• R500: 2-12AY7; 2-6V6; 1-5Y3GT
  • R500B: 2-12AX7; 2-6V6; 1-5Y3GT
  • R500C: 2-12AX7; 2-6BQ5 (EL84); 1-6CA4 (EZ81)
  • Speaker: 1-12″

The Lectrolab R200, R300 and R400 are all firmly ensconced in the “student” amplifier category.  They were single-ended designs with one 6V6 output tube, or similar.  They were similar to a Fender Champ in this regard.  

Lectrolab upped the ante with the R500, which sports a 12″ speaker.  Like the R400B it has individual first stage channels for instrument and mic. Each of these use half of a 12AY7, and have their own volume control.  The signal from the first stages are mixed and then sent through a paraphase inverter (another 12AY7). The inverter output drives a push-pull pair of 6V6 output tubes, with a shared tone control along the way. 

The Lectrolab R500B replaced the paraphase inverter with a concertina, or cathodyne inverter, perhaps in a quest for improved fidelity.  It also added a fourth input, which did not provide any further functionality, it still had two individual channel first stages, one for inputs one and two, the other for inputs three and four.  The control panel remained the same, two Volume and one Tone control.  I do not have a clear schematic for this one. 

The Harmony H305A is the same amp as the Lectrolab R500BSound Projects Company must have produced these for Harmony

The Lectrolab R500C adds another Tone control, so there is one for each channel. I do not have a schematic for this.  The tube compliment has changed.  You can see in the pictures below that the Lectrolab 500C has 9-pin tubes throughout, which means the 5y3 rectifier has been replaced with a probable 6CA4/EZ81, and the 6V6 output tubes with 6BQ5, 7189, or EL84.  In addition, the R500C is reputed to have fixed bias output tubes (see below).  All this makes it a very different amp than the R500 and R500B, although there is little cosmetic change.   It is very probable that the R500C is electrically identical to the S500.  You can see a schematic for that here, and a parts list here.

If a tweed Fender Deluxe is your kind of thing, you are probably going to like the R500.  Quite a step up from the R300 and R400.  

We have no schematic for the R500CPlease let me know if you have one pasted inside your amp!  However, it is very possible that, like other Lectrolab models, the RxxxC model is identical to the Sxxx model.  Therefore, the R500C may have the same circuitry as the S500. See the S500 page for photos of the S500 schematic and parts list.

Click on pics to enlarge:



Ex. 10 in the gallery above had this info on the page where it was listed for sale:

I purchased this amp from Robert A. Irvine he is a friend of mine and an inductee of the Rockabilly Hall Fame. He is still a great player and singer and a great person to be around.  Any case he was cleaning out his basement and found this old amp he used to use for small gigs and and jammin with friends.”

Robert Irvine was inductee #342 in the Rockabilly Hall of Fame.  He wrote and recorded ” Fastest Short in Town (Blue Fly) ” and “Lonely One ” for Chicago label Presto Records in 1964.  Listen to this video – It could be a Lectrolab you’re hearing!  The sonic signature is very much in the Lectrolab ballpark.



All those amps… But it’s the little brown Lectrolab making all that noise:


Q: Is it possible to get a Lectrolab R500C for free?

A: Yes, if you are lucky!

It’s a shame Lectrolabs don’t get the respect they so richly deserve, but it can be a blessing for the astute.  See an interesting article from a lucky man here:



Lectrolab amp breaks all the rules  

(from Robbie and Laura Reynolds at http://music.mylounge.com/t205184-lectrolab-amp-breaks-all-the-rules. 08-18-2006)

I had the pleasure of looking inside a Lectrolab R500-C and drawing a schematic of it. If you’re not familiar with this amp, it is one of the greatest super-blues-tone amps you can ever get. It has the deepest, thickest sound of any amp I’ve ever heard. Not only that, but it is also fairly loud before feedback.  

The strange thing about this amp is that it breaks several of the “rules” that you hear about good harp amps. The preamp consists of two 12AX7s. It sounds great this way, so there is no need to switch to lower gain tubes. Our buddy Gerald Weber states authoritatively that cathode biased power tubes are the way to go for a harp amp, but the Lectrolab uses a very strange -15 volt bias supply taken straight off of one of the high power leads at the rectifier.  

