Kay 752 amp - does this sound normal?

arcaxis said:
Attached is how I would likely mod the AC and switch circuit. It's kind of just removing one switch wire and moving it and replacing where it was with the white wire from the cord. If not sure I'll try to help further.
Click to expand...

This is very clear and looks very doable. Thanks! I'll let you know if I have any issues, but I think it should be pretty easy.


So ....

New switch is on the way. Once it arrives, I'll install it, making the wiring mod you described.

At that point, do you think it's safe to fire it up again? I can use my current limiter (100W light bulb in series) if that would help.

Considering the evidence, I'm pretty sure you're right that the spark likely came from the switch.
 
famous beagle said:
That looks right to me. I pulled the trigger. Should be here in two days.
Click to expand...

The one thing I'm not 100% sure about the pot is the diameter of the threaded part of the pot. I didn't notice a spec on the diameter. That one may be smaller than what is in the amp, but with some thin flat washers can be made to fit.
 
garww said:
A combo switch like that will be more common in radio parts. You can bypass the on/off as a temp measure if you have to.
Click to expand...
If he gets desperate I can dig through some of my parts bins....... :)
 

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One interesting thing I saw was that the wiring doesn't exactly match the schematic with regards to the rectifier tube.

The schematic shows the hot wire going to pin 3 of the rectifier (photo "Hot wire to pin 3 of rectifier").

But the wiring has the hot wire going to pin 4 of the rectifier (photo "hot wire going to pin 4 of rectifier - wide shot"). The gray wire (the original two-prong cord) with my red arrow drawn on it is the hot wire. The red circle shows where it enters the board.

The rest of it is kind of hard to make out in this picture, so I attached another photo ("hot wire going to pin 4 of rectifier - close shot") that clearly shows the pins of the rectifier and the path to pin 4 from that terminal.

Is this because pins 3 and 4 are internally tied in the tube or something? Is that why they're drawn that way (with the "V" shape connecting them) on the schematic?
 

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famous beagle said:
This is very clear and looks very doable. Thanks! I'll let you know if I have any issues, but I think it should be pretty easy.


So ....

New switch is on the way. Once it arrives, I'll install it, making the wiring mod you described.

At that point, do you think it's safe to fire it up again? I can use my current limiter (100W light bulb in series) if that would help.

Considering the evidence, I'm pretty sure you're right that the spark likely came from the switch.
Click to expand...

From the plug end of the cord, identify the prongs that are hot and neutral and use an ohmmeter to ensure the 'hot' prong goes to the switch and the 'neutral' prong to the 'common return' point.
The light bulb limiter wouldn't be a bad idea as well when powering up.
 
arcaxis said:
From the plug end of the cord, identify the prongs that are hot and neutral and use an ohmmeter to ensure the 'hot' prong goes to the switch and the 'neutral' prong to the 'common return' point.
The light bulb limiter wouldn't be a bad idea as well when powering up.
Click to expand...

I did do a continuity test when I first got the 3-prong and confirmed that, when looking at the plug with it facing you (as if you were going to plug it into your mouth), the black wire is the top left, the white wire is the top right, and green was ground.

I had the plug mapped correctly; I just wired it the wrong way like an idiot. :)
 
famous beagle said:
One interesting thing I saw was that the wiring doesn't exactly match the schematic with regards to the rectifier tube.

The schematic shows the hot wire going to pin 3 of the rectifier (photo "Hot wire to pin 3 of rectifier").

But the wiring has the hot wire going to pin 4 of the rectifier (photo "hot wire going to pin 4 of rectifier - wide shot"). The gray wire (the original two-prong cord) with my red arrow drawn on it is the hot wire. The red circle shows where it enters the board.

The rest of it is kind of hard to make out in this picture, so I attached another photo ("hot wire going to pin 4 of rectifier - close shot") that clearly shows the pins of the rectifier and the path to pin 4 from that terminal.

