50W Project

[Trem Abuser]

I'm doing a 50W lead project out of a Laney 30W combo. My plan is to be converting as close as possible to 50W lead specs except for the transformers at this point.

Question - for the 50W, do I examine everything as 1/2 of the 100W? For example, the 100W has 6 big filter caps; the 50W has 3. I'm following the 100W build instructions and I'm trying to figure out where all the wires will be hooking up on the 50W. I can follow along the build instructions on the 100W and if my project were a 100W project I'd have step by step instructions to follow.

I still have not disassembled the Laney head other than taking it out of the old combo enclosure. I'm worried about screwing things up irreparably or charging ahead when I need to be noting where everything was disconnected from so I could possibly reconnect it later.

Thanks,
Scott

[Trem Abuser]

OK, my first post was not a good question and wasn't specific enough to generate a response. But I still have many questions about my conversion project to convert my amp to a ptp-wired 50W lead, so I'll ask more specific questions, with separate subject headings, and hopefully I can get some help with my build.

1. Any specific things I need to consider for mounting the underboard filter caps? I see how they are arranged on the 100W - can I put one by the output tubes, and the other two between the transformers?

2. Which caps require the addition of ground lugs?

3. Which caps (if any) get a 56K 2 watt resistor between the two positive terminals?

[Trem Abuser]

My amp only had one 1 amp fuse.

1. Do I need to install an additional 4 amp fuse?

2. Do I need to run a wire from one of the filter caps (and if so, which cap) to the 1 amp fuse holder?

[Trem Abuser]

1. Do I need to install a 1.5K resistor connecting pin 5 of the output sockets? My amp previously did not have this resistor.

2. Do I mount a 1K 5 watt resistor between pins 4 and 6 of each output socket? My amp previously did not have these resistors.

3. My amp has only one speaker output jack and no impedence selector switch. The instructions show buss wire being run from the terminals farthest from the chassis over to the impedence selector switch. How can I mount the buss wire in this situation?

4. How do I determine which wire is the negative feedback wire? Since my amp has no impedence selector switch, where would I mount the negative feedback wire once I determine which wire that is?

[Trem Abuser]

My PT has DC wires colored as follows (as well as numerical markings I believe to be voltage indications):

(a) Yellow (2 wires), heater wires, 6.3 vDC.

(b) Orange (1 wire), -50 vDC.

(c) Blue (2 wires), 24 vDC.

(d) There is a black wire between the yellow wires and another black wire between the blue wires; markings on the PT seem to indicate that the voltage of these black wires is 0 vDC.

So my questions on hooking up the power transformer are:

1. Which wire attaches to the lower two terminals of the power switch?

2. Which wire connects to the nearby(?) filter cap and connects its positive terminals together?

3. Which wire attaches to the mains fuse holder (again my amp only had one 1 amp fuse, unless I also install a 4 amp fuse).

4. Which wires attach to the standby switch?

5. Do I have a bias tap wire which will attach to the circuit board?

[Trem Abuser]

With the three filter caps in my amp:

1. Do I need to make a jumper between two of the filter caps?

2. Which filter cap (if any) hooks with a blue wire to the board? (Obviously the color is irrelevant; I'm just following the instructions in the Installation Guide.)

3. Which filter cap gets red wires attached to the positive terminals that will be tightly twisted and attached to the board?

Thanks immensely in advance for any and all help you guys can give me. As you can tell I'm in over my head and need plenty help.

[Billy Batz]

Wow. I didnt read everything but a 50W is NOT half a 100w. Almost all the values are the same accept a few things. The 100 watters have 6 cans because the cans are wired together in series parallel to up the voltage rating. Its cheaper then using 1 large cap but the filtering amount comes out the same. 2 50/50 cans wired in series/parallel equals 50uF. In a 50 watter you dont need to do that. The mains filter section is a whole 50/50 can wired parallel for a total of 100u. You would jump the 2 positive terminals together. The rest of the filter sections are each a single 50uF. You wouldnt jump the positive terminals. You would run one wire from each of the terminals of the cans to a spot on the board where the 10k/2W power resistors are. Ive never built a 50 watter but IIRC the can in the corner is for the mains and would be wired to the rectifiers and the fuse. The can next to that is for a) the screens and it would attach to the same spot the choke is soldered to on the board and b) V3 which would have a wire from the other terminal of the can to the other side of the 2 2W power resistors the choke is soldered to. The final can is under the board and is soldered to either side of one of the 2 10K2W resistors that are next to the tone stack. Follow the diagrams.

The can with tightly twisted red wires is probably the one next to the one in the corner (right?) and those wires lead to either side of the 2 series power resistors the choke is soldered to. It doesnt matter which can you use. Its only an aesthetic thing and neatness thing. But it also is good not to have those wires running all over the place. The one under the board is right under the 10k2W resistors on the board that its wired to. The can for the screens and V3 is a bit far from the board but its the only good place to mount it.

