MarshallJCM800 2210 hum

[g1]

WHY THE LOW FILAMENT VOLTS??? That the filament volts are low with a new PT is indicative. They are not supposed to be 5.5V. Find that problem and you may find the hum issue also. In fact I think it's likely. I wouldn't bother with one more thing regarding the hum problem before I solved for the low filament problem.

Absolutely. No point chasing anything else til this is corrected. Resistance is futile.

[jvm]

I have 6.3 vac with output tubes removed and pre amp tubes still installed. I subbed a new set of output tubes with no change in hum or filament voltage. (5.9v with output tubes installed. same as original tubes) With no load, I have 6.3 volts. Shouldn't I see closer to 7 volts with no load?

[g1]

Yes, I would expect more than 6.3 unloaded, even loaded I would expect more than 6.3V with modern line voltages. That is why I suggested checking things mentioned in post #25.

[big_teee]

My 2204 Clone runs 6.6V loaded on all the heaters.
If it were mine I would rip the Heater wiring loose back to the source.
Build it back checking one tube at a time and make a nice neat tight twist pair as I went.
Most of the old marshalls had that sloppy loose heater wiring anyway.
You could correct the wiring and it may help on the hum at the same time.
I use a little heavier wire to the power tubes, then drop down if I want to.
T

[jvm]

Sorry, g-one.I had unhooked D2 and D3 from filament string. Forgot to post results. No change. B+ is 369 volts, filament 5.9V, PT primary is at 120V. I unhooked all PT secondary windings except the filament. Still 5.9v @ filaments. I can't find anything loading down the filament string but something appears to be doing just that. I am hesitant to rewire the heaters due to the fact that they were apparently fine previously. Something has to have changed since I last saw this amp. I just can't figure out what it is. I am pretty much stumped at this point.

[big_teee]

Sorry, g-one.I had unhooked D2 and D3 from filament string. Forgot to post results. No change. B+ is 369 volts, filament 5.9V, PT primary is at 120V. I unhooked all PT secondary windings except the filament. Still 5.9v @ filaments. I can't find anything loading down the filament string but something appears to be doing just that. I am hesitant to rewire the heaters due to the fact that they were apparently fine previously. Something has to have changed since I last saw this amp. I just can't figure out what it is. I am pretty much stumped at this point.

When I get to that point, I use the Napoleon Method.
Divide and concorquer.
My AC is 125-127V. I get 6.6V at a load 3.3v on each side.
6.3V may be ok with the tubes out at your location.
Try putting the tubes back one at a time and read the heater voltage.
Maybe you have a tube dragging it down?
Good Luck,
T

[jvm]

I can get 6.3v only by removing all the tubes. I reinstalled them one at a time and no single tube seemed to pull it down more than any other.

[big_teee]

I can get 6.3v only by removing all the tubes. I reinstalled them one at a time and no single tube seemed to pull it down more than any other.

If the wire is adequate in size, I would take the tubes out flip it over, and discharge everything.
Then re solder the heater wiring on the tube sockets, and see if that changes anything?
Maybe you have a cold solder joint in the chain.

T

[Joe L]

I have/had a 4212 which is the combo 50 watt version of that amp. I hated it. I still have it but it has been gutted and numerous circuits built into since around 1999. Right now it is a complete 2550 on the Lead side with a paralleled 12AX7 first stage. I temporarily disconnected it and am trying a EF86 on the clean side with only a volume control straight into the PI and it is very interesting.

But I am rambling.

V4a is the reverb recovery stage and if the connections to the tank get some oxide buildup they can go open. If you haven't, try reseating them and if that doesn't work, unplug the reverb return cable from the tank and short the shield and hot lead. If that doesn't work, try putting a ground at the junction of C29 and R42. Be careful and measure DC voltage first to make sure you are in the right place. It should be 0v. If any of these tests removes the hum then you should be able to figure that part out.

And check out that voltmeter if you haven't. You've changed a heck of a lotta stuff to still be getting a low reading on your filaments.

[Tom Phillips]

...And check out that voltmeter if you haven't. You've changed a heck of a lotta stuff to still be getting a low reading on your filaments.

+1 on that. You are getting the same low heater voltage reading from two different transformers with no load on the winding. The probability of both transformers having the same fault is very low. Try a different meter or use your meter to measure the heater voltage in a different amp. If you get the same low reading then you will know it's caused by a problem with your meter.

The low heater voltage reading may just be distracting you from going after the real cause of the hum. The fact that the customer finally told you that the hum started when someone else changed out the choke is the big clue.

[jvm]

Ok. I've resoldered all socket connections, done all of the reverb tests that Joe L suggested, and used another meter with no change.

[jvm]

I have been reading back through all these posts and saw one by Chuck H where he asked if I had checked every plate for hum. I apparently overlooked that part of the post. I have now done this and I have hum at every tube plate. I have been schooling myself on the hum in a good working JCM 800 50 watter I happen to have. I think I have been chasing my tail to some extent. Bottom line at this point, hum at all tube plates and low filament voltage (5.9v) Sorry if my inexperience has complicated the process, but I have never had to deal with this type of hum. Any suggestions based on the new findings?

[Tom Phillips]

...Any suggestions based on the new findings?

Right now I can only add that I don't think the low heater voltage is causing the hum so you should not worry about that. 5.9V is only 6% low and anything within 10% of the nominal 6.3V is acceptable. 5.9V may not be ideal but it is OK.

Seems like something happened when the choke was changed before the amp came to you. If you can post more photos maybe someone will notice a wiring change.

[Chuck H]

Have you taken an AC reading on the HV rail?

Have you checked for DC on the filament supply?

When you disconnected the IC from the filament supply, did you lift it from the junction of the diodes that feed it or did you lift the filament supply from the diodes?

Here's an idea! Remove all the tubes. Then lift the filament supply leads. Now solder a one ohm resistor between each filament supply lead and where it was connected. If you get a voltage reading across the resistor it means that something other than tubes is drawing current from the supply.

[Joe L]

Good suggestion Doug. I've been trying to figure out how to tell him to put a clamp on ammeter on the filament leads when I knew he must not have one. Same thing for the AC input with no tubes plugged in.