Fender Silverface Twin Hums after PT upgrade.

[AndrewM]

Help!

I have a non master volume silverface twin thats been converted to blackface specs and had a all caps and valves replaced in the last month or so. It was working perfectly until I decided to upgrade the tired old PT.

Now it has a loud hum that I cant find. It was a direct drop in replacement and I didn't change any other wiring. I put the old transformer back in and the amp still hums - Its got me stumped.

You can hear it hum with the volume set to 0 and it gets progressively worse as you turn it up. chopsticking the wiring doesn't seem to help.

The only thing odd I can find is 7.5VAC on the plates of the power tubes - could this be the issue? I cant find where its coming from. Its not on the 70uF caps in standby - only when the amp is running.

Any other ideas? I've been through the GEOFX debugging and nothing seems to solve my hum!

[AndrewM]

Fender Twin Reverb massive hum!

Found a similar thread - will try work it out!

[Amp Kat]

Does it hum with no cord in or just when you play. Do any of the controld effect the hum. What about the Heater CT wire is it grounded and do you have the third AC wire grounded by itself.

[AndrewM]

- It hums standalone with no guitar/lead plugged in.

- Hum is effected by pots, including eq, particularly the bass control. It is also present (but far less noticable) with all controls set to 0.

- Heaters have a virtual CT via 100ohm resistors - voltages are ok.

- AC earth wire is grounded by itself.


The chassis does seem to vibrate with the transformer but I'm not sure if its normal or worse than it was prior to the transformer upgrade.

[AndrewM]

Guys please help - this is driving me nuts!

Using the suggested fault finding from the other thread:

- Turning the reverb off/down / unplugging the tank doesn't help

- I pull all 4 power tubes and its silent.

- With the amp on with the power tubes out I get 455VDC and about 0.8VAC on either side of the standby switch.

- Each Pin 5 of the power tube socket has -43VDC and 0.1VAC .

- With the phase inverter tube out, and plugging each tube in individually, all four tubes have minimal noise.

- With the PI tube and all power tubes back in, none are redplating and are sitting biased at 36mA / 430VDC .

- With the amp running i also measure 7.5VAC on the plates - all other pins are minimal AC.

The hum is equally on both channels and doesn't respond to poking the wires around.

If I unbolt the transformer and leave it connected but loose from the chassis (the earth and CT connections are soldered on and secure) the the hum is less obvious but still not as quiet as I would hope..... Maybe I'm imagining things!

[Gibsonman63]

A couple of ideas, but now that I think about it, it would probally only effect one side unless you have more than one jack misbehaving.

Have you checked the input jacks. They should be shorting jacks. Sometimes, you have to take a burnising tool to clean the contacts and adjust them a bit where they get good solid contact. Otherwise it is like having a guitar cord plugged in with no guitar.

Are you testing with or without the footswitch. If the braided shielding comes loose on the RCA jack for the reverb, it will really hum, but just on the reverb side. I would test without the footswitch anyway until you get it sorted out.

[jrfrond]

I am going to be the first one to ask the stupidly obvious, but no mention of this has been made:

Have you tried adjusting the Hum Balance control? Or has that been removed in the conversion?

BTW- exactly WHY did you change the PT? There's no such thing, in my book, as a "tired' old" PT. It either works, or it doesn't. In my opinion, unless the old one wasn't working, you wasted time and money on this, and NOW you have a problem. This is a real "snake oil" upgrade. Those old Schumacher transformers were practically bulletproof.

The fact that you unbolted the transformer from the chassis and the hum was somewhat minimized indicates that you might have inductive coupling of the transformer into the steel chassis. Who made the replacement transformer?

[oc disorder]

As you have covered most of the normal faults maybe you have to assume that you have disturbed
something physically when replacing the transformer.
How about the earth at the bias cap & pot and check all those grounds that snake to the front of the chassis ... resoldering these may help particularly ones common to both channels.
Check any pots physical mounting that have a ground soldered to the back of the pots.
You have checked the bias caps ? tack a 100uF 100v over the existing cap and see if that helps..

[AndrewM]

All the jacks, grounds, pots etc appear to check out ok.
Tried the bias cap with no improvement.
No hum balance – has always had the fixed 100ohm Resistors.

Original old PT was 110V and I got sick of carting around a step down transformer. Not sure the brand of the 240V transformer but it’s a direct drop in replacement. Not a 'snake oil mojo' PT upgrade I can assure you.

I should add that I used the same mounting bolts for both transformers….. Is it possible I’ve damaged the transformer laminations inserting/removing the bolts? Could that be how the hum is coupling to the chassis?

[Gibsonman63]

More silly questions:

I have a florescent worklight on my workbench. It is great for working on amps, but I have to turn it off when testing them. Is it possible that you have something in your test area that in creating noise? (Florescent lights, dimmers etc...)Also, if you haven't already, you may try sticking it back in the cab and testing it so the top of the chassis gets shielded by the metal stapled to the inside top of your amp chassis.

Did you try pulling just the preamp tubes? That would isolate it between preamp section and power section.

The heater wires are sensitive in these amps. Have you tried moving them around with a chopstick. Also, Fender did not pay attention to the polarity of the heater wires on the power tubes. Make sure that the heater wires are not crossed between the power tube sockets.

[Amp Kat]

Agree with Gibsonman on pulling the preamp tubes one by one to isolate it. You need to find out if the hum is coming from the preamp or the power amp. I would even diconnect all the preamp tubes except the first one and send that coupled signal to the PI to see if I could get that first tube clean as if that is good the problem is downstream. Check the screen resistors also but it really is sounding like a loose connection somewhere. Also try pulling all but one powertube and swap them out to see if maybe a faulty tube is causing it and doesn't matter if they are new or not.

[Gibsonman63]

Can you really run on just one power tube? I was aware that you can pull two and in fact I have run some of my 100W amps this way for years at a time in the past. If you pull two, make sure it is two inner or two outer tubes so you will have a push and a pull.

[AndrewM]

Thanks guys - I've tried all your suggestions but the hum is still there.

Another strange thing happened when I was troubleshooting the hum - With just the 1st preamp valve, PI tube and power tubes in the amp started doing this low frequency thump thump thump noise (possibly motorboating?) when I push the volume past 6. Its the first time its happened and I'm not sure if its related to the hum or I've got a separate issue.. I'll try the first preamp stage decoupling cap as per GEOFX debugging page.

[Gibsonman63]

Interesting. Do the tremelo control change the intensity or cycle time of motorboating sound. There may be a clue there.

[Amp Kat]

Can you really run on just one power tube? .

Yes you can !