Beiträge von Luigi

Hi Kronat,
I think it's not necessary.

Although it is worth to include it, because R32 is probably intended to delay the output transformer break in case you don't have any speaker connected to the amp.

Hi,
did you check the power supply board is working well? You should have measured the voltages on blue lines before connecting them to the main board and to the output tubes.

Then, the first thing you can do is, with the amp turned off, to remove all the valves and measure the resistance across R37: it should be around 100k. If the resistance is significantly lower than 100k, start the search for some unwanted short-circuit.

Regards,
Luigi

Hello guys,
since I would like to try the traditional tone of a 33k slope resistor driven by a low impedance cathode follower driver, I would like to know if there is a technical reason why there is a resistors partitor in the cathode follower stage of M15MK1 or it is a pure tone adjustement related to the values of tone stack.

Thank you in advance,
Luigi

Hello,
I've just achieved a successful M15mk1 build.

Although, I noticed a lack of bass freqs on channel 2, at least regarding my tastes.

Then, I've looked at JCM800 and JTM schematics and I've seen that, in the first gain stage, the cathode capacitor (680 nF) is a bit small respect the 820 Ohm resistor. Probably, to match the response of original Marshall, a 2,2uF capacitor is needed.

Moreover, to get a bit more gain, I'm also thinking about adding a cathode capacitor to the second stage.

Do you have some advice about these points?

Regards,
Luigi

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Hello,
I'm building the M15mk1 and I'm facing the following issue when screwing the 2 boards to the chassis. The screw M3x8 seems to be too long and reaches the limit of the distance bolt before tighting itself. I use the following sequence looking from the top of the chassis: screw, locking washer, chassis, washer, distance bolt and so on.

I've tried to invert things, looking from the top of the chassis: nuts, locking washer, distance bolt, washer, board, lockinf washer, screw, but the screw seems always too long!

Someone can help me? Thank you in advance,
Luigi

FIXED: solved adding an additional nuts: screw, NUTS (screwed to the head of the screw), locking washer, chassis, washer, distance bolt and so on.

Hi Gaz,
thank you so much for sharing your experiments, I will take them in count. For now, I want just to collect some ideas.

I think that another possible mod to get more gain is to convert the cathode follower stage of ch2 into a gain stage, feeding directly the tone stack. In this case I would probably lose some volume before the PI, but, speaking of a relatively high gain channel, maybe this is not a real issue.

Luigi

Hello,
I would like to get the J5Mk1 sound in the 15watt format, because of rehearsal and live playing usage.

I think that a possible way to go is to start from M15Mk1 deluxe kit and try some mods. Obviously the easier mod that jumps in mind is cascading the two channels; probably also a tone stack re-shaping and an attenuation after the first gain stage (that was channel 1) are needed.

Do you have some guidelines or experience to share?

Thank you,
Luigi