Recent Posts

81

Tube Amp Building - Tweaks - Repairs / Re: Kay 703 grouding scheme and crazy squealing

« Last post by choosebronze on July 20, 2026, 12:20:53 am »

Okay I reread Rob Robinette's article on widowmakers for like the 4th time. And I read a couple more articles about converting antique radios to isolation transformers. I'm actually thinking I wired the PT the correct way.

I did some chopsticking and moving the wires around definitely changes the squealing, and turning the pots does too, big time. So I think a wiring problem is more likely. But anyway, here's what this craziness sounds like: https://limewire.com/d/oPfwo#T0VhQu8Isf

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Tube Amp Building - Tweaks - Repairs / Kay 703 grouding scheme and crazy squealing

« Last post by choosebronze on July 19, 2026, 08:42:53 pm »

I had most of the parts to build a Kay 703, including an old 6" combo amp willing to donate its cabinet and speaker. So I figured I'd build one for my daughter to have a little practice amp. And boy has it been a learning experience...

The original schematic shows T1 but I gather from prior posts that T1 wouldn't have been a true isolation transformer. I replaced that with a Triad N68X in my build. Other than that, adding a fuse, and deleting two of the input jacks, I built the amp just like the schematic. I read another post where Sluckey recommended adding an isolation tranny to a widowmaker and he showed what I would've basically expected, just dropping an isolation tranny between the hot and neutral wires with no other circuit changes.

But I'm getting stuck. I've built a bunch of amps and I feel like I understand that power scheme well, but the isolation transformer is really throwing me for a loop. The N68X has two secondary wires and a shield wire. Shield goes to the chassis/safety ground. The other secondary wires confuse me more. I originally had them wired the way I'd wire any other PT (they were wired as a series heater string and rectifier pin 5), but that gave me very weird voltages and no audio. So instead I wired a secondary wire to pin 5 of the rectifier. The other secondary wire sent to chassis ground, and redid the heater wires, just like it's drawn on the schematic. I don't.... exactly understand. It seems like that would've electrified the chassis? But in any event, that worked and the amp now plays. So something about that must be right even if I don't understand what it is.

Now I do have guitar audio, but I also have an incredibly loud squeal. Not a little hum. Not a "noisy" amp. Full on screeching that changes with the volume and tone controls. Happy to upload audio if you need a laugh. I went to ask AI about that (I know, sorry sorry) and it confused me even more!

It said that I don't want any part of my circuit touching the chassis anywhere - true "isolation," which I guess makes sense. That all "grounds" - my PT secondary return, input jack, all ground points in the circuit, everything, should go to one floating "ground" point that never touches the chassis. And the only things that should go to the chassis are the earth safety ground and the PT shield wire.

I'm now very confused, and very much not wanting to accidentally shock myself. Like I said, I'm comfortable working on a standard amp but I feel like I have to quadruple check everything with thxis thing because the electricity isn't necessarily where I expect it to be.

I am incredibly grateful for any help or advice you guys have.

83

Tube Amp Building - Tweaks - Repairs / Re: Single tube Reverb headache

« Last post by shaun on July 19, 2026, 11:56:53 am »

Hi Willabe,
I forget the book titles because I lent them to a guy who never returned them. But the link in acheld's post brings back memories, and one of the books I lost was by the Valve Wizard, so I'm pretty sure I followed his ideas. I guess we are all basically recommending what the wiz calls star grounds, or multiple star grounds.

84

Cabinets-Speakers / Re: a blown PA speaker

« Last post by acheld on July 19, 2026, 09:40:21 am »

Not for a PA speaker which has otherwise sounded good.

85

Other Topics / Re: Meme of the day

« Last post by shooter on July 19, 2026, 06:52:35 am »

....

86

Cabinets-Speakers / a blown PA speaker

« Last post by rake on July 18, 2026, 11:36:11 pm »

Hey guys I have an old pair of Carvin 805 Pa speakers 200w at 8ohm 10" + a horn.
While out on loan one of them lost the 10" speaker. (open coil) searched for a re-cone
kit or separate parts but no replacement for the odd sized coil. Located a suitable
replacement and ordered a pair. Also found new horn drivers. On a closer look the
Crossovers contain only two components. a coil and a ceramic wire wound resistor.
My question is, would you guys upgrade to a better pair of crossovers?

87

Tube Amp Building - Tweaks - Repairs / Re: 5E3 Build, Pre-PI Master Volume Question For The EE's

« Last post by sluckey on July 18, 2026, 05:43:25 pm »

I don't think so.

88

Tube Amp Building - Tweaks - Repairs / Re: 5E3 Build, Pre-PI Master Volume Question For The EE's

« Last post by Jazzcab on July 18, 2026, 05:33:47 pm »

Both caps are necessary. The 470K grid stopper is not needed but you may like it.

Thanks for your insight. Wouldn't the 1M master pot be doing the same sort of limiting to the V2b grid at lower settings as the 470K grid stopper would be doing with the 1M master at max volume?

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Tube Amp Building - Tweaks - Repairs / Re: 5E3 Build, Pre-PI Master Volume Question For The EE's

« Last post by sluckey on July 18, 2026, 04:42:34 pm »

Both caps are necessary. The 470K grid stopper is not needed but you may like it.

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Tube Amp Building - Tweaks - Repairs / 5E3 Build, Pre-PI Master Volume Question For The EE's

« Last post by Jazzcab on July 18, 2026, 03:26:30 pm »

Hey everyone, I'm finishing up an 5E3 build and needed some help and clarity from the more Electrical Engineering minded folks of the forum.

I've read many have had success taming some of the rudeness of the 5E3 (when pushed hard) by placing a grid stopper resistor before the V2b PI. Then some will add that on a switch to have the best of both worlds. I was originally going to do this mod but then...

I was reading there's a camp that instead of that will pull the 1M resistor before V2b PI and replace with a 1M pot to tame the harshness but also adding a master volume in the process as shown here.



That sounds like an upgrade to the resistor on a switch but some folks report this pot will have a limited response with most of the effect scrunched up into 1/4 to 1/3 of the range of the pot.

Then there's this one below. People report getting a full range from the pot. My question can I omit one of the .022uf caps or are they both needed, can and the 470k grid stopper resistor go? Seems like the 1M pot would be taming the harshness on its own?



TIA