Sound Projects R203 (Lectrolab)

[Sleepthieves]

I was fortunate enough to inherit my dad's old 60's Sound Projects model R203 tube amp, which he got when he was 12 years old from Montgomery Ward. After searching the Internet for about 5 years now, I have only seen it mentioned less than 10 times. I did stumble upon Lectrolab.wordpress.com however. It has a picture and talks about my exact amp at least. I have attached pictures of all the most important parts of this amp.

I want to make a clone of it so i dont have to beat up my dad's amp carrying it around. My problem is I do not know which transformers to buy in order to make a clone of this amp. It would be nice if there was a simple Hammond equivalent that would not alter the design much, but I don't know what is needed. Since I could not find a schematic for this model anywhere I made my own (attached below w/ all part values and tube pin voltages) to show the point to point layout. Sorry in advance for its unorthodox style as I’ve never made a schematic before and so it was extremely difficult for me to visualize it any way but in a semi-wiring diagram format.

Is there a way to test what the bigger transformer is putting out without being too invasive? I say bigger transformer because it is possible it is not a power trans but just an isolation trans. Also even though i know that the output trans is putting out 4 ohms, is there anything else i should know about it? I assumed its 4 ohms because the original speaker was labeled 4 ohm.

If you notice, the 120V coming from the wall goes straight to the on/off switch with all parts grounding to the chassis and no ground to the wall. I have heard this configuration along with the set of tubes be referred to as widow maker. I have since changed out the old two prong plug to a three prong with ground to the chassis on the transformer bolt in the hopes that it would make the amp safer. It appears that this amp had been modified previously as the 3 electrolytic filter caps are much newer in their construction than the other caps. The one picture I found on the web of the insides of this amp shows a multi-cap where the 3 newer electrolytic are in my amp. There is one value I don’t know and it is the smallest electrolytic, my guess is 25-25 but all that can be read is 25 vdc. I had to change out the speaker as the old one had a rip in it. I replaced it with a Jensen p8r as it was the closest thing i could find to the original.

I appreciate anyones input, thank you in advance.

[nashvillebill]

I've got a R200B, very similar in concept. I don't consider this a true "widowmaker" since there is at least an isolation transformer. (The amps that are real deadly, IMO, are the ones that don't have the isolation transformer, with one leg of the incoming AC directly connected to the chassis.) 120 volt to the on/off switch is typical, no big issue there IMHO.

You don't show it on your schematic, but mine did have a cap connecting the AC to the chassis, often called a "death cap" if it failed shorted. I removed that one from mine. Now let's think about the incoming AC: it goes through the on/off switch, then through the transformer, effectively isolating everything (once the AC cap is removed)....except for one thing on these. The heater filaments are all in series and run directly off the incoming AC. I don't sweat about that much, since I just play mine occassionally at home.

But if I were re-creating this amp--and indeed I've thought about it--here's what I'd do: Get a regular power transformer with a 6.3 volt filament tap. Then make all tubes 6 volt. That's easier on mine, perhaps, than yours, mine has 12BF6 tubes (available in 6 volt) and 50L6 (which becomes the common 6L6); the 35Z5 on mine becomes a 6Z5 or something like that. The output tranny would just be a common single-ended, maybe a Hammond 125SE? but I'm not sure.

[Sleepthieves]

Thanks nashvillebill, Alot of the stuff you put were ideas that i had, its nice to have someone back up my theories. You should consider putting info about your R200B on this webpage for lectrolab. The guy who runs it George is quick to respond.

[alexage]

"Widowmaker" discussion here:

Widowmaker? | Lectrolab Guitar Amplifiers

[Wittgenstein]

I am also looking to build a clone of the R203, but I am unsure which transformers to use. I would like it to sound as much as possible as the original (perfect for a practice / recording amp). The schematic included above lists the pin voltages, and I was wondering if anyone is able to work backwards to find replacement transformers? Is there a way to test original transformers with a multimeter to tell me what I need? Thanks in advance!

