Electronics > Beginners

Tracking back 100Hz hum in a tube guitar amp

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Tomorokoshi:
For current out of B+1, we can use the definition of capacitance:
C = Q/V

From the B+1 waveform, we see that the voltage drops by 10V. This happens in 7ms.

Using the nominal DC B+1 voltage of 320V, we can calculate the charge at the two points of the ripple waveform:
C = Q1/V1 and C = Q2/V2

With C = 47uF, V1 = 325V, and V2 = 315V, rearranging we get:
Q1 = C * V1 or Q1 = 47uF * 325V, Q2 = C * Q2 or Q2 = 47uF * 315V

The change in charge is:
Q1 - Q2 = (47uF * 325V) - (47uF * 315V) or simply C * (V1 - V2) = 47uF * 10V = 470uC.

The change in time is the 7ms directly from the waveform.

Therefore the current is:
470uC / 7ms = 67mA.

The average resistance of the amplifier circuit from B+1 is 320V / 67mA = 4776 ohms.

The power used is 320V * 67mA = 21W. Does this seem about right?

Tomorokoshi:
Getting back to the problem of hum.

The ripple on B+1 and B+2 are not in phase due to the low-pass RC filter between them. This is also true with B+3 to the preamp. This may be part of what produces the complex output hum.

Looking at the preamp, it may be that 12AX7 V3 is not perfectly matched for each element, R17 and R79 are not perfectly matched, or C22 and C78 are not perfectly matched. If so, the changing out-of-phase ripple between B+1, B+2, and B+3, when applied to the preamp input, would produce a small differential signal to the main power amplifier.

I looked at some other amplifier designs, and they don't seem to have the split between B+1 and B+2, avoiding the phase difference between them.

This all has to do with ripple rejection. An alternative is to reduce the total ripple.

Tomorokoshi:
From above, knowing that the equivalent resistance of the amplifier is around 4.7k, instead of having B+1 connected to the high-ripple output of the diode bridge, make another filter and put a resistor between the bridge and C17. Tune and calculate the value to achieve your target for B+1. Then reduce or remove R99 to eliminate the phase difference. You may need to change screen resistors R1 and R94.

What is the nominal DC voltage B+3?

floobydust:
The schematic shows at least six different grounds, it's easy to mess that up in manufacturing or it's a design problem, not easy to fix.

The EL84 heater wiring is usually a big source of hum, and best if they are a twisted pair away from any sensitive nodes. I would look for H1 and H2 to have the same ACV wrt HTR_GND. A hum-balance pot would be at R18/R19 to allow adjustment on audio gear that does not have a bypassed cathode (C21).

If you run it with V3 pulled, the output tubes are still high impedance inputs connected to B+3, as far as signal is concerned. Master Volume is weird because it only adjusts diff-mode impedance.
I'm saying with V3 pulled and Master Volume set to min. you would only be hearing imbalance in the output stage with tiny ripple on B+3 as common-mode.

I would needs pics of the grounding, there are at least two critical ones. Set your scope to line trigger to make it easier to trace the hum.


Interesting the input stage uses LND150 N-Channel Depletion-Mode DMOS FET, 500V IDSS 1mA as a triode.

spec:
Hi  Andreax1985,

It is very difficult to diagnose faults without having the faulty unit to examine, but from the symptoms that you describe, and looking at the first scope trace, it seems that the two EL84s in the push-pull output stage are oscillating at a high frequency, which would be the root cause of the problem. EL84s are high-gain pentodes, and are firery little devils, prone to parasitic oscillations.

If you are wondering why parasitic oscillations could cause hum, I can assure you that it is often the case and the reason is quite simple, but perhaps cover that in another post.

In order to stop the oscillation, try this:
* Are the EL84 valve holders of good quality and are the EL84s fitting snugly and fully home in the valve holders?
* Burnish all EL84 pins. Likewise burnish the valve holder sockets (even though the EL84s and valve sockets are new)
* Are all the solder joints on the valve base pins well made, especially if PCB type. Consider re-flowing the joints anyway
* Are R11/R12 (3K3 grid stoppers), connected directly to the grid pin on the EL84 valve holders with a short self lead on the grid side of the resistor?
* Change R11/12 to 5K6 and use good quality metal film resistors, connected directly to the valve holder pin, as described above
For initial fault-finding, you could simply try connecting a 100k metal film resistor in series with the power amp input connector pin.

If the problem does prove to be parasitic oscillations, we can discus a complete modification scheme, which will probably also involve some decoupling/frequency shaping capacitors.

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