Madamp M15MK1 build help....

Tube or solid-state, this section goes to eleven!
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bmxguitarsbmx
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Post by bmxguitarsbmx »

Alex's idea is a good one. For a quick solution, swap your tubes around until you get one that doesn't conduct any grid current. Grid current can start to flow at low plate voltages, but your design has plate voltages that should be ok for a majority of tubes out there.

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ryanuk
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Post by ryanuk »

Hey guys - bumping this thread.
I shelved this amp for a while as I moved and kinda forgot about it. Dusted it off the other day. Want to use this amp (Madamp M15MK1) as an opportunity to further my knowledge of tube amps with a view to working up to building my own.

Anyhoo, the amp still works but still has the issues of grid current, and my guitar volume is scratchy as a result. I note the suggestion of a coupling cap at the input.
However, looking at the Marshall 18w schematic, I can see they've utilised the design in the amp I bought. But the marshall plate voltages are a lot higher. With 280v before the 100k plate resistor. The Madamp has 250v at the same point. So the lower voltage on the plate (<150v) could be causing the grid current.

I've done some reading; I consider 2 approaches...

(1) could I raise the plate voltage by tailoring the plate resistor?
Would I need to raise or lower it? Schematic shows a voltage drop across the 100K plate resistor of 117v - so current of 1.17ma.
I'd read that 0.8ma and 150v was about right for the Marshall 18w so if I used a ~130K it would put me in the ball park?

(2) Or perhaps lowering the B+ node resistor (R16) to increase the B+ and ultimately plate voltage.

Can you guys comments on the implications of these suggestions?
Schematics earlier in this thread.

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bmxguitarsbmx
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Post by bmxguitarsbmx »

I'd stick the cap in. That will effect the tone the least of any solution. Raising the B+ 30V will change the tone a lot.

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Post by ryanuk »

bmxguitarsbmx wrote: 24 Nov 2021, 03:44 I'd stick the cap in. That will effect the tone the least of any solution. Raising the B+ 30V will change the tone a lot.
Thanks @bmxguitarsbmx.
I will absolutely try that.

But, for the purposes of learning/experimenting, I was keen to understand how to adjust the node resistor/plate resistors for adjusting the voltage on the plate.

My confusion largely comes from looking at the schematics, and using ohms law to work try and work it out. I can see the approx. voltage drops at each point, and the R values, to get an appreciation of the current in the circuit at each stage.

I can see how I could adjust the Rs to get a different voltage drop, but then this would change the current at each stage?
Would yo select an R value which gives the desire drop as well as keeping the current requirements broadly as they are now?

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Post by ryanuk »

I guess, asking my question a slightly different way...
Annotation 2021-11-24 145717.jpg
Annotation 2021-11-24 145717.jpg (16.79 KiB) Viewed 213 times
This is the preamp Ive built from the kit. Voltage at the 100K load resistor is 250V - voltage at the plate is 133V. Voltage at the cathode is 0.9V.

Annotation 2021-11-24 145718.jpg
Annotation 2021-11-24 145718.jpg (16.79 KiB) Viewed 213 times
This is taken from a Laney schematic. Voltage at the 100K load resistor is 200V - voltage at the plate is 140V. Voltage at the cathode is 0.9V.

How does the latter example start with a lower B+, but end in approx. the same plate and cathode voltage?

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Post by bmxguitarsbmx »

Individual 12ax7's will all bias slightly differently. Especially in this range where some 12ax7's will conduct grid current, which then causes more current to flow (as grid current makes the grid more positive in respect to the cathode, thus drawing more current).

Electrons from the cathode are drawn through the control grid to the more positive plate. As the plate voltage becomes lower, some of those electrons are able to accumulate on the grid and flow through the connected circuitry.

I have several amps that draw grid current at around this 140v plate volts area. I swap my tubes around to find the one with the least grid current. Grid current will cause pooping on relays too btw. That area can sound cool though, so I tend to ignore it. EH tubes seem to be able to get the lowest plate voltages without grid current and several other people I have chatted with agree with this observation.

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Post by Manfred »

If a valve in the idle mode draws such a noticeable grid current that the bias point is shifted, then the valve bulb is leaking and no longer has a perfect vacuum,
the air infiltrates and the tube is then defective.
The difference of the measured voltages comes from the current componet values and the operating voltage value at the measurement.
These values are different from amplifier to amplifier due to tolerances.
MADAMP.png
For example, Rk tolerance is -5% gives 780R.
MADAMP Rk Toleranz.png

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Post by bmxguitarsbmx »

Many tubes conduct grid current without being defective. Nothing wrong with the tube at all.

Those load lines are drawn with the grid connected to a low impedance bias supply that keeps the grid voltage stable. They will look different with high impedance's on the grid at low plate voltages.

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