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50 watt amp project
#1
Hey all;

looking for some help, i have put together the power supply kit PSU TPA and the power amp PA66, 50 watt version. i had the power supply working in a very prototype way, transformer and power supply board laying on my bench , but working. decided to add a power switch, it works, output transformer, power amp and front end. Hooked up an output jack and impedance selector, kept the options to a minimum at this point, master volume and presence control ( the ones available on the PA66 board ). Stuffed it in a hammond chassis, plugged it into the power limiting safety socket and turned it on, no tubes.

But, it doesn't work. I am trying to trace through the circuit to find the problem but thought I would ask on the forum if someone might know of likely suspects I should look at.

What I do know is, when I turn the amp on the light bulbs on the safety socket come on and stay on, fairly bright. The fuses do not blow, these are the fuses supplied with the kit and I am not sure offhand the wattage or blowage?? of the fuses. The panel LED does not turn on, nor does the 12v fan. Kind of afraid to leave it on long enough to check for voltages anywhere, I have left it on for a 10 count a couple of times, nothing hums or smokes or anything.

Is it possibly a messed up MOSFET?, any ideas on what to look at and how would be greatly appreciated. I have been trying to follow the AC supply but I also wonder if a short to ground would take out the fuses. What might draw power but not blow up the amp???

Greatly appreciate any help I can get...

Rick
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#2
Hey Rick

It sounds like you are doing the step by step testing that TUT3 says to do but you missed a step. For me that was a big lesson "baby steps" cuz I used to blow up stuff when I built it and just hammered on the power Smile

Did you connect the power amp to the PSU while it was all still just out of the box on your bench? You coulda checked voltages on the amp part to make sure things are cool there. Must be something in the PA or the wiring between the two parts since the supply already worked.

That's my guess.
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#3
Greetings!

Sounds like a big project so it's good you are taking your time to check things as you go along. From what I've seen and heard, most problems with electronics that does not work comes down to solder connections.

In a new build like yours, there is the added bonus of solder bridges. Some of those are so small you either have to use an ohmmeter - my personal favourite kind of meter - or just go brute force like a Cockney sailor and score around the pads and traces with a sharp but fine knife. In any case, you need good lighting to see the board properly to then see where bridges are that shouldn't be.

That sounded Shakespearean.

Cheerio
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#4
(12-06-2019, 10:59 PM)nauta Wrote: Hey Rick

It sounds like you are doing the step by step testing that TUT3 says to do but you missed a step. For me that was a big lesson "baby steps" cuz I used to blow up stuff when I built it and just hammered on the power Smile

Did you connect the power amp to the PSU while it was all still just out of the box on your bench? You coulda checked voltages on the amp part to make sure things are cool there. Must be something in the PA or the wiring between the two parts since the supply already worked.

That's my guess.

Thanks for the input Nauta, very much appreciated. I had hooked up the plate and heater supplies from the PSU but I didn't have the OT hooked up at the time. I borrowed a friends drill press and got busy with drilling the front and back panels and the bottom plate ect. The problem I was having at the time was the whole system gets a little out of control with the power transformer, power supply board, PA, and PAF board. The PAF board is especially small and light. The connecting wires were lifting the boards and everything was a bit out of control. I connected everything to the bottom plate of the enclosure and hooked up the OT, then finished the rest of the connections, yes, I should have tested when I hooked up the OT, before putting everything together. Maybe time to take it out of the box, easier to see everything, and go over the PA and OT connections. Thanks...
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#5
(12-06-2019, 11:25 PM)Sherlok Ohms Wrote: Greetings!

Sounds like a big project so it's good you are taking your time to check things as you go along. From what I've seen and heard, most problems with electronics that does not work comes down to solder connections.

In a new build like yours, there is the added bonus of solder bridges. Some of those are so small you either have to use an ohmmeter - my personal favourite kind of meter - or just go brute force like a Cockney sailor and score around the pads and traces with a sharp but fine knife. In any case, you need good lighting to see the board properly to then see where bridges are that shouldn't be.

That sounded Shakespearean.

Cheerio

Thanks Sherlok, you make some very good points. The older I get, the more important good lighting becomes. I have a nice magnifying lamp that helps me see all the small stuff on the boards but I have been thinking of improving the overhead lighting also. Perhaps that should be my first job. You make a very good point with the solder bridges. I have tried to see if anything looks wonky but I think it's time to pull each board out of the box and get a good view, top and bottom and like you said, make sure there are no bridges.

thanks...
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#6
Hi all;

So, to the best of my abilities, I followed some advice. The lighting was reasonable, now I have the lights I use to start veggie seeds over my workbench, full spectrum, very bright, no shadows, very nice. Checked the boards top and bottom, addressed anything I wasn't 100% confident in. Followed through the install notes again, fixed a couple things, changed one thing and fired it up. The lights on the plss worked as expected this time. Checked voltages, ok. Popped some tubes in, set up the bias. Plugged in a speaker and a synth to the input of the power amp and some sound came out. I already had two preamp boards ready, a Zen and Dumble kits. The Zen has fewer connections to the outside world so I hooked it up. it worked. The sound was very bitey, and aggressive, I assume that is the nature of the Zen preamp. Will take a bit of getting used to, never owned anything like this before. Still lots to do with adding options on the power amp, adding the Dumble preamp, effects loop, ect.