[Billy Batz]

Im sorry. I didnt notice your modding a Laney. In that case use the nearest cans or whatever way you can to do it nice and neat. The diagram is a chassis of a 50 watter. Just move the cans around however you need to to make it work.

[Trem Abuser]

Wow. I didnt read everything but a 50W is NOT half a 100w. Almost all the values are the same accept a few things. The 100 watters have 6 cans because the cans are wired together in series parallel to up the voltage rating. Its cheaper then using 1 large cap but the filtering amount comes out the same. 2 50/50 cans wired in series/parallel equals 50uF. In a 50 watter you dont need to do that. The mains filter section is a whole 50/50 can wired parallel for a total of 100u. You would jump the 2 positive terminals together. The rest of the filter sections are each a single 50uF. You wouldnt jump the positive terminals. You would run one wire from each of the terminals of the cans to a spot on the board where the 10k/2W power resistors are. Ive never built a 50 watter but IIRC the can in the corner is for the mains and would be wired to the rectifiers and the fuse. The can next to that is for a) the screens and it would attach to the same spot the choke is soldered to on the board and b) V3 which would have a wire from the other terminal of the can to the other side of the 2 2W power resistors the choke is soldered to. The final can is under the board and is soldered to either side of one of the 2 10K2W resistors that are next to the tone stack. Follow the diagrams.

The can with tightly twisted red wires is probably the one next to the one in the corner (right?) and those wires lead to either side of the 2 series power resistors the choke is soldered to. It doesnt matter which can you use. Its only an aesthetic thing and neatness thing. But it also is good not to have those wires running all over the place. The one under the board is right under the 10k2W resistors on the board that its wired to. The can for the screens and V3 is a bit far from the board but its the only good place to mount it.

Thank you very much for this and for the diagram. This is very helpful for putting the pieces of the puzzle together. Now if I can get a response to the power transformer question(s) I'll be in good shape!

[Guest]

OK you have a lot of questions here and I didnt read them all last time. Lemme try to answer them one at a time and maybe someone else can chime in on things Im not sure about.

3. Which caps (if any) get a 56K 2 watt resistor between the two positive terminals?

Ive never seen a 50W with bleeder resistors. You can put them on the mains filter.

2. Which caps require the addition of ground lugs?

You can put them on the mounting screws closest to each filter can lug for ground points or anywhere else you need to chassis ground something.

1. Do I need to install an additional 4 amp fuse?

No. The fuse does not effect the circuit. It just protects (or is intended to anyway) the parts. Ill defer you to someone else to see if you really should add one or not.

1. Do I need to install a 1.5K resistor connecting pin 5 of the output sockets? My amp previously did not have this resistor.

Again you dont have to but it has a minimal effect on tone and as is being discussed in another thread at this site right now helps to kill parasitic oscillation which manafests itself in different ways like weird squeels or mostly a weird artifact in the high frequencies.

2. Do I mount a 1K 5 watt resistor between pins 4 and 6 of each output socket? My amp previously did not have these resistors.

Again this is a good idea to do. Pin 6 is not used on EL34s so pin 6 on the tube socket attaches to nothing even when the tube is plugged in. Your actually wiring the 1K5Ws between the B+ and pin 4 but since pin 6 is sitting there doing nothing you can use that as a lug to attach and mount the 1K resistors and the wire from the B+.

4. How do I determine which wire is the negative feedback wire? Since my amp has no impedence selector switch, where would I mount the negative feedback wire once I determine which wire that is?

The OPT has 3 taps you can use- 16 ohm, 8 ohm and 4 ohm. With a selector you can choose which tap is connected to the output jacks. You can use multiple jacks one for each output. You can use one jack and solder one tap to it. NFB is just a term for feeding the later signal back to an earlier point. In this case your feeding back the transformer taps output to the phase inverter by means of a certain resistor probably 47k or 100k. In Marshalls they usually run a purple wire from one of the terminals on the selector switch the taps are connected to (which terminal depends on which tap you want to take 16 8 or 4) and solder that wire to the NFB resistor on the board.

3. My amp has only one speaker output jack and no impedence selector switch. The instructions show buss wire being run from the terminals farthest from the chassis over to the impedence selector switch. How can I mount the buss wire in this situation?

The buss is only for convenience accross the 4 terminals of the output jacks to the selector. You can simply use one jack and solder whichever tap from the OPT you need strait to it. If your cab is 8 ohms use the 8 ohm OPT tap. Now for the negative feedback you can choose to take it from whichever tap. If you use the 8 ohm on the jack you can simply solder a wire from the tip of the jack the 8 ohm tap is connected to strait to the NFB resistor on board. If you want to use a different tap from the one on the output jack (say 16 ohm is on the jack and you want to stick with getting the NFB from the 8 ohm tap) you can simply solder whichever taps wire strait to the NFB resistor on the board.

