Lectrolab R600: 2-12AY7; 1-6SJ7; 2-6V6; 1-6X5
Lectrolab R600B: 2-12AX7; 1-6SH7; 2-6V6; 1-5Y3
Lectrolab R600C: 2-12AX7; 2-6BQ5; 1-6AU6; 1-6CA4
Lectrolab R600B = Harmony H-306A
Up until now (October 2010) it has not been possible to find a Lectrolab R600B schematic on the internet. However, Wayne Baker of Virginia noted our plight and sent in a photo of the schematic glued inside his R600B. This is in the gallery below. As old and chewed up as it is, it is almost 100% legible. Looking at it, you can see that not only is the Lectrolab R600B circuit identical to the Harmony H-306A, but the drawing itself is identical (they did these things by hand back then). Only the title is changed. We can now be sure that Lectrolab aka Sound Projects Co. built the H-306A for Harmony. Case closed. Thank you, Wayne!
Small World Dept: It turns out that Wayne’s R600B (ex. 13 below) is the very amp shown in the header on every page of this site. I found that photo in an forum posting of his from a few years ago (yes, I troll and scrape for every Lectrolab photo that can possibly be found on the web). I thought his chickenhead-knobbed amp was one of the coolest looking things I’d ever seen.
Lectrolab R600 vs. Lectrolab R600B
- Different tubes. Both have a 6V6 output pair, but the R600 calls for two 12AY7’s in the preamp versus the R600B’s 12AX7’s. You can use one for the other, but the 12AX7’s have higher gain. Both use a pentode for the tremolo, a 6SH7 in the Harmony, a 6SJ7 in the R600 . The R600B uses a 5Y3 rectifier while the R600 uses a 6X5 (slightly less power capability).
- Higher filter capacitor values in the R600B.
- Different phase inverter designs. The schematics are deceptively similar in they way they are mapped on the page. But the R600B uses a cathodyne, or concertina, inverter while the R600 boasts a paraphase inverter.
- The R600 grounds one side of the 6.3V AC heater circuit. The R600B instead connects one side of the heater circuit to the output tube cathodes, atop the the cathode resistor and bypass cap. This point maintains a fairly steady DC voltage of approximately 20V and therefore “elevates” the heater AC, eliminating it as a source of hum injection to the audio path. This is a “trick” which many techs have used as a modification to reduce hum in cathode-biased amps. Note that there are many other sources of hum and this is not guaranteed to work unless you know it is coming from the heater circuit!
These differences aside, the similarity of the schematics suggest that the R600B was a revision of the R600.
The Lectrolab R600C is a significant redesign of the Lectrolab R600 and R600B. using 6BQ5 (EL84) output tubes, and a 6AU6 for tremolo. We finally found a schematic for this, thanks to Curt Hambright, who had a R600C sitting in his basement for years! Thanks, Curt, and I hope you get that thing fired up okay! Some notes on the circuit:
- 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 there are two preamp channels, identical except for the grid leak resistors. Inputs 1 and 2 share a 390K, inputs 3 and 4 share a 2.7M. This means the second channel, inputs 3 and 4, should have more gain than the first channel. This is virtually identical to the input stage of the Lectrolab S500, and likely the S600 also.
- The second gain stage is another standard half-of-a-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 12AX7 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 500 ohm resistor from B+, which according to the schematic is 345 volts. The screen resistor drops that to 340 volts on the screens.
- Output tube bias is fixed at -15 volts per the schematic, and derived from a voltage divider on the B+ winding of the power transformer. Most 2-EL84 amps are cathode-biased, as are the R600 and R600B Initially this because it was cheaper due to fewer components, and subsequently because so many designs 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 amp styles, but this is a significant factor in tone.
- One side of the 6.3 volt heater supply is “lifted” above ground by a voltage divider consisting of a 390K and 47K resistor connecting to the supply voltage at a point in the circuit indicated as 220 volts . I estimate this lift at about 27 volts DC (47K/390K*220V). Its purpose is to reduce hum from the AC heater wiring. Same concept as the R600B heater “lift”, but an alternative design solution.
- Tremolo duties are carried out by the sharp cut-off pentode 6AU6. Typical tremolo oscillator circuit design, with footswitch. The modulation signal is fed into the output tubes through their grids, making this is a “bias tremolo”. A somewhat unusual aspect of this circuit is the “bias tremolo” is fed into a fixed bias output stage. Sounds contradictory but it’s not. A similar arrangement is used in Fender’s brown Vibroverb and blonde Tremolux, which are prized for their tremolo sound. You can knock yourself out learning about bias tremolo circuits here.
Why do harp players love Lectrolabs? Look no further than here…