Is this because pins 3 and 4 are internally tied in the tube or something? Is that why they're drawn that way (with the "V" shape connecting them) on the schematic?
Click to expand...
3 and 4 are the heater for the cathode in the 35W4 (not internally tied). Not a big deal as the tube doesn't care which pins have the voltage. The guy that designed the circuit board decided it was easier to connect the way it is rather than go by the schematic which would have criss crossed the copper traces. Good catch to notice that :thumbs up:
 
famous beagle said:
Ha! And my wife thinks I have too much junk. :)
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That little bin of pots is just a small microcosm of the what I have under my workbench and the corner of the second floor of my barn. A New England mindset that you save everything thinking that one day you'll need it. :)
 
Just pointing this out because it took me a minute or two to realise..

The heater, like a filament bulb, doesn't care for polarity so, as arcaxis says, who ever put it together decided it was easier to flip them than jump one trace over the other.

Pins 3+4 of V2 are drawn at the bottom right of the schemo, away from the main tube diagram, because the two heaters are series wired and it's much easier to make it out that way.
 
arcaxis said:
3 and 4 are the heater for the cathode in the 35W4 (not internally tied). Not a big deal as the tube doesn't care which pins have the voltage. The guy that designed the circuit board decided it was easier to connect the way it is rather than go by the schematic which would have criss crossed the copper traces. Good catch to notice that :thumbs up:
Click to expand...

Oh, ok! Yes I see that now. (I know my example is a triode, and this rectifier is a diode, but still ... I can clearly see the triangle shape for heaters).

I don't know why I didn't notice that when I was building my Champ clone. (Or maybe I did at the time but forgot since then. That was 5 years ago or so.)
 

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Steenamaroo said:
Just pointing this out because it took me a minute or two to realise..

The heater, like a filament bulb, doesn't care for polarity so, as arcaxis says, who ever put it together decided it was easier to flip them than jump one trace over the other.

Pins 3+4 of V2 are drawn at the bottom right of the schemo, away from the main tube diagram, because the two heaters are series wired and it's much easier to make it out that way.
Click to expand...

Yes I see that now. Thanks for pointing it out. I do remember noticing "V2" next to that floating "3 V 4," but I wasn't sure exactly why they did that. What you said makes sense, though.

Learning more every day! And I'm officially feeling hopeful again for this amp. ;)
 
Update 5 (Is it 5?)

Ok, here's where we're at:

1. Replaced the filter caps with Sprague caps.
2. Replaced power cord with 3-prong using Arcaxis's modded schematic.
3. Replaced the switch (again wired with the mod).
4. Replaced the transistor.
5. Replaced the speaker with 8" Mojotone.

Tonight I fired it up with my light bulb current limiter. It didn't blow anything up, so that was good. At first the light lit brightly and then dimmed gradually, as it should, so that was good. However, then the amp started to hum, getting louder and louder gradually. It sounded like 60 cycle hum. No instrument was plugged in at all.

Here's what I've tried so far:
1. Swapped out the rectifier tube for a known good one.
2. Checked the ground connection of my 3-prong cord using continuity on my meter, and I can verify that the ground wire is making good contact to the chassis ground in the amp.

Still having the same problem.

Any ideas as to where I should look?

Thanks
 
Maybe a clue?

When it's humming, if I turn the volume knob down all the way, the hum gets more treblely. If I turn it up all the way, it gets more bassy. But the volume of the hum doesn't change.
 
the only hum Ive dealt with like that was a large cap solder (bad solder) on the power supply on a old Fender Bass amp.
i got all the tips from the gang here and online...and used a drum stick to tap around on the caps so as not to be shocked...and found the culprit.

then basically re-soldered the one spot and touched a few others....and that 100% fulltime buzz was gone, the amps amazingly lasted since without anything.

but it was buzzing with or without any gain. the dreaded hum.
 
CoolCat said:
the only hum Ive dealt with like that was a large cap solder (bad solder) on the power supply on a old Fender Bass amp.
i got all the tips from the gang here and online...and used a drum stick to tap around on the caps so as not to be shocked...and found the culprit.

then basically re-soldered the one spot and touched a few others....and that 100% fulltime buzz was gone, the amps amazingly lasted since without anything.

but it was buzzing with or without any gain. the dreaded hum.
Click to expand...

Thanks much for the tip. I remember someone a while back mentioning using a chopstick to tap on the components to check for bad solder joints. I assume that's the same idea. I'll give it a shot.
 
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