Name that buzz

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5dollazz
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Name that buzz

Post by 5dollazz »

Parasite? Bad ground? Leaky cap? Tequila? Man I hate tequila. And I hate this buzz. I've attached a crappy phone recording of it. It's kind of a fuzziness that abruptly "squelches" out as the note decays. The amp is a beat up 64 Bandmaster.

A brief history of my buzz: it only presented itself after changing the filter caps. I changed em back to the originals after that and it still buzzed. I also installed a different set of new filter caps after that and it still buzzed (they're still int here). I've since swapped/cleaned/tightened lots and lots of parts, one at a time (and put them back) hoping to track it down. It did disappear for a few months (MIRACULOUSLY) but now it's back. Any idea of what I should look at, which direction to go? I'm pretty sure it's somewhere in the PI or after as I can pull pre-amp tubes on either channel and it has no effect on the buzz.

Thanks for any insight or advice, I know this is vague. I'm just going nuts and when it was running cleanly I really dug this amp.
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billyz
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Re: Name that buzz

Post by billyz »

If you hate tequila it's because you never had good tequila! Try El Tesoro Anejo, or Gran Mayan extra Anejo.
As for your bad buzz, have you tried different speakers?
Stevem
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Re: Name that buzz

Post by Stevem »

Does it do it thru a different speaker?
I have had many preamp tubes do this.
Does the normal channel do it,if so than like you said,it's likely to be. In the PI section and you need to scope it out to see if it's oscillating on the PI input,or coming in on one of the grids to the output tube(s).
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
5dollazz
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Re: Name that buzz

Post by 5dollazz »

Thanks guys. Yeah I've tried different speakers, no luck. It does do it on the normal channel as well as the vibrato channel. I don't yet have a scope (working on that) but a while ago I listened with a signal probe at the PI input and heard it there. I'll go over that section again tonight.

I actually emailed a dude over the weekend about getting an old tektronix 2215 but it hasn't come through yet. Maybe this is the amp that pushes me to finally do it.

I've got some buds that swear by good tequila and I definitely like it more than that evil Cuervo from my 20's. Too many bad decisions. You're right though, I probably should expand my horizons and learn to love yet another kind of quality liquor. That can't possibly go wrong.
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billyz
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Re: Name that buzz

Post by billyz »

Could be a lead dress issue. I recently had a fender doing something similar , finally cured it with a 1.5K grid resistor. Might try it on the PI and see what happens.
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johnnyreece
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Re: Name that buzz

Post by johnnyreece »

I'm listening on crappy laptop speakers, but it sounds similar to what I had going on in my little 2W build. Here's my clip:

https://soundcloud.com/john-lamb-25/amp-distortion

We tracked it down to the PI. The verdict there was that my PI was driving the power tubes too hard, and I think I've got it sorted out. I'll try to listen on a better set of speakers soon, but I'm no expert (which is why I was asking for advice on mine :lol: ).
Stevem
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Re: Name that buzz

Post by Stevem »

Poor connections in the PI tube socket can make for oscillations due to increased resistance onto the gids acting like a antenna.
Find a round wound guitar string that will pass with a little force into each reciver on that tube socket and run it in and. Out with some cleaner, retention the recivers some and then see if the problem remains.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Blackburn
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Re: Name that buzz

Post by Blackburn »

johnnyreece wrote:I'm listening on crappy laptop speakers, but it sounds similar to what I had going on in my little 2W build. Here's my clip:

https://soundcloud.com/john-lamb-25/amp-distortion

We tracked it down to the PI. The verdict there was that my PI was driving the power tubes too hard, and I think I've got it sorted out. I'll try to listen on a better set of speakers soon, but I'm no expert (which is why I was asking for advice on mine :lol: ).
Very interesting! I just had an issue yesterday with a SE amp that was doing that same buzzy thing and I swapped chassis to find it wasn't the speaker. New tubes in the buzzy one and old tubes in the good one. Stevem mentioned pre tubes and I'll roll a few to see if this cures it.
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johnnyreece
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Re: Name that buzz

Post by johnnyreece »

Good luck! Yeah, I made all kinds of mods trying to kill it before focusing on the PI. Think it's a winner!
5dollazz
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Re: Name that buzz

Post by 5dollazz »

Thanks for the leads everyone. I'll definitely try cleaning the PI tube socket with a string before getting out the soldering iron tonight. I got some tattoo needle brushes for this purpose a while back but they don't have the constitution to get the job done.

