1973 50W JMP - 2563E

[neikeel]

Quick Update:

Stuck tubes in tonight and put guitar in channel 1, input 1. Turned power on, then flicked standby off. Vol I and Vol II turned completely down to zero. Lots of hissy noise. Much worse as I raised the volume to about 1. Change guitar input to Channel 1, input 2 and hiss went RIGHT UP. Then started building up into something that sounded like Mic feedback at a gig. Quickly turned off power switch and squealing continued to build up for a moment before dissapating.

Shame, should check your joints

Woke up one of the children so had to pack it in. Going to spend a good afternoon one day soon with everything setup and to hand (I dont have a proper workspace at all and have to second the dining room whenever I need to do this stuff, then pack it away before the morning! ).

I know the problem, that is my best place, my music room is too out of the way and too dark.

Just been through the wiring with my Bass head next to it for comparison. Input Jacks, pre-amp sockets, power tube sockets, filter caps, all wired the same. Continuity testing shows heater wires connected all the way from PT to V1 and all wires from sockets passing continuity test back to board, or between sockets. Choke wiring checked for reverse polarity too (well....the red goes to the same power tue socket as on my bass head, and the white respectively)

That is good. Choke polarity should not matter.

Bit stumped. I suppose it's got to be a bad solder joint? Or bad earth somewhere?

Sounds most likely bad joint (or possibly a component getting too hot during soldering if you are absolutely sure of your wiring). Make sure your input jacks are earthed to the bus bar and V1 cathode pin. I would also take a new earth from the bus bar (near the presence cap and run that to the tied earth point on the filter cap bolt that you have the PT commons hooked up to. Check your presence pot wiring and the purple wire runs - so that they are no way near anything else. The scream you got can be heard if the OT wires are transposed from V4 to V5 but your wires will only be one length so you must have that right.

Quick question too. I was getting about 457v on the tip of the HT fuse holder. Is this the B+ voltage? If so, is this a little high considering the Marstran specification for this PT states that it produces 420v ? Or is that 420v the expected plate voltage? (pin3 on V4?).

It is the voltage on the pin 3 of the output valves. Voltage sounds fine.

EDIT****** The amp was NOT in it's head case when i did this. Would this really make a difference? I've kinda got it set up like the amp in this vid at the moment: http://www.youtube.com/watch?v=ltD2B34_jNQ

Should only make a difference if the presence and the volumes are all the way up.

[OdgeUK]

Quick Update:

Stuck tubes in tonight and put guitar in channel 1, input 1. Turned power on, then flicked standby off. Vol I and Vol II turned completely down to zero. Lots of hissy noise. Much worse as I raised the volume to about 1. Change guitar input to Channel 1, input 2 and hiss went RIGHT UP. Then started building up into something that sounded like Mic feedback at a gig. Quickly turned off power switch and squealing continued to build up for a moment before dissapating.

Shame, should check your joints

But there are so many! LOL! ........

(or possibly a component getting too hot during soldering if you are absolutely sure of your wiring).

This is what I think is most likely. Being a bit of a soldering klutz, there were a few times when I thought I had probably had the iron on a component lug for too long (ie. connecting wire was becoming very hot).

Thanks, as usual, for the tips Neil. Next update in a few days!

[OdgeUK]

These OT Primaries......Red, Brown and White. I wired these as per the McGadden diagram and using photo's of how my amp was wired before I tore it apart. Red to V4, White to V5 and the brown to a filter cap along with one leg of the choke.

Noticed that on another thread, some guy fixed his squealing problem by reversing white and red. Although this then was the opposite of the McGadden diagram, it seemed to work for him.

Is it possible that in wiring up a different PT, I've somehow changed the required position for red and white?? I'll swap them tonight to see, if I get time.

[neikeel]

Before you do that (it's fiddly playing deep in the amp around th e valve sockets) do this:

Switch the amp on as before but lift the selector side leg of the 47k NFB resistor. If this abolishes the howl then swapped OT primaries was the problem. Your OT wires should have bben the originals and the white wire usually goes to V5 and is a bit longer than the red wire as it loops round over on the chassis side of V5 whereas the red wire is shorter and goes to V4. That is probably not the problem. I had that problem on a new build where the manufacturer (Heyboer) had reversed the colour codes inadvertently. Should not be an issue with your amp.

