Monarch Tube stereo power supply spikes

[samgraysound]

Hi,

I rebuilt a Monarch SMX-50a, tube receiver. New filter caps, coupling caps, audio tubes, and sadly output transformers. It worked well for a while

It was brought back recently. After playing for 5-15 minutes there is popping sounds, the volume decreases, and it gets tinnier.

I opened it up, and monitored voltages while it was running. Curiously the behavior coincides with a fairly sudden 20v (270 to 290, 8%) jump in DC voltage at the output of the rectifier tube.  This is reflected all down the line, with power tube plate voltages 40v (220 to 260, 18%) high. The AC voltage off the Power Transformer secondary jumps 10vAC from 480 to 490.

This change happens suddenly over 10 seconds or so, after the amp has been running for a while. The power transformer feels very warm to the touch.

I was thinking maybe as the power transformer heats up some winds of the primary are shorting together? How can I test this hypothesis? What else could it be? Why would this jump in power cause /lower volumes, shouldn't it get louder, or more distorted? Does the popping corelate?

Will post a picture of the relevant bit of the schematic.

Sam

 

[samgraysound]

power supply schem

 

[samgraysound]

audio schem 1

 

[samgraysound]

audio schem 2

 

[mjrippe]

Definitely sounds like the PT is failing.  Yank the tranny and send it to CJ for tear down and rewind.

 

[scott2000]

From mercurymagnetics.com

One of the original uses of a variac was as a diagnostic tool. When an amplifier is repaired, you turn it on using the variac starting at 0 volts. As you increase the voltage, the current draw on the "Amperes" meter (or ammeter) on the variac lets you know if there is a short within the amplifiers. On a tube amp with a tube rectifier, it'll give an indication of current draw relating to the bias/voltage relationship. If your ammeter is slowly going through the roof, you have a bias problem. If you advance from 0 volts and the ammeter pegs itself, you have a shorted tranny, capacitor or power tube.

I was thinking maybe as the power transformer heats up some winds of the primary are shorting together? How can I test this hypothesis?

Sounds like you definitely narrowed it down...

Good Luck!

 

[Gene Pink]

I was thinking maybe as the power transformer heats up some winds of the primary are shorting together? How can I test this hypothesis? What else could it be? Why would this jump in power cause /lower volumes, shouldn't it get louder, or more distorted? Does the popping corelate?

If your power transformer had enough shorted primary turns to raise the secondary voltage that much, it would also draw a lot more current and likely blow the power fuse. Is the fuse the correct size, and can you measure the current draw from the mains? If it goes way up when the problem happens, then you may be right.

But I doubt it. When the problem happens, the transformer secondary goes higher by a small percentage, and B+ goes higher by a larger and larger percentage as you work your way from rectifier tube to the output tube plates. Do I have this correct?

This indicates that when the problem happens, these class-A output tubes are drawing much less current, allowing B+ to float higher.

The one thing that jumps out that could cause this, is the common cathode resistor for both outputs, that 200R, 2W. If this thing drifts to higher R when hot, the relative bias voltage would increase, pushing both output tubes towards cutoff, drawing less plate current.

I would take a good hard look at that resistor.

With the amp cold and unplugged, clip meter leads across the resistor so you can monitor it. Heat it in some way, maybe reflow the solder joints a couple times at each end. Watch the meter for sudden large increases in resistance.

Good luck with it.

Gene

 

[ruffrecords]

If the HT supply suddenly  increases it could be because the load has been reduced. The lower load would also cause an increase in measured power transformer secondary voltage. The fact the sound becomes tinny suggests some bias has gone wrong. I would be suspicious of the balance pot.

Cheers

Ian

 

[CJ]

pull the power tubes and see if it still does it,

 

[samgraysound]

If your power transformer had enough shorted primary turns to raise the secondary voltage that much, it would also draw a lot more current and likely blow the power fuse. Is the fuse the correct size, and can you measure the current draw from the mains? If it goes way up when the problem happens, then you may be right.

But I doubt it. When the problem happens, the transformer secondary goes higher by a small percentage, and B+ goes higher by a larger and larger percentage as you work your way from rectifier tube to the output tube plates. Do I have this correct?

This indicates that when the problem happens, these class-A output tubes are drawing much less current, allowing B+ to float higher.

The one thing that jumps out that could cause this, is the common cathode resistor for both outputs, that 200R, 2W. If this thing drifts to higher R when hot, the relative bias voltage would increase, pushing both output tubes towards cutoff, drawing less plate current.

I would take a good hard look at that resistor.

With the amp cold and unplugged, clip meter leads across the resistor so you can monitor it. Heat it in some way, maybe reflow the solder joints a couple times at each end. Watch the meter for sudden large increases in resistance.

Good luck with it.

Gene

Hey Gene, thanks!

I replaced both the cathode resistors, and not only did the problem go away, but the bias voltages across the whole audio section became much closer to the ones on the schem. Thought it was just simpler to replace them then try to get them to reproduce the problem without power.

Thanks again!
Sam

 

[Gene Pink]

Thought it was just simpler to replace them then try to get them to reproduce the problem without power.

Thanks again!
Sam

Happy to help.

Personally, I like to confirm a bad part by testing before replacing, to ensure that I have really found a fault. Less come-backs that way.

Gene