Another weird 69 SVT Problem

Did you try different basses?

 

Can you isolate the cab away from the amp and try it with the volume down really low, to see if it still does it?

I'm wondering if it's something physically resonating because of the sound coming from the cab being at just the right frequency.

If you isolate the amp from the sound of the cab and it still does it, I suppose it could be anything in there. If it only does it when the sound from the cab is getting to the amp, then I'd be more likely to suspect a tube getting microphonic.

Maybe you could try putting a piece of high density foam around one tube at a time to see if you can identify which tube it is? If it's happening because of resonance with sound from the cab, putting foam around the problem tube might lessen the problem or change the frequency (i.e. which note causes the problem).

 

JimmyM having a question about something Ampeg??? What has the world come to

 

What did they do to the amp.?
Could be a lot of things. Have the Elytics been changed in the last few decades.?
Best

 

Does it do the sme thing with a Gb?

 

Assuming your bass setup is good, only doing it on F# does indicate some wierd acoustical/mechanical resonance thing. Some bad solder joint/bad connection, loose tube or socket that only vibrates and shorts at that one particular frequency.

Were it me, I'd try checking as much simple stuff as I could hoping it was something simple first, but it may be the needle in the haystack. Check jacks, wiggle tubes one at a times, see if you stumble into it. Using a long speaker cable to separate the amp from the cab, even into the next room would confirm it's the amp if the static stops, but you may have already confirmed that.

 

I used an SVT a while back and it did this when on top of a cabinet. Turned out to be loose tubes. I put it on a milk crate and it was fine.

 

I know you've had yours apart before and you know about voltage/safety, etc. Can have a friend keep playing F# and poke around with a chopstick/plastic stick or something insulated and see if putting a little pressure on some connection/resistor/cap makes the noise stop....with your other hand in your pocket of course. Might save some hours ($$$) at the tech, or just waste a lot of your time. Sorry, can't be of any more help.

 

This is what you need Jimmy....a nice milk crate for it.

 

I'd go Mesa if I were you - hahaha!!!


- georgestrings

 

you might want to re-tension the tube sockets, also make sure all the screws and bolts are tight, so you have good grounding through out both chassis, also check the Molex connections between the preamp chassis and the power amp chassis

 

Sounds like a similiar issue I had with my VR....which turned out to be a tube in the preamp (V1.) I think it might have been a mica spacer within the tube that lost it's composure on certain "power" notes (F#). Might be worth a try before you get into the deep stuff.

 

Just had a similar thing a few weeks ago, where a low A from my bass, if played loud, would cause severe distorted crackle, but coming out of the PA/Keyboard amp. Turned out to be a bad tube in the vintage tube amp that was in the chain from the Hammond B3 to Mixer to PA. Dunno why there was a tube amp in that rig in the first place, but there was, and the tube was resonating at the frequency of the note I was playing.

And, ever thankful to TB, someone here suggested the tube might the problem, and they were right.

 

By resonating, I do mean something loose in the amp that just happens to vibrate along with an F#. It would do that with any speaker, but should'nt do it with the amp away from the speaker. Tech may not have caught it because the amp was on the bench and the speaker a few feet away, or hooked up to a dummy load.....no speaker at all....nothing to caus the vibration.

Most techs do play them to make sure everything sounds good, but it may not have been placed directly atop an 810, or turned up loud enough to cause it.

 

+1
Just had this with my Hiwatt head. Open E or highest octave E produced similar results. Took the back off and started taping the EL34 power tubes with the eraser end of a pencil - nothing. Sat there seemingly stumped. Thought okay maybe a pre tube? Had to remove the chassis to get the tube shields off plugged it in, hit the first one and knew what the issue was.
Ironically I thought for sure it was the 34's. Each one has been bumped around enough that the center plastic locater pin has snapped off exposing the bare glass tip in the center of the pins.

 

Hi.

The chopstick test never fails.

My vote is on a microphonic pre-amp tube as well.

You can (and IMHO should) also feed the amp ~46Hz sine while doing it, way easier -not to mention safer- than to play/have someone else to play the instrument.

Easy to do with Your computer and either on-line signal generator or with one of the freewares floating around the 'net.

Regards
Sam

 

+1 on a preamp tube, although output tubes can do it too, but it's less likely.