1979 Fentder Twin, Output hum.

[catstrat]

This twin has an ultra linear o/p transformer. It has a bad hum until you disconnect the ultra linear taps. I did this and moved the screen resistors to the plate tap for the screen voltages and now the output is probably about 35 to 40 watts. The hum is gone though. I know i've overlooked something. I did a filter cap service so that's not it unless the caps i used are bad. Help.

[g1]

Does the "output tubes matching" pot have much effect on the hum?

Edit: Even though the hum is gone with screens tied to plates, that low power 40 watts still indicates something very wrong. What are your voltages at the power tubes with UL taps and with resistors?

[catstrat]

Hi G-one. This one has a pot for tube heater hum control. I clipped that out of the circuit and soldered 2 100 ohm resistors off the pilot light holder to ground like the older Fenders. It's my understanding the hum balance trim pot causes more problems then it cures. The bias pot controls bias to all of the output tubes instead of balancing the tubes. Is this amp suppose to have the bias balance circuit instead. The only way to get the bias to quite down is set the bias cold as you can get it. All the plate voltage seem good. The o/p tube plates are at 445 volts. No ac ripple on the plates but some on the screens. I'm thinking maybe just add some filtering to the screen and see what happens.

[g1]

How about some history? What was the original fault? Was the hum there before the re-cap?
Have you tried different power tubes?
As far as I know, those UL amps came with the bias/balance circuit so this has probably been changed over, is the pot labelled? The matching/balance circuit did make a big difference for getting rid of hum.
Maybe try to work with a pair of power tubes at a time.
You mentioned biasing colder made it quiet, what kind of idle current are you measuring with and without hum?

[catstrat]

One amp history coming up. This amp was bought as a fixer upper by the customer. It had a bad hum and the reverb was really weak and it needed a retubing. I've replaced all the tubes that tested bad on my tester. The hum was originating in the power section hence the filter cap replacement. It still had the original Mallory filter caps. After the cap change it still hummed but i didn't find any ripple on the plates either with my scope or a voltmeter. I removed the P I tube to make sure it was the power section. That kind of puzzles me why is there ripple on the screens but not the plates. They both get their filtering from the same caps. But G one , let me go out and make sure the screens do have ripple. I'll get back to you. I work on multiple amps at the same time and i get them mixed sometimes.

[Enzo]

Don't forget the bias supply, a weak filter cap there will also put hum into the output tubes.

If you see ripple on the screens, look back at the screen B+ node, I wonder if it is there as well. The screens act like little plates, sorta, so with the screen resistor acting like a load, and hum amplified through the tube will appear at the screens. If the B+ node remains clean, then this is what is happening.

You removed the PI tube, but the PI circuit is still connected, the plate resistors and coupling caps. COuld be involved.

[catstrat]

Hi G one. I just checked for ripple on both the plates and the screens and i was mis taken. I didn't find ripple at either point but i did find when i checked it against the schematic,the o/p tube balance circuit is tied into the vibrato circuit via a tap on the balance pot. It goes through a 3.3k resistor and connects to the vibrato circuit at a 100 k resistor. The tap was disconnected when i got the amp. I don't know why but i'm going to reconnect it and see what happens. If i would have checked all this before i probably wouldn't have had to bother you guys. But, i don't know if this is going to fix it ether. I'll let you know. Thanks.

[catstrat]

Hi Enzo. I replaced the bias caps already. The more i think about it, it may the fact the vibrato circuit is disconnected, i'm not sure why it would cause a hum. I'm about to find out. I'll let you know. Thanks

[catstrat]

It looks like some one in the past tried to rewire the o/p tube balance circuit. After comparing this to the schematic, I think they were trying to mod it to the basic bias circuit. Is it best i restore it to the schematic circuit or should i leave it as is? And which circuit is better, The tube balance circuit or the standard biasing scheme? Thanks,

[JoeM]

.... It has a bad hum until you disconnect the ultra linear taps. I did this and moved the screen resistors to the plate tap for the screen voltages and now the output is probably about 35 to 40 watts. ...

Are you sure it's wired correctly? You haven't inadvertently wired the output tubes in triode mode?

[g1]

Suggest you get the bias circuit back to stock first. Nothing worse than trying to work around some unknown mod. Once you get it working right you can change it to conventional bias adjust if you want.

[catstrat]

Hi JoeM. I'm sure it's not wired in triode mode even though the output acts like it. Actually the Plate voltage is at about 460 volts and not 445 as i said earlier and the screens are at about 455/458 or so. I did find the bias balance pot to have a problem with the solder lug contact points on the pot. They have a small copper brad that secures the lug to the conductive surface and that brad was loose. I bet that's the reason they rewired the circuit. I'll try to wire it to the original and go from there. That pot has a center tap that goes to the vibrato circuit and that was loose also. I checked with Antique electronics to buy one and they didn't have them. Thanks everybody.

[Jazz P Bass]

Try any of these suppliers.
Note: Not too sure what yours looks like.


Fender Amp Pot By CTS 10k Bias Pot

Potentiometer, Original Fender, Bias 10K Linear - Screw | Antique Electronic Supply

Opentip.com: Potentiometer, Original Fender, Bias 10K Linear - Screw

[catstrat]

Thanks Jazz P. This amp has one of the bias pots that has 3 lugs like a normal pot but it also has a fourth lug on the opposite side of the pot. You can check out the schematic and get an idea of what i'm talking about. Since the original 4 lug pot had the fourth lug disconnected anyway i replaced it with the normal 3 lug bias pot that Fender normally uses. The rewired bias circuit is working fine ,it just has the hum that i'm trying to shut up and hums, for me, are my arch enemy. I hate a "hummy" amp. Maybe i should put extra filtering on the bias voltage. I'll check it again for cracked solder joints. I've already changed the caps and the bias diode.
I guess it would help if i figure out if it's a 60 cycle hum or a 120 cycle hum. As i remember a 60 cycle hum indicates a ground issue, and a 120 cycle is a filtering problem. I may have that backwards though. Thanks folks.

[Jazz P Bass]

Didn't this ultra version have a hum balance circuit?

If it was removed, try putting it back.

The few that I have worked on, the control definitely helped reduce hum.