Marshall Major measurements, early 200W

Bakersound

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p { margin-bottom: 0.1in; direction: ltr; line-height: 120%; text-align: left; widows: 2; orphans: 2; } [FONT=Courier New, serif]I recently acquired a Marshall Major and while it runs fine I plan on giving it some TLC. This specimen is serial #2126 plexi panel (windowed serial number), early in the series after the change to 6 knob design and steel chassis. Aside from the “windowed” serial number, other age distinguishing characteristics: plexi panels front/rear, appear original; final chassis component locations for tubes/transformers/caps, steel chassis; 4 top-mounted caps (vs 5 for later Majors), they appear original; christmas tree located next to OT (vs rear of chassis as in later Majors); perforated circuit boards with mustard caps and Dublier coupling caps.[/FONT]


[FONT=Courier New, serif]Circuit is common cathode 820ohm/250uF V1A/V1B, no bright caps across ChII volume, 470 + 390 cathode V2A, 56K + 250pF tone stack, and 0.047uF coupling caps to output tubes. Original 0.022uF/400VDC mustard caps and 0.047uF/600VDC Dublier coupling caps, but the 47K V3 plate load resistors and 68K grid-bias resistors in the output section aren’t original, nor the 100K power supply drain that lives next to the original 25K 10W “UK Made”. Doorknob screen grid resistors are gone, 270/5W instead. While the main filter caps 2x 400uF + 2x 50uF+50uF (top mounted) look original, the remaining preamp caps (now 47uF/450V) and bias caps have been replaced. The rectifier diodes are changed. The standby switch is not original, but very old; the fuse holders are also not original (why change these?). The Presence pot has been changed, though still has the original mustard cap attached to it. The OT is original (all wiring looks untouched and stock); the PT looks to be a stock unit but I believe it has been replaced: wiring is not neat, there is some tape on outside that says “Marshall 200 PT”, and the internal “christmas tree” for mains voltage selection is missing and the primary is hard-wired to the mains wires. Due to appearance it would have been swapped long ago. The mains wiring is not done correctly (neutral to fuse), also indicating changes were made here. Lastly, some of the circuit wiring - oddly, just the blue wire - does not appear to be original wire type. The yellow and green say “Southhampton” “England” on them, the blue says nothing and has a much thinner sheath – possibly not rated for 600V.[/FONT]


[FONT=Courier New, serif]My guess is the amp suffered a significant problem, possibly blowing the PT, taking out one or more of the doorknobs, the rectifier and possibly some of the plate and bias resistors (the ones replaced) on the circuit board, plus maybe some of the wiring. And the fuse holders??? So maybe someone plugged it into 240V accidentally? The plastic octal sockets are original, with no signs of arcing or other damage. The 9 pin sockets are ceramic as usual.[/FONT]


[FONT=Courier New, serif]I have been gathering photos of other Majors, and tracking dates/serial-numbers against the pictures to piece together a timeline of the different builds Marshall for the Major family. Since these amps are 45-50 years old, nearly all have undergone some type of modification/maintenance/updates/tweaking, making it hard to find a reference for what is truly stock componentry. I plan on keeping original what I can, but this amp is not close to “all-original”, and so in some cases I will lean more toward functionality. [/FONT]


[FONT=Courier New, serif]Last owner had it for 10+ years, sold it because “it eats output tubes - I can’t afford a new set every year.” It had a relatively new set of JJ KT88s.[/FONT]


[FONT=Courier New, serif]The Major has a colorful reputation. Some see it as the Holy Grail of tone, others recognize its UL design means it does not sound like a “regular” Marshall (which might be good or bad - in the ear of the beholder, I say), still others think it is a temperamental ticking time bomb ready to “smoke on the water” the moment they run it hard or insert any type of distortion box upstream in the signal chain.[/FONT]


[FONT=Courier New, serif]While I believe the Major is more reliable that most think, I agree the Major is hard on output tubes. First basis for this statement, the previous owner’s experience with this particular amp (the bias looked to be set appropriately). Second basis for this statement, the measurements I’ve taken show the amp runs the output tubes over spec. To mitigate this somewhat I plan to run a quad of KT90’s I have, along with a screen grid resistor change from the present 270ohm to maybe 680 or even 1Kohm.[/FONT]


