Sound Projects R203 (Lectrolab)

[martin manning]

One more... There's a ground lug under the tone pot too. It grounds the tone pot CCW lug, the 50C5's cathode bypass cap, the power supply filters, and the power transformer secondary. No sound at all without that! I find that drawing the circuit in schematic form helps to spot these details.

[nashvillebill]

Considering the amps were as low-cost as possible, I can entirely imagine this scenario:

Amp manufacturer: I need a price on a quantity of a thousand, output transformers for a 50C5 single-ended.
Transformer supplier: Sure, that'll be a total of $1000 for the lot.
Amp mfg: Too expensive, after all this is a budget guitar amp.
Transformer vendor: Well how about these surplus doorbell transformers? They're kinda close. $200 for the lot.
Amp mfg: Great! I bet I can use them with a 50L6 too....

Maybe, just maybe, part of these amps sound/charm is their cheap cardboard baffle, flimsy enclosure, cost-cutting engineering, and perhaps even components that don't even match the circuit requirements.

Unfortunately, around here anything with the word "tube" in it is being sold (well, advertised at least) for big bucks. Whether old radios or non-working Harmony amps. Buying anything to cannibalize for the PT and OT likely won't be cost effective. And having Mercury do the OT sure isn't gonna be cheap.

Antique Electronic, Triode, and Angela all have some fairly inexpensive 5 watt OT's. I believe Mojotone has a special on one for $10.80 Mojo Surplus » Surplus SE Output Transformer

Maybe buy the Mojotone item, and try temporarily substituting it into the existing amp, with the existing speaker and enclosure and all? This would let you know if it's an acceptable replacement without spending a lot of scratch.

I'm certainly no expert, but with onyl 3 watts or so of output, I can't see the OT as being saturated enough to add a whole lot of tone. Same with the power supply, this amp doesn't draw much current so I suspect the power supply isn't sagging much--therefore most any PT that supplies the correct voltage would be OK.

[martin manning]

Here's a schematic drawn from the layout and photos. Much easier to understand than a point-to-point layout, IMO. I'll update it if the missing component values can be determined.

[Sleepthieves]

Thank you martin, like i said i had never drawn a layout or schematic before and Wittgenstein found a decent program to do it but he had never done one before either. I cant thank you enough for saving me the time and the hassle of redrawing our layout into a true schematic. The speaker is 4 ohm and i will spend some time figuring out the other "?" on your schematic and give you the values. What program did you use to draw this?

[martin manning]

Thank you martin, like i said i had never drawn a layout or schematic before and Wittgenstein found a decent program to do it but he had never done one before either. I cant thank you enough for saving me the time and the hassle of redrawing our layout into a true schematic. The speaker is 4 ohm and i will spend some time figuring out the other "?" on your schematic and give you the values. What program did you use to draw this?

You're welcome; I got curious as to what is in these things and made a hand sketch from your layout. I'd consider converting the input stage from grid-leak to cathode biasing at some point, at least as an experiment. The schematic drawing I made is in PowerPoint.

To fill in the missing pieces I suggest the following:

1) Measure the resistance across the outer lugs of the volume and tone controls in-circuit. This should be a good measurement since they are isolated from the rest of the circuit by capacitors.

2) With the amp plugged in and the 35W4 removed, measure the AC voltage across the power transformer's primary and secondary. This will confirm that it is a 1:1 isolation transformer.

3) If you really want to find out what the output transformer's primary impedance is (and I believe that matching it will be important if you want to duplicate the sound of the original), that will require some effort.

First you will have to disconnect (un-solder) the primary lead that goes to the power supply (at A in my schematic, so at pin 7 of the 35W4, or disconnect the jumper that goes from pin 7 at P3 on the terminal strip in your layout). Be aware that it will require some skill to do and undo this neatly. You will need some solder wick, or a solder sucker thing. Having done that, disconnect the leads from the speaker and connect them to a small AC voltage. A 6.3VAC transformer, say, or a low-voltage AC wall wart. Measure the AC voltages at the primary and secondary of the OT (be careful, there will be a couple of hundred volts on the primary). The ratio of these voltages (Vpri/Vsec) is the turns ratio, and the square of this ratio is the impedance ratio. Multiply the impedance ratio by 4 Ohms (the speaker impedance) to get the primary impedance.

Having said all that, the book value for a 50C5 is 2500 Ohms as noted already, and the Mojo transformer linked above seems to be a good bet and inexpensive too.

[nashvillebill]

The cathode cap on the 50C5 is 100mfd ??? Seems awfully big.

