1. Getting Started

Plan

Document the “As-Is”

  • Disassemble all components from front panel (all connections are not visible)
  • Separate stacked circuit boards
  • Trace circuits and document
  • Create a schematic of the amp in the state it is in now.
  • Note date codes and brands where possible
  • Determine reverb tank input/output impedances
  • Determine how the reverb pan was installed in cab, and what size it was

Define As-Is Circuit Issues

  • “Disconnects”
  • Obvious changes to original circuit, i.e. filter cap values, added components, etc.

Create “To-Be” Schematic

  • Obtain or infer the original architecture and values of the power supply.   This area of the amp has been altered.
  • Preamp seems to be intact and probably will not change much from As-Is
  • I’m hoping the power amp circuit (with exception of power supply connections) will not change from As-Is
  • Obtain replacement reverb tank,. If not available: determine cabinet fit and circuit changes that may be required to accommodate an available tank.
  • Confirm tube types
  • Refine heater circuit as required
  • Insure grounding is safe, three-prong cord, etc.
  • Review schematic for viability and mistakes
  • Post schematic for feedback

Test components for functionality:

  • Transformers
  • Tube Sockets
  • Transistors
  • Resistors and caps (in circuit)
  • Pots
  • Jacks

Repair & Refurbish

  • Source and obtain parts as needed
  • Clean Chassis, circuit boards, knobs and cabinet covering
  • Install To-Be Components
  • Add gaskets for chassis through-hole wiring
  • Test heater circuit
  • Test B+ power supply
  • Test power amplifier
  • Test preamplifier
  • With entire amp powered up, check and note voltages
  • Sound check
  • Debug and Retest
  • Verify and publish final schematic
  • Publish sound sample files

Document the As-Is

TAKE IT APART!

Disassembled all components from front panel – Removed jack nuts, knobs and pots nuts, switch nuts, unsoldered and removed pilot light and fuse nuts. The panel now removes from the chassis giving us a “head-on” view of the circuitry, below.  Note that each row of pots attaches to a circuit card.  The two cards are stuffed from the “outside”, and oppose each other on the “bottoms”.  There are wires soldered into the bottom of the cards, access is a challange.  The separate rectification card issues are easier to see.

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We can see it face-on with the front panel removed.  The “Rectification Card” will need to be rectified.

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Front view with the panel removed (Click any of these images to enlarge):

The circuit cards are held in place by the mechanical connection of the jacks and pots to the front panel.  Note the input stage cards, with the input jacks soldered to them, are adrift with the panel out.  The two cards with most of the circuitry on them, including the pots, are more stable without the front panel because of numerous connections to the tube sockets.  This is not the most repair-friendly approach one might imagine.  The more you work on stuff like this, the more you appreciate Leo Fender.

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Pot leads attach directly to circuit boards:

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Not much room to fiddle around between these two boards:


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Pots and Dating

Pot codes are now readable. They are

  • 304-6538 (Qty: 1)
  • 304-6552 (Qty: 7)

This indicates the pots were made by Stackpole , most of them in the last week of 1965 . The pots all look original since their connection to the circuit board appear to be original solder joints. In that case, the amp could not have been made before 1966. Assuming good inventory control practices on the part of both Stackpole and Lectrolab, we may estimate that the amp was manufactured in 1966. If Lectrolab bought, or Stackpole made, very large quantities at one time and/or sold fewer than anticipated, the date of manufacture could be much later. For example, Fender used Stackpole pots dated from 1966 on amps constructed into the early seventies So “assuming good inventory control practices”, especially as far back as the sixties, may not be a good assumption. Stackpole was founded in 1926, and appears to be in business as of this writing, 2010 (http://www.seielect.com/), although they no longer sell potentiometers. I do not know when they stopped selling them. I do not know when Lectrolab ceased to exist, but judging from a number of sources they may have lasted into the early seventies.

Pictures of  a Lectrolab R700 in remarkably good condition are posted at http://www.flickr.com/photos/pacoblues/3362254449/in/set-72157615399206203/ This amp shares many attributes with the S 950, including style of circuit board construction, identical plastic and electrolytic capacitors, chassis construction, and more.  The R700 chassis is clearly marked “AUG 18 1966”.  Perhaps the S models were made concurrently with the R models, perhaps the S succeeded the R.  Who knows?

All we can infer with available data is that the amp was made between 1966 and the early seventies.

I am amused to see Stackpole pots from this period for sale on eBay for over $200 USD as I write this. This is because they were used in Fender guitars of that period. Sadly, the pots used in this amp have circuit board vs. eyelet connectors and so are not collectible, else I would gladly sell the lot of them for over $1500, and replace them with perfectly good new pots. Going through this exercise makes me realize that, given the prices for old components that were used in Fender and Gibson guitars, people must be seeking out any type of old equipment that housed these components (table radios, hi-fis, organs, test equipment, car radios, etc.) and harvesting them.

