Help to identify a capacitor in this schematic

Hi guys,

I am working on a Marshal 1974X-20-20 reissue 18 watt tube amp hand wired point to point.
http://www.ampwares.com/schematics/marshall/marshall_18watt_schem.pdf
Capacitor C3 connected to the cathodes of the EL84's has a stated value per the schematic as 50-500uf/50V. The actual cap in the amp measures 439uf and is marked as 470uf 100V. What does the 50 indicate? This cap and a 125 ohm 5 watt resistor (resistor in the amp measures 107 ohms) are in parallel and control the bias.

Just as a side bar, there are other things on this schematic and actually installed in the amp that I don't understand how it is possible that they could actually work. Looking at the Normal input J1 Low and J2 High both have a 1M resistor from the positive input to ground and on to the grid of the 12ax7. This resistor would have the effect of creating a large impedance mismatch which would limit the current to the tube. If one wanted to have a low gain and a higher gain input how is it possible that the two resistors could have the same value?

Signed....confused in Miami...again...lol

Billy

Edit...One other clue is that there are 50uf 25V caps indicated which says to me a value that has not been made for a good many years....47uf yes...50uf no. The actual value printed on that cap is 22uf 100V, so there are things on the schematic that do not match what what is in the amp in the first place.

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jim hardy
Gold Member
Dearly Missed
What does the 50 indicate?
check note 2 lower left ?

Calculate corner frequency for the two cap values. At what frequency would each be 125 ohms ?
What's frequency of lowest string on a bass guitar ?

nsaspook
check note 2 lower left ?
View attachment 97140

Jim, just how much punch could a guitar amp have with no C4?

82.41 hz for the lowest note on a six string guitar. I guess that's what you were asking?? Note, this is not a bass amp.

Running like crazy today....be back to this issue later today.

Trying to contact Marshal to get the correct schematic for this model/ serial number amp.

This redraw of the Marshal schematic by who ever did it makes no sense to me.

Thanks,

Billy

Merlin3189
Homework Helper
Gold Member
... Looking at the Normal input J1 Low and J2 High both have a 1M resistor from the positive input to ground and on to the grid of the 12ax7. This resistor would have the effect of creating a large impedance mismatch which would limit the current to the tube. If one wanted to have a low gain and a higher gain input how is it possible that the two resistors could have the same value?
...
I don't think these 1M resistors have anything to do with matching nor gain. They are just bias (grid leak) resistors.
I also can't see any difference between the two inputs.

jim hardy
Gold Member
Dearly Missed
Jim, just how much punch could a guitar amp have with no C4?

I guess that'd depend on its RDX ?

wirenut and Averagesupernova
Back into this amp.
The amp has been playing fine for years then last week dropped in volume by at least 50%.
There is no distortion. No issues with the power supply. No speaker issue. Plate voltages are all correct. Both output tubes are running at around 45ma. All tubes are for sure good. I injected a 1000hz signal and it is clean through all grids and at speaker out. It clips in a normal way at the output jack to the speaker at 95% of max volume. I can find no resistors or caps not working. There is point .5VAC at 60hz passing through the filter caps...ok so the DC is not perfect but....

I am starting to think this is another output transformer gone bad. Jeezs...two in one week!!! I have not ever seen an amp play with no other symptoms other than low volume and the output transformer be the cause. I guess it is possible, perhaps...???

At $170 for a new output transformer I want to be damn sure I am not missing something. Based on the above any ideas?? Cheers, Billy EDIT...Well I got Mr Simpson out and the transformer is open on one side of the center tap.....CRAP!! Is there a Emoticon for the word crap??...lol...I think I need to buy stock in Mercury Magnetics Last edited: jim hardy Science Advisor Gold Member Dearly Missed Well I got Mr Simpson Simplest test equipment is the best. Plate voltages are all correct. Both output tubes are running at around 45ma. That observation doesn't agree with open transformer. I print a schematic and write on it all observed voltages as i measure them, then do my circuit analysis from the marked up schematic.. Saves a lot of 'senior moments' -"Did i measure both plates or same one twice?" Hi Jim, I assume all plate voltages are correct. I know the schematic I have is much in question. People on the Marshall forum commented on other amps of the same type and reported plate voltages of more or less the same values I measured. No one seems to ever reports plate current...not sure why...it's not that hard to measure. I measured V1 131V pin 1 and 6...V2 204V on Pin 1 and 207V on pin 6....V3 149V pin 1 and 90V pin 6...V4 316V pin 7 and 308V pin 9...V5 329V pin 7 and 308V pin 9. With the amp off I measured apx 350 ohms from pin 7 V4 and pin 7 V5. From V4 pin 7 to center tap it was 250 ohms. From V5 pin 7 to center tap was open. I don't understand how it is possible that the two output tubes both could be running at around 45ma with a open V5 pin 7 to center tap. Where is the current going from the plate if one side of the primary is open????? I have to assume I have got something wrong at this point. I also did not expect the primary to read 350 ohms. I sort of thought it would be less than 200 ohms Tomorrow I am going to de-solder the output transformer leads from the tube sockets and center tap and re-measure. I am not quite ready to put$170 on the pass line and roll the dice just yet...lol