[Steve Conner]

This is a cheap, nasty amp with a particle board cabinet and a tube complement that was surplus from radio manufacture. But these old amps often work great for practice or recording, the puny output power gives you rocking distortion at relatively low volume.

If you want to make a "clone" I'd suggest taking some liberties. Use a transformer set intended for a Fender Champ clone, and tubes with 6.3 volt heaters. 6CA5 or 6X5 rectifier (or a 5Y3 or whatever is usually in a Champ), 6AQ5 or 6V6 or EL84 power tube, 6AU6 preamp tube etc.

The Champ transformers aren't exactly the same, but they're easy to get hold of.

[nashvillebill]

Isn't it funny, when these and their similar Champs/Harmony/Kaye amps were being produced 40 or 50 years ago, they were considered cheap, crappy "beginner" amps. Few serious musicians would even contemplate using them, much less gigging with them., due to their flimsy construction and low power.

Now they're in high demand and boutique amp manufacturers are reproducing them for big bucks! Go figure. (Not slamming these amps or people who own them, as I mentioned above I bought an ancient Sound Projects amp....)

[Wittgenstein]

Thanks for your suggestions, however I am really looking to stay as close as possible to the original design and sound. I am currently building a 5F1 Champ and my buddy is working on the 5C1; great little amps, but they sound nothing like this little Sound Projects R203. I especially would like to use the same 3 tubes in my clone (50C5, 12AU6, 35W4) as I have about 10 extra sets, most being NOS. I am not worried about small changes such as carbon film vs composite or which brand caps, but I do not want to stray so far as to change tubes and transformers that bump up all the voltages to make a Champ hybrid. Especially since the Champ had some beefy transformers to begin with for only being a 4-5W amp. They are almost as big as on my 5E3 Deluxe which is 14W.

Is the problem that it is hard to find a similar transformer that is made today to fit this amp? I am just unsure what to look for when searching Hammonds products. I am also unsure how to even go about figuring this out, aside from mesuring all the pin outs. Is there a way that I can test the taps coming off the transfomer to see what I need? Something like 110v to 110V single ended with 3 taps (5v, 13v, 110v)??? Is there more information that is needed from me to figure this out? Is there a book or somewhere someone can point me to find out more about this? I have been reading every tube amp and electrical theory book I can get my hands on, and the more I learn, the more I realize that I need more/better books. Some of the best information I have found is on this forum thanks to active knowledgeable members (Bruce, Loudthud, Tubeswell, martin manning, etc... ), and it has been greatly appreciated.

As you mentioned these old amps are great for recording. It produces a beautiful clean almost audio amp sound until you turn it all they way up to get great tube distortion. Even at full volume with single coil pickups, this amp produces no hum! Nothing I have played sounds like this amp, and I can only assume that the magic is coming from this circuit / tube configuration, and I would hate to change that. Help me bring Lectrolab amps back to life.

[52 Bill]

There are two transformers in this amp, one power and one audio output. The audio output will probably be found from an radio restoration source, as a lot of old tube radios used the 50C5 output tube. My guess the cheaper the better, to match the original one.

To measure the original power transformer, read the ac voltage at the input side and at the output side. If my guess is correct, you should read about the same voltage on both sides of the transformer. As for current draw, look up the tubes in a tube manual and add up the plate current specs for each tube. Then add some for a safety factor and that's the current rating that you need to look for in a replacement.

The power supply transformer is probably only a low current isolation type, probably rated at no more than 75-100mA at 115v. Mouser lists a Triad isolation transformer stock #553-N48X that is rated at 130mA. I would imagine that it would work in that circuit.

[Wittgenstein]

Thanks Bill! Sounds similar to the fix I found here, for adding a isolation transformer to an old amp that never had one.

