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

12″ Speaker


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.


Lectrolab R600C

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.



Ex.2 sold on ebay 9-20-10: $334.69 USD



Why do harp players love Lectrolabs?  Look no further than here…




36 Responses to R600

  1. Wayne Baker says:

    Nice website. I see you found some pics of my R600B. Do you still need a schematic? I can’t find an email adress on the site to send it too.

    • alexage1 says:

      Wayne – Thank you VERY much for the schematic pics you’ve sent. The are posted in the gallery above, another fine addition to the Lectrolab knowledge base!


  2. rob roberge says:

    I have a R600 (and have had a 306A)…a GREAT amp. I also had a Lectrolab with 2 EL84’s and a single 12″ (that was labeled Panaramic, which is usually a Maggie off-brand). Sold a 950 to a friend (with 4 7189s’s as I recall?).

    And a fabulous twin 8″ speaker model with one EL84. Love these amps. The 600 is my favorite dual 6V6 amp in my collection. Light, loud enough to gig with…cranked it’s very Neil Young & Crazyhorse…turned down, it’s clean and sweet (can get that rich early Jimmy Bryant tone on a Tele neck pickup). Just incredible amps.

    Thanks for the cool blog. I’ve written about a couple of Lectrolabs over at myrareguitars.com. Still haven’t review the 600/Harmony 306A…wonderful amp. I like it better than a friend’s 57 Tweed Deluxe. Honestly.

    Great page–thanks


    PS: I also had one of those VERY rare (and very crappy) “re-issue” Lectrolabs that guy was doing about 5 years ago on ebay. Did you see those? I bought one once I saw the name, and at least GOT the amp (a lot of people were totally screwed and never received an ordered amp). But, other than as a conversation piece, it had nothing going for it.

    • alexage1 says:

      Rob – Thanks for the kind words. Your article on the S400 was inspirational when I started this blog – it was hard to find cogent thoughts about Lectrolab anywhere. Looking forward to your R600/H306A review. Please feel free to send a sneak preview here!

      All – in addition to being an accomplished writer, Rob plays guitar and sings with the Los Angeles area garage/punk bands The Violet Rays, The Danbury Shakes and LA’s legendary punk pioneers, the Urinals. He also restores and rebuilds vintage amplifiers and quack medical devices. For more info, visit http://robroberge.com/

  3. curt says:

    i have a r600c with a perfect schematic who do i email pics to

  4. Pingback: Schematic for a Regal R-1160 tube amp

  5. gmoon says:


    I’m the owner of the “lime green” R600–the one posted on Musicians Electronics Forum. [Ex. 7 above]

    I’m sending you a link to images of the original R600 schematic, cleaned up.Due to a computer meltdown I’ve been unable to log on to MEF, and they don’t respond to any password recovery queries. Otherwise, I’d have posted there first. But you’ve got a great little Lectrolab website going, and it belongs here…

    (I’ve got the amp largely rebuilt and it’s sweet…Now to remove the contact paper and restore the case.)


    • alexage1 says:

      Thanks, Doug. Really appreciate the schematic. Good job cleaning it up.

      Of all the ways that people have messed up Lectrolabs, and there are many, your lime green R600 with the flannel pajama grill cloth and matching green handle is my favorite. It will be a little sad when you take off the contact paper! Hopefully the contact paper protected the original covering over the years, but If you ever decide to recover it, the original material was referred to as “Irish Linen”. I think this may be a common material used in bookbinding (see it on book binding supply, or fabric supply sites) and still available today in different weights. It may also be call “Airline Linen” or something like that.

      • gmoon says:

        Thanks for the info on “Irish Linen”, George. There’s only a little remaining behind the schematic. It is definitely not typical tolex. Sourcing bookbinding supply houses is a great idea. I’ve seen similar treatments on old Kay or Harmony amps–more paper than tolex.

        It will be a little sad removing it’s green, cheesy glory…luv the tablecloth grill! (or pajama, as youcalled it.)

  6. Steve Hill says:

    Wonderful website on these low cost, undervalued amps.T hank you for putting up all the important info.. I have a R600B that I bought at a rummage sale in mint condition. It had two Bugel Boy 12ax7’s in it. Not many times do you buy an amp that pays for itself in two tubes. I experimented with 50’s P12R and Q’s which progressively made the amp much more robust and loud. Also tried some big ceramic speakers that pushed a lot of air. Very quiet and stompbox friendly. Jumping the channels works fantastic.

    • cnevins says:


      Hey in your opinion which of the AlNIco Jensens had better sound and response? My R600C has the original C12R and is a tad rattley above 7. Gonna set her aside and put it in either a special amp for ‘clean’ use only, or a 4 watt Supro Trojan. Don’t want to blow it. Still deciding on an alNIco or ceramic Jensen.