Can you post two good quality pictures one of the board includind the valve bases and control pots and another of the power section to include the power transformer, switches etc. I take it that all you wires are as short as possible but without tension and you have used other original amps (especially on Marstran site) for a guide on the PT wiring runs.

Throbbing screams/howls building up gradually sound a bit like oscillation and may be a poorly functioning NFB circuit.

Did you go over the earthing wires?

[OdgeUK]

Did you go over the earthing wires?

Yeah, I continuity tested every ground on the amp, to the chassis, and the common ground on the PT. All good. So all grounds are contiguous.

Will post some pics tonight, although I was wanting to avoid that! When you see my soldering, the vintage amp police will most likely turn up and take the amp into care!

Lead dress is OK i guess. Heater wires are twisted and routed around the PT and edge of chassis. Choke and OT common are twisted together, and the 333v HT feeds are also twisted and fed around the edge of the chassis.

The only other thing I can think of, is that my PC was on and right next to the open chassis of the amp, when I was testing. Also, the speaker jacks , input jacks and control pots were only 'finger tight' to the chassis (although all earths on these continuity tested to ground ok.)



Will also test the NFB / OT Primaries. Although I too can't see how anything I could have done would result in these now needing to be the reverse of what was previously wired.

NFB is wired to 8ohm (yellow) tap, rather than 4 ohm tap. As was previously wired.

Should also mention that I, temporarily, changed the way the power section was wired to the switch. Neutral from the mains socket going stright to switch rather than your recommended way Neil. I had originally wired it with Neutral to the voltage selector common etc but I seemed to have power on continuously that way.

[neikeel]

Did you go over the earthing wires?

NFB is wired to 8ohm (yellow) tap, rather than 4 ohm tap. As was previously wired.

Should also mention that I, temporarily, changed the way the power section was wired to the switch. Neutral from the mains socket going stright to switch rather than your recommended way Neil. I had originally wired it with Neutral to the voltage selector common etc but I seemed to have power on continuously that way.

I think that NFB suits these amps best. The way the neutral/live feeds is wierd. The wires to the PT are either side of the primary, you break it with the switch and fuse on one limb and the selector usually goes on the other limb. As long as the switch is in line you should be able to turn off either way. Wonder if you have the common and one of the other primary wires mixed up?

[OdgeUK]

I think that NFB suits these amps best.

Where would you connect the purple NFB wire? On my Bass head, it's on the speaker jack. On other amps, it's on the 4ohm tap. On mine it's on the 8ohm.

[neikeel]

On the bass and organ heads speaker jack is normal.

On a lead amp of your era 47k onto 8ohms is usual, mid 70's (I thought at pcb intro???) saw change to 100k on 4ohm tap. What is your resistor value?

[OdgeUK]

You can probably see the lead dress in this photo, will upload some better ones tonight:



I have changed the routing of the feeds from the neutral of the mains socket since then.

[neikeel]

Your board looks fine (not sure about inputs/volume wires but it is probably just the way the wires lie).

The earth wire (green/yellow should go to its own dedicated earth bolt/star washer closer to the chassis edge, not the earth from a filter cap I cannot see where the filtercap is wired to (?tip of HT fuse is correct). The choke OT brown wire are correct on HT body. Another wire from her should go to the board diodes post rectification.

The live wire from socket should go to the tip of the mains fuse and the body of fuse to two lugs of the on off switch. The other lugs ('on position') of the on/off switch goes to the common wire of the PT primary (not sure of colour-am in between cases at work!).

The other cluster of wires from the PT primary go to respective lugs on input selector switch. Neutral wire from socket goes to middle lug of selector switch with no links or bridges across any terminals.

Are the snubber caps from the board also earthed either to the bus rail or star washer?

I cannot make that out from your pic, is that what you have got

[OdgeUK]

Your board looks fine (not sure about inputs/volume wires but it is probably just the way the wires lie).

Yup. I've triple checked this area. Pretty sure this is ok.

