Trouble-shooting hum in a Fender 65 Twin Reverb

[AMPREPAIR]

How do you find the source of undesired hum in a Fender 65 Twin Reverb? This amp seems to work fine except for a residual amount of buzz. The buzz is hardly noticed during loud use - but I would like to reduce (or eliminate) the hum.

When V5 is removed, the hum remains. However, when (V6) is removed, the hum disappears (I assume this eliminates the output tubes and DC power supply from possible causes). For what it's worth, this model does not have the hum switch and I have tried to adjust the hum pot (per recommended procedure) with little success. Do guitar amps like the 65 Twin Reverb simply buzz no matter what you do? Is there anything I can do to reduce the hum?

I would like to know is how much hum is acceptable when measured with a 'scope? In this case, scope measurements at the speaker jack (with 4 ohm resistive load) show spikes as high as 180mV peak to peak.

[Jazz P Bass]

When monitoring output hum on a tube amp, I start to worry if it is over 9 mvs.
4mvs is really good.
But you are talking buzz.
Those 180 mv spikes are definitely cause for concern.
Do any front panel controls reduce or aggravate the buzz.
If so, then the problem would be before that control.
Check the interstage coupling caps for any leaking Vdc.

[g1]

65_Twin_Reverb_schematic.pdf

V5 is the tremolo tube, so removing it doesn't tell much. Removing V1 will take the normal channel out of the circuit. Removing V4 will take the vibrato channel out of the circuit.

[Billy R aka DynaFreak]

Is your amp an original made in 1965 Twin Reverb or is it the current production Reissue "65 Twin Reverb"?

If pulling V6 quiets the hum then it is a good bet that the problem is in the preamp or phase inverter. Sub out each preamp tube one at a time with a known good tube to help narrow it down.

[R.G.]

Question #1 on hum/buzz issues is to ask whether it's a hum (i.e. all bass) or a buzz (spikes at the hum/buzz frequency.) Hum is from AC power line leakage or coupling, buzz is from some high frequency or switching process that recurrs at line frequency.

Question #2 on hum/buzz issues is to ask is the noise 60Hz and therefore powerline frequency, or 120Hz, in which case it's coming from the rectification system in some way.

The cures are quite different.

[AMPREPAIR]

This is a 65 Twin Reverb reissue (about 10 years old). I just discovered the output tubes were biased way to hot! I believe a 6L6 maximum plate current should be 30 Watts. However, they were set to 38 Watts! Although a subjective value, I like to use the 70% rule - so I will adjust the plate current to 21 Watts. The output tubes seem to have "thrown-in-the-towel" with a measured output (at clipping) of 9 Watts! I will install a new set of output tubes to see how it performs. With any luck, the buzzing will "go away". I will also try all the other suggestions provided in this thread. This is a great place to "bounce" ideas off others.

[km6xz]

You did not say whether gain or tone controls influence the noise amplitude, nor the frequency of the noise.
The "watt" measurements do not make sense, where and how are you measuring watts, is it cathode current, plate dissipation or milliamps?

Does the amp show even clipping at 9 watts(?). I doubt the power tubes are involved in this. What does the waveform look like on the scope?
Have you for sure isolated the source to the power amp section with no control having any effect on the noise?

[J M Fahey]

I believe a 6L6 maximum plate current should be 30 Watts. However, they were set to 38 Watts! Although a subjective value, I like to use the 70% rule - so I will adjust the plate current to 21 Watts. The output tubes seem to have "thrown-in-the-towel" with a measured output (at clipping) of 9 Watts!

Please define "watts" and how did you arrive to the numbers posted.
Thanks.

[AMPREPAIR]

I don’t understand why you want me to define “watts”, but here it is one definition: A unit of power, equivalent to one joule per second and equal to the power in a circuit in which a current of one ampere flows across a potential difference of one volt.

I’m not sure what “numbers” to which you are referring; but if you review the specs for a 6L6, the datasheet will disclose the maximum plate dissipation is 30 Watts. As I mentioned, I like to run 30% below this maximum value (the popular 70% rule).

If you are referring to how I determined 9 watts output, I measured output voltage at clipping was 6 Vac. I squared 6 Vac then divided by 4 ohms (load) which equals 9 Watts).

If you are referring to how I determined 38 watts output per tube, I multiplied the measured plate voltage by cathode current. (407 Vdc multiplied by .094 amps equals 38 Watts). Keep in mind the screen accounts for a small portion of that power. In the case of this amplifier, the bias setting that produces 94 mA at 407 Vdc is more than triple the tube’s maximum value.

