Tweed deluxe spitting at me !

[maxkracht]

That little terminal strip next to the tube socket also looks real crusty and is being used as a ground point. Tightening that screw might help. Better yet, unscrew completely, scrape off any rust/oxidation, and reinstall. Make sure there is a star washer under there so it gets proper contact, or run another wire to a better ground point. Cleaning and reflowing solder joints could also help. Looking closer, seems like there is a fair amount of oxidation. That might have seeped into the socket itself. Everything should be shiny, not fuzzy and white/green...

[sluckey]

Those caps on the grids of the input tube serve a purpose. Many amps (maybe even all) that have some sort of VVR or power scaling will cause a dc voltage to appear on the grid whenever the B+ is reduced to a low level. The symptom is the guitarist will complain about scratchy volume pot on his guitar. Adding a cap and a grid leak resistor between the tube and the input jack is the fix. I doubt they are causing the amp to spit.

[maxkracht]

Many amps (maybe even all) that have some sort of VVR or power scaling will cause a dc voltage to appear on the grid whenever the B+ is reduced to a low level.

Thanks, I haven't ever messed with low voltage preamp designs or whole amp power scaling and didn't realize this was a problem.

I doubt they are causing the amp to spit.

My thought was, if the pair of grid leaks at the tube socket, or just one of them, were intermittently connected due to corrosion or something else, you would get volume fluctuation. Could be spitty.

[hebaton]

Since both sides (A and B) of your 12AY7 are idling , runnng off of the same cathode resistor and bypass cap that would leave these 3 possibilities for your issue in terms of the preamp section or the (12AX7 ) driver / pi section.

1) a bad 12AY7.

2) a bad .1 blocking cap on the brite output
A simple swap of these two .1 uf caps would prove that out.

3) a bad 100k plate load resistor.



Once again a swap would prove that out.
And yes you could read good voltage at the tube pin yet still have a issue when the tube is being driven with audio.

These are the easy things I would start with.

Ps.

I would start out by soldering a wire or placing a clip lead across the .1 uf caps off of the 68K resistors.

When you say ".1 blocking cap" are you refering to the ons on the inputs or the last thw on the board before the filter caps ? ?

[hebaton]

That little terminal strip next to the tube socket also looks real crusty and is being used as a ground point. Tightening that screw might help. Better yet, unscrew completely, scrape off any rust/oxidation, and reinstall. Make sure there is a star washer under there so it gets proper contact, or run another wire to a better ground point. Cleaning and reflowing solder joints could also help. Looking closer, seems like there is a fair amount of oxidation. That might have seeped into the socket itself. Everything should be shiny, not fuzzy and white/green...

ThIs amp was obviously exposed to high humidity for a long period... I hope it doesn't need every solder joint redone ! I see oxidation everywhere !

[maxkracht]

He means the caps on the board, not the ones hanging off the tube sockets.

If it were mine, I would focus on cleaning. Make sure the amp is unplugged and doesn't have more than a couple volts on the big filter caps then give it a good once over. An acid brush or cotton swab and some alcohol should clean up most of that. Cleaning inside of the tube socket and input jacks is also important. Corrosion can cause intermittent connections and do some weird things. Shouldn't have to resolder everything, but even if you do, it shouldn't take too long.

[sluckey]

He means the caps on the board, not the ones hanging off the tube sockets.

He said, "I would start out by soldering a wire or placing a clip lead across the .1 uf caps off of the 68K resistors." This is the caps on the INPUT. The layout shows them on the board connected to the 68Ks and an alternate jumper wire across them. But they were moved off board to the tube sockets.

It's best to leave the caps and grid leaks as is if you will keep the power scaling. The soldering on that socket and terminal strip look suspect to me too. Makes me wonder if acid core solder was used?

[maxkracht]

maxkracht wrote: ↑Sat Aug 12, 2023 8:07 am
He means the caps on the board, not the ones hanging off the tube sockets.
He said, "I would start out by soldering a wire or placing a clip lead across the .1 uf caps off of the 68K resistors." This is the caps on the INPUT. The layout shows them on the board connected to the 68Ks and an alternate jumper wire across them. But they were moved off board to the tube sockets.

He was referring to two things in that post. First, swapping caps on the bright OUTPUT second jumping the caps on the INPUT. Pretty sure jumpering was just a quick temporary make sure there isn't something going on, not a permanent thing. Wouldn't hurt anything. I think any of those caps being bad is unlikely, unless a lead just snapped.

[hebaton]

HI everyone,

It looks like oxidation is the bad guy here. After cleaning the pins of the 12ay7 and "wigling" it in the socket for agood minute, I got a clean voice ( well a normal one ) from the normal channel.
I think I'm going to scrub a few things in there, try to remove tha oxidised material off the soulder and in all the tube sockets + the input jacks...

Thank you all for te support and suggestions. I will report on this in a few days and let you guys know if it solved the issue.
Any suggestions as to how to get this done, and what to prioritise ( I don't want to start re soldering everything ) will be much appreciated !

Claude

[maxkracht]

Tube sockets are the priority. making sure the terminal strip connected to the tube is clean and properly grounded is also important. After you get most of the gunk off the outside, you can drip rubbing alcohol through the tube socket, then use a toothpick to scrub the contacts where the tube pins go. Then some more alcohol and it should be good. Holding a clean paper towel under the socket while put alcohol through it will stop you from making a mess, and you can tell when the liquid coming out is no longer dirty.

[Stevem]

Your whole issue could very simply be a poor ground connection of the input jack on that channel, or a poor solder connection on the ground side of that 1 Meg resistor.

If each 1 Meg resistor is not getting a solid ground then the input grid of that tube does not know what the hell to do with the audio signal voltage getting applied to it, to describe it in a simple way.

Also Alcohol is not a good flux remover.
Get a spray can of the real stuff if that’s what you’d looking to do.

Many times on builds with issue related to the tube sockets and tube pin connectivity is simply that flux has flowed down into the tube pin revciving area and made for a bad connection.

In a pinch I have used a non toxic automotive brake cleaning spray to clean things out, but this stuff can remove paint and harm the control panels so be careful with the stuff.

[maxkracht]

Also Alcohol is not a good flux remover.
Get a spray can of the real stuff if that’s what you’d looking to do.

I have a mostly full can of what I thought would be good, MG chemicals, flux remover. Has been sitting on the shelf for years. I much prefer 90% IPA and some scrubbing. I dilute my liquid flux with that as well...

I could be wrong, but Heard someone say brakleen was similar chemistry to the crc electronics cleaner but electronics cleaner has less active ingredients. Alcohol also strips some paint, so be cautious with everything...

[TUBEDUDE]

I use IPA and a tooth brush. For stubborn crust Acetone saves the day. Use caution and work fast with it as it evaporates with the quickness.