Marshall JMP 50W Voltage Question

[mikeboone]

I am looking at a 50W JMP MkII that has an issue with low output and it sometimes cuts out.

Question: I was looking at the schematic and measuring voltages through the amp and noticed that the plate voltages on the PI measure much higher than the schematic calls for. I am measuring 190 on one and 204 on the other. The schematic shows 87V for each this point. Which is correct? Schematic seems low compared to some other schematics I have looked at with similarly configured PI's and the voltages I am seeing are in that range.

Thoughts?

[Chuck H]

Are we talking plate voltage??? No Marshall JMP has 87V on the PI plates. It has to be a typo or some other error. Perhaps 187V and modern wall voltage (120+, sometimes 127V in my home) has them high from THAT. A little common sense looking at the schem will tell you if the current at the PI should drag the V down that far. Compare to the preamp volts and consider.

Chuck

[Mr.coil]

Hey People's,

I am working on one of these...It only has 350 on the uphill cap...is that right? Shouldn't it have about 450-500

[Chuck H]

Not sure what "uphill cap" means (guessing the first filter) but one of mine had about 380Vp (newer one, 80's?) and the other 420Vp (older one, early 70's). At 350 I might think something is dragging the voltage down. What is the bias current???

[Mr.coil]

Sorry ...Uphill= the first cap after the rectifier..

After reading on line a bit....I see where there are many post about this issue.. apparently there was a few years that this is normal...something about Marshall redesigning...and EL34 vs 6550 import or not...Blah, Blah, Blah..

Any how...Marshall never expected this unit to do 50 watts with EL34's, they were imported here with 6550's, and seeing as how the bigger tube will do twice the current...walla walla bing bang 50 watts!

Someone has plugged in EL34's in over the years...because they thought they sounded better....well if you want clipping at lower power ....that'll do it.

I see where some folks even put in 6V6's because the voltage is so low...

Mystery solved ....

Thanks

Mr.Coil

[g1]

Well, yes they were intended to put out 50W with EL34's. As far as I know, the only ones from the factory with 6550's were JCM800's for the US market only. At the time, Marshall was having trouble with EL34 supply/reliability. So they changed a few components in the bias and screen circuits in the US models and fitted them with 6550's.
The British models used EL34's and put out 50 watts.
The transformers and power supplies were the same. If your plate voltage is 350V then something is wrong. Could be a fault, could be a non-stock power transformer.
And installing power tubes that deliver twice the current will not double the output power of an amplifier.
I think you still have a mystery to be solved.

[Mr.coil]

so lets check out your theory...

I have a 4 tube 100 watt amp each tube delivers 25 watts..by drawing X current.

when I remove two tubes (by activating the 50-100 watt switch..

LOW AND BEHOLD...it only does 50 watts!

Lets look at what happened....The voltage stayed the same...but ...Wow there is only 1/2 the current going through the output transformer...Holy cow 1/2 the current = 1/2 the power...

Or twice the current = twice the power...hum..

There goes your theory.

[Chuck H]

ooooh... I'm just going to watch this one....

[Alex R]

[pulls up comfy chair and opens popcorn]

[g1]

So what you're saying is anyone with a 100 watt Marshall running EL34's can have a 200 watt amp simply by installing 6550's.
This is quite a revolutionary discovery, I think someone could get rich with this.

P.S. I was unaware that there are thousands of 50W JCM800's in the UK which are only putting out 25W because they use EL34's rather than 6550's.

[Mr.coil]

No I am not saying that at all, ....and I do not want to get into a discussion about "well in 1974 when chuck got the flu..Barnie came to work and put in the wrong resistors for a few weeks, and that's why those amps sound like hot buttered okra...

What I am saying is: if everything remains the same..doubling the current will double the power. You can extrapolate what ever you want.

This particular amp has a power transformer big enough to be in a Marshall Major!...(11 lbs)......maybe 50Hz?, poor iron?, contractual requirements?, can't guess yet. And the output is bigger than a fender twin...That's a lot of iron for a 50 watt guitar amp. They look stock.

I have 9 different output transformers on a chassis with a 8 pole, 14 position, selector switch and permanent clip leads...with the click of a switch I, and my customers, can hear, see, feel, taste, and smell the difference between this former and and that former..in milliseconds. It is a great myth busting device. But I am going to use it to see if this amps former is somehow defective, while still passing signal.

