Sound Projects R203 (Lectrolab)

[Sleepthieves]

On my R203 the hum changed considerably depending on which way i had it plugged in. The plug was symmetrical so i had to mark it on the side that when plugged into the larger hole in the socket had way less hum. Wish i had a silvertone amp in case. Thought about making one of those too but its so close to my R203 which im gonna clone, so no need to make two old amps that are damn near identicle.

[Sleepthieves]

Thanks Martin for the updated schematic. No interest hasnt waned just time and money. Already building other amps and trying to get the funds together to complete them first. Hope to have time soon to try some of the tests if possible to figure out more info on that isolation transformer. Thanks for everyone else's continued interest.

[ddr]

Thanks Martin for the updated schematic. No interest hasnt waned just time and money. Already building other amps and trying to get the funds together to complete them first. Hope to have time soon to try some of the tests if possible to figure out more info on that isolation transformer. Thanks for everyone else's continued interest.

Hi Folks,
I just came home with an R203 and googled it. Mine has the original electrolytic, so I will see if I can get the value off of it. I have not tried it yet, the guy who sold it to me told me it hums very badly and needs repairs. I will see what information I can add to the discussion once I pop it open next weekend.

[ddr]

Hi Folks,
I just came home with an R203 and googled it. Mine has the original electrolytic, so I will see if I can get the value off of it. I have not tried it yet, the guy who sold it to me told me it hums very badly and needs repairs. I will see what information I can add to the discussion once I pop it open next weekend.

I popped it open and the original electrolytic cap is:

50uf 150V
30uf 150V
20uf 25V

I could not get any good pictures because the wires going to the speaker are so short I cannot slide the chassis out enough. When I unsolder the wires to remove the chassis I will take some gut shots of the cap if anyone is interested.

Mine does not have a Jensen labelled speaker in it. The marking on the rear plate of my speaker is:
S13996
Z67749

And on the cone itself it has:
0812-017

If anyone knows how to decipher these codes can you please let me know?

Thanks,
Dan

[martin manning]

Thanks for adding the info re the original multi-cap.

While you are in there, measure the AC voltage at the PT primary and secondary secondary with the 35W4 removed, and the resistance of the elements of the two pots. That will complete the schematic details for the next guy that finds this.

[ddr]

Thanks for adding the info re the original multi-cap.

While you are in there, measure the AC voltage at the PT primary and secondary secondary with the 35W4 removed, and the resistance of the elements of the two pots. That will complete the schematic details for the next guy that finds this.

Power Transformer:
Primary: 124VAC
Secondary: 204VAC

Tone Pot: 524K
Volume Pot: 566K

I am going to replace the power cord before I do anything else

[martin manning]

Power Transformer:
Primary: 124VAC
Secondary: 204VAC
Tone Pot: 524K
Volume Pot: 566K

Ok, thanks ddr, here's the revised schematic. So the PT is not just an isolator; mystery solved. Now if you get really ambitious, you can figure out the OT primary impedance ;^)

[alexage]

Martin - thanks to you, and to everybody on this thread for the schematic and thoughts on this circuit. This is a fantastic community of helpful people. I've posted the rev2 schematic at the lectrolab.wordpress.com R203 page, along with a link to this thread.

[ddr]

Ok, thanks ddr, here's the revised schematic. So the PT is not just an isolator; mystery solved. Now if you get really ambitious, you can figure out the OT primary impedance ;^)

Thanks for that Martin.

Note: I am leaving this text here for history, but I replaced the electrolytics and am playing away now. The amp is humming a little, but I kind of expected that with the way things are grounded and heaters running straight off of the AC line. The voltage at B+ point A is now 148VDC and 4VAC instead of what I report below. I used a 47uf 400V, 22uf 250V, and a 22uf 50V to replace the original can cap. I will read the specs for the rectifier tube and see if I can increase the filtering to reduce the hum.

I replaced the cord with a 3 prong and removed the death cap. Now I am trying to get rid of the horrible hum (the guy who sold it to me told me about it before I bought it, so I knew what I was getting into). As soon as it starts warming up it starts humming very loud, even with the volume all the way off.

I figured there is some ripple in the DC coming off of the rectifier, so I measured both AC and DC between pin 7 of the rectifier and ground. It is reading 83V DC and 84 V AC. Shouldn't it be all DC? Would this be a rectifier problem or the filter cap? I hope it is the filter cap as I have some coming in the mail today.

How much AC is normally at pin 7 on a 35W4?

Thanks,
Dan

[ddr]

This little sucker is more and more interesting

According to the data sheet the max filter capacitance for the 35W4 is 40uf, but we are using 50uf. Here is the sheet:

http://www.r-type.org/pdfs/35w4.pdf

I think I might try it with a diode rectifier instead of the 35W4, then I could bump the first filter cap way up. This would change the heater voltage to 62V, but I have a transformer to get there.

[martin manning]

ddr, I didn't see that this thread had been updated until now...

Anyway, I think you have now replaced the filters and the 50C5's cathode bypass cap, and what was an intolerable hum is now much reduced?

Since this thing is single ended, it can be expected to have some hum. Changing to a solid state rectifier will increase the DC voltage quite a bit, maybe too much. As far as the first filter's capacitance goes, it is probably within tolerance. You could try inserting another 47uF cap followed by a choke (like one from a Deluxe Reverb, say), between the 35W4 and the existing first filter; you won't mess up the heater string voltages that way. Not sure how the tube rectifier's effect on sound would be affected, but probably not as much as going to a solid state rectifier.

[ddr]

ddr, I didn't see that this thread had been updated until now...

Anyway, I think you have now replaced the filters and the 50C5's cathode bypass cap, and what was an intolerable hum is now much reduced?

Since this thing is single ended, it can be expected to have some hum. Changing to a solid state rectifier will increase the DC voltage quite a bit, maybe too much. As far as the first filter's capacitance goes, it is probably within tolerance. You could try inserting another 47uF cap followed by a choke (like one from a Deluxe Reverb, say), between the 35W4 and the existing first filter; you won't mess up the heater string voltages that way. Not sure how the tube rectifier's effect on sound would be affected, but probably not as much as going to a solid state rectifier.

Hi Martin,
Sorry, by rectifier I meant the tube rectifier. The issue was with the filter caps, they were shot. I replaced them and the hum is low, about what I would expect from a tube amp. I did replace the three input jacks with a single Neutrik (plastic) jack and that also helped. I may twist the heater wires and see if that helps more. While I was at it I removed the 22K resistor on the input.

Finally, I replaced the 12AU6 with a 12AW6 for a bit more gain.

If I can get enough gain out of this I will rebuild it with modern components, the one resistor I checked was off by 35%...

Have a good one,
Dan

[alexage]

Original Schematic FOUND

Thanks to a generous reader, Phillip Lemay, you can now see a factory schematic for the R203 on my Lectrolab site here:

R203 | Lectrolab Guitar Amplifiers

[martin manning]

Interesting... values for the control pots are not shown, and there is no dropping resistor in the filament string. Nice addition to the archive.

[nashvillebill]

Thanks to a generous reader, Phillip Lemay, you can now see a factory schematic for the R203 on my Lectrolab site here:

R203 | Lectrolab Guitar Amplifiers

Alexage, I can send you the schematic I drew up for the R200B if you PM me with a good email address and you are more than welcome to post it on your site as well.