With new tubes installed and biased to 21 watts on each output tube, the amplifier works great. However, the darn buzz is still faintly there.

Now that I have answered your questions, I hope you can provide guidance on how to find the source of undesired hum in a 10 year old Fender 65 Twin Reverb.

[AMPREPAIR]

I think I may have goofed-up. I thought providing values in Watts would be helpful instead of just mA. It seems to me that providing watts provides the complete "picture" instead of just half. Anyway, all the tone/volume controls have no effect on the undesired "buzz" that is occurring at 60 Hz. Viewing the "buzz" on a 'scope, you can see the spikes, although they are fairly faint - requiring the brightness of the 'scope to be turned-up. Sorry for any confusion I have created.

[Tom Phillips]

Check the integrity of the hum balance pot and make sure it is properly adjusted. You can also measure each side of the heater voltage with respect to chassis ground and verify that each side is approximatly equal at ~3.15Vac.

[MarkusBass]

I thought providing values in Watts would be helpful instead of just mA.

Yes, it was helpful but you confused other people by calling it "plate current" and specifying a value with Watts unit. Clearly current is not measured in Watts. You ment "plate dissipation", which is plate voltage (Volts) multiplied by cathode current (Amperes).

Anyway, all the tone/volume controls have no effect on the undesired "buzz" that is occurring at 60 Hz. Viewing the "buzz" on a 'scope, you can see the spikes, although they are fairly faint - requiring the brightness of the 'scope to be turned-up.

Are you sure that the valves that you used are matched pair? Have you measured the cathode currents?
It looks like the problem is related to the power amp or to the power supply but I would check the new valves first.

Mark

[R.G.]

IF the buzz is happening at 60Hz, that limits the causes quite a bit.

- does this amp have a toroidal power transformer? If so, the cause may be DC offset on the AC power line. Toroidals are very sensitive to offsets and can saturate in one direction from a trivial offset.
- does the amp do this in another building?
- Does the hum change with other tubes being pulled?
- does the hum change with other controls being changed?

If the answer to the above is all "no", there are a few other things to check.

[Jazz P Bass]

Anyway, all the tone/volume controls have no effect on the undesired "buzz" that is occurring at 60 Hz. Viewing the "buzz" on a 'scope, you can see the spikes, although they are fairly faint - requiring the brightness of the 'scope to be turned-up.

Where are you "scoping" the buzz?
The output at the dummy load?
If so, work your way back into the preamp.
-Is it riding on the B+.(check all of the preamp plate nodes)
-Is it riding on any of the grids.
-Is it riding on any of the cathodes.

[AMPREPAIR]

This is great! I am receiving a lot of nice helpful advice. Thanks to everyone!

I measured both sides of the heater voltage wires (referenced to ground) and balanced the voltage to 3.14 Vac on each side to verify the control is functioning normally. This did not resolve the undesired buzz so I then listened to the amplifier and adjusted the hum control to provide the best “null” as per the Owner’s Manual (minimal adjustment was required).

Someone asked: “does this amp have a toroidal power transformer?” Answer: No. It is your typical iron core transformer (I don't think Fender uses Toroidal transformers in any of the 65 Twin Reverbs).

Someone asked: “does the amp do this in another building?” Answer: This will be very difficult to try right now as the amp is disassembled on my bench. However, I did plug the amp into another outlet on the opposite side of the room with similar results.

Someone asked: “Does the hum change with other tubes being pulled? Answer: Yes. When V6 is removed, the buzz disappears (previously mentioned in this thread)

Someone asked: “does the hum change with other controls being changed?” Answer: No. (previously mentioned in this thread)

Someone asked: "Are you sure that the valves that you used are matched pair?" Answer: Actually, this amp requires four valves. I installed two sets of matched pair Mesa Boogie output tubes. One matched pair was installed in the V7 and V10 positions, the other matched pair was installed in the V8 and V9 positions. Measuring across the 1 ohm resistor from cathode to ground, the current varies slightly between tubes 50mA +/- 4mA. The plate voltage is 426 Vdc thus producing about 21 Watts each from each valve. Incidentally, how much variation in cathode current is considered acceptable between valves?

Someone asked: “Where are you "scoping" the buzz?” Answer: ‘Scope measurements were taken at the speaker jack (with 4 ohm dummy load) - as mentioned at the beginning of this thread.

As suggested, I will take measurements working my way back toward the preamp stages. I will be sure to check grids and cathodes as well.