Marshall M1989 Popping Noise

[pdf64]

359V seems rather low; what is the HT voltage, eg at the standby switch?

[patlaw]

359V seems rather low; what is the HT voltage, eg at the standby switch?

361V DC. The voltage on the HT transformer is 556V AC.

Without the light bulb limiter:

396V DC on each plate
PD1 = 15.36W
PD2 = 16.65W

[Randall]

Things are a bit unclear to me. You cannot bias an amp on a light bulb limiter, but I'm sure you already know that. Does this amp have bias pot RV1, as indicated in the above drawing? If so, you haven't indicated what bias voltage range you see, or if it is turned fully.

[patlaw]

Things are a bit unclear to me. You cannot bias an amp on a light bulb limiter, but I'm sure you already know that. Does this amp have bias pot RV1, as indicated in the above drawing? If so, you haven't indicated what bias voltage range you see, or if it is turned fully.

I serviced the amp with the light bulb limiter, but I neglected to remove it when I started on the biasing step. It has now been removed. It does have the 25K bias pot. It is set about midrange. I haven't adjusted it yet.

[pdf64]

It's bias time. As is, with the 6550EH's, the idle currents are 27 mA and 31 mA. The plate voltage on each tube is 359 volts. These numbers yield PD of 9.7 watts and 11.1 watts. The maximum dissipation of this tube is 42 watts. Half of that, assuming cold biasing, is 21 watts. Things seem way low here. Am I calculating the numbers correctly?...

Yes, you're calculating the idle plate dissipation correctly, but as Randall and you have mentioned, after the initial power ups have proved that the equipment under test is free from fault current issues, the light bulb limiter needs to be removed from circuit.

When setting bias, I think it's helpful to consider the whole amp as a system, rather than focusing on the power tubes to the exclusion of all else, eg don't neglect to consider the amp's likely intended conduction angle and capability of the its iron.

I think that the 'bias to idle around 70% of the power tubes plate dissipation limit' was intended as a guideline for the upper max, rather than an absolute target, see http://www.aikenamps.com/index.php/i...why-70-percent
It seems to assume that, as is typical for most tube guitar amps, the amp has been designed to operate in warm class AB1, eg conduction angle would be around 270 degrees, and get the max possible audio power output from the power tubes in that mode.

Whereas your amp has apparently been designed around 25 watt tubes and then been fitted with 42 watt tubes. Hence with those 42 watt tubes it could operate in warm AB1 at fairly low idle plate dissipation.

So I suggest that you set the bias so that the 6550 idle at around 17 watts, as would be appropriate for amps with 25 watt tubes.

I hope you can see that it would be a bad idea to set the bias so that the 6550 were idling at 70% = 30 watts, as the transformers may not cope with that much continuous current draw, the amp was almost certainly not intended to operate like that.

The 'bias to idle at 70% of the power tubes plate dissipation limit' guideline was almost certainly not intended to apply to your scenario.

[patlaw]

The 'bias to idle at 70% of the power tubes plate dissipation limit' guideline was almost certainly not intended to apply to your scenario.

Fantastic explanation. It makes a lot of sense. Thanks for taking the time. 17 watts it is.

[drewl]

359V seems rather low; what is the HT voltage, eg at the standby switch?

Mid to late 70's Marshalls had some lower High Voltage.

[patlaw]

I just put it back into the cabinet. There's a light burning smell. It's been sitting a while, so hopefully it's not a problem. The tubes are rather close to the wood of the cabinet. The amp sat on the bench running without issue for hours.