You Guys are the greatest!

Mr.Coil

[pdf64]

If the power tubes truly could deliver twice the current (all else being equal), then the power across the load would be quadrupled, power being related to the square of voltage or current.
My understanding is that normally for a 4 tube power amp, compared to a 2 tube power amp, the signal current doubles but the load halves (ie OT primary impedance). As P=I2R, that results in twice the power output.
A B+ of 350V does seem unusually low, though I remember seeing 50 watt Marshalls running below the 400V minimum that I'd expect.

[g1]

Mr. Coil: Sorry if I misinterpreted what you were saying but the 2 examples used were like apples and oranges.
First we were talking about a push-pull 50W amp with a pair of tubes. If the power supply were designed for a 50W output and the amp was working properly, the amp will be limited by the power supply. You will not get more than 50W output no matter what tubes you put in. (I will be using approx. figures, not exact numbers).
The second example you gave was a 100W push-pull with 4 power tubes, then 2 of the output tubes are removed. This is a completely different scenario, we are cutting power in half, not doubling it, and the supply is designed for 100W output.
If the "50W" Marshall were limited to 25W due to power supply issues, installing 6550's would not help.
As you mentioned "if everything remains the same..doubling the current will double the power". This means the power supply must be capable of the doubling in current (a 50W amp with a power supply capable of delivering 100W worth of current). Also the circuitry adjusted so we have the different tube types working properly (bias etc.)
As I stated previously, the 50W Marshall's that had 6550's installed for the US market were not limited to 25watts in their EL34 configuration. Maybe not quite 50W, but close.
Possible problems in the amp you are dealing with could be:
a) low B+ to the power tubes due to power supply problem or power transformer (as you stated, B+ is only around 350V)
b) worn out EL34's
c) large impedance mismatch at the output transformer (or incorrect one)
Schematics are available for both the 6550 and EL34 versions of affected amps, appropriate changes should be made for whichever tube type you go with.
Good luck with the amp

[Mr.coil]

Well we subbed a dozen other OT's...nothing, replaced the old power diodes, nota, subbed all the tubes, nota, subbed all the big caps...nope, all the normal tests were preformed. It seems that this amp is destine to produce a week 30 watts, after all it does not say "JMP 50" ...just "JMP". Keep in mind EVERY JTM-45 that we have ever tested, with the same equipment, always did more than 50 watts! So times 65!

However we did find an extra winding in the stock OT ..that was soldered to the imp. sel. in an odd way... we disconnected it and taped it up and now it seems to sag the power supply a little less. (Now it does 30 watts instead of 25) We think it was a dedicated "feedback winding" for other model or a 70 volt winding? I am thinking it was bucking the main winding somehow, anyhow the soldering was not stock. BTW the OT is labeled...( "Replace with 6550's only" ) and measures 3.5K. We tried a pair of the big bottles...after re-bias...not much change.

The biggest discovery is that the power transformer's best talent is for converting 110 to 220 or vise-versa. It would appear that this original power transformer is actually a 2 to 1 "Line Auto-former" with a pair of wimpy aux. windings.

We were confounded as to why anyone would employ a 500 VA auto-former to do a 30 watt audio amp job???

Here is a wild thought....Maybe Jim came up short on PT's at this particular time and found these "2 to 1 auto-formers" for a good price... had the manufacturer wind the extra high voltage winding, used in this amp, after the fact but before final assembly? Usually high voltage windings are about 15-30 ohms ...this one is 70 ohms!!!...that's pretty high compared to the heft of the primary winding and the general BULK of the former. No wonder it sags down to 300 VDC from 355 VDC ...approx. 15 % at full power! And whats up with the 500 VAC ct. rating anyhow? Maybe he was going for the "simu-tube" rectum-frier characteristic.


Anyhow ..I close with...There is no way a pair of EL34's will ever SAFELY do 50 watts with a slack 300 VDC, power supply. So I am moving the primary tap to the 110 VAC. and calling it a day.


Mr.Coil

[pdf64]

I noticed the above Marshall voltage chart in the GT book, which shows the expected power tube plate voltage of 380V for el34, 395V for 6550, 50 watt amps.
It seems bizarrely low, especially compared to the 530V plate voltage of the 100 watters. Pete.