Fender Twin Reverb massive hum!

[Shannon]

I have a '69 Fender Twin Reverb Silverface that has worked fine for the past year. A few days ago, after warm up and then switch on, it emitted a strong hum which started to build into a howl. I switched the standby switch off immediately. It feels as if the hum will eventually destroy the speakers (starts soft and then builds).

Started replacing all the smaller tubes (preamp tubes) but no luck so far.

Amp had 7205/12AX7's in and 2 x 12AT7's in the correct positions.

Any clues as to what the resolution could be?

Should I replace the output (6L6's) tubes as well?

[darrellcheng]

Hello! Does it hum only when standby is on? is the hum from the speakers? I'm guessing it is. Touch the output transformer, does it get very hot? also look at the tubes are they red plating? Do you know if any work has been done? You may have a bad filter cap.
However I think it will likely be a bad ground somewhere, where a ground wire got loose or a short somewhere. dont unplug the speakers if you are turning on the amp! Also Check the pots, does it increase with volume? Does it hum when all the knobs are down to 0? Are any of the pots loose? The circuit uses the lugs on the pots to ground to the chassis. Make sure nothign is loose and be safe while poking around in there! never use your hands use a chopstick! very possibly a bad ground, check solder joints. Check filter cap pan if your filter caps are stil original, i had one original blow up and the fluid was everywhere.

Good luck and be safe, drain caps and unplug before poking

[Enzo]

Turn off the reverb, turn the control down. Does it still do it? Your description sounds like a reverb pan feeding back.

Other than that, the power tubes are WWAAAAYYYYY more likely than the small tubes to be the problem.

You could also have a failed filter cap, and don;t forget the filter cap on the bias supply.

[Shannon]

Thanks for the input thus far:

1) The hum does come from the speakers. When the amp is idling (warming up) there is no sound. The moment it I flick the standby switch (turning the amp on) the sound builds and gets really loud!

2) Not sure what "redplating" means (no real experience with valves) but a few of the original 7025's had quite a lot of glowing on the top parts of the tubes. However with the replacement 12AX7's this has gone away. The 6L6's do glow a fair bit when idling. I noted that the relative glowing of the tubes (all of them) does not really change when the amp turns on and the hum gets really loud.

3) With regards to the output transformer, can I touch it when the amp is powered up? Also. I normally kill the power (put it into idling/standby mode) quickly as I fear the hum will destroy the speakers. Not sure if that is sufficient time to note if the transformer is heating up.

4) The hum occurs when all pots are set to zero. It happens rapidly (around 3 seconds before its unbearably loud - hurts the ears)

5) I haven't unscrewed the cap cover but suspect these are the originals, not replaced. I'm busy hunting for replacements as they are quite rare in South Africa. Most electronic shops don't carry such high voltage ratings. Contacted the previous owner and he confirmed that no work was done on the caps. He replaced the original Jensens speakers with Goodmans and left the electronics untouched. Will inspect and possibly replace caps this weekend.

6) Not sure about the 6L6's. Might as well replace them since I've already replaced all the other valves! Just worried that whatever afflicts this amp will destroy these new valves!

[Enzo]

The 6L6s are FAR more likely to be the problem than the small tubes, especially if the hum is there with all controls at zero.

Aside from anything I might suggest, also please go over to RGs Geofex web site and go through the amp debug procedure.

DO this: pull all four power tubes. turn the amp on. It should be silent. If the hum is still present you have a serious problem, probably a bad output transformer. This is not the most likely thing though.

If that is OK, leave the amp turned on and not in standby, as if it were about to be played. Now set your meter to AC volts and measure what is on either terminal of the standby switch. If you are a tech, then you could also scope it. But the meter works fine. There should be something like 450v of DC, but there should be not much AC, maybe 10 volts. If there is a LOT of AC, then you have a failed filter cap.

Please do look inside the cap vault. If the ends oc the caps show little bulges or if white stuff is leaking out, they are bad.

