Andy40's Mojotone Tweed Deluxe build diary

[DrNomis_44]

Hi Guys,

Thanks for that guidance.

Okay, I'll look into the best point to put a bleed resistor in but for now I've been successful on draining the filter caps, and pulled the chassis out and buzzed out the circuit.

I found the culprit being the bridge between pins 8 and 3 on the 12AY7, there was a loose connection there.

It also gave me a chance to check and recheck all of the solder joints and reflow a couple that I did not like.

All voltages are reading within their tolerances except for one being the PI grid leak from the 1M resistor going to pin 7 of the 12AX7.

Apparently its supposed to be 19V and its reading 2.71V which is quite low.

I can't for the life of me figure out why, except that the carbon comp resistor may be not within specs. I was actually quite surprised at how inaccurate the carbon comp resistors were wiht tolerances. I think if I build one again it would be with a turret board and metal film resistors.

Anyway, all input channels are working perfectly and the amp sounds and feels amazing.

Most carbon composition resistors are usually marked with either a gold tolerance band (+/- 5%) or a silver band (+/- 10%), occasionally you might get a carbon composition resistor marked with either of those bands but when you measure them with a multimeter they measure significantly out of tolerance, in that case I'd personally replace it with one that measures within tolerance, it sounds like that 1M resistor might be partially short-circuited if the voltage reading is that low, there could also be something causing a leakage-path in parallel with that resistor, I seem to remember reading somewhere on the internet that some old Fender amps had eyelet boards in them where some parts of the vulcanized fibre-board got a bit damp and a leakage-path developed between some of the eyelets, that's more likely to occur in old vintage Fender amps rather than modern reproductions, sometimes solder flux residue can become slightly conductive and create a leakage path too, sometimes a valve can develop a leakage-path between two of it's pins, this is caused by a process called Ion Migration, and tends to happen between two pins at different voltages, try temporarily replacing that 1M carbon comp resistor with a 1M/5% 1W carbon film resistor and see if that restores the voltage to it's correct value, hope that helps.

You could also try substituting another known good valve for the phase inverter valve, and see if that fixes it too.


It is fairly common for carbon composition resistors to be a bit inaccurate with regards to tolerances, a lot of that is caused by the way a carbon comp resistor is manufactured, carbon film resistors do tend to be a bit more accurate and metal film resistors do tend to be pretty accurate (within +/- 1% or less of the marked value), I also remember reading somewhere on the internet an article about a type of harmonic distortion generated by carbon comp resistors due to the voltage-dependent nature of the resistive carbon material used to make them, supposedly the distortion caused by carbon comp resistors is part of why vintage valve guitar amps sound the way they do, because the distortion adds pleasing harmonics, though whether that is actually true or not I don't really know for sure.

https://www.diystompboxes.com/pedals/rgccmp.html


When a carbon comp resistor overheats they tend to drift up in value and will measure out of tolerance with respect to the marked value, incidentally, the reason why a resistor is made to a certain tolerance is because it's very difficult to make them exactly spot-on in value, most electronic circuits are normally designed so that the component values aren't too critical for normal circuit operation, if a circuit calls for a particular value of component and the person building it doesn't have it in stock, they can substitute the next higher value and the circuit should operate as it is supposed to, unless the circuit is for some particular application where high-precision is important, so say for example you're building a circuit and one of the components is a 10k/5% carbon composition resistor, and you don't have any in stock, but, you do happen to have some 12k/5% 1W carbon film resistors and you pick one, measure it, and it actually measures about 11.75k, if you use it as a substitute for a 10k resistor in the circuit, the circuit should still operate as it is supposed to .