[FONT=Courier New, serif]Here are some measurements. I also own a Fender 300PS, and tested it at the same time since I consider it in the same class as the Major (i.e. big tube amps for bass guitar). If you don’t know what the 300PS is, it is a Fender preamp with their goofy (but functional) 5 band boost/cut EQ (also found on their Super Twin and Studio Bass amps - both of which happen to be UL like a Major, but the 300PS is not UL) with an output stage having similarities only to its bigger, older brother the 400PS. It is rated for 300Wrms into 8ohms using 4 GE 6550A (or the TungSol special part number with the same spec). I obtained these measurements on both amps using the same quad of KT90s. (I didn’t think to run all the same measurements with the KT88’s - oops).[/FONT]




[FONT=Courier New, serif]MM/300PS Measurements[/FONT]
[FONT=Courier New, serif]date 20160117[/FONT]
[FONT=Courier New, serif]117VAC; 7.8Ω dummy load; 440Hz test signal; EH KT90 quad[/FONT]
[FONT=Courier New, serif]NOTE: first voltage is Idle, second voltage at max output prior to clipping (% droop)[/FONT]


[FONT=Courier New, serif]PT Primary Major 496VAC 481VAC (-3.0%)[/FONT]
[FONT=Courier New, serif]PT Primary 300PS 260VAC 253VAC (-2.7%)[/FONT]
[FONT=Courier New, serif]--> 300PS power supply uses a voltage doubler circuit[/FONT]
[FONT=Courier New, serif]Bias Vg1 Major -83V[/FONT]
[FONT=Courier New, serif]Bias Vg1 300PS -40V[/FONT]
[FONT=Courier New, serif]B+ OT CT Major 666V 626V (-6.0%)[/FONT]
[FONT=Courier New, serif]B+ OT CT 300PS 696V 665V (-4.5%)[/FONT]
[FONT=Courier New, serif]Vg2 (screens) Major 660V 605V (-8.3%)[/FONT]
[FONT=Courier New, serif]Vg2 (screens) 300PS 342V 320V (-6.4%)[/FONT]
[FONT=Courier New, serif]--> Major Vg2 always measures < Va but usually within 5-10V, even when in full clipping; 270Ω screen grid resistor[/FONT]
[FONT=Courier New, serif]Vrscreen Major <0.1V 8-9VAC[/FONT]
[FONT=Courier New, serif]Irscreen Major <1mA 30-35mA[/FONT]
[FONT=Courier New, serif]Pscreen Major <1W ~20W <-- yikes![/FONT]
[FONT=Courier New, serif]Iac Major 0.75A(standby) 1.28A(idle) 4.25A(full power)[/FONT]
[FONT=Courier New, serif]Iac 300PS 1.00A(standby) 1.70A(idle) 4.20A(full power)[/FONT]
[FONT=Courier New, serif]Voutput Major 44.3VAC(full power)[/FONT]
[FONT=Courier New, serif]Voutput 300PS 44.1VAC(full power)[/FONT]
[FONT=Courier New, serif]Poutput Major 252Wrms (7.8Ω load)[/FONT]
[FONT=Courier New, serif]Poutput 300PS 249Wrms (7.8Ω load)[/FONT][FONT=Courier New, serif] [/FONT]


“[FONT=Courier New, serif]Idle” is off standby but no signal[/FONT]
“[FONT=Courier New, serif]Full” is max output prior to clipping[/FONT]








[FONT=Courier New, serif]KT88 testing (limited)[/FONT]
[FONT=Courier New, serif]Iac 300PS 1.40A(idle) 4.00A(full power)[/FONT]
[FONT=Courier New, serif]Voutput Major 41.4VAC(full power)[/FONT]
[FONT=Courier New, serif]Voutput 300PS 41.5VAC(full power)[/FONT]
[FONT=Courier New, serif]Poutput Major 221Wrms (7.8Ω load)[/FONT]
[FONT=Courier New, serif]Poutput 300PS 221Wrms (7.8Ω load)[/FONT]




