A Harley Benton GA15 in the spotlights?
It seems like a bit of a mistake: our previous posts dealt with cool 1960s amplifiers and guitars, made in the USA or Italy. How on earth does a dirt-cheap Chinese made Harley Benton GA-15 fit in that list?! Please bear with me. First of all, it was all too easy to get lured into buying this little amp because of its sweet retro look and, ahem, very low price. Harley Benton is the house brand of Thomann, Europe’s largest on-line music retailer and, if I’m not mistaken, I bought a GA15 for some 140 Euros from that web shop in 2010. One shouldn’t expect a top-quality amplifier for just 140 box, but hey, how wrong could you go with a nice-looking little amp powered by two EL84 tubes? And even it would be rubbish, all the basic ingredients for a great tube amp (chassis, transformers, tubes) are there. The GA15 could be spiced up, modified and improved… and that’s exactly what this post is about!
Once the brand-new Harley Benton was unboxed and plugged in, a fairly dominant mains (50Hz) hum immediately caught the attention, even at low volume… this wasn’t exactly a quiet amp! The 1962 Gibson Skylark and 1968 Gibson Hercules discussed in previous posts were designed nearly 50 years earlier but A LOT quieter… Furthermore, even though unboxed, the GA15 still sounded very ‘boxy’ (pun intended), with an overly strong emphasis on the mid-range frequencies and little bass and sparkle. Also, the amp seemed to accentuate some more unpleasant aspects of the guitar sound (odd harmonics perhaps?). Trying to correct all this with the three-band EQ generally yielded insufficient improvement. It seemed that the stock circuitry of this amp definitely did not bring out the best in a guitar.
To identify the cause of the perceived ‘boxiness’, we had a look at the inner workings of the GA15. As the GA15 is quite a popular amp to mod, finding and downloading a schematic was no problem at all. Looking at a fresh printout, the first surprise was that, after the first 12AX7 preamp tube (V1), the signal goes straight into a series of two TL072 ICs (integrated circuits). These boost the signal for the three-band EQ and buffer the on-board effects loop. The second 12AX7, which is configured as a long-tailed-pair phase inverter, only comes into play after the two ICs. It splits the audio signal in a positive half for the one EL84 power tube, and a negative half for the other. Together they cooperate to form a so-called push-pull power amplifier. Anyway, the bottom line is that the GA15, with an important role for the two ICs, is by no means a pure tube-amp! One might say that, instead of gaining cosmic qualities by flying through ethereal tube vacuum, the signal crawls through some earthly semi-conductors, getting duller in the process…
The second surprise was that one of the cathode bypass capacitors of V1 had a very small value (2.2 uF). Capacitors block DC, but transmit AC. The larger the capacitance, the lower the AC (or audio) frequencies that can be transmitted. The stock GA15 cathode bypass capacitor had a very small value, hence much of the bass frequencies are effectively filtered out at that point, contributing to a boxy sound.
The cause of the annoying hum became only apparent when opening up the chassis. Circuit board traces supplying 6.3V AC to the filament heaters of the tubes were running directly parallel to traces that conduct the feeble input signal that comes directly from the guitar. Not a good idea and, very likely, this is where the hum comes into play!
Lastly, I noticed that V1 (the first preamp tube) had no grid leak resistor, which may cause the grid to become gradually charged and amplification becoming less efficient… potentially even leading to a runaway condition that might destroy the tube!
What to do?
Based on all the above, one might conclude that the GA15 was not very carefully designed. Might this intentionally have been the case however? Harley Benton products come from OEM-manufacturer Saein in China. This factory also produces for Ibanez, Epiphone and Peavey; one can imagine that it is necessary to create a difference in quality between these ‘A’ brands and the lower-priced Harley Benton line. For instance, the GA15 bears obvious similarity with the Epiphone Valve series amplifiers in terms of switches, knobs and exterior design. If their quality were also similar to that of the Harley Benton amps, people might start preferring the latter, of course. Might this also have been the reason why the GA15 was discontinued at a certain point?
In any case, we decided to improve the sound of our GA15. To that end, we developed a number of mods (implement them yourself with our GA15 Mod Kit plus instruction manual):
The ICs and all related components were removed from the circuit, hence creating a 100% tube-based amplifier. The first preamp tube would in principle be connected to the phase inverter tube by a high-voltage coupling capacitor, if no tone controls were present.
As the original EQ was IC-based, an alternative tone control/EQ was needed. With no ICs to rely on anymore and only one preamp tube for achieving most of the gain (apart from some phase inverter gain), the tone control circuit should cause minimum signal loss. A Baxandall tonestack seemed a good candidate therefore. A point-to-point wired incarnation of a Baxandall tonestack was soldered to the back of two tone control potmeters (treble and bass).
Not knowing beforehand how much gain would be left or would be needed, a gain control was introduced between the first and second triode of V1. This created a possibility for overdriving the second triode or, reversely, preventing it from being overdriven in case of a very loud input signal.
The cathode bypass capacitor that caused the boxy sound was swapped for one with a larger value, to allow the bass frequencies to pass the capacitor. The bias of the preamp tubes was also changed by employing slightly different cathode resistors.
To completely get rid of the hum, the 6.3V AC traces on the circuit board were cut and new wiring was installed, staying clear from the circuit board and hence from sensitive audio traces that could pick up hum.
In addition, the regulated power supply that used to power the ICs to supply DC (instead of AC) was now employed to supply power to the tube heaters, to further reduce hum. As the heater filaments draw much more current than the two little ICs previously did, a heat sink of sufficient capacity was attached to the voltage regulator and mounted to the chassis.
A grid leak resistor was added to the first triode of V1.
Some switchable negative feedback, a presence control and a proprietary tone-shaping control (which will probably be featured on Cosmic Ampworks’ own line of amplifiers later on) were added to the flipside of the amp. Two potmeters were mounted in the drill-holes that previously accommodated the jacks for the effect loop (the idea of having an effect loop was abandoned, as we rarely use an effect loop and there were no tubes or places left to power it).
Lastly, the stock 10’’ speaker, which substantially contributed to the boxy sound, was swapped for a 1980s Jackson Redline 10’’ speaker. A nice bonus of this speaker is its red cone, which improves the front looks of the amp. In terms of looks, two black knobs were replaced with red ones, to match the red speaker cone, and as an indication that this is not a stock GA15!