When I shut it down after playing the first time, the amp didn't seem to want to turn off, kept playing. Turned it back on, back off, but still seemed to keep on playing. Back on, realized the power light was going on and off as expected. turned it off and kept playing, this amp plays for a while after it is shut off. is the the power supply caps or did I mess something up. My other amps all shut of pretty quick when I kill the power.

now I can rest a bit and just play...
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#7
Hi TiCatFan

Yes, our PSU has a lot more energy storage than the average guitar amp supply does.

I always keep bashing on the strings after power is turned off, as this actively drains the caps and you can hear the progress. It is especially important to do this if you are in the process of doing mods or repairs or incremental building. You don't want to be surprised by a charged cap.

Even though our supplies have bleeder resistors that will dissipate the cap charge over time, it will take a while for this to happen since the bleeder R value has to be a compromise between wasting power and heat, and discharging the caps in a "reasonable" time. The PSU has 440uF of filtering as a minimum - it actually works out to quite a bit more - and this is 4-8 times what is in many guitar amps.

So, always turn the power off and keep playing until the sound fades away.
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#8
Hi all;

newbie needs help again. so far everything is working. but there is another preamp to install into my build. I have the D-PRE board assembled, but not connected to switches or the rest of the amp. Since this amp has the Z-PRE already installed i need to install a channel switching mechanism. that would be cool.

So, Q1 - I have a C&K 7101 switch on the panel to control, and a RLY (relay) to execute the switching. I think I can wire the RLY okay but am unsure how to wire the switch. When coming off the coil which of the 3 lugs do I solder to, which goes to ground and which lugs get the cap installed on, kinda assume the two lugs used above.

Q2 - so I want to add foot switch to the panel switch, how do I wire the jack to make everything work well, and where do I put the cap on the footswitch??? From the schematics of RLY it appears I would connect coil side of the Panel switch to the tip contact of the Jack, and ground to ground between switch and jack. is this correct? do I need to add a capacitor in the footswitch or am I covered??

Got more questions probably, but I think this will be enough to get the D-PRE up and running

thanks all
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#9
Hi ticatfan

For the panel switch, you use the middle terminal and one of the others since all you need to do is turn the relay on or off. It does not matter which side goes to ground.

Connecting a foot switch via a 1/4" jack requires the use of one with a closed contact on the tip, just like the input jack for the guitar amp. The relay coil ties to the jack tip. One side of the switch ties to the closed contact. The other side of the switch ties to the jack sleeve which is ground.

The cap goes from the jack tip to ground so that both the remote switch and panel switch benefit from its presence.

Note that when the foot switch is plugged in, the panel switch is disabled. This is the standard way to wire such an option.

An alternative is to wire the panel switch inparallel with the remote switch simply by using only the jack tip instead of the closed contact. This introduces an ergonomic snag where if both switches are connected , one of them has to be in the open position for the other one to work. So, to use the foot switch say, the panel switch has to be in the 'off' position and the FSW can do the channel changing as you want. There's a reason this wiring is rarely used and mostly only by accident.
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#10
Thanks Kevin, really appreciate the feedback. I have read the sections on switching in TUT in the past but didn't understand much of it as I hadn't done anything with switches. This project has been quite the learning curve for me. I do have one more switching question. I have a 2 button footswitch and a stereo or Tip/Ring/Sleeve 1/4" jack. One switch we have dealt with, changing from Z-Pre to D-Pre, the other switch would be the D-Pre overdrive. The notes that came with the kit are excellent, but I can't figure out where or how to tie in the footswitch for overdrive control. And do I then put a cap from ring to ground also???

Once I get this done the basic amp will be complete, Of course there are still all the extras to try like switchable tube biasing, Triode/Pentode/Ultralinear control, BFX, power scaling . Got all the kits and parts, but I might want to just play for a while.

Thanks...
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#11
Hi TiCatFan

Yes, wire the Clean/OD switch to the ring contact of the FSW jack and the end of the relay coil(s) to the Ring. Add a cap from R to S.

The 2-button foot switch then has functions divided between the two switches: one selects which preamp (Z or D to use, while the second sets how D will function. When Z is selected, depressing the D footswitch just "presets" D to how you want it and you hear nothing from it until the other button is pressed to select D.

When D is selected, the Clean/OD button makes live changes to what you hear.

Overall, this is an ergonomically compromised switching approach but it is what 99% of manufacturers provide in their multi-voice amps. It may happen that it suits you Smile
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