succes with changing the old transistors. A good substitution is 2N6488. Musicman amps comes in several sizes and variants, some with traditional tube driver circuit, and this one with transistor driver circuit. It's probably the best amp fender never made... be aware, I'm not a guitarist, just a poor tech. I'm also a bit worried about the insane high anode voltage...
What a terrible scheme! I've never seen anything like this in my life. Some kind of perversion. There, essentially, lamps are not needed at all, they are there more for beauty. It’s not clear why this was done, to save five 12AX7? This is a very small saving; instead of an anode transformer, they could install a pulse generator, it would be much cheaper. I would never buy such a device..
That's strange to hear, because this thing was made for a person, who helped with amps and effects Rolling Stones, Alvin Lee, EVH, Mark Nopfler, ZZTop, Erick Clapton and many others. I'm not talking about Mr.Fender, he also had a great team. Some of them work in NASA and made a lot for the USA. For sure, it can sound for you not good, but not for others.
Sweet video, Stuart. I did one a few years back on the HD150, which is the head version of this amp pretty much. I went into some detail about the design quirks.
Ok great I'll chack it out. Love your channel.
It's a common grid configuration. Essentially it runs very close to class B, hence the low bias current. The very high HT is ok as the bias current is low. As a result, output valves last a long time. It a design that goes way back to the 1920s. It was used in a lot of RF power stages. The down side is that the input impedance is very low and needs high current to drive it, so that's what the power transistor is doing.
Interesting for sure thank you for the info!!
Interesting thanks. I wonder WHY they wanted this unusual design though.
@@stuartukguitarampguy5830 I think it was a bit of a gimmick, although it would allow you to get lots of watts without overcooking the valves, as would have been the case with a standard AB1 push pull or needing more valves, plus a long life. It was a well known technique in RF world for boosting the output of a transmitter by simply strapping on a common grid stage to the output. The original transmitter output would be 75 ohms so low enough impedance to drive the common grid add-on but I think Music Man were the only people to use it in audio.
Ok thanks.
What sort of wattage do you get out of these amps?
I’m getting 56Vpp at the output which is a bit low. I estimate that I should be seeing 76Vpp at the output to get 130 watts
Well, I have now watched every single Video over the last 2 months and I am now a fully qualified Amp Tech!! Ha Ha!! Very very informative videos and have loved every minute of them AND repaired my burnt out Trace Elliot Velocette 10, Very Happy.
Thanks Stuart, Keep the Videos coming.
Well done you!
Great video Stuart. I had one of these back in the day. Here in the US these amps came with Sylvania 6CA7 valves, the American version of the EL34s. Crazy high plate voltages. The screens are at 350 volts. The 6CA7s had no problems operating under these conditions.
Thanks David.
Another superb and very informative video Stuart, not only in theory but also excellent display of the practical skills needed to do the job
Thanks for the great feedback!
Thank you Stuart. I have the HD-130 Music Man head, I've never had to get into it but seeing your video has given me courage should the need ever arise to open it up. Great and informative video!
Thanks!
Had no idea Leo had a part in these. I've been swimming in Music Man's lately & honestly, they've been great to work on
Stuart, thanks again for the clear explanations as you work through this amp. The diagrams are exceptionally helpful. I find myself understanding much more of what you are saying.
Always entertaining as these amps are fascinating to see. I have played one of these before and it did sound good. I was looking for a clean amp at the time and decided on a Roland JC-120 instead.
Keep up the good work!
Yes they sound great that's for sure.
I had an HD130 head in the mid 80s and it was a GREAT sounding loud amp. I also recall having a real printed schematic for it because I brought it to the local TV repair shop to fix a broken tube socket.
I do now have the schematic but it was hard to get hold of.
Great job Stuart and very thorough video. Much appreciated 👍👍
Thanks Paul.
Old Leo sold Fender to CBS. Then he went and formed up MusicMan. It was good deal. Everybody was happy.
Then later Leo formed G&L Instruments.
I've never heard anyone complain about these MusicMan amps.
Thanks Mr. Stuart.