  7. cnevins says:

    Wow! Cool site…glad I found it. Just acquired a R600C and I’m absolutely in love with it. Been trying to find out as much about this beast as I can. It was kind of a fluke that I got it. Went to purchase another amp (Supro Trojan) and saw that sitting there. The guy said it needed work to fix the tone controls and tremolo, and would sell it for a quarter what it was worth to him. He said in top condition was worth $1200 (to him…not sure what it’s really worth) and he’d sell it to me for $300. Then he fired it up and there was nothing wrong with it…AT ALL!!! Needless to say, he was pretty upset, but said he’d honor his quote. I am super happy! It even has the original Jensen C12R driver. Since the top bottom and speaker panel are fiberboard (major tonerobber, IMHO), I’m planning on making a brand new cab out of high-end 9-ply 1/2″ plywood, or solid clear pine to see if I can make this girl sing even sweeter. I’m a cabinet maker so plan on making this identical to the original. I just need to try and find the same covering. I looks like a waxy linen or light canvas. Any input or advice in that? Thanks.

    • alexage1 says:

      Thanks for the comment! The covering was called “Irish Linen” in the mail order catalogs that sold many of these amps (See the Branding and Advertising page). “Airplane Linen” may be another description. Similar material may still used in bookbinding. I haven’t tried it, but bookbinding supply sites may be helpful, and would be the first place I would look. Pretty sure they thinly laquered the cabinet after covering, probably a very dilute mixture, probably nitrocelluose. Hope that helps.

      Good luck, and send photos when you’re done!


      • cnevins says:


        Hey thanks for the tips. I actually found a binding supply company that carry a couple types with different thread counts and durability. Here’s the link:


        $55 bucks a yard!! Whatever it takes I guess.

        What a gem, this Lectrolab is, and even better that they were a stand-alone company. Just makes them that much more unique and special with regard to sound. I’m still amazed at the quality of growly tone I can get with this old girl, and the tremolo…to die for. Any info on the foot switch? Mine was absent. How does that work being a 1/4″ phono plug-in? Have you come across another foot switch that works? Hope this wasn’t already answered in the site. Thanks!


      • alexage1 says:

        Foot Switch: I don’t have any photos of the original footswitch, but it is as simple as can be, and essentially standard, the same as almost any other amp you will find with a tremolo footswitch. Two wires/conductors travel from the 1/4” jack to a single-pole single-throw switch (SPST). When you close the switch it connects the two wires, and the tremolo turns off. This is because you just grounded the tremolo signal. If you have no switch plugged into the amp the tremolo is always on. There are many switch on the market that will work in the $20 to $30 range. Here’s one: http://accessories.musiciansfriend.com/product/Roland-FS1-Single-Footswitch?sku=421358
        Speaker: The C12R is not a high-power speaker, but “rattley” may be the result of something else. Be sure to connect the amp to a known high power speaker to see if the issue goes away before spending your money. Everyone has their opinions/tastes about speakers, so what to use is highly subjective. Here is a good, if old, shootout:
        Eminence makes some good speakers. If you click on the model name on this page it will take you to a page for that model that has sound samples:
        Finally, for what it’s worth, if there is any general consensus on 12” speakers for 15 watt amps, it is going to be that Celestion Blue is fantastic. They are pricey, but I love them!

  8. cnevins says:


    Hey, FYI, I also discovered another ‘Blue’ AlNico speaker contender that many forum posters and amp builders say is as great or better than a C.B…the WGS ‘Black and Blue.’ It’s been out for a couple years now and uses the same Mueller paper cone assembly used in UK C.B’s. WGS dubs it as a ‘PERFECT UPGRADE’ for UK AC15’s/30’s! It’s $75 cheaper, too. https://wgs4.com/content/BnB


    • alexage1 says:

      Hi Chris – Thanks for the speaker tip. The footswitch question bothered me a bit because, well shucks, I don’t have any pictures of an original Lectrolab footswitch! BUT, I dug around a bit and found an excellent pic of a Harmony H-306A footswitch, which is now posted in the gallery above.

      As we know, the Harmony was built by Sound Projects (Lectrolab) and is identical to the R600, so we may guess that the Lectrolab footswitch was the same as well, although probably painted a different color. In any case, one must dig this switch, it is so art deco, and the tension relief spring looks like the same sort of thing used on old clothes irons. Very cool.

  9. Paul says:

    here is a Lectrolab 600 S which I haven’t seen before , it is on ebay.com now item #


  10. Chris Nevins says:

    Hey, has anyone on here ever cloned one of these amps? From what I’ve read, these amps were built with ‘middle of the road’ components. I’m considering building my own version of an R600C I currently own, using premium components, just to see if I can improve on an already amazing amp.

  11. sam babbitt says:

    The photos of the “messed with” one is mine. I bought it, rebuilt with a new pine cab and raw tweed covering it. It had a real dog of a 12 inch in it. I didn’t really see the point of having the controls on the opposite side of the speaker so I pulled out the shit 12 inch, built a new baffle board and installed two 8 inch speakers I got from a basket case Premier Super-8. Then I varnished the raw tweed cloth and now it sounds like Dave Davies personal weapon of choice and it looks old enough to know the difference.

  12. Colin Odden says:

    Hi. I’m looking for advice on capacitor values. I’ve got an R600C (happy to share pics, but mine’s not as nice/clean looking as the pics you’ve already got), and I’m trying to figure out which caps are likely replacement candidates (the amp’s not been powered on for at least a decade).