[martin manning]

Looks like you are getting some good help here.
I believe there are two grounds missing from your layout:
The terminal strip lug labeled P1 is grounded to the chassis (to ground the 50C5's cathode resistor and the speaker), and there is a ground lug under the volume control that is soldered to its counter-clockwise lug. It is needed to ground the volume pot and the output transformer's secondary (the green enameled wire). Also, C8 is a "death cap'" which you probably want to omit. It will make the chassis hot if it shorts.

A couple more observations...
The small electrolytic that you can't determine a value for is the cathode bypass cap for the 50C5. It wants to be about 100uF to be effective throughout the range of audible frequencies.
The 5W dropping resistor in the filament string is labeled 75 ohms in the layout, but it looks like it's 130 in the photo.

[nashvillebill]

I was going through the layout this afternoon to compare it to my R200B and came to the same conclusion, a couple of grounds weren't shown.

This thread is a little confusing, since there are two posters who want to clone the amp--the original poster (Sleepthieves) and wittgenstein.

edited: Oh, let me add something for anybody who is unfamiliar with these amps but wants to make one using the layout here: The switch on the Tone pot is a separate switch used to turn the AC power on. The line power also runs the filaments, all the filaments are wired in series, and there may also be another resistor in series to drop everything down (I didn't look at that part of the layout shown). I think I'm pretty safe in saying that running the heaters in series off of the incoming line power is not the preferred approach in new amp designs. That's why I suggested to Sleepthieves that he use the 6 volt version of the tubes, if possible, and use a conventional filament heater tap off the PT.

[Sleepthieves]

Just for reference Wittgenstein and I are friends IRL and live in the same town. We would both like to have one of these amps that we can bang up and carry around instead of destroying my vintage one. Props to 52Bill for the idea to go for radio transformers since this is about 3/5 of an "All American 5" tube radio, sounds like the easiest way to get the most true transformers. In fact not sure why i didn't think about that before, seems obvious now. Thank you very much to Martin Manning and Nashvillebill, your insights are the kind of thing that Wittgenstein and I just don't have yet. Drawing that schematic was hard enough and even after looking at the amp for over a week straight i would have never known about the two missing grounds. If you come across anything else, thankyou in advance. If you need any more info just ask and it will be promptly posted.

[nashvillebill]

I think the Lectrolab site may even have mentioned it, but these amps were so cheap partially because they used radio set tubes. At the time, transistor radios were becoming popular and tube radios were suddenly obsolete. So radio set tubes were being dumped on the market for pennies a tube. Hence the somewhat unusual tubes (by today's standards), like the 12BF6 in mine. They may not have been "optimum" by hi-fi standards but they worked well enough.

[alexage]

Some random thoughts on the output transformer:

1. A HUGE number of radios and other audio gear were made with the 50C5 or its close relatives, and it was probably a radio (or phonograph) output transformer they used in the guitar amp. You might find an old radio for cheap (which uses the 50C5) and take the transformer from that.

2. The load resistance of a 50C5 is 2500 ohms, the speaker was probably 3.2 or 4 ohms, so you'd be looking for a impedance ratio of about 625:1 which means a turns ration of 25:1. [Somebody check me on this, I'm a electronics moron]

You can Google ways to measure the "turns ratio" of an output transformer, here's one example: Output Transformer Impedance

3. If you find a transformer which you know was used for a 50C5 (or similar tube) with a 4 or 8 ohm speaker, and it is about the same size/weight as the original, then it ought to work just fine.

4. There's a bunch of vintage radio nuts around (of course, we amp maniacs are totally normal). I'd bet that by tapping into that network via forums, retailers, etc. you could find something similar.

5. Or cannabalize another guitar amp like a Harmony H-400 or many, many others. Probably cheap if not in working condition.

6. Call Paul at Mercury Magnetics. They've surely had this question before, and those guys ALWAYS have an answer, although it may be an pricey one - They can clone anything.

This is a solvable problem. I bet the guys that made these amps would be rolling on the floor laughing if they knew these discussions would be happening 50 years later.