Chandler
“Guitar city, Sydney
”
I found this one as a chassis and case in the explosives store behind an old sawmill.
The only difficulty was the large python wrapped around it.
Contains:
40 watt head
The valves were gone but the remainder looked valuable if not intact. The very lumpy transformers and white chicken-head knobs made it instantly interesting even covered by years of grot. The python's interest was because some native mice had filled it with straw and set up house inside.
The more I cleaned it up, the better it looked. The Henderson power tranny checked out okay, then the A&R 2653 60W output tranny too. When all the HT caps reformed okay using the Megger the prospects of bringing it back to life started to look really bright.
I traced and drew out the circuit, at the same time testing every component from input to output using an ohmmeter or Megger. This identified a number of failed components which were duly replaced. These are marked (*) in pencil on the circuits.
It took a bit of detective work to figure out what the original valve line-up had been, but after checking base wiring and output transformer impedance it resolved to all twin triodes to EL34/6CA7's.
Circuit
No surprises here (unless you happened to touch the uninsulated can of one of the electros in the HT voltage doubler! - naughty, naughty). A very conventional arrangement including passive Baxandall tone controls. Tremolo cathode coupled to an early preamp stage. Two EL34/6CA7's in cathode bias.
View preamp circuit (200kb jpg).
View main amp and power supply circuit (250kb jpg).
One curiosity is the use of cathode bias on the 6CA7 output pair. This alone seems to limit the output power to around 40 watts, while the valves, power supply and output transformer all seem capable of 50-55 watts. The power transformer in particular is very generous with an estimated 230 watt core. So a conversion to fixed bias may be in its future.
Other curious items include a HT triple electrolytic with only two sections connected; the output transformer having a ground on its tap rather than common; only a fuse between active and the power transformer; the standby switch only switches the preamp HT.
The phase inverter is a split-load type, but it uses an uncommon form of biasing called contact bias. Using cathode bias the highest allowed grid resistor is 1M ohm, but the 12AX7 data allows this resistor to be up to 10M ohm to develop contact bias. This bias is due to the tiny current that flows out of the grid through this resistor. Another way of looking at it is that the grid collects electrons, negative charge, which tend to repel other electrons (bias) and leak away via the 10M, maintaining an equlibrium.
While this saves a resistor and works when new it is particularly suseptable to bias upset by leakage through the input coupling capacitor from the anode of the preceeding stage. Also to age instability in the 10M resistor.
HT filter choke left front; live electro in black; original speaker connector front center; RCA tremolo footswitch connector front right; valve cans scrounged from an old VHF TV transmitter.
Four input sockets, two for straight, two for tremolo channel. These are the same sensitivity in each channel, but the straight channel has about three times more gain.
Controls are:
Intensity; Speed; Vol1,2; Vol3,4; Bass; Treble; Standby; Power; Pilot.
Back panel: (retro-fit 6.5mm sockets) 8 ohm speaker, 0dBV line output.
The Case
The ply case clearly isn't the original. The chassis is now secured by screws through the end plates, but in the view above two of four holes through the chassis show where it was originally hung, Fender-style, from the top of a different case.
The case front plate was missing, and remaking another one revealed the existing case is a nice one-off owner built effort.
The cloth covering I used is worth mention. These days cases are often covered with carpet. This is in fact boat carpet and priced accordingly at $15-$25 a square metre.
The local plant nursery stocks “root balling cloth” (don't ask, it's Secret Horticulturists Business ;), a black synthetic fabric so tough I literally have to use tin-shears to cut it. It comes 2m wide and costs around $2 a running metre, or a buck a square. Four years on the road on various bins and it still looks great.
The original speaker connector and its awkward position is a strong clue. This type were commonly used to connect to the speakers below in a combo unit, and from just this position on the chassis. The rear 6.5mm jack sockets are a retro-fit, so it looks like it has been de-Combo-ised at some point.
But at this power output level and vintage it is unlikely to have had only one speaker in the cabinet yet it would have been no wider than the 18 inch chassis.
So this is an “artists impression” (guess) what it might have originally looked like.
A little under three feet, or one metre, high.
Covered-up power wiring, and strapped down filter caps, but otherwise pretty much original.
Detail showing the old style small-pin/large-pin speaker socket, and the chassis marking in pencil “No2”. The matching plugs were just an insulating disk with two pins, very minimal.
It is not uncommon for a manufacturer of a product like guitar amps to make a special batch for a particular customer to their requirements, even if that is only a front panel with their name on it - “badge engineering” it's called - the only bit they engineer is the badge. Tandy's “Realistic” brand is a classic example with many of “their” products being made by others and bought-in.
So the “Guitar City” front panel logo strongly suggests that this is one of a batch made for a music shop, perhaps Chandler their own brand, and that this is number two of a short run of perhaps ten or a dozen such combos.
But it doesn't look like a backyarder job, the chassis metalwork is properly done, even the flimsy light purple anodised front panel, so my hunch is that it was built by an existing amp builder like Moody, and badge engineered for Guitar City.
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