Yes there is some excellent feedback in these comments. People seem to love them. I really liked the sound.
Absolutely brilliant video
Many thanks!
After buy a Music Man 212 Sixty-Five that needs maintenance, I spent a lot of time reading and studying the schematic. I was able to successfully repair the amp and it sounds fantastic again. I replaced all e-caps, all dioes, all op-amps. A couple of resisistors that were out of spec. The 2 PNP transistors on the heat sinks too, these are on the phase inverter board. A couple of rotary control pots. These are wonderful amps. The other two small blue adjustment trim-pots that you mentioned on the pre-amp circuit board are for adjusting the tremolo.
Hi Keith Wow a huge amount of work, well done.
Hey Stuart, back in the day I used to use a Music Man 112RD65, what an amp..! Just fab sound and powerful. Alas, it blew up eventually. Great video and nice to see one of those Music Man amps on your bench, getting some TLC, nice job...Ed..uk..😄
Cheers Ed
Somebody told me years ago that the output stage on these amps is run basically in class B which is why the plate current is so low. It would be interesting to know if pin 6 is used for anything; an American 6CA7 doesnt have a suppressor grid and doesn't use pin 6 (a 6L6 doesnt use it either), but a true EL34 brings the suppressor gris out via pin 6 and it usually gets tied to ground (or to the output tube cathode in a cathode-bias amp). Those 1.5k resistors are tied to pin 4 and are obviously screen resistors ( it would probably be better to have one for each tube rather than sharing a resistor per tube pair).
The KT77 (kinkless tetrode) would probably be the closest European equivalent to an American 6CA7 ( beam power tube).
Whoops, a correction ---- the EL34 brings the suppressor grid out at pin 1, not pin 6, and in a fixed-bias amp it gets tied to ground (or to pin 8, the tube cathode, in a cathode-biased amp).
Useful info thanks.
I have the exact amp in head cab form. The board with the dual op amp and the 2 3 leg transistors is the phase inverter/ driver board for the power tubes. The blue wheel on that board is what you use to set bias. My amp was factory wired to run either 6l6 or el34 tubes. And right now it's actually running both at same time. One of each per side. I'm just having a ongoing issue with the speed and intensity section now. At one point it was perfect, then a xener diod went out, after that I could swap the metal can op amps around and either I had reverb and no trem or I'd have trem but no reverb. New transistors for the trem circuit, the black half round 3 leggers, it's all there but if you don't hit the strings hard. Hit the strings hard then you get full sound and as it lowers the trem starts working again. Kinda stuck on this
Hi Jeremey. Ok well done for getting so far. I can;t add anything useful to help you I'm afraid. I hope you get it sorted!
I played one of these amps for 40 years on an average of 5 nights a week and it never went wrong on me . the only thing I did was change the tubes 3 times.
best amp ever , when I bought this amp all my friend went for the cheaper Peavy amps , none of them lasted 10yrs on the road and had constaint problems
Ugh, Peavey...
I really like Peavey amps, they were affordable but seem to be pretty rugged. Thousands still going strong years later and both the solid state and valve amps have a great tone.
Possibly not rugged enough for 40+ years on the road, being slung into vans and constantly joggled about, or possibly not easy to repair, is that why you added the 'Ugh' comment Stuart?
Hi Rob I can;t recall why I went ugh tbh. But there's a difference between how a musician views an amp and how a tech views one.
E.g. people love their Mesas but they are a SOB to work on so techs hate them.
Stuart, glad to see, with your experience, that you were just as bewildered with this design as I was last year working on a 212RD65--it took me quite some time with the poor schematic pages.
It's certainly an odd one.
Beautiful condition 1976/77? I wonder how many people (like me!) were screaming at you "the standby switch is on the back"!!! ha ha! I have the near identical amp 1976 but 65x 210!! Lovely clean amp and have NEVER had a problem with it!. Nice one Young Stuart!
Yes I've been fooled by that before. My other favourite is completely dead amp until you plug an instrment in!
My old guitar teacher's main amp was a Peavey Deuce which was solid state preamp / valve poweramp and he always got lovely tones out that (and with EMGs in his 60s/70s partscaster to boot!)