[FONT=Courier New, serif]--Major has plenty of voltage from V3 to cleanly overdrive output tubes[/FONT]
[FONT=Courier New, serif]--300PS PI clips same time as output stage - PI possibly insufficient drive to obtain max output power? Same result with 6L6 vs 6V6 as driver tube[/FONT]


[FONT=Courier New, serif]While the two amps produce nearly the same power using the same output tube set, the Fender runs the output tubes within their spec’s. KT88 are max-rated Va=800V and Vg2=600V (some less than this – see the KT88 svetlana/genelex datasheet!), KT90 are max-rated Va=850V and Vg2=650V. The Major, via the UL connection, puts 640-660V on the screen grids most of the time, and even under max power they are at or above 600V, but the biggest worry is the power dissipation for the screen grid when the amp is pushed hard. At clip, by measuring the average voltage across the screen grid resistor (remember, the screen grid supply is coming from the UL transformer tap, and so has output signal riding on the B+) we can determine how much average current is flowing through it, and then determine the average power the screen grid in the tube has to dissipate. KT88/KT90 screen grids are only rated around 8W max, and so if they are pulling ~20W you can know tube life is compromised. When driven into clipping the screen grid dissipation continues to go up, and at some point the screen grid will start to glow and eventually melt.[/FONT]


[FONT=Courier New, serif]The KT90 is not a cure-all, since even its screen specs are being (slightly) exceeded in the Major, but it should live longer than a KT88. I’ve included a photo of a KT88 with a KT90, you can see the electrical elements inside the KT90 are substantially larger (taller by about 50%) so this tube has more current carrying capacity at least.[/FONT]


[FONT=Courier New, serif]Here are a few pre-refurb pictures.[/FONT]


[FONT=Courier New, serif]I need help finding a place to fabricate a reproduction plexi rear faceplate. Ideas?[/FONT]


[FONT=Courier New, serif]
More measurements (220Hz):
[/FONT]


[FONT=Courier New, serif]Poutput Major [/FONT][FONT=Courier New, serif]7.8Ω load on 8Ω setting: 44.1VAC = 249W[/FONT]
[FONT=Courier New, serif]Poutput Major [/FONT][FONT=Courier New, serif]7.8Ω load on 16Ω setting: 45.0VAC = 260W[/FONT]
[FONT=Courier New, serif]Poutput Major [/FONT][FONT=Courier New, serif]7.8Ω load on 4Ω setting: 32.0VAC = 131W[/FONT]


[FONT=Courier New, serif]Poutput Major [/FONT][FONT=Courier New, serif]5.5Ω load on 8Ω setting: 37.5VAC = 256W[/FONT]
[FONT=Courier New, serif]Poutput Major [/FONT][FONT=Courier New, serif]5.5Ω load on 16Ω setting: 35.4VAC = 228W[/FONT]
[FONT=Courier New, serif]Poutput Major [/FONT][FONT=Courier New, serif]5.5Ω load on 4Ω setting: 31.0VAC = 175W[/FONT]


[FONT=Courier New, serif]Note the increased power at the “mismatched” impedance selection. This lines up with my expectation the KT90s have a lower ideal primary impedance than the KT88s (the larger KT90 tube has more surface area overall). Basically the KT90s can conduct more current (lower impedance) and therefore the impedance match to the load is improved by also using a lower load impedance that indicated by the transformer setting.[/FONT]


[FONT=Courier New, serif]Then I decided to measure the OT:[/FONT]


[FONT=Courier New, serif]120VAC on primary gives 4.97V/4Ω, 7.06V/8Ω, 9.86V/16Ω[/FONT]
[FONT=Courier New, serif]Windings ratio is (primary to secondary): 24.1:1/4Ω, 17:1/8Ω, 12.2:1/16Ω[/FONT]
[FONT=Courier New, serif]Impedance ratio is: 583.0:1/4Ω, 288.9:1/8Ω, 148.1:1/16Ω[/FONT]
[FONT=Courier New, serif]Calculated primary impedance for this OT: 2332Ω/4Ω, 2311Ω/8Ω, 2370Ω/16Ω[/FONT]


[FONT=Courier New, serif]So the ideal primary impedance would seem to be about 2.3KΩ. This fits with my understanding a pair of KT88s should be loaded between 4-5KΩ, so a quad should be half of that.[/FONT]
 

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tschrama

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Good info, thx!

