Tubes: 2-12AX7; 2-6BQ5; 1-6CA4
Click to enlarge:
Ex. 1 sold on eBay for $285 9/15/2010. The seller described it as:
Vintage early 60’s LECTROLAB tube amp We have owned a bunch of Lectrolabs over time and have two left… This one is a just a great sounding small room or recording amp… 2 X EL84 in the power section into a 12″ Alnico magnet speaker… 4 inputs so you can jump them and blend the two channels… This has been gone over by American Music here in Seattle and is ready to play… Speaker is vintage ROLA low powered Alnico 12-15 watts…check out VG article this month on alternative speakers… Tubes are new power tubes with a mix of new and possibly original preamp tubes… Great raw sounding EL-84 amp…way cooler than a new Chinese AC4
Way cooler than a lot of things, my friend…
Example 2 was a Craigslist posting brought to our attention by Jim Kachel, a Lectrolab S600 owner. I thought it was possible that examples 1 and 2 were the same amp until Papa (who now owns example 2) set us straight – his comment is below.
And finally, Phil, the third S500 owner discovered here, has sent us a schematic. Thanks to all you!
Now that we have a schematic we can comment on the design:
- 1st gain stage is a 12AX7, standard Fender-type design but without a cathode bypass capacitor, so lower gain. Jacks 1 and 2 feed one triode, 3 and 4 the other triode. So two preamp channels, identical except for the grid leak resistors. 1/2 share a 390K, 3/4 a 2.7M. This means the second channel, which is input jacks 3 and 4, should have more gain than the first channel.
- After the first stage the signal passes through a simple tone circuit (treble attenuation) and volume control to the second stage. I think the schematic is missing a ground symbol that would connect to the junction of the tone and volume controls as well as the second stage cathode resistor.
- The second gain stage is another standard 12AX7 triode amplifier, again with no cathode bypass capacitor.
- That uses up 1-1/2 12AX7’s so far. The second half of the second tube is a standard concertina phase splitter which feeds…
- a 6BQ5 (EL84) push-pull output pair, the power amplifier. Again, a typical arrangement with 100K grid leak resistors, no grid stop resistors. The screens are fed through a single 680 ohm resistor from B+, which according to penciled notes on the schematic is 307 volts. The screen resistor drops that to 296 volts on the screens.
- Output tube bias is fixed, at -10.7 volts according to pencil marks, and derived from a voltage divider on the B+ winding of the power transformer. Most 2-EL84 amps are cathode-biased. Initially this because it was cheaper due to fewer components, and subsequently because most of these designs now slavishly copy AC15’s. Fender Blues Jr. is a current 2-EL84 amp that has a fixed bias. Neither approach is inherently better or worse, they both sound fine, and tone differences between them are subtle.
- There is no negative feedback loop (NFB) around the power amplifier. This is not subtle. NFB reduces both gain and distortion. The Fender “sparkly clean” sound is dependent on NFB, and the Vox “grit” relies on its absence. Other design choices are at play in both amps, but this is a significant factor in tone.
- One side of the 6.3 volt heater supply is “lifted” above ground by another voltage divider shown as “A” on the schematic. I estimate this lift at about 22 volts DC (47K/390K*182V), and it is there to reduce hum from the AC heater wiring.
All in all, this is a fine, respectable circuit that should last a few lifetimes and is in the same class as a Vox AC15 or Matchless Spitfire or any number of 2-EL84 amps. Well… one difference is that you can buy it (if you can find it) for under $300! And a caveat here is that I don’t know how the transformers, which are important components, compare with other amps. All the blather you read about resistor and capacitor types is a just that, any changes (except for replacing bad ones) in that area is at best a very, very thin layer of icing on the cake. But it’s religion for some folks.
Can you tweak this amp? Sure! If it’s like every other Lectrolab I’ve seen it’s not hard to work on it, assuming you know how to work on amps. If you don’t, then send it to someone who does. Otherwise, the people who rely on you being alive may come to me for damages! Choices are limitless but some mods might be:
- Add a three prong AC plug. Always a good idea.
- Goose the gain by adding bypass capacitors to the first and/or second stages
- Add a resistor for each 6BQ5, replacing the one that feeds both. These resistors can be increased in value to provide greater protection to the tubes, and should be flame resistant in either case.
- Add a grid stop resistor to the output tubes to increase high frequency stability. 10k will probably do it.
- Possibly get rid of C5, which shunts high frequencies around R14 in an effort to improve HF stability. It may or may not be required.
- Replace R26 with a resistor and trimpot that will give you come control of the bias voltage.
- Go crazy – rewire the two channels of stage one to be in series, thus adding another gain stage to the circuit and creating a metal-worthy gain monster. You’ll need another volume control in there.
But seriously, folks. None of this is necessary. There is probably enough gain in the amp to drive it into some very sweet power tube distortion, and it should just fine if you back off the volume and play it clean. If the amp is working then it’s been working for over 40 years. If the electrolytics are good, the amp should sound great.
S500 owners: What do they sound like? Please post a comment below.