S950 Rehab Project

This is the story of an effort to rescue an old, forgotten guitar amp.  I’ll add to this page as the work progresses.  Happy ending will be a schematic and functional amp.

ORIGINS

I acquired this amp from Jim Christopher of Encino, California.  Jim is an interesting guy with a passion for old guitars and amps, Lectrolabs in particular, and this one specifically.  He got it in non-working condition, took it to a tech, but the tech was unable to get it repaired.   Lack of a schematic was one issue, and someone had messed with the power supply circuit so that the stock configuration was unable to be inferred by looking at the amp.

Jim said this may be the only existing example of this model, and wanted it to find a “good home”!  His enthusiasm was contagious, and I couldn’t resist the challenge of getting this amp documented and fixed. Learn more about Jim, and see his interesting Craigslist Vintage Guitar Hunt site here: http://craigslistvintageguitarhunt.blogspot.com/

A Google search reveals no available schematic for this model. In fact, the only S950 references I could find was on this page, and it was just a photo.  For all I know it is a photo of the amp in my possession.  Other Lectrolab products are documented, but almost nothing on the “S” series, which may be the last series produced by Lectrolab before they ceased to exist.  In fact, there is very little data regarding Lectrolab company or its products.

I’m going to dissect this amp, but before i do, here are the first impressions:

Front panel:

  • 2 Channels, Each with input jacks, Volume and Tone pots
  • Tremolo Speed and Modulation pots
  • Reverb pot
  • Fade pot. I have no idea what “Fade” means.
  • On/Off toggle
  • Standby toggle
  • Pilot Lamp
  • Fuse Holder

The picture at the top of this page is a composite photo of the panel, the Frankenstein aspect seems appropriate…

Internal:

  • 8 miniature 9-pin tubes sockets (it came with no tubes)
  • Handwriting on the chassis near the tube sockets indicates 1-12AU7, 5-12AX7, and 2-EL84 or 2-7189.  Both are indicated in different handwriting for the same sockets.
  • Power transformer may or may not be original
  • Original output transformer has been replaced with a TF110-48-UL from Triode
  • Two small open-frame transformers that appear identical
  • No Choke
  • Eyelet construction on rigid fiberboard circuit cards/boards, no printed traces, all hand-wired eyelet-to-eyelet.
  • Five circuit cards– Two of them contain the first-stage preamp components, 1 card for each channel.  These are closest, and connected, to the input jacks.  Each of these cards also have a transistor in a TO-5 package, a Texas Instruments 2N1273-5 which is a germanium small signal transistor.
  • An assortment of carbon comp resistors and miscellaneous capacitors.
  • Silicon diodes, no tube rectifier.   One board appears dedicated to rectification for both plate and bias voltages.
  • The rectification board is hanging loose, unbolted from the chassis.  The entire power supply area seems to have been the victim of a repair job that was never complete.  There are loose wires and “temporary” solder connections.
  • Two newer  multi-capacitor cans, JJ 500V 40/20/20/20, chassis mounted.  Holes have been chopped into the chassis to accept these.
  • 2 RCA jacks, one has been hand-inked red, probably to indicate it connects to the reverb tank input.  I’ve seen this in blackface Fenders.  The other would be for the reverb tank output.
  • The reverb tank is missing.
  • Chassis is  U-shaped bent steel with the face plate bolted across the top of the “U”.  Circuit boards are parallel to the face-plate and stacked two-deep.  Access is from the “sides”.  Photos Below.

It appears we have a push-pull EL84 output stage that should put out approximately 15 watts, coupled to a preamp with six tubes and two transistors.  That’s a lot of hardware for an old guitar amp.

One mystery is two small transformers, in addition to the power and output transformers.  They appear identical, have an air gap, and have two leads from each side, so I’m guessing they are both 1/2 to 1 watt single-ended output types.  One is certainly for the reverb amp, it is tied to one of the RCA jacks.  Initially I thought this might be an inter-stage transformer used as a phase inverter.  I have seen this on old Gibson amps, radios, and hi-fis) but never repaired an amp using one.  But, the transformer-phase-splitter circuits I’ve seen use a center-tapped transformer, which would have five leads, and these have only has four.  The suspense builds…

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Rear face:

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Looking up into the chassis:

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Detail of the rectifier board from the above – I think someone tried to work on this without taking the front panel off.  That means they would have to solder from the side, from this angle.  Challenging.  Note the “hanging” resistor, and components soldered high off the board.  Pretty sure someone was shooting in the dark with this job.

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Next Page:  Getting Started

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