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John Hornby Skewes Amplifiers


JHS C50PM.


C50PM Guitar Amplifier



If your enthusiasm was fired by our December review of the Vox AC30, and you rushed down to your local music shop thinking a 30 watt amplifier wouldn't cost much, your excitement was probably dampened considerably when you were asked for over £300. By way of recompense, therefore, this month's amplifier is more affordable.

The JHS C50PM is a 50 watt solid state amplifier with two 8" speakers, costing £139 including VAT. Have I said enough to confirm your prejudices? Cheap oriental rubbish, right? Wrong! For a start, the label on the back says 'Made in U.K.' and there are definite signs that someone has been thinking about the details of this unit. Take the cabinet, for instance; this is covered in a very handsome deep grained brown PVC material, which looks commendably hard-wearing. With its brown and yellow control panel, the whole amplifier looks pretty smart in fact. There is a plastic corner cap for every corner — so many manufacturers only fit four, or even none at all — and these are soft, thick plastic that is unlikely to break easily.

The cabinet measures 21 x 16½ x 9½ inches, and although the lightweight construction (½" chipboard with 1" square framing) doesn't do much for the sound, it does make the amplifier easy to carry single-handed. The sort of person who is likely to buy this amp would probably take it to gigs in the back of a car, or even on the bus; so it doesn't have to be built to battleship standards, but the lack of weight will save a lot of cursing from those who have to do their own roadying.

Inside the C50PM.

There are two input sockets — 'bright' and 'normal' — sharing a single input volume control, and a master volume control, called 'gain' on this amplifier; more of this later. As well as the normal bass and treble controls, which work well and have sensible turnover frequencies, the C50PM also has a parametric midrange tone control. An unusual feature, this, on a budget amplifier, and despite one of the controls being noisy this is one of the most useful facilities on the unit. There is plenty of range on the gain control — up to 20dB cut or boost — and the frequency knob covers 80Hz to 1.25kHz; in other words, exactly the same four octaves spanned by a 24 fret guitar! Obviously that engineer has been thinking again, and has taken into account the special requirements of the electric guitar.

It is, in fact, impossible to set the tone controls to give a flat response, not that it really matters on a guitar amplifier as long as the final sound is acceptable; which it is. As well as a wide range of tonal possibilities, some special effects are available; I turned up the volume to get feedback from my guitar, and found I could control the pitch at which feedback occurred by sweeping the mid-frequency control. Conversely, this control should also be good at reducing feedback.

The final item on the front panel is a socket for a footswitch; this introduces diodes into the preamp signal path to give clipping. The effect is a fairly harsh fuzz sound, unfortunately also accompanied by a drop in volume. The depth of fuzz can be controlled with the input volume control, whilst the volume is regulated with the gain control which is situated at the preamp output.

Although it's nothing like the good old valve sound, at full steam the fuzz sounds better than some effects boxes, I must admit, and is perfectly adequate for single note lead lines. On chords, however, the sound is very discordant and jangly, and any attempt to tone down the distortion by backing off the input volume just makes the fuzz sound as if it were 'laid on top' of the original signal; that is, when the notes decay, the fuzz suddenly goes away leaving the unaltered guitar sound.

With a reasonably powerful guitar, however, it's possible to overload the preamp section itself, leaving the fuzz switched off. Simply turn the input volume up to near maximum, and turn down the gain; the sound is roughened up slightly, but not so distorted as to be a nuisance on chords, and I much prefer this effect to the 'genuine' fuzz. If you turn everything up so that the power amplifier clips as well, the harshness comes back; I'm a valve amp fan, mind, and the JHS is better than some transistor amplifiers in this respect, possibly due to the speakers used (two anonymous 8" units). On the other hand, if you're a punk and enjoy sending your audiences home with their spectacles cracked, this could be just the amp for you. Otherwise, be careful to avoid overdriving the output stages.

Because the tone controls have so obviously been designed to work best with a guitar, I've referred to this unit as a guitar amplifier throughout the review. In fact, the power supply and heatsinks aren't really big enough for the continuous sounds coming from an organ or synthesiser, and under these conditions only about 30 watts are available before clipping sets in.

In the studio, background noise — hiss and cabinet rattles — would probably be too obtrusive; on stage, however, in a pub or club this amplifier should be ideal, and I imagine this is the market it's aimed at.

If you've only got £139 to spend on an amplifier, you can't expect perfection; give this one a try, it's British and gets full marks for effort.

JHS C20B.


C20B Bass Amplifier



Inside the C20B.

This is quite a different kettle of fish; it's a 20 watt bass amplifier, measuring 19½ x 15½ x 9½ inches, and fitted with two of the same 8" loudspeakers in a sealed compartment. This amplifier is intended solely as a practice amplifier for bass guitar, and as such has virtually no competition. Such devices are few and far between. As you would expect, such a compact speaker arrangement is not especially efficient at reproducing bass; to get a decent sound the amplifier has to work quite hard, and there is no chance of competing with a drummer (is there ever?). Whether or not you are willing to pay £105 for a private practice amplifier is up to you, although this unit may have a use in a primarily acoustic group, playing folk music for instance. A bass guitar and one of these amplifiers would be a much more portable alternative to an acoustic bass in these circumstances.

Portability is helped by the lightweight construction, which is identical to the larger unit: ½" chipboard with 1" square framing, brown PVC covering and corner caps. All the electronics except the transformer and mains switch are mounted on a single PCB: there is no chassis, the PCB and transformer are mounted on the front panel, which is a length of aluminium angle screwed into the front of the case. The C50PM is built the same way, incidentally.

There are two input sockets — why? — a volume control, and four tone controls. Treble and bass have had their turnover frequencies chosen sensibly once again, although the bass control has a rather limited range, being capable of only 10dB cut or boost; this may have been done to protect the speakers. The other two are fairly sharp band-pass/stop filters, which act like a two section graphic equaliser. The lower one is centred on 65Hz — C above bottom E on the bass — and has a range of +18dB, while the upper filter is centred on 125 Hz, an octave above the lower one, and has a gentler range of +12dB. If anything, the 65Hz control has too much range; things need to be turned up quite a way to get a meaty sound but it's easy to go too far with this one, and horrendous distortion is the result.

Apart from that, the sensitivity has been kept low on this amplifier, at 150mV as compared to 15mV for the C50PM; possibly for this reason, the noise level is much lower and the unit is commendably quiet.

I must admit to having reservations about the usefulness of this device, but it's well made, the tone controls are well thought out, and at the price there's nothing else like it.

Test Results

C50PM

Power:
30W RMS continuous 35W @ 10% distortion
Sensitivity: 15mV for 30W out
Signal to noise ratio: 40dB
Bass control: +10dB @ 65Hz
Mid control: ±20dB, 80Hz to 1.25kHz
Treble control: ±10dB @ 2.5kHz
+15dB @ 7KHz (through bright input)


C20B

Power:
21W RMS continuous 28W @ 10% distortion
Sensitivity: 150mV for 20W out
Signal to noise ratio: 63dB
Bass control: ±10dB @ 25Hz
65Hz control: ±18dB, 20Hz bandwidth
125Hz control: +12dB, 40Hz bandwidth
Treble control: ±15dB @ 10KHz

Measurements by Dave Goodman.



Previous Article in this issue

Roland TR606

Next article in this issue

Fostex A-8


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - Feb 1982

Scanned by: Stewart Lawler

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Gear in this article:

Amplifier (Combo) > JHS > C50PM

Amplifier (Combo) > JHS > C20B

Review by Peter Maydew

Previous article in this issue:

> Roland TR606

Next article in this issue:

> Fostex A-8


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