Another thing is the coupling caps. Everybody seems to agree that .1 uF capacitors let more bass through, but the Lectrolab uses .01 uF.  Regardless of what anybody says about .01 uF caps, this is a very deep-sounding amp. Another rule that it breaks is the one about ground loops caused by using the chassis as a ground path. Things are grounded willy-nilly all over the place, and the filament leads aren’t twisted together. This is supposed to cause a hum nightmare, but the amp is almost silent.  

I have an amp of my own design of a similar power rating and
configuration. It’s a pretty decent amp, but not as good as the ones I’ve been building lately, so it’s probably destined to be cannibalized sooner or later anyway. I’ve been thinking that I ought to turn it into a Lectrolab clone just to see if I could recreate the amazing sound of the original. 

A note about “rules” – There are none.  Here’s a few facts:

  • Both cathode-biased and fixed-biased amps can sound great, or terrible. 
  • Reducing the coupler cap values can make low notes sound better, more defined.
  • It’s possible to have an acceptable signal-to-noise level with many different grounding schemes.

11 Responses to R500

  1. cv says:

    Hi –
    I’m glad you found my blog post on my Lectrolab R500c – I love that amp! Also glad to find your blog in return – there is so little info available about these amps; this is the best resource I’ve stumbled upon. Nice job!


  2. Pingback: NAD! (NOAD?) 1961 Hamonry H305A - My Les Paul Forums

  3. Don N says:

    I bought an ElectroLab, Model, R500B in 1959. From Left to right, It has two pickups (1 & 2) with one volume and one bass control and a second two pickups (3 & 4) with one volume and one bass control plus a foot control pickup with a volume and base control. on the right side of the control panel was the on/off red light and fuse. Below the light is the toggle switch. The picture you have posted (R500B or Rr00C) have fewer controls then this R500B.

  4. Don N says:

    Posting Correction: The Amp purchased in 1959 was an ElectroLab R600B.

  5. Dwight Corella says:

    I recently acquired a Harmony H-305C . It appears to use 2 ea. 12AX7’s , 2 ea. 6V6’s and a 5Y3. Do you know of a schematic? Is is comparable to a Lectrolab R500B? Thanks for your time!

    • alexage1 says:

      A quick look around the net reveals little or no external difference between the Harmony H-305A and H-305C. And no schematics. The H-305A is the same amp as a Lectrolab R500B. Perhaps some minor changes were made for the “C” designation, or perhaps the model name change was made for costing or customer differentiation purposes. The cirecuits are probably very similar if not identical. If you haven’t yet unscrewed the back panel (assuming it still has one) and looked at the interior-facing side of it, you may find a schematic pasted there. Let us know. If not, perhaps an enlightened sould can alert us by posting the answer here!

      – George

  6. Hawkeye Kane says:

    With the difference between the R500 and R500B’s inverters, would it be prudent to try running AX7’s in an R500 rather than AY7’s? Granted, the inverter won’t care what it’s running really. But the preamp halves could be gained up for some more driving tone by either an AX7 or a 5751.

  7. B Cauthen says:

    Hi. I recently purchased a R500B and love it! I use it for blues guitar and it has a very organic, warm tone. Fantastic value at $350!

    I had never heard of Lectrolab before buying the amp, and so I was very excited to find this web page. Thanks for the great information and photos.

    My amp seems to be a variant of the 500B. The biggest difference is that my amp has only three inputs: two on channel one and only one on channel two. Other than that, the on/off switch is located below the power indicator light and fuse, instead of being on the same line as the other two.

    I have photos of the back, as well as a clear photo of the amp schematic, that I am happy to share.

    A question for those with more technical knowledge than I: Is there an easy way to brighten the tone of the amp? I am plying it with the tone turned all the way up and an eq pedal, but if there’s an easy way to improve the tone, I’m happy to ask my amp tech to do so. I thought I would ask here first, however, since he probably hasn’t worked on many amps like this. In advance, thanks!

    All the best.

  8. george gavaza says:

    I have an R500C with schematic in tact. Is anyone interested is pictures? I will need directions on how to post.

  9. Erik Kundrats says:

    I recently purchased a Harmony 305A amplifier. Looks like it’s all original and it sounds great to me. The schematic on the back panel reads “Schematic Circuit of Model R-1150 Amplifier”. Someone crossed out the R-1150 and hand wrote “H305A” above it. The R-1150 designation follows the Lectrolab model numbers, but I cannot find any reference to a model R1150. I have a picture of the schematic I can email if you are interested.

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