Check Continuity on Tube Amp Power Transformer

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How do you test a Power Transformer that is disconnected from circuit?

• Thread starter louderock
• Start date Jul 6, 2012

• #1

Helping a friend diagnose an amp. The amp powers up but the tubes aren't receiving any heater voltage. He pulled the PT and is taking measurements. What sort of resistance should he see across each of the wires? Should any connection be open? Is it easier to just apply 110vac to the primary and measure voltages on the secondary?

• #2

Resistance measurements will only show if the winding is open. A shorted winding can not be found with an ohm meter. Unless it is shorted to other windings.
Since you have the transformer out, measure the heater winding for 6.3VAC when the primary is connected to 110. If this voltage is ok, put the transformer back in the circuit and measure the voltage at the heater winding again. This should be 6.3 VAC. If that voltage is ok, look for an open between the transformer and the heater pins on the first tube. If you do not have any voltage at the heater winding of the power transformer when it is in the circuit but it measured ok when it was out of the circuit, something is loading and pulling down the voltage. Start by pulling tubes one at a time and remeasure. When your heater voltage returns. Troubleshoot the circuit of that tube you just pulled. I hope this helps. Let me know what type of Amp and I may be able to help more.

• #3

Quote:"Is it easier to just apply 110vac to the primary and measure voltages on the secondary?"

Yes.

Do that to find out if the transformer is working.

What amp do you have?If it's a modern amp like a Marshall or Peavey it may have DC heaters for the preamp tubes and then you have a whole different ball game.

And.....it may have internal heater fuses and such wonderful things.It would help to know what you have.

• #4

The amp belongs to a friend and I believe it's a late 70's Marshall JMP.

• #5

First thing, get rid of all the "I believe" stuff and find out exactly what you are working on.
Then get a schematic for that amp.

• #6

First thing, get rid of all the "I believe" stuff and find out exactly what you are working on.
Then get a schematic for that amp.

I don't have the amp in front of me but I have asked him the model and year of the amp.

• #7

Hi, I just signed up for TGP. Its my amp we are discussing here. Its a 1980 JMP 2204(?). 50 watt Master Mark 2 Lead. I believe the power transformer is toast. I followed the Orange and Black thick leads to the tube socket and measured no voltage on or across either wire. Also no voltage on or across anything on the board. 120 read at the switch and both fuses test good. I pulled the Tranny and read dead short on all input wires, open reading between all input and all output wires and varying resistance on all output wires. Hope this helps...

• #8

Oh, and the amp is no longer powering up.

• #9

Oh, and the amp is no longer powering up.

What happened immediately before it stopped working? Did it blow a fuse, hum loudly, flash of light, lightning,...?

• #10

Popping sound and ozone smell and blown fuse.

• #11

And before that? Tube swap, speaker swap, no speaker connected...?

Anyway, measure the heater windings. If they're producing anything over 5 V AC they're good. If not, they're not.

• #12

"I followed the Orange and Black thick leads to the tube socket and measured no voltage on or across either wire. Also no voltage on or across anything on the board."

Isn't that what I did here?

No speaker connection. Up to this point it would power up, but the tubes would not come on.

• #13

Do you have any DC voltage? Measure the plate voltage at one of the tubes. Does any tube have voltage?
If all are missing Plate voltage. Check the 4 diodes (usually 1N4007) in the bridge rectifier circuit. Also check your filter cap (large capacitor after the rectifier diodes. This capacitor may actually have 2 caps in the same can, check both sections).
With the power OFF and the amp unplugged. Discharge the Filter cap and measure with an Ohm Meter across all four diodes and from each section of the filter capacitor to ground. A good diode will show about .7 ohms in one direction and open in the other. A shorted diode will measure almost 0 ohms in either direction.
If the diodes or the capacitor are shorted. The amp will not power up and continue to blow the 2 Amp fuse. If you do have Plate Voltage on at least one tube but not all, let me know which ones do not and I can help you troubleshoot that section of the circuit. But with blowing fuses, mostly likely none will have Plate Voltage and your problem is in the recifier circuit.

