Gibson GA50 clone issues

[Duck Dodgers]

Wanted to build a Gibson GA50 clone. Back in August '22 I found didit's original “Amp Garage” post and sent a pm to him for some info. Thanks Ian! Used his schematic with minimal changes and added a tube PS. This is the first amp I've built in about 20 years and it's giving me some grief.

First issue; on startup LOUD snap, crackle and pop noises. Once it's warmed up a bit it quiets down except for the second issue. I suspect a tube but don't have any spares.

Second issue; the infamous 120Hz buzz. Does not increase with volume or tone controls. Preamp components (everything to the left of the inverter) and controls are grounded at the input jack. All other components are grounded to a lug near the transformer.

Re-arranged the grounds. Filter caps, transformer CT and mains grounded near transformer. Everything else, preamp and all parts between inverter and power tubes, are grounded together at the input jack. No change. Put it back the way I built it.

Scope traces are from V2 plates. The rounder one is P2. The other, squarish one is P5. Scale is 5mv vertical and 2ms horizontal. Also the traces are very jumpy. It was hard to get a decent shot of them.

Good-ish news; seems to have very mellow jazzy tone. If I can get rid of the buzz it will be interesting to try to optimize it.

Thanks for any help.

[xtian]

First issue; on startup LOUD snap, crackle and pop noises. Once it's warmed up a bit it quiets down except for the second issue. I suspect a tube but don't have any spares.

You get loud noises immediately on startup? Or after 15 seconds as tubes start operating?

[Duck Dodgers]

Right at start up.....

[LOUDthud]

With two chokes the amp should me quiet as a mouse unless the ground is not right. Running the center tap straight to the chassis is not a good idea despite what Leo did. I have edited your schematic to show how I would do it. Some will argue with my method, follow it as much as you can starting with the center tap past the two chokes. Past that, it's not so critical.

Noise on startup: Hard to explain. Try turning the amp on with the power tubes not installed. Are those 6L6 {nothing}, the old metal tubes ? You should ground pin 1 on each 6L6 socket to ground the metal case.

GA50 LTG.GIF

[Stevem]

Place a sheet of aluminum foil bigger then the chassis on a small section of wood.

Sit the chassis open side down on it such that it makes a good connection ( use a clip lead if need be) does the 120 hz buzz then mostly go away?

[xtian]

Right at start up.....

I would suspect the rectifier. Got another to try? Or you could just use silicon diodes instead, to test the hypothesis.

[Stevem]

Place a sheet of aluminum foil bigger then the chassis on a small section of wood.

Sit the chassis open side down on it such that it makes a good connection ( use a clip lead if need be) does the 120 hz buzz then mostly go away?

To track down your other noise issue I would say it’s a tube or a noisy plate load resistor.

I would first disconnect the audio input at V2A and see if it’s still there.

If that does not do it then I would swap in new plate resistors in the PI section .
Also make sure you have no DC leaking back to the 1 Meg on the input jack.

Sizzling frying bacon sounds are aways a resistor, constant sizzling like light rain on a steel roof with no popping is more then likely a tube.

Your condition since it get better with heat could be a tube socket to tube pin connection issue.

Even low resistance value resistors like on the cathode of a tube can introduce noise.

[bepone]

i can agree+small correction on the upper diagram, tone stack contains also grid leak for V2 tube, and must be grounded to V2 cathode resistor

GA50 LTG.gif

[didit]

Hello --

Completed my prototype over five years ago with well faded recall of various bring-up frustrations. Hummed initially, but the worst was fairly quickly eliminated with some attention to grounding. It had an isolated jack day-one and the preamp+PI ground on the strip terminal for the pair of filter caps downstream of the 2nd choke --- a quasi star. Uncertain if this was necessary. Have a few dusty notes to myself about redoing power amp layout and a few parts to replace/upgrade including beefier brick for the cathode resistor. Life mostly got in the way and it sits cardboard-boxed on a shelf awaiting freer time. Another planned tweak is to use the 5V "rectifier" winding to feed a regulated DC filament supply for preamp and perhaps also PI. I'll have time and be better motivated later this year. Will update with any fresh insights.

Definitely recall having sort of small set of 6SJ7 to find some well behaved --- microphonics and in an instance of one tube some form of nasty static noise. Suggest you pull that and see if your startup noise and/or hum is reduced or goes away.

Don't give up.

Best .. Ian

[Duck Dodgers]

Doing the easy things first...

Pulled a GZ34 out of another amp. No load noises on startup but the 120Hz was still there. Put the original 5V4 back in. No load noises after 2-3 tries. Possibly a poor connection in the socket?

Tried shielding with a piece of aluminum. Also brought the amp into the house so it would be on another power circuit and away from our solar inverter. No change in the noise.

Trying to get my head around the schematics. Rather than sharing a common ground with the filter caps, the components in each section are tied to their respective filter cap then the filter caps are on a dedicated "ground" from the center tap. Looks like the input jack sleeve would still be grounded to the chassis as well as the treble pot or, should I isolate the input jack and tie the pot (RP2) into that section? I currently have a buss wire soldered to the back of the pots and to the sleeve of the input jack (you can see it in the photo) should I disconnect this? This won't be too difficult to implement (you can see the grounding and power busses in the photo), but, the center tap and circuit ground not being tied to the chassis has me a little baffled.

Thanks for the help. Be patient, I'm a (retired) mechanical engineer.

Cheers

[Phil_S]

I suspect a ground loop or a missed/dry ground solder. It is difficult for me to see the ground buss(es) in the photo. Are able to post maybe 2 or 3 up close, slightly overlapping photos of the open chassis? I'd like to get a better look.

[Duck Dodgers]

Photos Attached. You should be able to open photos and then click on them to enlarge.

Preamp section and pots are grounded at the input jack.Input jack is not isolated. Remainder, filter caps and inverter and main amp parts are on a buss that goes to the chassis and attaches where the Transformer CT and line ground attach. Ground and power busses are twisted pair of 22ga buss wire. Pretty easy to follow.

[bepone]

i dont understand this layout, looks like scrambled. critical thing is where is grounded capacitor minus terminal for first tube?

must be on the star point made with these wires>
-V1 g1 grid leak resistor,
-V1 cathode resistor and bypass cap,
-ground of the input jack / moreover it is critical that input jack gnd and cathode V1 gnd wires are short as possible and go to the same point/because V1 tube where is biggest impact to Signal/Noise ratio, is amplifying Ugk voltage, means Uinput - Uk, measured from the same gnd point!

-ground of the chassis

all this must go in one point with minus pole of capacitor supplying this point. all other options are not acceptable... because all those points are referenced to gnd of V1 capacitor which is providing signal current for this first stage.

then after it is the same story for V2 tube. etc. if this is correct there is no any humm for some normal valve.

[Duck Dodgers]

I thought a sketch of the current Power Distribution might be helpful.

Pretty self explanatory.

Cheers

[LOUDthud]

Break the ground between C3 and C4 and connect C3's ground to the ground that connects to the input side chassis ground.