WHile you are at it, probe pin 5 of a power tube socket. There should be -40v or something of bias there, but there should be zero AC. If you read substantial DC< then the bias supply has lost a cap or shorted a rectifier.

If that is OK. Remove the small tube next to the power tubes and set it aside. It is the phase inverter tube. Put just one 6L6 in a socket and turn the amp on. If you do this on a normal amp, there will be some hum and maybe noise, but not super loud hum. SO with one 6L6, does the problem remain? Try each of the rest of the tubes one at a time. This is a simple test of the tube. If one makes the loud hum, but not the others, then it is probably bad. If all four act the same, they are probably OK.

Possibly the socket you chose has a problem, so try the 6L6s in another socket. If each socket behaves like the others, then the amp is probably OK. If one socket behaves diffferent, then that socket has an issue.

Once we have found any bad tubes or sockets, then install any good tubes in any good sockets. Does the amp now still hum? It shouldn't.

If we found a problem, was it corrected? If so does the amp work? If not proceed.

None of that fixed it? At this point power off and reinstall the loose phase inverter tube. The hum come back? Probably. If you already changed small tubes, then they are probably not at fault. And since the amp does this with the controls at zero, there is only so far back the circuitry the problam can be. Let's chase after that if we get that far.

All tubes have a filament or heater inside, that is the orange glow you see sticking out. All tubes shsould have that, but don;t read anything into them looking different. Some stick out more than others, but the only part that matters is the part inside where you can't see. This won;t happen ion the small tubes, but every tube has that gray metal structure hanging inside. IN the power tubes, sometimes that netal thing gets red hot - literally glowing red hot. We call that red-plating, since that structure is the "plate" of the tube. That is a sign of trouble.

[Bruce / Mission Amps]

Adding to and getting back to what Enzo said about the reverb at first... I'd pull the third preamp tube from the right first... it is a 12AT7 and is the reverb driver tube.... then if it still does this howling, pull the fourth tube a 12AX7, that is the reverb recovery tube and preamp mixer.
If removing one of those tubes fixes it (specifically the 12AT7)... you might either have a bad preamp tube or more likely, the 25uF-25v caps across the cathode biasing resistors of either of those tubes is going bad.

[Shannon]

Went through Enzo's steps:

1) Pulled power tubes - no hum when amp is switched on (relief )
2) Couldn't measure the voltage on the standby switch (without dismantling).
3) Opened the capacitor tray
- original caps - 2 x 20mmF 500V Mallory, 2 x 70mmF 350V Mallory and
4 x 10mmF 350V (newish caps) connected in a weird configuration - looks like a series parallel connection.
-No leaks visible.

4) Skipped the pin 5 test
5) Pulled out the inverter V6 and inserted one 6L6 at a time. Small steady hum. Tried all 4 - no issues. Tried all sockets - no issue.
6) Went back to pin 5 test (reinserted V6) and got a surprise - voltage is 515V dc and 0.275 V ac.

Based on the direction given thus far, i may have a problem with the bias circuit and/or rectifier? This probably involves opening up the amp entirely?
Can't see if that circuit tray slides out easily.
Should I hand this over to a tech to follow through?

[Enzo]

Um, you do realize, I hope that from the underside, the tube pins count clockwise from the key. But that means up top from the outside, the pins count counterclockwise. You sure that 500 volts wasn't really on pin 4? ON pin 4 I do expect that kind of voltage.

[Shannon]

Measured the voltage on pin 5 from "underneath" looking directly at the socket. Not from above. Counted clockwise from the key (as per a pinout I saw of a 6L6") to the 5th pin.

Is this right? Guess I should check the adjacent voltages as well to confirm.

[Shannon]

Enzo,

Re-Checked all the voltage measurements since I did probably get the pins confused...

Amp is upright. Measuring from underneath directly into the tube socket.
Pin 1 - 0V
pin 2 - 0V
pin 3 - 515V
Pin 4 - (-56.8V)...value at both standby and full power
Pin 5 - 515V
Pin 6 - 515V
Pin 7 - 0V
Pin 8 - (-57V)..value at both standby and full power

AC voltage at around 0.3 V

According to the pinout diagram of the valve, shouldn't pin5 and pin 4 be at the same potential? (http://tdsl.duncanamps.com/show.php?des=6L6GC)

[Mark Black]

Seems like there is still some confusion on the power tube pin numbers.

