Gibson EH150 Amp

Japan fighting the Chinese, Russia fighting the Finns, the British Empire fighting the Germans and Italians, Churchill not even PM yet, the USA nervous of Japan, Tom Jones born… It feels very strange to have your hands in an amp that was made when all this was going on. Sure we have troubles now but I guess it really is all relative. This is an early 1940-built amplifier in surprisingly nice nick for its 72 years, with even its original field coil (ie it doesn’t have a permanent magnet) speaker still in good shape. The EH150 first appeared in 1936 (only 24 years after the amplifying valve itself had been invented), and with a couple of quick evolutions of circuit became the first really effective production electric guitar amplifier, resulting in a big commercial success for Gibson, particularly after it was seized on by the great Charlie Christian to develop and power his ground breaking lead runs.

Playing this amp now, it’s too dark and indistinct to be more than adequate on the regular ‘Instrument’ channel (the guitar pickups of the day were substantially different in design to those we’re used to now though), and maybe it would be nothing special if not for that historical connection with a guitar genius. However, it’s the higher gain ‘Microphone’ channel which is really startling on this amp. While many instrument amps from around this era and through much of the fifties had a low signal level input for microphone (often used for acoustic instruments such as accordion), what makes this one stand out is that it is brilliantly usable for gain with a guitar. That’s because it’s virtually unique in using a transformer as a phase splitter rather than the primitive valve circuit topologies usually used during that time. This means it is capable of utilising all of the gain offered by the mic channel (and there is a lot) without so severely overloading the phase splitter that it constrains the overall volume the amp is capable of (the “sounds good at ‘3’ but can’t go louder” syndrome).

This is all good, but the real problem with this amp is its tendency to being microphonic. Sure the cab is pretty and well crafted with its ‘Airline’ tweed and luggage styling (it is actually lined, for goodness sakes), but it is just too small in volumetric terms, and too flimsy, especially with the back removed as it must be to power and play it (do not sit on one of these while you are strumming away). Yes it is a design almost from electric guitar pre-history, but it’s still got a pair of 6L6 power valves (state of the art technology at the time) in there with a perfectly capable 12″ speaker. It can easily go to 20 watts or more, and that is a lot of vibration (and the pierced metal speaker grill probably reflects a good deal of energy back through the speaker cone too). There is an external speaker jack, but plugging anything in to it just adds another speaker with the output transformer impedance being changed to match (hence ‘Echo Speaker’, there’s no reverb on board); the amp’s own speaker will still be thundering away. This example was particularly bad in this respect, since it had a pair of later ‘coke bottle’ 6L6GA types shoehorned in there. These are nice valves, a technically improved version of the metal clad first generation 6L6 the amp would have originally had, but they are physically quite a bit wider and taller. Space is limited, so it’s no good fitting them if it means that then the speaker is connected to the right valve, and the right valve is connected to the left valve, and the left valve is connected to the rectifier valve… there is nothing at all spiritual or righteous with that:

(Note the rotated speaker – it can be a good tip to re-position speakers every once in a while, especially if left unplayed for any length of time, as it reduces the likelihood of gravity induced cone sag leading to voice coil rub.) This amp was designed for the older spec metal valves like this, which do not touch:

Now with these valves the problem is that, while they do have great shielding from electric and magnetic fields, this method of metal construction for valves was soon superseded, because they are themselves notoriously microphonic, especially in a combo amp.

Fortunately there are electrically equivalent subs benefiting from all the experience gained by the time the third and fourth generation valves appeared through the ’50s and ’60s that can be used instead to address the microphonics issue. For example, these JAN-Philips 6L6WGB are mil-spec, with thick glass and beefed up internal supports to counter vibration (and G-forces in Dr Strangelove B52s, or something), and they substantially do address the microphonics issue in this amp, while still being musical (these are favoured for Fender Bassmans, both vintage and re-issue, but some mil-spec or industrial grade valve equivalents, especially small signal ones used for preamps, can be harsh-sounding):

The metal clad preamp valves (a 6F5 and three 6C5s) have electrically equivalent later evolutions too, and these may be tried in those preamp positions if there are microphonics issues in this area, however, this amp functioned fine with the original metal preamp valves so they were left in place.

Further, the amp chassis itself can be better isolated from the vibrations transmitted directly from the cab, and some decent rubber grommets at the mounting screws are of help. The valve sockets also can be isolated from the chassis with small grommets, as Fender often did with its ’50s combo amps.

As all the original large box-type power supply filter and bypass caps had already been replaced, as had the original beeswax coupling caps, the only remaining problem with this particular amp was that oddly it was the much lower gain Instrument channel that was far noisier than the Mic channel. This was chased down to a grounding issue, fixed by physically changing its location to remove the ground loop. Unsurprisingly given its age, the amp also suffers from relatively high levels of background hiss and some intermittent static; if it were to be used as a recording amp the 75 year-old carbon comp resistors would need to be individually checked and offenders replaced as necessary, or all just replaced as a matter of course to ensure future reliability (this is where the ‘vintage’ discussion takes place). I also replaced the 2-prong power cord so the amp is safely grounded. And, it’s always good to check for loose conductive things in amps that may cause short circuits or arcing (old wire clippings, clumps of solder spatter etc), and this paid off. Literally, as I found a quarter dollar coin dated 1965 wedged inside…

Charlie Christian 1916-1942