I would feel a lot better if I had a schematic that really matched the amp.

Well, tomorrow is another day.

Cheers,

Billy

wirenut
Gold Member
If one wanted to have a low gain and a higher gain input how is it possible that the two resistors could have the same value?
Correct me if I am wrong, but if you look closely at this part, when plugged into the lo jack (J1) you have a 1M resistor from pin 7 to ground. When you are plugged into the hi jack (J2) you will have two 1M resistors in parallel, in effect you will have only 500K there? Everything else is just 2 halves of the ECC83 in parallel. Pin 1 connects directly to pin 6, and pin 3 connects to pin 8.
Or am I just seeing things?

Merlin3189
Homework Helper
Gold Member
No, that's what it looks like.
But it would be a very iffy way of providing different sensitivity inputs. It would depend on the input devices having well defined source impedances, so would probably only work for specific components (including leads.) If we're talking of magnetic inputs on guitars, I don't think they were that high impedance. A source under 10k is hardly going to notice the difference between 1M and 0.5M.

It looks like it may be a misprint. The second circuit I found when I searched, showed a slightly different, symmetric, input.

wirenut
Gold Member
As I look closer if you plug into J1 (lo) you send the signal only to pin 2 with a 1M resistor to ground, and pin 7 is grounded through J2.
However when you plug into J2 (hi) pin 7 becomes ungrounded with a 1M resistor to ground, and signal goes to both pin 2 and pin 7.

My eyes are not what they used to be and i cannot really see the hand drawn one so well

I think the hand drawn schematic was drawn missing the detail of using both halves of the ECC83 on hi and one half on low. I usually find that if it was drawn in an odd way, there is a reason for it.

Merlin3189
Homework Helper
Gold Member
You're right! I hadn't noticed that.
That would double the gain for the hi input, which is not a lot, but something.

OP could check that bit of the cct on his amp. It was so much easier on those hand wired systems! But switches are always a problem, with makes and breaks it can get very confusing.

Of course, none of this helps with his transformer. I can't see any way he can have power on both anodes with part of the transformer primary oc.

jim hardy
Gold Member
Dearly Missed
With the amp off I measured apx 350 ohms from pin 7 V4 and pin 7 V5. From V4 pin 7 to center tap it was 250 ohms. From V5 pin 7 to center tap was open.

I don't understand how it is possible that the two output tubes both could be running at around 45ma with a open V5 pin 7 to center tap. Where is the current going from the plate if one side of the primary is open????? I have to assume I have got something wrong at this point. I also did not expect the primary to read 350 ohms. I sort of thought it would be less than 200 ohms

Indeed that's a confusing set of observations.
Unplug the tubes and repeat resistance measurements?
Sounds as if you have an intermittent connection on V5's side of transformer. Hopefully it'll be where you can see and fix it.

How did you measure plate current ? at Cathode ? Electrons emitted from cathode current are perfectly happy to exit via screen grid. Does V5's screen glow red?

In your shoes , I'd start tacking 100 ohm 2 watt resistors across all my transformer secondaries to protect them against operation with no speaker. Because solid state amps have now dominated the market for forty years we have two generations of users unaware that an output transformer relies on secondary current to prevent destructive voltage excursions in its primary windings.
So they are not attuned to check for sound before cranking up the volume knob.

I'm sure glad wirenut and Merlin figured out that front end !
Has me stumped, and i still dont understand phase splitter....is there one?

Thanks for pointing out how the front works as I now understand it but.....it actually never worked when the amp was new and everything was working well. As least not in the way one would see a big reduction in gain on a Fender amp for example. There was never any read difference when plugging into J1 or J2.