The earth wire (green/yellow should go to its own dedicated earth bolt/star washer closer to the chassis edge, not the earth from a filter cap

Ah.....ok. Well, I've not changed that at the moment. I notice that on the tube rectified JMP's, there is a washer and bolt on the inside edge of the chassis. Where do you suggest on this particular amp? Transformer bolt?

I cannot see where the filtercap is wired to (?tip of HT fuse is correct). The choke OT brown wire are correct on HT body. Another wire from her should go to the board diodes post rectification.

Yup. As described.

The live wire from socket should go to the tip of the mains fuse and the body of fuse to two lugs of the on off switch.

It certainly does.

The other lugs ('on position') of the on/off switch goes to the common wire of the PT primary (not sure of colour-am in between cases at work!).

I've sort of modified this. I've got Live and Neutral, from the kettle socket, on one set of lugs, and PT common and a wire from the voltage selector (which is jumpered to yellow 240v of PT secondaries) common lug on the 'switched' side. Seems to work.....?

The other cluster of wires from the PT primary go to respective lugs on input selector switch.

I taped these up for now.

Are the snubber caps from the board also earthed either to the bus rail or star washer?

Earthed to a filter cap star washer.

[OdgeUK]

RIGHT....UPDATE.

I put the tubes in tonight, but no guitar, or guitar cable. Switched her on. Usual background hum in standby, tubes all glowing a little. Then flicked to 'on' and a much louder hum/hiss from the speakers.

Checked voltages. Seem pretty good, except ONE area on V2. I couldn't get a measurement from Pin1 and the popping noise over the speakers as my probe touched this pin was very loud and noisy. Similarly pin 7 was too, although I got a reading here. Might have to check connections, but this is the area where there is a 100K resistor (which I've checked and is reading 97k). On one side of the resistor at pin 1, it's very noisy putting the probe there. And pin 7, the same side of that resistor, noisy too. Pin 6 though, the opposite side of the resistor is ok for a reading.



Should that wire from pin 1 to pin 7 be touching the metal centre or not? At the moment, it is not.

Ok. Some things of note. The Mullard XF4 in v4 was slightly microphonic I think. Tapping on it with a chopstick when the amp was 'on' resulted in a tapping through the speakers. I replaced this with an EL34 from my Orange amp, and that has cured that (but not the loud hum/hiss through the speakers).

Of even bigger note.......My NFB resistor is a 100K resistor, currently going to the 4ohm tap. However, placing my multimeter on either side of that resistor results in only a reading of 6.7k?! Could this be the cause of my issue?


*******EDIT************

Actually, it's not really a HISS.....just a much louder 'hum' than when on standby. It might be alright, but I don't think my bass head hums this much with no input.

Might try a guitar in it tommorow, once I know that that buggered 100K feedback resistor is or isn't causing the issue.


*************VOLTAGES*************

V1:

Pin1: 133 (expected: 180)
Pin3: 1 DC or 1.5VAC (expected 1.6)
Pin6: 193 (expected 160)
Pin8: 3 DC or 2.6VAC (expected 1.6)

V2:

Pin1: No reading, bloody noisy when probe touched (expected: 140)
Pin3: 1 DC (expected 1)
Pin6: 284 (expected 250)
Pin7 (noisy): 179 (expected 140)
Pin8: 180 (expected 140)

V3:

Pin1: No reading, bloody noisy when probe touched (expected: 140)
Pin3: 1 DC (expected 1)
Pin6: 284 (expected 250)
Pin7 (noisy): 179 (expected 140)
Pin8: 180 (expected 140)

V4:
Pin3: 441 (expected 380)
Pin4: 442 (expected 390)
Pin5: -32 (expected -30)

V5:
Pin3: 444 (expected 380)
Pin4: 443 (expected 390)
Pin5: -33 (expected -30)

[neikeel]

You never get a true reading of the NFB resistor in situ.

It is fine

Does not matter about V2 resistor touching centre post either

Sounds like you might be getting there

Good luck!

[OdgeUK]

You never get a true reading of the NFB resistor in situ.
It is fine

Phew...

Does not matter about V2 resistor touching centre post either

So it doesn't matter if it does or doesn't?

How do voltages look to you? Alright?

I guess, I'll resolder V2 and then plug in a guitar. Tommorow hopefully.

[neikeel]

And.....