Might want to compare that schematic with the base R200 on the Lectrolab site mentioned earlier, there's a lot of similarities and a couple of differences to confirm.

[martin manning]

The cathode cap on the 50C5 is 100mfd ??? Seems awfully big. Might want to compare that schematic with the base R200 on the Lectrolab site mentioned earlier, there's a lot of similarities and a couple of differences to confirm.

Thinking on it some more, I'd already changed the schematic. I'm guessing that whomever replaced the multi-section can with the radial caps that are in there now chose values that were close, so it was likely 40-30-??.

Edit: A photo of another R203 on alexage's Lectrolab site that still has the original filter in it seems to confirm that it was a three section cap. My best guess right now is that a 40-40-20, with the 20uF section serving as the cathode resistor bypass, was in the R203.

[loudthud]

From the layout diagram with the voltage chart it shows 170.5VAC on pin 5 of the 35W4. So that's pretty much the secondary voltage of the power transformer. The one bad thing about this amp is the heater string is operated straight from the line voltage. It would be a lot safer to run it from the secondary of the power transformer.

Because the 35W4 is only a half wave rectifier, the transformer will have to be somewhat bigger than you would otherwise think because of the imbalanced current.

For a power transformer it's easiest to use two transformers in series. The heater string is 150mA and a 50C5 takes about 50mA. The 12AU6, less than 1mA. So something like 170V at 300mA would be a starting place. See if you can find 120V @ 35VA and 48V @ 10VA. Because I used the 120V unit to power the heater string, the 48V unit can be smaller than 300mA but 500mA might be the smallest you can find. You will have to experiment to get the phase correct on the 48V transformer.

[martin manning]

From the layout diagram with the voltage chart it shows 170.5VAC on pin 5 of the 35W4. So that's pretty much the secondary voltage of the power transformer.

This 170.5VAC on pin 5 puzzles me because the voltage on pin 7 is only 143.6VDC. That's about what is expected with 117VAC on the plate, per the curves in RC-30.

[loudthud]

This 170.5VAC on pin 5 puzzles me because the voltage on pin 7 is only 143.6VDC. That's about what is expected with 117VAC on the plate, per the curves in RC-30.

I agree with you and certainly think that measurement should be verified. The measurement of -12.7 VDC tells me that the waveform is clipped on top pretty hard. It would be good to know the resistance of the primary and secondary of the transformer as well as the voltage with the 35W4 removed.

[nashvillebill]

On my R200B, just for reference, I measured 155 VAC on pin 5 of the 35Z5. Pin 8 I measured 160 VDC.

Plate on the 50L6 I measured 151 and screen 87.

[martin manning]

Don't know if interest in this project has waned, but here's a revised schematic with best guesses at the missing component values.

[nashvillebill]

Great job! I've also taken the R200B schematic that's visible in a photo on the Lectrolab site and redrawn it using CAD, I'll post it next week when I get back to work. The 203 is different in that it uses a sharp pentode 12AU6 rather than a triode, so it may impart a slightly different color to the preamp.

I notice one aspect of my 200B that others may encounter. Mine has an audible 60 hz hum as the volume gets above 5 or 6 (even though I've recapped it and gone to star grounding). I believe it's because the filaments are strung on the 120 VAC line. I twisted the filament wiring which helped a little, but IMHO this would be another reason to change to a traditional 6V heater. The 12AU6 is available as a 6AU6, and the 50C5 is available as a 6C5, so the circuit would be otherwise unchanged. The rectifier could also be changed to a 6V version or get a PT with a 5 volt rectifier tap to use common 5V rectifier tubes.

[loudthud]

So the hum goes away when you turn the volume control down? You might try turning the AC plug (power cord) around in it's socket. The Silvertone "Amp in Case" uses almost the exact same circuit as the R203. I think the model number is 1448. That amp uses a small transformer to isolate the heater on the 12AU6. Nobody really knows why they did it that way but speculation is to get rid of the hum. You could use any 12.6V filament transformer and substitute an 82 ohm 3W resistor in place of the 12AU6 heater.

[nashvillebill]

Yep, it only becomes audible when the volume knob is cranked more than half ways. I've tried reversing the AC plug to no avail. Note I've removed the AC line-to-chassis capacitor, though maybe eventually I might replace it with a type XY cap.

My R200B uses a pair of 12BF6 triodes rather than the 203's 12AU6 pentode.

I don't need this amp for any serious work, so I'm not going to bother with adding another filament transformer, though that's interesting about the Silvertone. If I do anything, maybe I'll break down and try shielding some of the internal wiring (from the first gain stage to the volume control and second gain stage) but this is just something to fool around with in my spare time (which I have too little of).