Technology

All Lectrolab amps are tube amps.  The last Lectrolab model we are aware of, the S950, used a transistor as the first stage preamplifier but the rest of the amp had eight vacuum tubes. Except for diodes used as rectifiers, this is the only known use of sold-state devices in a Lectrolab amp.

Design

Lectrolab amps were well-designed for the standards of their time and their intended purpose.  From today’s perspective we would see safety issues such as questionable grounding, lack of a three-prong cord, and easy access to tubes (is that really so bad?).  See more about old amps and saftey at Widowmaker?.  Most Lectrolab designs were similar to many other guitar amplifiers produced at the time.  They are similar to many amplifiers produced today!  Some designs are unique in my experience with guitar amp circuits (see S950). 

Lectrolab never made what we would consider a high output amp.  They focused on single-ended “practice amps” on the low-power end, up to push-pull dual 6V6 or 7189 (EL84) amps on the high end.  They claimed output ratings ranging from 3 watts to 25 watts.  To get 25 watts out of a Lectrolab, the sun would have to be shining just right, you’d need the wind at your back, and your electric company would need to be running the grid a tad on the hot side!  18 or 20 optimistic watts is probably a better expectation.

  • Many Lectrolab amps had tremolo, injected at different points in the circuit depending on the model.  “Bias tremolo” was most often used.  Only one model (S950) had reverb.
  • Most amps were “self-biased” which is to say they were cathode-biased.  Later models applied a fixed bias voltage to the output tube grids.
  • A variety of tubes were used in the preamp sections including 12AX7, 12AY7, 6SQ7, 12AU6, 12J5, 12BF6, 6SN7, 6SH7 and 6AU6.
  • All earlier models used tube rectification, later models used diodes. Tubes in this function included 5Y3, 35W4, 35Z5, 35X5, 6V4, 6CA4, AND 6X5

Electrical Construction

In the Lectrolab amps I have seen, circuitry is of sturdy construction and excellent workmanship, with point-to-point or eyelet circuit board construction.  None of their amps used a printed circuit board.  Heater wiring was loosely twisted and run along the chassis.  Grounding schemas appear random, although there was probably some experience-guided (hum reducing) reasoning behind it.  Assembly was high wuality. It would be unusual to see a solder failure not caused by trauma, or a misguided attempt to modify or repair the amp.  Chassis construction was partially enclosed steel, with the wooden cabinet often completing enclosure of the electronics.  As most Fender amps prove, there are sturdier albeit more expensive approaches to this.  Tube sockets were riveted directly to the steel chassis.  Pots were chassis mounted and wired to the other components, except in later models where long pot leads were directly soldered to circuit board eyelets.  Even this arrangement is much easier to service than today’s “modern” amps, such as a Fender Hot Rod Deville and others of it’s ilk.

Cabinetry

On earlier Lectrolab amps the cabinet material is lightweight fiberboard, sometimes ridged on one side.  This is not a particularly sturdy material, and causes some reviewers to complain that the amp is made of “cardboard”!  Suffice to say you would not want to take one of these out on a rainy day.  Later models were made of solid pine or plywood, and are quite sturdy.

Cabinets were covered in a variety of materials and colors.  Smaller older models have a greyish (paper?) covering with wood-like textured finish. Late 1950’s, early 1960’s models had tweed style covering.  Mid-1960’s models featured “Irish Linen” per catalog descriptions.  Late models came in a thin black tolex type of covering.

Transformers

Many people say the most sonically important components of a tube amplifier are the transformers and speakers.  American transformer quality was generally excellent in the 1950’s and 1960’s.  Heavy duty high voltage transformers were widely used in consumer products.  The iron was high grade and there was a large and skilled labor market.  The Chicago area was home to many transformer companies.  Transportation was expensive so manufacturers sought local suppliers, especially for heavy or bulky components.  Lectrolab used transformers that were manufactured nearby in the Chicago area.

Speakers

Speakers from that time and place are also widely respected to this day among guitar amp aficionados.  Lectrolab used Chicago-made Jensen speakers in most of their amps. These, not the current Italian Jensens, are still prized by guitar players.  Quam was another speaker provider, located in Quincy, Illinois.

Other Components

After transformers and speakers, tube amps are comprised of rectifiers, resistors, capacitors, and tubes of course.  Lectrolab used the typical materials of the day, which were all good, though some have life spans that may need to be managed in your Lectrolab amp.  Capacitors, especially electrolytics, can leak and require replacement, occasionally a resistor will be faulty, pots can wear out, etc.  Every Lectrolab was fully functional as designed and built, and should be able to function perfectly for the indefinite future, given some basic maintenance.

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5 Responses to Technology

  1. I’ve been browsing online more than 3 hours today, yet I never found any interesting article like yours. It is pretty worth enough for me. Personally, if all website owners and bloggers made good content as you did, the internet will be much more useful than ever… Johnetta Riola

  2. mj says:

    Hi I just bought a Lectrolab 200B amp on ebay….almost by accident ! I made a low bid for fun and it was enough to win. Not having yet seen it I think my plan will be to restore it as close as I can with the neccessary safety improvements.

  3. Dave says:

    Looking for tube specs for a K100. I recently purchased one that has been restored. I’d like to find another set of tubes but a few of the tubes do not have any identification which I can find a source for. Suggestions for schematics or tube identification. This site is great! Thank you.

  4. James Dawson says:

    Just picked up ’54 (based upon date code on the Jensen speaker) R500C at a vintage music store in St Paul Minn. It has the cardboard cabinet. Wear and tear is nominal, considering its age. No hum, but there is some intermittent crackle after it gets warmed up. All the tubes glow orange, not blue, and the pilot light dims when the crackling occurs. Wondering if I should try new tubes or just learn to live with the crackle?

    • alexage1 says:

      Pilot light dims when crackle occurs. That’s concerning because it suggests something is drawing a lot of current, enough to drag down the power transformer, when it is “crackling”. Someone may have better, or more ideas, about the cause of this, but I suggest:

      1. Take it to a reputable repair person who will go through the entire amp and sort out anything that needs it. Likely you’ll want to futz with it before parting with cash, so…
      2. Make sure the fuse is the correct value, so if something bad is happening the fuse will (hopefully) self-destruct before something more expensive does.
      3. Try replacing the output tubes – 6BQ5 or EL84 – There may be an intermittent internal short in one, which will draw much current and stress the power transformer, dimming the pilot. If that fixes it, still take it to a qualified person to have a look at the screen resistors, which may have been stressed by this as well.
      4. If that doesn’t change anything try replacing the rectifier tube – 6CA4 or EZ81
      5. It’s almost certainly not the preamp tubes, these rarely fail in a way that would dim the pilot light, but you never know, stranger things have happened!
      6. All manner of other problems could be the cause, like dirty or arcing tube sockets, but you’d need to open it up to deal with that, and you should not do that if you are not experienced because it could kill you.
      7. If it’s still crackling, go back to step one and stay there.

      Good luck!

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