This post talks about how I did an initial exploration of my Marshall JMP 50 Watt ‘Master Volume’ (model 2204). As a continuation of my previous post, here’s an update:
- The multimeter arrived! It’s incredibly basic (it’s even a manual ranging device, which means I must be careful) but very well built.
- I couldn’t get hold of a 1k Ohm wire-wound resistor to make my own capacitor discharge doohicky, but unless I was going to be desoldering components I left that for later.
So my goal was to use the voltage chart contained on page 178 of the Tom Mitchell book to check my own amp. I was hoping that my amps values would be “within 10%” of the published values. To do this I needed to measure voltages “at the pins” of the various tubes in the amplifier. That meant I needed access to those pins in a safe manner. The last thing I wanted to do was injure/kill myself so I followed accepted procedure by making sure I only ever used one hand to place the red probe onto the pins. The black probe was clipped onto the chassis (so I didn’t even have to use my other hand to hold the black probe, which was behind my back, and therefore unlikely to touch anything).
I unscrewed the back panel of the amplifier, and then removed the 4 screws that secured the chassis to the cabinet. When the last screw came out, the chassis kind of plopped forward due to the sheer weight of the transformers. Probably 90% of the weight of this thing is those transformers. I then carefully picked up the chassis from inside the cabinet and placed it upside down on top of my 2×12 cabinet (which was itself on it’s side). This gave me easy access to both sides of the chassis, and non of the tubes were touching the supporting surface, and therefore not being strained mechanically. It occurred to me that I really should build a support frame if I was going to do this on a regular basis; something that would allow me to flip the chassis about easily without having to risk touching components inside.
There was more risk involved as the inside of this particular amplifier wasn’t exactly pretty. After looking at the insides of other people’s custom builds, my 2204 looked like it was thrown together correctly but with little regard for “craftsmanship” or “functional beauty”. I was hoping for this kind of attention to detail, but was disappointed to find a rabbit’s nest of wire and seemingly shoddy soldering. Firstly I made sure the black probe was safely clipped to the chassis, and that anything and everything was out of harms way (including some of my pets which were messing around on the floor. I made sure they were out of the room). Secondly I attached the speakers, as it’d be very bad to power up the amp without them; the speakers are “part of the circuit”. I then turned on my multimeter and dialed it to the highest DC voltage setting.
The assumption here is that all the voltages measured are “in relation to ground”. It’s not like testing individual components where you might measure the voltage drop across a resistor, or test for continuity. It seems all voltages appearing on schematics or charts like the ones in the Mitchell book, are all about their relationship to ground, hence clipping the black probe to the chassis and simply touching the red probe to the appropriate pins.
So, now I was ready to measure something! I thought I’d start with the amplifier on standby as I knew from reading that all this did was provide power to the pilot light (in this case, inside the rocker switch) and power to the heaters (filament) inside each tube. I could see that each tube was indeed glowing and the pilot light was on, so with my red probe I headed for pin 9 on the first ECC83 with excitement. Nothing. Nothing at all. Zero. I tried pins 4and 5 , which turns out are soldered together. Nothing on those either. So, first lesson of the day….the book goes on to describe the voltages expected on the heaters of a 12AX7ECC83…3 volts. Ok, so where’s my 3 volts? Well, Mitchell is very clear to point out that the voltage on heaters is AC, not DC. That requires a different multimeter setting. Once set correctly, I got a nice steady 3 V AC on all my tube’s heater pins.
Right, so back to the other pins.This was where it got weird. I was getting great numbers as I made my way around the ECC83s. It all made sense until I got to the EL34 tubes. According to the Mitchell chart I was to expect a voltage on pin 6. But it was clear from looking at the pins that nothing had EVER been soldered to those pins. I did some poking around and found that pin 6 on an EL34 is officially “no connection”. Mitchell’s own diagram in the book agrees with this page.
I was able to measure DC voltages in my 2204 that were within 10% of the ones in the book, which I took to be successful. I also took this to mean that nothing was fundamentally wrong with the way voltages were in the amplifier, so my odd metallic resonance may be coming from something mechanical. I decided to take a break so I turned off the amplifier and unplugged it to have a think and perhaps read some more.
One particular test in the Mitchell book describes the act of tapping the tubes with an insulating object (such as his much touted Sharpie). So after getting the amp the right way up and turning it back on, I plugged in a guitar and fiddled with it to get it loud enough to reproduce the errant noise. I then found a nylon pen of suitable length and proceeded to gently tap the output tubes. Sure enough, one of the EL34 tubes made a very distinctive “thonk” when I tapped it. The other was completely silent. I believe I have found the problem and I think it simply comes under the term “bad tube”. My plan today is to buy a matched pair of EL34 tubes. Mitchell unconditionally recommends Groove Tubes or Mesa Boogie tubes. I’m going to shop around for a decent price obviously, but will stick to these recommendations for this first effort. That’ll be another post as this one’s way too long already.