    The big paper cap is a single 40MFD 250V. I’m guessing MFD here means microfarads, not milifarads, but is that right?

    There’s also a set of caps that are axial-lead, red, and are also marked mfd. Some are .01mfd, some .05mfd, and are rated either 200v or 400v. Are these also mfd=microfarads?

    Many thanks, in advance, for any advice.


    • alexage1 says:

      Hi Chris – Capacitor value abbreviations have changed since the 1960’s:
      Old: MFD = New: uF = Microfarads = Farad/1,000,000
      Old: MMFD = New = pF PicoFarads =Farad/1,000,000,000,000
      nf is also in vogue, = Farad/1,000,000,000
      So 1MFD = 1000nF = 1000000pF, or perhaps more usefully:
      1000pF = 1nF = .001uf or MFD
      It can get confusing. There are also many other cryptic ways capacitors are marked!
      In any case, 40MFD is a large capacitor, I think a paper cap of this size might need to be the size of a beer can, or larger! Are you sure you’re not referring to an electrolytic cap in an aluminum can sitting atop (not inside) the chassis, like a tube?
      A paper capacitor would look like this: http://antiqueradio.org/art/recap05.jpg
      The only 40MFD cap in the R600C is part of the power supply, feeding the screens of the EL84 tubes (per the schematic above) at ~340 volts. That would be in a can with two other capacitors labeled 40/20/20mfd of something like that, and would be rated higher than 250V, should be at least 400V.
      Perhaps there’s a hard-to-see decimal on the paper cap?

      The axial lead caps marked .01mfd or .05mfd are expressing the value in microfarads.

      I suggest you look around the web to find advice on powering up old amps, or even better, take it to a tech who has experience. Sometimes they power up just fine, often there are issues. Replacing caps just-in-case can’t hurt (IF you know what you’re doing, it can hurt a lot if you don’t!) but will not insure the amp will light up without a problem.

      Hope that helps,


      • Colin Odden says:

        Thanks for the reply. It’s true that tube amps of this era are new territory for me, but I’ve got a variac and have a little experience bringing somewhat newer stuff back from years of disuse. The mf=uf thing is what had me worried.

        On the other hand, I wasn’t smart enough to turn the cardboard cover on the “paper” cap enough to see that it clearly says “Sprague ‘Atoms’ Dry Electrolytic.” So, shame on me. 40MFD (=40uf), 250V, surge 300V.

        Incidentally, I found/scanned the schematic to my R600C, which I’m happy to share if there’s a place to send it.

        Thanks again,

      • alexage1 says:

        Hi Colin – glad you solved the mystery! If you replace that cap I suggest you go with something closer to a 450 volt rating. -George

      • alexage1 says:

        Oh yes! Please send that schematic to me at lectrolab@gmail.com or simply click below “Contact Me” at the upper right of the page. It will be great to get a better resolution R600C schematic than the one already posted. Curt – I’ll get that posted for you ASAP.

  13. curt says:

    Well after 2 yrs and some help from the best tube amp guy I know at JP Electronics http://m.facebook.com/JpElectronics?v=info&__user=100003393278965 my R600c lives
    again !! Thx Jesus

    I was wondering if I could get a copy of the schematic Colin has for the R600c to compare to mine we found a few diff things in the amp than on schem and looking at pics on site here theres a few things diff on mine than those shown

    Thx Curt

  14. Colin Odden says:

    Dragging this out a little further: I replaced all the electrolytics in my R600C, cleaned the jacks, pots and tube sockets, then promptly snapped one of the speaker wires right at the OT such that it’s beyond repair. At George’s excellent suggestion I bought a Hammond 1760E (with 4/8/16 ohm taps, whereas the 1750E has just an 8ohm tap). The screw spacing is identical and size, otherwise, is basically the same. Plenty of lead on the 1760E to reach everywhere it needs to go.

    • Colin Odden says:

      I’ve decided to let go of my R600C, purely out of financial need and because I know there’s probably someone out there who can afford to get it in the shape it deserves to be in. The second channel’s out and the 6AU6 tremelo driver tube might be bad. It’s got cabinet wear commensurate with age. Otherwise pretty great, and the aforementioned 1760E means you could run alternative (ie different impedance) speakers or cabs with it. Cheers!

  15. Ck says:

    Any idea where I can get on of these tone beasts?

  16. Hawkeye says:

    I found a Harmony 306A in my local music store. It was paired with a matching outboard reverb unit. I’ve sent you an email with pictures!

  17. Brad Linzy says:

    Just wanted to inform you all that the Harmony H415, I have found out, is a Lectrolab, and is pretty much the equivalent of an R600C or S600 Lectrolab model. There are a few slight differences, such as a 5Y3 rectifier in the Harmony, but essentially they are the same – same tube compliment otherwise, same exact tremolo circuit, same 390k and 2.7M grid load resistors on V1! So the Harmony H415 – touted by Ken Fisher of Trainwreck amps as a “sleeper” – is a 212 Lectrolab R600C.

  18. Jared Stephens says:

    Thanks for this info. I had an R600c actually given to me. Great condition. This really helps me out! Great site.

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