Since we're talking about a Fender amp here it might be easier to relate some of the pins to typical component layout for absolute identification (unless someone has changed something).

Pin-1: a wire and one end of a small 1.5K resistor. This pin is unused in the 6L6 so Fender used it as a tie point. It will carry the negative bias voltage.

Pin-2: Two (usually green) wires. One end of the tube heater. Should have little or no DC voltage. May read around 3V AC to ground.

Pin-3: One or two wires depending on which socket. Tube plate. Should have high DC voltage.

Pin-4: One end of a 470 ohm large (2 watt) resistor. Tube screen grid. Should have high DC voltage.

Pin-5: The other end of the 1.5K resistor. Tube control grid. Should have negative bias voltage (delivered through the resistor).

Pin-6: One or two wires (depending on which socket) and the other end of the 470 ohm resistor. Another pin not used by the tube, used as a tie-point. Should have high DC voltage.

Pin-7: Two (usually green) wires. The other end of the tube heater. Should have similar voltages to pin-2.

Pin-8: A wire (usually braided with no insulation) going from there directly to a solder blob on the chassis nearby. Tube cathode. Should have no measurable voltage.

Just wanted to help clarify...

[Enzo]

Still looks like you are counting them backwards. All the numbers make sense then. If they are really on the pins you chart, then the amp is totally wired wrong, which I doubt.

The drawing on the TDSL page is looking from the bottom.

No, pins 4 and 5 are not connected together, they are independent elements. I assume you refer to the dots on the wires as they cross the glass outline. Those dots should not be drawn like that.

[bnwitt]

Actual readings are:

Pin 8 - 0VDC
pin 7 - 0VDC which is probably 3.1VAC
pin 6 - 515VDC this will go down with power tubes inserted
Pin 5 - (-56.8VDC)...value at both standby and full power this will go down with power tubes inserted
Pin 4 - 515VDC this will go down with power tubes inserted
Pin 3 - 515VDC this will go down with power tubes inserted
Pin 2 - 0VDC which is probably 3.1VAC
Pin 1 - (-57VDC)..value at both standby and full power

There is nothing wrong with your bias circuit. Pins 2 and 7 when read to ground with meter set on VAC should show about 3.1VAC. When read from pin 2 to pin 7 you should get the 6.3VAC heater (filament) supply.
Without power tubes in, (and therefore no current flowing) those voltages seem ok.

Go back to Bruce of Mission amps post and follow those tube pulling instructions starting with all tubes in place.

I've attached the two layouts and schematics of the silverface era twins in pdf for your review.

Just so you understand the pins on the power tubes,

Pin 1 = No internal connection in the power tube. In your amp it is being used as a dead terminal to connect the 1500 ohm resistor between the phase inverter output and pin 5 the control grid

Pin 2 = one end of the power tube's heater (filament) element

Pin 3 = The plate or anode of the power tube

Pin 4 = The Screen grid of the power tube

Pin 5 = The control grid of the power tube

Pin 6 = No internal connection in the power tube. In your amp it is being used as a dead terminal to connect the 470 ohm resistor between the power supply and pin 5 the screen grid

Pin 7 = the other end of the power tube's heater (filament) element

Pin 8 = The Cathode of the power tube.

[Bruce / Mission Amps]

Measured the voltage on pin 5 from "underneath" looking directly at the socket. Not from above. Counted clockwise from the key (as per a pinout I saw of a 6L6") to the 5th pin.

Is this right? Guess I should check the adjacent voltages as well to confirm.

No that is not correct.
When looking at the tube side of the socket, from the notch of the key hole, you count the numbers backwards, as 8 through 1 going clockwise.... see the art work attached to this reply

[bnwitt]

Tube pin out.

Edit: Sorry dupe