diode resistor master ?

[jamme61]

I have a marshall studio 15 amp
looking at the schematic it has a diode in parallel with a resistor (R6 10k) just before the master volume. My question is what does that do? thanks for any time on this.

[matt h]

(deleted)

[martin manning]

What it does is attenuate the bottom half of the PI output waveforms (the part that isn't important) when the MV is turned down low. I have one of these things, and it is a really nice little amp with a number of interesting features.

[gingertube]

What it does is attenuate the bottom half of the PI output waveforms (the part that isn't important) when the MV is turned down low. I have one of these things, and it is a really nice little amp with a number of interesting features.

Sorry Martin, You are on the right track but not quite correct.
Looking at the schematic - note there is a diode across each 10K (both phase splitter outputs) and they are wired with diode anode to the tube anodes. What that will do is clip the TOP (+ve peak) of the phase splitter outputs, the side that DOES matter, at low Master Volume settings. Probably adds some "grunge" at low volumes.

Cheers,
Ian

[matt h]

(deleted)

[martin manning]

What it does is attenuate the bottom half of the PI output waveforms (the part that isn't important) when the MV is turned down low. I have one of these things, and it is a really nice little amp with a number of interesting features.

Sorry Martin, You are on the right track but not quite correct.
Looking at the schematic - note there is a diode across each 10K (both phase splitter outputs) and they are wired with diode anode to the tube anodes. What that will do is clip the TOP (+ve peak) of the phase splitter outputs, the side that DOES matter, at low Master Volume settings. Probably adds some "grunge" at low volumes.

Sorry, I disagree. The voltage across the 10k doesn't get high enough to forward bias the diode unless the pot is turned down, which happens on the positive swings, bypassing the resistor. Then, when the diode turns off on the negative swings, it puts the 10k between the coupling cap and the pot wiper which increases the voltage division.

[jamme61]

So is it just there to add a little dirt at low volumes? I had one of these masters on my plexi. At low settings it sounded terrible- wish I knew about this- maybe it would have helped the sound. Thanks to all for the info

[martin manning]

I don't think we know exactly what its purpose is yet since it's attenuating the unused portion of the waveform. My suspicion is that it is there to eliminate some nastiness associated with driving the PI hard (preamp volume up) when the master is set very low.

[jamme61]

Thanks Martin is your studio15 stock?

[martin manning]

Thanks Martin is your studio15 stock?

It is. I've seen some mods to coupling cap values that others have made, but since this is a pretty rare amp (only ~500 made), I don't want to do any more than maintain it. For example I replaced the original (1988) filters when I bought it, and that perked it up a little. It has been in regular use for about six or seven years now.

[martin manning]

So the thing attenuates the lower part of the signal in a sim with a sine wave input (as suggested above), but if I hook two of them up to a LTP there is just a small amount of attenuation on both the top and the bottom of the output waveforms, presumably because the two triodes talk to each other via their common cathodes. I'm stumped as to what benefit it brings.

[jamme61]

So the thing attenuates the lower part of the signal in a sim with a sine wave input (as suggested above), but if I hook two of them up to a LTP there is just a small amount of attenuation on both the top and the bottom of the output waveforms, presumably because the two triodes talk to each other via their common cathodes. I'm stumped as to what benefit it brings.

Thanks martin- I just got the amp today and it got dropped_ I guess UPS threw it around. the speaker baffle pulled out of the cab. I checked out the amp and its all stock( no mods) plugged it into another cab and it sounds really good IMHO. I think I'm going to try and see if I can screw the cab back together. If I keep it, I'm just changing the filter caps like you did makes sense - it sounds great thanks

[martin manning]

Wow, that's a shame. Hope you can repair it.

I'm beginning to think that the diode and 10k combination might only be there to maintain some AC load at low MV settings, but just 10k? Why not flip the pot around and wire it like the KF Type 2/LarMar and solve that problem, as well as eliminate the shift in frequency response with volume setting? I might be tempted to swap the wires in mine and see. It's a PCB amp, but there are flying leads on the MV pot.

[matt h]

(deleted)

[martin manning]

I recognized the effect it would have on loading early-on, but what I'm coming around to is that that might be its only duty!