Recent Posts

71

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

« Last post by choosebronze on July 20, 2026, 10:55:51 am »

Ooh, okay. Thank you Steve, that makes sense. So then 2 questions.

1. If I'm building the stock circuit with the Triad in front of it, do you have a suggestion for what I should use as T1? Thus far I'd been using the Triad in place of T1, not in front of it.

2. When built as you've shown, I assume the chassis remains my ground, and all of the floating ground stuff AI mentioned is bunk.

72

Tube Amp Building - Tweaks - Repairs / Anyone care to help me finish my build ?

« Last post by antolc37 on July 20, 2026, 10:37:10 am »

Hello all, I'm having trouble finishing a custom build I've put together on my spare time, it's basically a Marshall 1959 preamp (bright channel only) in front of a ECC99 in push/pull configuration, all fits in a 1U 19" rack I had lying around.

The amp is working "ok", quite loud and crunchy as expected, the only thing that is bothering me is a loud squeal or hiss (oscillation?) when the volume is pushed beyond 8/10 and I can't seem to be able to pinpoint the source of this (it seems to be around V2a~ when poking with a stick the area after the first coupling cap seems sensitive up to the V2 grid)

So far I've tried switching the 33k grid resistor on V1 with a 68k, I've also redone the shielded cable from the input jack, I've tried grounding V1b too (because it is not used at the moment) without much success.

Amp is split in two boards, the "power" board with diode rectifier bridge and capacitors/self/resistors network for providing high voltages, and the "sound" board with coupling capacitors, tone stack, cathode polarization etc.. each board has a ground bus and the ground buses are connected together to the chassis in a "star" manner. Main plug has its own wire screwed to the chassis also.

Power transformer is toroïdal 30VA, 220v primary (I'm in Europe), 0.085A@200V sec1, 2A@6,3V sec2. I can provide all the voltages if need be of course.

Output transformer is hammond 125C wired for 12,5kohm primary under 8ohms speaker.

As it is my first build, I'm not completely satisfied with the cable management and how some parts of the design ended up (namely heaters), I'd be happy to read your advice on that matter, I don't know if its bad enough to create that hi pitch hiss however, or maybe the choices I made in some component values are bad, anyway I hope you can help me finish this project, I can provide any information one would need for "debugging".

On the picture you'll see that both OT and self are not tied to the chassis yet, OT is supposed to be on the far right corner but I wanted to test the amp before screwing it on.

thanks all

73

Tube Amp Building - Tweaks - Repairs / Re: Princeton Reverb circuit refinements

« Last post by tristanc on July 20, 2026, 10:12:56 am »

An update from me - I've done little-to-no tweaking and just used the amp.

What I did do was listen to this video: https://youtu.be/CMps8xUBC2E?si=lwB2VXWQnwNyEN4e

Specifically these sections where the amp is on 10.

Strat: https://youtu.be/CMps8xUBC2E?t=1811&si=eG0lHkTMcCMhaVDI

LP: https://youtu.be/CMps8xUBC2E?t=1918&si=oZ0681HI2L_hF9NB

Tele: https://youtu.be/CMps8xUBC2E?t=2030&si=Sk4LZ4Rhrec5Qms9

Playing along at home, living room volume, through my attenuator, I think I get a very similar sound / distortion characteristic. I'll try and record some parts to do a comparison. So perhaps this is just The Sound of the Princeton and I'm over thinking it. And maybe the sweet spot of the amp is at 5/10 and drive should come from somewhere else.

To that extent, I bought a BD-2 clone to add a bit of drive at lower volume and the combo sounds great - especially with the Celestion Neo Creamback speaker I swapped in.

One finding was that at 'playing with a drummer' volume (4/10) I had to turn the gain on the pedal way down, compared to playing at home through an attenuator - perhaps the speaker coming in to play.

I'm still loving the amp. Love the form factor, the cleans at the point that things start compressing, and the reverb / trem. I wish I had built some of my other amps in to this form factor - particularly the higher gain ones.

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

« Last post by Willabe on July 20, 2026, 09:43:37 am »

star grounds, or multiple star grounds.

Those are 2 different things. Not the same at all. 

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

« Last post by sluckey on July 20, 2026, 09:41:59 am »

The attached file shows how to connect the Triad isolation transformer to provide complete isolation. Notice the two primary windings are connected in parallel.

There is no need to change any wiring on the original schematic.

76

Introduce Yourself / Hello everyone

« Last post by EL34 on July 20, 2026, 09:39:27 am »

Welcome to the Hoffman Amps forum

There are many different boards on this forum for different topics.

The Tube Amp Building - Tweaks - Repairs board is where you can post amp questions.

That board is listed here.
https://el34world.com/Forum/index.php?board=13.0

Thanks,
Doug Hoffman

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Introduce Yourself / Hello everyone

« Last post by antolc37 on July 20, 2026, 09:39:27 am »

Hi everybody, I'm new here, thank you for accepting me, I'm here to get help and hopefully provide some in the future !

Thanks !

Anthony

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Other Topics / Re: Meme of the day

« Last post by shooter on July 20, 2026, 06:46:43 am »

...

79

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

« Last post by AlNewman on July 20, 2026, 02:06:04 am »

I don't think you want your heater transformer to connect with your B+ at all.  I do think you need a ground reference for your heaters. 
Do you have a schematic as you have it wired currently?

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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