" Pscreen Major <1W ~20W <-- yikes!"

Yeh that Vsg2 is far to high. Easiest solution would be to replace those 270Ohm screen resistor with 5K/5W resistors. Or redo the Vgs power supply.
 

Codyjohns

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Thanks for the info. :yesway:

& with a couple little tweaks my 1968 Major sounds like this. :)











 
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Bakersound

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Michael, thanks for the feedback. I've spotted your Major before, mainly because it is very close to mine in age - only 4 top-mounted caps, perf board (and type of resistors/caps where original), christmas tree near OT. I can see some of the updates/mods done, but yours has the original Dublier 0.47uF coupling caps (big) whereas mine are 0.047 coupling caps. I had already estimated a build date of early 1968 (mine does not have a build signoff label on it, not sure when those started being put on the amps).

Couple of questions, if you don't mind. Your doorknob screen grid resistors are also gone, I can't see in the photos, what are you running there now? You mind sharing what your serial number is? (and is it in a "window" on the back, or printed, or stamped somehow?) Lastly, I am trying to figure out how to replicate a rear panel, since mine is half broken off. The biggest issue is getting a good photo of that original old-style "Marshall Major" logo on the left in the rear (since mine is completely missing). The rest of the panel matches up pretty well to most of the other 50/100's of the ear, so that parts easy. You have a decent resolution photo of the back?
 

danfrank

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Bakersound said:
[FONT=Courier New, serif]"B+ OT CT Major 666V 626V (-6.0%)[/FONT]
[FONT=Courier New, serif]B+ OT CT 300PS 696V 665V (-4.5%)[/FONT]
[FONT=Courier New, serif]Vg2 (screens) Major 660V 605V (-8.3%)[/FONT]
[FONT=Courier New, serif]Vg2 (screens) 300PS 342V 320V (-6.4%)"[/FONT]



Good info, thx!

" Pscreen Major <1W ~20W <-- yikes!"

Yeh that Vsg2 is far to high. Easiest solution would be to replace those 270Ohm screen resistor with 5K/5W resistors. Or redo the Vgs power supply.

Redoing the power supply in the Major is the way to go... Fender did it right with a voltage doubler for the power supply. The original Marshall 200 "PIG" did it right by using 2 B+ supplies; one for the plates of the power tubes and another for the screens, in "pentode" operation. The original "PIG" used supply voltages of 720v for the plates and around 420-430v for the screens. The way the "PIG" is designed, I doubt that it ate OTs unless a power tube shorted out and overheated the primary winding of the OT. This is easily fixed by installing fuses in both B+ power supplies. The Major ate OTs because the harmonics produced in the amp when overdriven would cause voltages in the OT be 3-4 times what the B+ voltage is. The winding insulation in the Drake(?) output transformers would break down and windings would short out.
Both amps "eat" power tubes because of the high voltages involved.

If I had a Major, I would install a small isolation transformer, configure it as a voltage doubler for 320-330 volts. I'd disconnect the power tube screens from the OT and connect it to this new supply. I'd also keep 1K 5 watt resistors on the screens.

Bakersound said:
"[FONT=Courier New, serif]Vg2 (screens) Major 660V 605V (-8.3%)"[/FONT]

Original G.E.C. KT88s may have been able to handle the very high screen voltages used in the Major, but with today's KT88s; no way I'd chance it. Definitely asking for trouble.

One more thing, Bakersound, Thanks for the data on your Major. Any way you can measure the Primary impedance of the OT? I think it's 2.25K ohms, plate to plate, for the Major OT.

Also, take a look at "Pleximaster's" posts in this thread:

http://www.marshallforum.com/showthread.php?t=78770

for an idea how to create your back panel.
 