Nice. I haven;t come across one of those yet.
This amp most likely has the series filter cap. system where the mid point is attached to the secondary winding centre tap thus not requiring the voltage dividing resistors.
Ok thanks.
The springy tube clamps are known as "bear traps" here in the states. The Mullard EL34 outputs are almost certainly not original; in the US this amp would have been factory supplied with Sylvania STR "fat bottle" 6CA7 tubes which is the American "equivalent" to the EL34, the 6CA7 being a beam power tube and not a true pentode, but with the same pinout and somewhat similar characteristics to the EL34. "STR" stood for "special test requirement".
PS, If memory serves correctly the Bakelite base of the fat bottle 6CA7 would have been wider diameter than the base of an EL34 and that's probably why the bear clamps weren't touching the Mullard tubes.
Looks like its effective for low noise and high clean headroom. Nice video
Thanks Daniel
Screens are at ~350V. Earlier model use 12ax7 PI and standard grid drive/bias setup. Joan Jett uses one on stage.
Ok thanks.
Well done. I had wondered (complete guesswork on my part) whether the transistors in the cathode circuits were automatic bias (as on tubecad) but I may be completely wrong. Probably an illusion, what seems to be invisible soldering technique on the white-red wire (5:22).
I had another look but couldn;t see that.
I have one of these and they are absolutely fantastic. Takes pedals likes nothing else I have ever played.
Thanks Rafael.
I loved your wee song :)
Cheers!
Last time I saw Johnny Winter live he was running two of these amps in stereo and he sounded amazing
I used to love Johnny Winter back in the day. An amazing guitarist.
The vintage Mullard el34 I the gold standard and have the highest reliability-and worth big $$😎
Stuart your a good teacher
Thank you.
I still have my Music Man HD 130 head , its been all over the country with me. I did have to have the power transformer replaced once.
Great. Bit of a lump to cart around though.
Very strange output valve configuration. After a lifetime of involvement in electronic equipment maintenance, testing and design, I’ve only seen common base configuration used in weird impedance matching circuitry or in h.f. amplifiers as a method of reducing inter-electrode capacitance. I share your intrigue. Really interesting vlog, many thanks for posting it.
Yes it's super rare even in transistor circuits.
Still own one of these, purchased in1977. Done hundreds of gigs and sometimes used as a bass amp. Very robust.
Thanks Dennis.
If you watch the Last Waltz Eric Clapton plays through a music man amp , when he joins The Band onstage. He endorsed them for several yrs.
Ok nice. I really liked the sound of this amp actually.
well im in my hd130rev. 2275 coming into a reverb issue having some distortion and effecting the trem system. take the reverb out of the loop and trem works perfect, verb in loop and normal playing on guitar the trem doesnt like it, easy on strings and trem likes that. swapped the ic cans around and tried nos so i know its not in them, looking for a better more eligible schem , and help be great
Hi I have the schematics. Email via the link in the splash screen at the end of each video.
Very interesting Stuart.. I used one of these several years back ( loaned to me by a friend for a gig) and I was very impressed with it. Had I known what lay under the bonnet at the time, I'd have been very nervous 😅.. Music Man were eventually bought up by Ernie Ball . I can't help think that those FETS that substitute 12AX7's might be hard to find these days should that become necessary.
There's probably an equivalent somwhere. Someone else said they weren't FETs but normal power transistors.
@@stuartukguitarampguy5830 Apparently Fender made something similar .. It was called The Showman (not to be confused with the 1960's version). Solid state pre-amp and valve output.
What type of amplifier class or biasing is it called when using a FET to modulate the cathode and keeping the grid at + 15vdc? How did you calculate the balancing resistor values? I'm not sure why you used 220K instead of 10K or 1K
Hi Wayne. I'm not sure pf the technical term. In transistor it would be common emitter or common base. I've never seen this configuration before. No you couldn;t use 1k! Do the power calculation for 350V across a 1K resistor! 220k is common. It dispates about half a watt so a 2W resistor is fine.
