Before we can apply mains voltage to our amplifier, it’s paramount that we double check our wire connections, both physically and with a continuity meter. This allows us to be safe in the knowledge that all of the crucial high-voltage connections are in the right place, and also the ground connections too. If I’d mistakingly connected a high-voltage wire to the eyelet on the board, catastrophe could occur.
Using a digital multi-meter set to continuity mode, I check every connection against my layout diagram to ensure that nothing is amiss. All of my grounds are in place, my signal wires connect to where they should, and nothing is out of place. Good.
With the chassis now mounted on the test bench, I can now apply voltage to the amplifier and check that voltages appear in the places that I want them to, and not where I don’t. I use a current-limiting device made using 100w incandescent lightbulb that can be switched in and out of the circuit. It sits in series with the live mains wire and will protect the amplifier should a fault current flow.
The filament of the lightbulb increases resistance as more current flows through it, and therefore self-regulates how much current the amplifier can draw. With 240v of mains voltage and a 100w bulb, simple Ohm’s law will tell you that no more than 416mA of current can flow at any time (current = watts/voltage). The bulb also gives a useful visual indication of current flow, as it will glow brighter as the amplifier tries to draw more current.
As I apply power to the amplifier, the indicator light illuminates on the front panel and I can hear the power transformer humming away. Using a voltmeter, I check that the three secondary windings are producing the expected voltages and when all is confirmed I can switch the amplifier off in preparation for installing the valves.
New tubes and old valves
The EH-185 uses valve types that are no longer produced, so I got to spend a few hours browsing internet auction sites and specialist suppliers for our set. I found that langrex.co.uk had a trio of RCA 6SQ7s, all dated to 1965, and a Haltron 6N7 from 1966 that’d be a great lineup for this amplifier. The 6L6 output and 5U4GB rectifier valves are modern production units from TAD and JJ respectively.
The NOS valves from Langrex are tested on arrival and I find that the 6N7 results are unclear, so a matching replacement is quickly dispatched to me. Quite a service! I start by installing just the 5U4GB rectifier and fire the amplifier up again. This will energise the high-voltage rail and allow me to take further voltage measurements. The lightbulb limiter starts to glow slightly, indicating the amplifier is starting to draw a substantial current. With everything checking out, it’s time to install the rest of the valves and flip the power switch for the final time.
Our EH-185 chassis is ready to play, but before I plug a guitar in, however, I check the operation of the amplifier on the oscilloscope, checking that it is passing signal and that I cannot see anything of concern. Thankfully, all seems to be okay, so I can connect a speaker to the output and grab a guitar. The 2×12” bench cabinet is somewhat overkill for this small 20w amplifier but it has both a wide bandwidth and lots of clarity, allowing for close examination of nuanced details.
We’ve got two channels on this amplifier, one ‘microphone’ and one ‘instrument’. Plugging in an alder-bodied, rosewood-board Fender Stratocaster loaded with ’54 spec Monty’s pickups into one of the instrument inputs, the resulting clean tone is warm, detailed, and full-bodied. There’s certainly a degree of harmonious marriage going on here as the tone and feel just seems to work. The bottom end is understandably loose with some pliability in the response, but this is common to many pre-1960 amplifiers. Death Metal thud and grunt, it is not.
It’s actually quite easy to understand why jazz players love these amps; there’s instant gratification from nailing a fast run of notes as everything is clear and concise, whilst simultaneously delivering a complex, detailed harmonic content. The sustain is there and quite impressive but crucially doesn’t it get in the way of the next incoming note. This channel rewards precise playing and fans of rich cleans. I try adding a little slap-back delay and a smidge of reverb to open up the sound and boy, we are rewarded. Noodling around the fretboard is a joy and arpeggiated chords allow the harmonics to build up wonderfully. So far, this thing is a hit.
Switching over to the mic channel, things are a much different affair. This channel has two extra gain stages and even with low-output pickups, it’s quite obvious that this channel is spicy hot. I grab a P90-loaded Les Paul and discover that clean headroom is practically non-existent. This, however, is a good thing as what you get instead are thick, creamy, and throaty overdrive tones that can easily be dialled to suit your taste by tweaking the guitar’s volume and tone controls.
I try connecting a 1×12” pine cabinet loaded with a 35w American-voiced speaker and find that the note edges now have a slight fuzzy character. This is what I’ve heard on many YouTube videos of original examples. Mission accomplished.
I had an email from a reader asking what I plan to do next with this EH-185 project. I plan to keep refining the circuit, ironing out some of the idiosyncrasies and move the design towards being more player-friendly. The instrument channel is almost perfect, but onboard spring reverb would really make it shine. The mic channel is fun to play but could do with a wider range of drive sounds, quite simply it needs to be cleaner at lower settings, gradually accelerating into its current level of madness as you turn it up. Playability is key these days, so that’d be my primary goal.
I’ve enjoyed this project, it’s been fun to get stuck into something different and I’ve got a good starting point for development. Thanks for reading!
Find out more about Rift’s range of boutique amplifiers at riftamps.com