Biasing Musicman HD-130 amp?

[Mark]

The amp in question is the one that uses transistors on the cathodes of the output stage and is driven by IC’s.

What I don’t understand is there is no more than 16VDC on the grids and the cathodes have to be more positive than the grids to stop the valves red plate and going up in smoke.

I gather that TR-1 is used to bias the transistor. However the notes I have said adjust pot so there is 25mV across the 3.9ohm resistor. Unless my maths is off that’s 3.2mAs per valve which seems crazy.

If anyone has the time to explain how the bias works to me I would greatly appreciate it.

https://www.thetubestore.com/lib/thetub ... ematic.pdf

MM-Service-Bulletins-All.pdf

IMG_8780.jpeg

[martin manning]

What I don’t understand is there is no more than 16VDC on the grids and the cathodes have to be more positive than the grids to stop the valves red plate and going up in smoke.

I gather that TR-1 is used to bias the transistor. However the notes I have said adjust pot so there is 25mV across the 3.9ohm resistor. Unless my maths is off that’s 3.2mAs per valve which seems crazy.

The outputs are driven at their cathodes and the grids are fixed at 16V, so it's a grounded grid configuration. The BJT is biased for very low current so it will cause the cathode voltage to rise well above the grid, like a large cathode resistor.

[ampdan]

http://www.pacair.com/discus/messages/6 ... 1203019135

[FUCHSAUDIO]

Due to the high plate voltage (around 700 volts) the screens are at 50% of that, and the bias is low (about 20 ma per tube).
The bias is measured at the 3.9 ohm emitter resistor and the Pacair schematics and tech notes explain what you are looking for.
Matched tubes is a good idea, and quality tubes (I like JJ's in these), are imperative.

[martin manning]

Due to the high plate voltage (around 700 volts) the screens are at 50% of that, and the bias is low (about 20 ma per tube).

As Mark notes above, each pair of power tubes gets 25 mV over 3.9 ohms, so just 3.2 mA each. It's very nearly running in Class B.

[Mark]

What I don’t understand is there is no more than 16VDC on the grids and the cathodes have to be more positive than the grids to stop the valves red plate and going up in smoke.

I gather that TR-1 is used to bias the transistor. However the notes I have said adjust pot so there is 25mV across the 3.9ohm resistor. Unless my maths is off that’s 3.2mAs per valve which seems crazy.

The outputs are driven at their cathodes and the grids are fixed at 16V, so it's a grounded grid configuration. The BJT is biased for very low current so it will cause the cathode voltage to rise well above the grid, like a large cathode resistor.

Thanks for your reply Martin, I appreciate your help. I should have stated what I know. I knew the output valves have no real gain, and the signal is fed into the cathode. I gathered it was a grounded grid arrangement, but I didn’t understand why the grids are at 16V as it achieves the same end as having them grounded.

The low current bothers me as bias calculators are saying the current should be 17mA to 20mA for class AB. The current going through each seems almost insignificant and I wonder what will be outputted.

[Mark]

http://www.pacair.com/discus/messages/6 ... 1203019135

Thanks for the information, it’s good to know.

[Mark]

Due to the high plate voltage (around 700 volts) the screens are at 50% of that, and the bias is low (about 20 ma per tube).
The bias is measured at the 3.9 ohm emitter resistor and the Pacair schematics and tech notes explain what you are looking for.
Matched tubes is a good idea, and quality tubes (I like JJ's in these), are imperative.

Hi Andy, good to hear from you again, thanks for your reply.

Various online calculators have AB bias at 17mA to 25mA, however with the suggested 25mV across a 3.9 ohm resistor I l’m looking at a current of 6.2mA, there are four 6CA7’s so that is 3.1mA per valve which I can’t say I have encountered operating current as low as this before, as this is the first MM amp I have had to repair (maybe I lead a sheltered life )

I vaguely remember reading something many years ago that this design was done to maximise valve life and reliability, but that’s the extent of my knowledge of MM amps.

[martin manning]

I knew the output valves have no real gain, and the signal is fed into the cathode. I gathered it was a grounded grid arrangement, but I didn’t understand why the grids are at 16V as it achieves the same end as having them grounded.

When the drivers are fully turned on the cathodes are driven nearly to ground, Vg-k is positive, and it operates in AB2.

The low current bothers me as bias calculators are saying the current should be 17mA to 20mA for class AB. The current going through each seems almost insignificant and I wonder what will be outputted.

Bias it as described in the service notes and it'll be fine :^)

[ampdan]

http://www.pacair.com/discus/messages/6 ... 1203019135

Thanks for the information, it’s good to know.

You're welcome. It definitely helped me when I had one come through here.

[Stevem]

While you have have it change out those filter caps in the 15 volt supply otherwise it will come back just like Murphy’s law predicts.

[maxkracht]

Thank you for the explanation! On the handful of these I have worked on, I assumed they liked to be biased cold and didn't think too much about it. Following the MM directions always worked fine for me.

[FUCHSAUDIO]

Due to the high plate voltage (around 700 volts) the screens are at 50% of that, and the bias is low (about 20 ma per tube).
The bias is measured at the 3.9 ohm emitter resistor and the Pacair schematics and tech notes explain what you are looking for.
Matched tubes is a good idea, and quality tubes (I like JJ's in these), are imperative.

Hi Andy, good to hear from you again, thanks for your reply.

Various online calculators have AB bias at 17mA to 25mA, however with the suggested 25mV across a 3.9 ohm resistor I l’m looking at a current of 6.2mA, there are four 6CA7’s so that is 3.1mA per valve which I can’t say I have encountered operating current as low as this before, as this is the first MM amp I have had to repair (maybe I lead a sheltered life )

I vaguely remember reading something many years ago that this design was done to maximise valve life and reliability, but that’s the extent of my knowledge of MM amps.

My pleasure. I think I may be thinking of the tube driver version as 20 ma per tube..? I never ran the math on the solid state versions, just followed instructions and set to the 3.9 ohm test point voltage the schematic asked-for.
The positive grid voltage is required to make things work (I know, because when that zener supplying that voltage dies, the waveform gets messed up and biasing isn't correct).

[Mark]

Given the time of the year, I have been somewhat preoccupied and haven’t looked at the amp for a while. I tried to address tonight.

The plate voltage was 660VDC, the screen voltage was 330VDC, the control grid voltage was 15.6VDC, the cathode voltage was 52VDC.

The mains voltage was 230vac instead of 240vac.

I put 1 ohm resistors on the plate and measured around 3mA of current.

IMG_8888.jpeg

I posted this to help people who haven’t seen this type of Musicman amp before and could use a heads up on what to expect.

[Mark]

The amp is putting out about 140 watts output. I should do more measurements to assist others.