@@stuartukguitarampguy5830 What do you mean by Vacuum Tube Flashover? The Flashover is when the cathode voltage is Equal = to the Plate voltage?
Hi Wayne. It's a bit of a catch-all loose term really. I'm not totally sure what causes it, possibly slight loss of vacuum. But it's when a high current flows from anode to cathode. It usually takes out the HT fuse etc. It's almost always a faulty valve.
As much as these amps can sound great, (think Brian Too Loud Macleod), I've heard some of these amps depending on year are "tube hungry" with the particular bias setup, or lack there of. Thanks to your video, it becomes quite clear why this can be the case.
I know alot of amps have their design quirks though.
One tech's dream can be another's nightmare.
I've been looking for one as of late, and if I do get one, the resistor mod across the caps would seem crucial.
Ive actually heard other techs mention it as well, so seems like a good idea.
Hi. Ok thanks for that, I hope you manage to find one.
14:10 - Sounds like it used by Andy Scott's amp! :)
I was wondering if you have a video on a Marshall JCM2000 DSL 401 Dual lead Combo? I have one with all good valves and no volume
No sorry. I hope you get yours sorted. Obviously try the usual stuff. Is HT there etc.
2 questions: When you were "setting the bias" did you have the amp in high power mode? Did you measure the actual values of the 3.9 ohm emitter resistors before starting to see if that could be the cause of the imbalance between the 2 sides?
From memory it was in high power mode. No I didn;t actually measure the 3.9ohm res. Probably should have but I'd be surprised if they were much different from each other.
I had the 2x10 65 combo but it was earlier and had a valve rectifier. A decent enough amp that I swapped my silverface deluxe for. I don't miss the Music Man but I do miss that deluxe. Any way thanks for the video Stuart.
Thanks for sharing!
I have one of these I bought over 30 years ago from the original owner. Here in the US mine came with Sylvania 6ca7 fat bottle tubes! The earlier version was really like a bassman, no reverb ,no tremolo but had a 12ax7 phase inverter tube. you can see the block off plate where the tube would’ve been in this newer version that’s on the bench here . I have one of those two and it had 6ca7 Sylvania’sis in it too! I would say the earlier version sounds a bit better.
Ok thanks for the info.
Hallo! I also have in my MM 130Hd US Sylvania. Can't say are they better or not because I had only 115RP with Siemens EL34 and 130hd head with EVM 12L, both of speakers have their benefits. Another story is Bob Luly... I've noticed "RL" on the back side. Maybe that's just another guy or woman. I had also "RL" but inside wood box. Thank you for video and sorry for my englisch.
Hello thanks tor that detail vid, I had many MM's and they came from the factory with Sylvania 6CA7 tubes, Blue and Yellow marks ;) please correct me if someone has some info about MM's 1974-78 first run tubes topic
Hallo. My had Sylvania 6CA7 USA with green marks. Nothing was changed, still working excellent. Thanks for info.
@ so they put yellow blue and green, ancient fact unearthed :)
I believe Roger Fisher used Music Man amps when he was recording with Heart.
I'm so old... I have never heard of either!
It is very very strange indeed i have 1974 hd130 reverb head it to me is a weird cathode bias clearly in the states op amps and transistors were the “new thing” i have the gb-1 board 1/12ax7 phase inverter and four el34 and the bias pot is near the r45/ 46 your are required by music man per the listed schematic to read 0.5 VDC and yes the high power side is the benchmark setting of course no signal applied it took my a whole night to trouble shoot my red plating tubes and two 5 amp fuses blown before i could figure it out as well sheesh its rough!! But i got it stable and it functions rather loudly and consistently at full level its 130 dB!!!
Just looking at my boards schematic it looks class a cathode bias for sure still confuses me terribly 😂😂
After looking at my schematic again, it appears to be a grid bias set up, but somehow you read point “y” which is tied to the cathode to read 1/2 vdc but the input to the boas trim is near 46vdc inverted like it should to the grid (g1) and trimmed to spec to me that weird also, I have done research on Music mean in the relationship to Leo Fender. From what I understand he just ran. The count me in was very upset that there was a solid state pre amp from what I understand. He wanted tube, preamps and solid state output but mini Fender isms appear to be found like the filter caps under the Doghouse on top of the chassis the overall look, and of course the loudness 😂😂
So with my issue, long story short, I’m sure I’m too late is that the tooth red bleeding because of the grit not being biased correctly with the right amount of negative voltage which makes sense Excellent work Stu!!!