Bakersound

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I've thought about rebuilding the power supply as Danfrank suggests. The upside is the improved reliability and possibly a sound quality shift more in the direction of "Marshall", though maybe the Major's sound is something Major owners like as-is. The downside is the modification is significant and will have an impact on the potential resale value the amp has due to its age, rarity, and (what is left of it) originality. Were it something less revered I would likely make the alteration, but I'll probably keep the Major gene pool intact even knowing it has its flaws. For example, I didn't hesitate to gut a perfectly functioning Laney Pro 50 head because I didn't like the way it sounded --- guess what it sounds more like today? :)

I did do some OT measurements, and came to the nearly the same result Danfrank said about primary impedance. He said 2.25Kohms, my measurements say it is about 2.3Kohms (basically the same thing withing measurement tolerance). Here were the figures I got:

120VAC on primary gives 4.97V/4Ω, 7.06V/8Ω, 9.86V/16Ω
Windings ratio is (primary to secondary): 24.1:1/4Ω, 17:1/8Ω, 12.2:1/16Ω
Impedance ratio is: 583.0:1/4Ω, 288.9:1/8Ω, 148.1:1/16Ω
Calculated primary impedance for this OT: 2332Ω/4Ω, 2311Ω/8Ω, 2370Ω/16Ω
--> So the ideal primary impedance would seem to be about 2.3KΩ

My last thought here, and this is a curiosity for me, the subject of (paraphrasing) "Major's fry output transformers when overdriven hard because the harmonics produced cause voltages to be 3-4x the B+". Many have echoed a statement using the acronym "PRV" and "1800V" and so forth. Two points I'd like to pose for you guys to ponder:

A) The OT in the Major is identical in size to the OT in the 300Wrms Fender 300PS; the OT in a 200Wrms Fender Super Twin/Studio Bass (also UltraLinear like a Major) is somewhat smaller. How come these amps aren't known to blow up their OTs? Is it by design, is it by better manufacturing, or is it by usage (i.e. Major players are more likely to dime their amps and/or use distortion pedals)?

B) Check out the OT in a Super Lead. A whole lot smaller than the OT in a Major. Given the Major was "top bill" in Marshall's mid/late 60's lineup, you have to believe Jim Marshall wasn't telling the tranny manufacturer (Drake? Dagnell? I don't recall) to cut corners in building the OTs for his biggest, baddest amp. Why is it the smaller Super Lead OT can take the brunt of a 100W amp at full clipping (and then some) yet the big Major "can't take any distortion pedals - you'll blow it up!"? Doesn't seem to add up.
 

kips1963

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Michael, thanks for the feedback. I've spotted your Major before, mainly because it is very close to mine in age - only 4 top-mounted caps, perf board (and type of resistors/caps where original), christmas tree near OT. I can see some of the updates/mods done, but yours has the original Dublier 0.47uF coupling caps (big) whereas mine are 0.047 coupling caps. I had already estimated a build date of early 1968 (mine does not have a build signoff label on it, not sure when those started being put on the amps).

Couple of questions, if you don't mind. Your doorknob screen grid resistors are also gone, I can't see in the photos, what are you running there now? You mind sharing what your serial number is? (and is it in a "window" on the back, or printed, or stamped somehow?) Lastly, I am trying to figure out how to replicate a rear panel, since mine is half broken off. The biggest issue is getting a good photo of that original old-style "Marshall Major" logo on the left in the rear (since mine is completely missing). The rest of the panel matches up pretty well to most of the other 50/100's of the ear, so that parts easy. You have a decent resolution photo of the back?

Michael, what speakers are in your cab? Greenbacks?
You've got the best sound on the internet.....!:hbang:
 

danfrank

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I've thought about rebuilding the power supply as Danfrank suggests. The upside is the improved reliability and possibly a sound quality shift more in the direction of "Marshall", though maybe the Major's sound is something Major owners like as-is. The downside is the modification is significant and will have an impact on the potential resale value the amp has due to its age, rarity, and (what is left of it) originality. Were it something less revered I would likely make the alteration, but I'll probably keep the Major gene pool intact even knowing it has its flaws. For example, I didn't hesitate to gut a perfectly functioning Laney Pro 50 head because I didn't like the way it sounded --- guess what it sounds more like today? :)

I did do some OT measurements, and came to the nearly the same result Danfrank said about primary impedance. He said 2.25Kohms, my measurements say it is about 2.3Kohms (basically the same thing withing measurement tolerance). Here were the figures I got:

120VAC on primary gives 4.97V/4Ω, 7.06V/8Ω, 9.86V/16Ω
Windings ratio is (primary to secondary): 24.1:1/4Ω, 17:1/8Ω, 12.2:1/16Ω
Impedance ratio is: 583.0:1/4Ω, 288.9:1/8Ω, 148.1:1/16Ω
Calculated primary impedance for this OT: 2332Ω/4Ω, 2311Ω/8Ω, 2370Ω/16Ω
--> So the ideal primary impedance would seem to be about 2.3KΩ

My last thought here, and this is a curiosity for me, the subject of (paraphrasing) "Major's fry output transformers when overdriven hard because the harmonics produced cause voltages to be 3-4x the B+". Many have echoed a statement using the acronym "PRV" and "1800V" and so forth. Two points I'd like to pose for you guys to ponder:

A) The OT in the Major is identical in size to the OT in the 300Wrms Fender 300PS; the OT in a 200Wrms Fender Super Twin/Studio Bass (also UltraLinear like a Major) is somewhat smaller. How come these amps aren't known to blow up their OTs? Is it by design, is it by better manufacturing, or is it by usage (i.e. Major players are more likely to dime their amps and/or use distortion pedals)?

B) Check out the OT in a Super Lead. A whole lot smaller than the OT in a Major. Given the Major was "top bill" in Marshall's mid/late 60's lineup, you have to believe Jim Marshall wasn't telling the tranny manufacturer (Drake? Dagnell? I don't recall) to cut corners in building the OTs for his biggest, baddest amp. Why is it the smaller Super Lead OT can take the brunt of a 100W amp at full clipping (and then some) yet the big Major "can't take any distortion pedals - you'll blow it up!"? Doesn't seem to add up.

I think the power supply alteration could be done with very little alteration to the original amp parts and chassis. That way it could be changed back to stock without damaging resale value.
The 3 knob Marshall 200 has partridge output transformers rated at 2.25K primary impedance also, like the Major, even though the "200" is operating in "pentode" mode and not ultra-linear.
As for the Major blowing outputs... The reason is pushing square wave signals into the OT at full power. It's not the size of the OT that is the problem, it's the quality of the insulating varnish (enamel) used to coat the magnet wire used in the OTs windings. McIntosh tube amps used around 440-470 volts as the B+. The OTs, which they wound in house, used either double coated or quadruple coated magnet wire to wind their transformers. Fender probably used similar wire in their OTs. I guarantee that the Dagnalls and Drakes that Marshalls use, don't have this type of wire in their OTs. Marshall typically used the cheapest suppliers for their amp's components. Marshall has always been this way. The 3 Knob 200s used Partridge outputs that Partridge provided as "samples". Once the Partridges ran out, Marshall never used them again in their amps. I'm pretty confident that if Partridge provided the OTs for the Majors, the Majors would have had far fewer OTs blow. Partridge was a very good quality transformer, much better than Dagnall or Drake.
The Marshall 2000 250 watt Lead amp also blew many output transformers. This was a "pentode" configured output stage but this particular amp used around 650v on the plates and screens of the output tubes. Marshall used Dagnall OTs in these amps.
It's vary late and I'm rambling...
In summary, the Majors had crappy transformers (inadequate wire insulation used) and these amps used much higher voltages than the Superleads.

Oh! And thanks for taking the impedance measurements of the Major's OT. Much appreciated.
 

Bakersound

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Yes, I am thinking the same as Danfrank here. I don't have a good dataset of OT failures to draw from to determine if they are mostly shorting failures (arcing) or open failures (over current). I suppose extreme over-temp could cause shorts or opens, but given the large size of the OT I can't see it getting that hot even with the Major running overdriven. If the Major OT failures were heat-caused, then we should see the same for the similarly-built SuperLead OTs, because on a power per size basis they are run just as hard (meaning how much heat they must dissipate).

Now, voltage-based breakdown, if the same material and construction were used for Major & SuperLead (we assume this is true), and if the ~550V B+ on SL's were right at the end of the OT's safety zone, then the ~650V in the Major might push it over the edge.