• #14

Found a schematic for the power transformer. Hooked AC up to the black and blue input wire and read the following:

Small Black Output with either red wire: 309 VAC
Large Orange to Brown 6.5 VAC
Large Orange to Black 3.1 VAC
Black to Brown 3.1 VAC

These are all a little low, but should add that "sag" factor, right? There is no break in connection between the transformer and the tube socket, the transformer wire goes directly to the socket. What should I check next?

• #15

Your Red wires with the 309VAC should run to on/off switch then to the rectifier circuit. Look for approx 250VDC at the .5 Amp fuse or at the Output of the diodes. If you do not have a 2nd fuse, this DC voltage can alse be measured at the filter capaicator.
That voltage is the plate voltage, and the amp's operating DC voltage. Track down where you are missing this volatge and you will find your problem.

• #16

I tested the filter caps. Can't get any reading but open with any combination of the 3 pins on either cap. There are 2 of them. They read (Top to Bottom):

DALY
R50 uF Y50 uf
500v D.C. WKG
J4 75/28 BTD
U.K. MADE YL

There is another cap near the preamp tubes that reads:

DALY
ELECTROLYTIC
R50 Y50 UF
500V
SS 254 500
J04BSS
U.K. MADE 8005

These look to be the originals. I read somewhere these Caps have a life expectancy of about 10 years? Should I replace all 3? Whats a good brand to get this amp as close to original as possible?

Thanks!

• #17

If you read an open across the caps, that's good, the capacitor is not the cause of the blown fuse. As far as them needing replacment because of age. I would not replace any cap until they cause hum, static, or somehow degrade the signal.
But that is a preference, either way a dried out capacitor will not cause a dead amp unless it shorts.
So with the amp plugged in and turned on, did you measure any DC voltage at the capacitor? (Between the + and ground). Or at the Plate of the tubes? Or at the .5 Amp fuse? (Not the 2 Amp fuse in the AC circuit, the smaller fuse between the output of the rectifiers circuit and the rest of the amp, it may be located near the filter caps). What reading did you find?

• #18

Off to the weekend gigs... I'll put the transformer back in and get those readings Sunday...

• #19

Great news (at least for me!) I put the transformer back in, and initially had the same result. I unplugged the amp to double check my solder work when I notice a small spark between a couple of the solder connections on the first tube sockets. Im assuming this was the filter caps discharging. I put a piece of tape between the pins, plugged the amp back in, and saw the tubes light up. I connected the amp to a cabinet and no sound. About ten minutes later, tube #2 started a light show and blew the fuse. 2 fuse replacements later, I decided to try swapping the tubes in placement (tube 1 in socket 2 and vice versa) Tube 1 now giving the lightshow and tube 2 acting normal. I put tube 1 back in its original slot and left tube 2 out... VIOLA! Works like a champ!(with one tube, pun intended. ) Valuable lesson learned: I was told these tubes had "very low hours" on it when I bought the amp (it was not working when I bought it, BTW) and assumed they would be good. I know to always start with the simplest solution, but skipped that step. The amp has a couple of mods, but the low input seems to be the unmolested high input, and this thing sounds just like I remembered! (I bought a JMP just like this one in Germany new in 1978 after blowing up my Plexi...) The one small snag in my pulling and replacing of the power transformer is that the power switch light stays on all the time. Switch works as it should, powers off and all, but light goes on as soon as the amp is plugged in. Ideas?

The other question I have is about tubes. It has EL34's in it (by some company called ARS, which only seems be missing an "E" on the end, following my introduction to them...) I bought this amp as a trip down memory lane, and want it to sound like my original. Anybody know if the European version of this amp would have shipped with EL34's?

Thanks for all the help!

• #20

JMP 2204 amps were shipped with either 6550's or EL34's in the output. You may be able to determine which one your amp left the factory with. Look for a schematic or chart inside the cabinet showing a list of the original tubes. The 6550 and EL34 tubes are similar enough that they will work, BUT if you change from one to the other, you must set the tube bias. Even if you replace the tubes with the same type it is still a good idea to check their operating bias current.
For your other question about the power light always on. That doesn't sound good. If the light is getting power when the switch is off, something is not connected right. That means your transformer is connected to the AC current all the time. Do you also have heater voltage with the power switch off?

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