Ok thanks for that. I found it frustrating too.
Those power tubes should be 5881's or 6l6gt's, 6l6's were stock on those amps. From my personal experience with them.
Ok thanks.
I'm in the process of repairing another MusicMan amp from this time period, also 6CA7s, internet research says they changed to 6L6GC eventually, but 6CA7/EL34 is probably correct for when this was made.
Mullard valves - if they're anywhere near functional... worth a pretty penny. Especially in yankee land.
Yes. Bit of Emperor's New Suit of Clothes' in my opinion!
Ooh, nice. The stripey bell bottoms are the originals, I've been told.
You needed to get a circuit diagram before you start servicing this amp. There was a lot of stuff you got wrong. There are two 150K resistors on a board on the right side of the chassis. So the 220K resistors you added weren’t required.
Not to be mean, just giving feedback.
This might get you a good schem: 'It seems MM list their schematics by chassis number rather than model'...
Well done, Stuart!
Thanks Alex.
Great video Stuart ⁉️
Thanks 👍
Common base, common emitter? No that isn't the point at all. The output stages are a "cascode" hybrid design. It's a great improvement over the usual stages in many ways. (I expect the purists who "know amps" will disagree violently with that".) The transistor is much better at controlling currents, bipolars are current controlled devices by their nature, and as it sits under the valve's cathode it runs at a very low voltage allowing a higher frequency lower voltage transistor to be used. Heat dissipation is also pretty much not a big issue. The valve only serves to handle the high voltages of the power supply and transfer the BJT's current through to the transformer. You get much better bias stability, lower distortion which is what Music Man want, hugely improved upper frequency range. It's an absolutely brilliant "clean amp" solution to keeping the sound "valve" while letting the far superior transistor act as the amplifying element. If you don't know it you really should look up "cascode".
Thanks for the explanation Albert very useful.
Che io ricordi gli ampli MUSIC MAN erano equipaggiati con 6L6 io ho avuto un RD12 65 e RD212. 100 entrambi con 6L6 acquistati nuovi nell' 80 e 82 - grazie della risposta
Sì, sono ottimi amplificatori quando funzionano. Grazie per il tuo contributo
😄😎
Cathode driven valves methinks.
Yes something like that. I wonder WHY though???
Hi Stuart, I enjoy your videos. You should defenitely spend a bit more time on that ugly schematic, there is a lot of usefull information to be found. I have spent hours.. The amp already has balancing resistors across the big capacitors, its the 150k resistors R16 and R67 on the schematic. The driver transistors are bipolar NPN transistors, not FET's. If there are problems with adjusting the bias or distorted output signal (and the tubes are ok), I have had succes with changing the old transistors. A good substitution is 2N6488. Musicman amps comes in several sizes and variants, some with traditional tube driver circuit, and this one with transistor driver circuit. It's probably the best amp fender never made... be aware, I'm not a guitarist, just a poor tech. I'm also a bit worried about the insane high anode voltage...
Very useful info thanks.
Peavey made a similar abomination.
I bet it wasn;t as good!
What a terrible scheme! I've never seen anything like this in my life. Some kind of perversion. There, essentially, lamps are not needed at all, they are there more for beauty. It’s not clear why this was done, to save five 12AX7? This is a very small saving; instead of an anode transformer, they could install a pulse generator, it would be much cheaper. I would never buy such a device..
That's strange to hear, because this thing was made for a person, who helped with amps and effects Rolling Stones, Alvin Lee, EVH, Mark Nopfler, ZZTop, Erick Clapton and many others. I'm not talking about Mr.Fender, he also had a great team. Some of them work in NASA and made a lot for the USA. For sure, it can sound for you not good, but not for others.