Other Stuff

  • Jacks – All are Switchcraft, both the one open frame jack (Reverb/Tremolo footswitch) and four “Patent Applied For” enclosed input jacks.
  • Switches – Both the On/Off and Standby switch are SPST, and appear to be the “standard” Fender type from the sixties. One is marked “Carling” and the other is marked “C”. They appear identical.

No new anomalies have appeared. Except for the power supply, which has been the victim of an abandoned repair job, the balance of the circuitry still appears to be original. There is one component that has been snipped out of the Channel 2 preamp board, its cut leads are still in place. It appears to have been replaced with a resistor that was not installed through the existing eyelets, but rather “tacked-on” to the leads that were already there. While certainly “tacky”, this type of repair approach can be understood in the context of time. If the tech wanted to just “get the amp working” as fast as possible, then the front panel would have stayed on, access to boards would be limited, solder removal from existing eyelets (for proper replacement and resoldering of components) impossible.

Channel Two First Stage Preamp Card detail.  You gotta love sockets for transistors (the brown base below the can).  You can pop them in and out like tubes:

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Top view of the same board.  I’m guessing that stamp of Texas was put there when Stevie Ray Vaughan was about twelve years old.

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Most of the components are original and the construction is of high quality.  Note the two “mystery” transformers mentioned earlier.  Well, one is surely for Reverb, whichever one isn’t is a mystery:

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Trace Circuits and Draw Schematic

A lot of work still to do here, but a few preliminary observations:

The amp is fixed bias, which indicates (beyond a design choice) the willingness for a greater investment, since these cost more to build than cathode-biased amps do. All the Lectrolab schematics I’ve seen to date are cathode-biased. I’m guessing this was a top of the line amp for them.

  • The rest of the output section looks very conventional, like most push-pull pentode guitar amps.
  • 1K grid stops
  • 220K grid/bias resistors
  • cathode straight to ground,
  • a single 1K resistor for both screens, I think.

The output tube screens were not attached to any voltage source. Their circuit terminated in a 1K resistor with one end “flying”. A result of a previous repair attempt abandoned, I hope. We want some voltage on the screens!

A literal “hack job” on the chassis to install two electrolytic cap cans. In some places too much chassis was hacked away. In others, positive terminals carrying high voltages are dangerously close to the gounded chassis. Yikes.

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Bottom side of the filter capacitor cans viewed from inside the chassis:

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The original output transformer had four mounting bolts. Judging by the placement of the original bolt holes on the chassis, that would have been a pretty good-sized transformer, probably larger than the replacement that is on it now.

I noted that the heater wiring at one of the output tubes was grounded to the chassis, apparently at the factory.  I also noted someone added two 100k resistors to ground at each end of the heater secondary where it leaves the transformer. This is a common mod to reduce hum in heater circuits without a center tapped transformer, like this Lectrolab, although they are best placed furthest from the source, usually off the first preamp tube. BUT , given that the circuit goes on to be grounded on one side, I don’t see how this could have reduced hum! Hmmm.

There is a component I didn’t recognize in numerous places on the boards. It’s a hollow tube, with two leads that each coil around the tube but don’t touch each other. So it looks like a two little side-by-side coils, each with one end disconnected. All were color-coded the same.  Judging by their placement in the circuit, I was pretty sure they were low value capacitors.   I posted a picture and question about them at diyaudio.com and promptly received an answer.  They are capacitors, and without going into the color coding details (there was little standardization on this for capacitors), I can tell you the consensus is they are 40pf.

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Old style capacitors:

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The circuit board layout is a hairball. Hard to unravel because of the number of connections made on the underside (blind side) of the boards. These “undersides” face each other and cannot be pulled very far apart without unsoldering a number of wires that connect them, as well as stressing the wiring from the board to the sockets and pots. So I’m in there with a tiny light and a magnifying glass, secure in the knowledge that if I go blind, the Lectrolab Preservation Institute and Foundation will provide disability payments.

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Can I complain some more?  Hard to get in there:

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As with most old amps, the thing is dir-tee, sporting a fine layer of grease-embedded dust on everything throughout the inside of the amp. Even after spraying it with compressed air, the black wires look white, and I need to wipe off resistors to make out the color code. My advanced technique for this last task is to lick the end of my finger and rub the resistor. After doing this many times in succession,  I’ve developed a taste for this amp, and can report that it delivers a palette of old solder, resin, wood glue, and pressboard out-gassing, shaded with a just a hint of tobacco, musty-factory, and old-clothing-closet overtones. It finishes with a bouquet of assorted vintage cooking oils and fried animal fat dispersed atmospherically for thin layer deposition throughout the interior. So you know it’s going to sound good.

The boards themselves are 1/16” material, stiff and hard like fiberglass, not the flexible tarpaper stuff in old Fenders.

At first glance, the phase inverter is starting to look like something off the beaten path.  May not be the usual long-tailed pair or concertina splitter.  More later.

Next Page: Circuit Tracing and Block Diagram

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