Jim, I measured the current on the output tubes with a bias probe connected to a DMM. It is just a socket saver with the plate pin disconnected with two leads coming off it to hook up the DMM.

nsaspook....I will have to read the link in detail as I really do not have a clear understanding of the " common cathode differential pair"

Need second cup of coffee..lol

Thanks,

Billy

The output transformer issue. OK..I de-soldered the center tap lead from the filter cap . I removed the tubes.
Across the primary 344 ohms i.e. pin 7 to pin 7...I have not de-soldered the leads from the tube sockets yet but the tubes are out
one side to center tap 326 ohms
the other side to center tap 18 ohms

Unless I really don't understand how this amp/ transformer works these values can not be anywhere near correct.

I would expect some value across the primary...say 200 ohms and a more less equal value of say 100 ohms from each side to the center tap.

Unless I am really lost this transformer is bad.

Also I have seen on several web sites people commenting about how Dagnall transformers in these 1974X amps fail pretty often.

Cheers,

Billy

Update: I spoke with the folks at Mercury Magnetics and they confirmed that the values I measured indicated the transformer was bad. They also said that I could always return a transformer if need be which I thought was pretty nice of them. It is pretty easy to screw up a output transformer so I really don't understand how or why one could return one but.....their only request was not to shorten the lead wires. Really nice folks to work with. Their transformers are pretty pricey but they make good stuff.

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wirenut
Gold Member
Update: I spoke with the folks at Mercury Magnetics and they confirmed that the values I measured indicated the transformer was bad. They also said that I could always return a transformer if need be which I thought was pretty nice of them. It is pretty easy to screw up a output transformer so I really don't understand how or why one could return one but.....their only request was not to shorten the lead wires. Really nice folks to work with. Their transformers are pretty pricey but they make good stuff.
Could they have meant if you bought one thinking that was the problem, and found the issue was with something else, and did not use the output transformer?
That way if the leads weren't shortened they could resell it?
Either way that sounds like great customer service.

jim hardy
Gold Member
Dearly Missed
The output transformer issue. OK..I de-soldered the center tap lead from the filter cap . I removed the tubes.
Across the primary 344 ohms i.e. pin 7 to pin 7...I have not de-soldered the leads from the tube sockets yet but the tubes are out
one side to center tap 326 ohms
the other side to center tap 18 ohms

Unless I really don't understand how this amp/ transformer works these values can not be anywhere near correct.

I would expect some value across the primary...say 200 ohms and a more less equal value of say 100 ohms from each side to the center tap.

Unless I am really lost this transformer is bad.

Sure sound like a short between layers on that one side of primary
which is how they fail when run at high volume with no speaker attached
when its magnetism snaps from negative saturation to positive saturation the volts per turn is huge and the volts per layer of turns is huger yet....
Mother Nature will find a place where the insulation is not robust and a layer to layer short is probable right there ,
I've unwound industrial coils about a foot in diameter and found multiple layer to layer shorts. They spot weld themselves and make the coil run hot. Just like in your tube amp these coils were fed by a controlled current so they didn't burn up as would an AC transformer.

If there's a place for some secondary current to flow it'll protect the primary, remember primary and secondary amp turns cancel....
First time i encountered 100 ohm resistors across inside of speaker terminals i said "Aha! How thoughtful of them!" and imitated on my audio projects.

I have the new transformer on order. I assume it will be here in a few days. I am pretty sure that will solve the problem with the amp. I am less sure how the new transformer will change the sound of the amp. The Marshall 1974X is a great sounding amp so I hope things stay the same sound wise.

I don't have a lot of space to work in so taking on too many projects at one time causes some issues of how to store things until the parts arrive. I have two other amps in the "fix me" pile but they are Carver M-1.ot stereo amp and pre amp and I really don't understand the power supply on them. Got some studying to do!!

Have a fun day,

Billy

Update
I installed the new Mercury Magnetics transformer which solved the issue with the amp.

It did change the tone of the amp some. Not better or worse just different. I may like it better than the old one as I get use to it. There does seem to be a difference in the way the amp now responds to the guitar. I have a hard time to try to explain that. All in all I am happy with the new transformer.

Thanks to everyone for all the help. Many of the post have caused me to dig deeper into several subject and that is always a good thing.

Cheers,

Billy

jim hardy
Gold Member
Dearly Missed
NOMB, but

do you think it worthwhile to put say a 100 ohm resistor across secondary for "Speaker Wire Fell Off" primary winding protection ?

1oo ohms won't load down an 8 ohm speaker much

800/108 = 7.41 ohms

around 1/3 db which is imperceptible

wirenut
Gold Member
Jim your suggestions are always valuable.
I think that is a good idea.

Yep...good idea...just happen to have a 100 ohm resistor...lol