5. Do I have a bias tap wire which will attach to the circuit board?

Typically not on a 50W. You tap the bias supply from the secondary AC B+. Those are the wires running from the PT to the standby then the rectifier diodes. You want to take it BEFORE the rectifier diodes so solder a wire to on of the diodes (the opposite of the side they join together) and run that wire to the Bias supply input on the board.

4. Which wires attach to the standby switch?

The 2 secondary AC B+ supply wires. Im not sure which wires they are for you because I dont know which transformer your using but you can email or call the manufacturer or if its a common one someone here may know the color codes. The AC goes to the SB switch, then to the rectifier diodes to be recitified to DC.

3. Which wire attaches to the mains fuse holder (again my amp only had one 1 amp fuse, unless I also install a 4 amp fuse).

When the power cord come into the chassis one lead will go to the fuse holder and then to the PT primary. Its different on different amps. Some amps have the same 120VAC lead going to both the Fuse then the AC switch and other have each lead giong to either the fuse or the AC switch then the PT. You PT has primary leads that attach to these lines.

1. Which wire attaches to the lower two terminals of the power switch?

2. Which wire connects to the nearby(?) filter cap and connects its positive terminals together?

3. Which wire attaches to the mains fuse holder (again my amp only had one 1 amp fuse, unless I also install a 4 amp fuse).

Maybe some of the other guys here can help you identify the leads from the PT. I think the 2 blue wires are probably your AC line to the standby then to the rectifier diodes. If thats the case the black wire between them is the CT and is grounded. The black wire between the 2 yellow wires is the 6.3VAC CT and is also grounded to help with the noise from the 6.3VAC heater supply.

[Billy Batz]

That was me.

[Trem Abuser]

Billy,

Thanks a lot for that detailed response to my questions.

My amp did not have a standby switch previously, but I plan on installing one with the new board (seems a better idea to have one rather than not have one).

Looking at the schematic for the Laney, I'm wondering if the blue wires (+12v and -12v) can be attached to the new standby switch? They weren't attached to the standby switch previously as there was no standby switch (obviously). The blue wires were attached to a separate set of rectifier diodes from the diodes the primary tap was attached to and appear to power the "treble boost" and "bass boost" functions of the Laney. (BTW, the Laney also had reverb which I do not plan on keeping at this point.)

[Billy Batz]

Oh and Trem the bleeder resistor doesnt go between the 2 positive terminals. It goes between the positive and ground terminals. Its possible your Laney amp has the resistor between Filter sections wired directly to the caps but thats not a bleeder.

I looked up a few laney schems and I dont see any with the color code scheme you talked about on the PT. The orange sounds like it may be the primary and the blues the AC secondary. You may be able to call laney. Their still around right? Maybe someone here knows. The primary shouldnt be hooked up to any diodes. As far as I know anyway. Im looking at a Laney schem and it has the primaries as a set of brown and blue wires and the AC secondary a pair of red. Its obviously not the same.

[Trem Abuser]

Oh and Trem the bleeder resistor doesnt go between the 2 positive terminals. It goes between the positive and ground terminals. Its possible your Laney amp has the resistor between Filter sections wired directly to the caps but thats not a bleeder.

I looked up a few laney schems and I dont see any with the color code scheme you talked about on the PT. The orange sounds like it may be the primary and the blues the AC secondary. You may be able to call laney. Their still around right? Maybe someone here knows. The primary shouldnt be hooked up to any diodes. As far as I know anyway. Im looking at a Laney schem and it has the primaries as a set of brown and blue wires and the AC secondary a pair of red. Its obviously not the same.

Billy, thanks as always. OK, you said that you had not seen a 50W with a bleeder resistor on the mains filter cap. If anyone knows whether the 50W needs or should have the bleeder resistor, please let me know.

On the PT, on the DC side, the red wires are marked as "310v" (Question - is this the "primary"?). The blue wires are marked as DC voltage of "+12v" and "-12v". Then the heater wires are yellow, at 6.3v DC. I believe I know where to hook up the red and yellow wires, but I'm confused on the blue wires.

On the AC side, the amp has brown and black wires (2 each). I probably have the wrong idea about what is the primary and what is the secondary, etc. Any help on this would be greatly appreciated.

Scott

[Billy Batz]

I would think theirs no reason not to use a bleeder. I think in this situation though you need to up the value to 220k.

The AC side is the primary. Those 2 wires connect to the power cord via the AC switch and fuse.

OK so on the "DC" side the Red sounds like your B+. So you would connect the red to the rectifier diodes. As for the others I really dont know. You got your heater supply but I cant help with anything else.

Try posting these questions on another page. Heres something I overlooked the whole time Trem. I dont think you can use that transformer for a 50W Marshall circuit. You should either get in touch with a manufacturer and ask them about it or think about buying a set of Marshall transformers.