Here's what else I am thinking. Sure, heavily distorted (clipped) signals approach square waves, and inductors (transformers in this case) fight sudden changes in current flow (as happens when square wave transitions from low to high and high to low), but with proper loading on the output I don't think there would be sufficient voltage spiking to break down nominal insulation seen in most transformers. Plus, I don't see any spikes on my scope when I run a test - thought to be completely fair my testing is done with a mostly resistive load whereas a loudspeaker load may be doing who knows what impedance-wise under clipping. I have a 50MHz scope (low tech these days) but that should be more than sufficient to see some kind of spiking/back-EMF/"PRV" going on at the square wave edges, but to the contrary they look mostly rolled off to me. Fact is, if all it took to kill a Major OT was running them hard into clipping then we'd all be on our 2nd (or more) OT by now. Could there be some other trigger?

So here's my theory --> sure, the Major OT may not have the margin we like, and the Major runs more voltage than other amps, and clearly there is enough empirical evidence to convince everyone the OTs only die when the amp is run well into clipping (or maybe run reasonably hard with a square wave signal coming in - say from a distortion pedal). The "trigger event" I am suggesting is a temporary break in the output load. The speaker cable gets unplugged, or the connection is flaky, or a speaker blows and opens the circuit, etc. Now THAT, given the OT is conducting a lot of current, will generate a BIG back-EMF spike on the primary. It is basically the same as what a spark coil does in your car ... store a lot of energy in the coil (transformer) then suddenly stop the flow of current and BAM huge voltage generated (secondary on the car, primary on the OT) enough to jump a spark gap. Note the car coil has lots of turns on secondary whereas amp has lots of turns on primary, and car coils has fewer (lower voltage, higher amperage) on its primary just like the OT's secondary. But it matters not - the transformer has energy stored in it that it wants to get rid of, and will do so to the path of least resistance. In our amp, if the speaker outputs are now open, the primary impedance looks to be a better place to dissipate the energy.

I have seen so many flaky speaker cables and jacks and connections over the years I am pretty confident this is likely the cause for most SOTW situations. And if we think about why Majors might be more prone, a Major not only runs higher voltage, but it also is driving a lot more POWER into the transformer, and it is the amount of energy (power) stored up that will determine how much spark-gap can be jumped.

While I do exclusively use speaker cables, I don't subscribe to the "Never use a guitar cable as a speaker cord" because a 15W amp setup is just fine using the wiring size in a guitar cable, however using a Major with a guitar cable could be a problem. Figure typical guitar cable center conductor might only be 24ga, which is rated only to 3.5A safe current in free air, less if insulated (remember it is in the center of the coax cable - a nice warm blanket!) and 200W into 8ohms is 40V / 5A. So if the Major is run hard and if maybe one end of the cable is fraying ---- it "fuses" and now that 200W+ pushing through the OT has no where to go except back through the primary. BBZZZZTT.

Last thought if you are skeptical a flaky speaker connection is what SOTW is all about, ever wonder why Speakon connectors were invented? Well, not because of tube amps and their specific failure modes, but simply that high-powered audio signals are not handled well by the old-fashioned Phone Plug that was originally designed to handle milliwatts of power (telephone signal switching). And that's been known for decades - even before Speakon was created any reasonably high-powered amp used banana plugs instead of 1/4" phone plugs. Speakon and banana connections can conduct more current, with better contact area (lower contact resistance) and are fundamentally more robust - certainly the Speakon connector. I'm not saying one needs to convert Majors to Speakon, but I am careful to use well-built, heavy speaker cables (in my case, using military plugs) whenever I run a big tube amp.
 

Bakersound

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While I was thinking on the subject I did a little Internet research --- check out this document from Hartley Peavey. Probably written when I was a kid ... and I just now found it. And he used the same analogy to the automotive spark coil!

http://peavey.com/monitor/pvpapers/Chapter7.pdf

Actually, go up a level and most of the chapters look to be an interesting read.
 

Codyjohns

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It's not rocket science. :)

Just beef up the mylar insulation between the primary and secondary windings of the OT and high prv voltage is contained.

Problem solved. :yesway:

It's how I'm able to make my '68 Major so usable. :D
 
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