help needed

[dk2]

when i was usin my ma50c last night on the boosted channel i noticed even when the channel volume wasoff istill could here it the crunch balance was on half,whats goin on? can anyone help please

[Nebulozny]

There is that bleed on gain channel, I think Steve explained it on this forum , it's normal, well not on every amp but it's not a faulty amp you have, don't worry, I have that too on my MA100C.
Clean channel doesn't come trough at zero volume though.

[Phano36]

Hello DK2
Yep ! Same thing for me. On the overdrive channel only. It's the way this amp is design.
Lolll I told you on another post you need top crank this volume to enjoy loll not to put it down.

Enjoy ciao !

[jagermike66]

phano is 100% correct lol

At band practice the volume level is directly proportional to the size of the smile on my face. I get a noticable 'jump' when the OD channel hits about volume 5-6. LOVE IT!

[KevinOConnor]

Hi Guys

To my eyes, I think the OD selection could have been improved and the feed-through problem eliminated by design.

The first relay in the signal path sends the first-stage signal to either the clean EQ or to the OD Gain control. This is a series switch. The load of the EQ is disconnected from the first stage when OD is selected, but the amount of gain loss imposed by that loading is usually negligible. Marshall might have decided that extra 10% was necessary, though.

An alternative use for the relay would have been as a shunt switch. The clean EQ would be wired to the first stage all the time, as would the feed to the Gain pot. The relay contact has its wiper to ground and shorts out either the output of the gain pot or the output of the Clean volume. Shunt switching is always quiet and the feed-through around the OD Gain pot would be minimised.

With any circuit layout, you try to place the switching elements where the audio signal already is. Depending on the control layout the signal may have to traverse more distance than with a different control layout. This mechanical hurdle can impose a susceptibility in the PCB or wiring arrangement where electrostatic coupling of signals occurs. On a PCB, use of a ground plane goes a long way towards eliminating this problem. otherwise, normal spacing of traces and use of guard traces is the usual means implemented.

Variation from one assembly to another - where one sample has feed-through and another does not - reflects variations in the PCB manufacture and cleaning, or variations of wire lay in hand-wired amps. TUT3 details proper layout of wiring and for power and grounding feed for hand-wired amps, but the principles apply to PCB layout, as I do for my own amps. The high impedances in tube circuits make even small inter-trace capacitances problematic, which is where the initial board layout and subsequent soldering and cleaning become critical.

Have fun
Kevin O'Connor