I've got one question regarding a grid stopper on the PI input: do I only need to install it on the upper/non AC grounded grid? The other half gets its signal from the cathode right? Does the NFB play into this? OK that was more than one question, but with my limited understanding they all seem related. Thanks.
5dollazz
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Re: Name that buzz

Post by 5dollazz »

Also - johnnyreece, how did you end up fixing your buzz? It does sound similar to mine. Thanks!
5dollazz
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Re: Name that buzz

Post by 5dollazz »

Hey all, here's an update on what I did last night. Spoiler alert: the buzz is still there. Also I tested for the the buzz in between each step.

First I cleaned the PI tube socket with deoxit and a round wound low e string and then retensioned it. Then I tried a 100k resistor on the grid (soldered directly to pin 2) of the PI. I removed that after discovering it didn't change anything. Then I individually replaced each cap from the 500pf cap feeding the PI to the end and put them back in after nothing changed. Then I shotgunned all the resistors in the PI. No luck. The new resistors are still in there, I'll put the originals back in tonight. Then I changed the 470ohm screen grid resistors (I had already swapped out the originals when I did the cap job, they looked pretty bad and had drifted way high, I thought maybe one of the replacements was bad or something). Then I replaced the grid stoppers on the power tubes for the same reason (they had both drifted to 2.5k), no luck. Then my fiancee insisted that I stop making so much damn noise. It was pretty late so I guess I understand where she's coming from and I do have a habit of listening to loud guitar rawk when working on guitar projects.

There's not many other parts for me to try swapping after the PI. I could try new bias splitter resistors tonight. I realize that this probably isn't the most prudent way to find the issue, but I'm kind of at a loss. Any other ideas? I know I know I gotta get a scope. Working on it. Oh I also chops ticked like crazy last night. Nothing made a difference. I'm thinking of trying some shielded wire from the input jack as well. Thanks again.
Stevem
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Re: Name that buzz

Post by Stevem »

One thing you can do without a scope and pin down if it's a oscillation issue at least with the preamp tubes is to take a long enough length of wire that will let you solder it to a tube plate Andy than rap it 3 or 4 times around that same tube sections grid wire.
If it's a oscillation problem this will greatly reduce it or kill it altogether and at least you will know what the heck stage it's coming from!
Be sure too not let the non connected end of the rap wire touch anything else.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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johnnyreece
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Re: Name that buzz

Post by johnnyreece »

Well, my problem was a design issue; it was wrong from the get-go, and didn't get better until I biased the PI colder (along with some other things). Since yours is an intermittent issue, I'd guess something else might be going on.
5dollazz
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Location: NYC

Re: Name that buzz

Post by 5dollazz »

Thanks again for the tips everyone. Big news - I got a scope over the weekend. It's a Tektronixs 2215, it's a little beat up but works fine, and it even came with a Tek 10x probe. All for super cheap from a old school Russian dude with an old school TV repair shop, deep in Brooklyn. Playing with it has been a blast. SO NERDY. As an aside - does anyone have any links or recs for a place that explains just how to analyze waveforms in a guitar amp?

Anyway, I've determined that there my amp does have some kind of parasitic oscillation. Looking at the scope, it doesn't present itself until I get to the plates of the PI and only shows up on low guitar strings that are picked pretty hard. I couldn't get it to appear using a signal generator.

I tried a 500pf cap across the plates of the PI to no avail. I've all tried other 12AT7s in that slot. Any other ideas? Before getting the scope I disconnected the NFB and heard the problem, though I'm going to do that again and confirm it on the scope. Could it have something to do with the socket itself? I've already cleaned it well and re-soldered it, but I'm thinking of throwing in a new 9-pin because hells bells I've subbed in new parts for everything else around there without any luck.
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