Enjoy the benefits of a premium membership!
Upgrade Account
-
TalkBass has been independent since 1998. Add your voice.
Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
Join free Log in
Want zero display ads or expanded classifieds tools? Compare plans.
Another weird 69 SVT Problem
- Thread starter JimmyM
- Start date
I assume that the noise is coming out the speakers, that is, it is electrical and not a mechanical vibrating noise due to something loose that is vibrating. Although a mechanical vibration can be causing an electrical problem. Unfortunately, it could be one of many things including a bad tube or component.
If you have extra tubes, the easiest solution is to swap them one at a time, test, and see what happens. Damping a tube by pinching it, thereby adding extra mass, can stop it from vibrating and help in isolating which tube has a problem. As B-String mentioned, that pesky 6C4 tube is a prime candidate.
Next, I would check for any mechanical problems that might be inducing an electrical problem. The '69 version normally has vibration dampers on the feet. The same type that holds the amp tray on a B-15. This helps isolate the amp from any vibrations from the cab. Inspect the feet and mounts or at least wiggle the feet to check them out. Then check that the power amp and pre-amp are mounted securely. That nothing is loose. As will33 said, isolate the amp from any source of vibration. Don't put it on or in front of a cab. Test the amp and see if the problem persists.
Here is an example of how a bad component can cause this sort of problem. Seans had a buzzing problem with his blueline SVT. In the power amp, he changed the 10 ohm 5W plate resistors, as well as the 22 ohm 1W screen resistors and the problem was resolved. These are wire wound resistors. Sometimes the winding can loosen and vibrate. It is sometimes simpler just to change these resistors.
If you have extra tubes, the easiest solution is to swap them one at a time, test, and see what happens. Damping a tube by pinching it, thereby adding extra mass, can stop it from vibrating and help in isolating which tube has a problem. As B-String mentioned, that pesky 6C4 tube is a prime candidate.
Next, I would check for any mechanical problems that might be inducing an electrical problem. The '69 version normally has vibration dampers on the feet. The same type that holds the amp tray on a B-15. This helps isolate the amp from any vibrations from the cab. Inspect the feet and mounts or at least wiggle the feet to check them out. Then check that the power amp and pre-amp are mounted securely. That nothing is loose. As will33 said, isolate the amp from any source of vibration. Don't put it on or in front of a cab. Test the amp and see if the problem persists.
Here is an example of how a bad component can cause this sort of problem. Seans had a buzzing problem with his blueline SVT. In the power amp, he changed the 10 ohm 5W plate resistors, as well as the 22 ohm 1W screen resistors and the problem was resolved. These are wire wound resistors. Sometimes the winding can loosen and vibrate. It is sometimes simpler just to change these resistors.
Won't be tonight. Already woke my wife up once tonight and I don't dare send amplified static through the house. She has to work at 6 am. Yikes!
I'll bet you I check it and it's the 6C4. First one I'm checking. I've bought like 6 or 7 now and not found a good one yet. So I settle for "doesn't suck as much as the others," but they all suck about the same
I'll bet you I check it and it's the 6C4. First one I'm checking. I've bought like 6 or 7 now and not found a good one yet. So I settle for "doesn't suck as much as the others," but they all suck about the same
Ok I had a similar story with just plain old 12AX7.
gonna save they boring details...
Lets just say certain preamp tubes are more susceptible to microphonics if the gain is high like usually V1 or closest to the input jacks. So out of all the crappy brand new out of the box microphonic tubes i seem to get. I have to roll them around and tap on them and try to find the least noisy one. Then stick that one in the socket that has the most gain.
otherwise out of the 6 or so 6C4 the plates might still be good but the heaters seem to fade out. So try to find the ones with the brightest heaters. and then find the best sounding ones with bright heaters.
Beans is right, on careful inspection of the 10 ohm wire wounds, I found one had a crack in the ceramic and the wire inside was vibrating, I presume.
JimmyM you can check these with a DMM in situ quite easily, if you find one reading out (mine measured 60 ohms), replace them. There is a kit on ebay for the svt, but the wire wounds are not the same spec?, I had no trouble finding the correct value, one point, there are two different lengths and one is too long if I recall
I have a military spec 6C4 in mine
Know where to get more of them? My local guy gave me his best ones. Doesn't seem to affect the tone in a bad way when they work, though. Still haven't got to take a look, but if it's beyond tubes, it's beyond me
Antique Electronics Invalid Link Removed has them for $4.50. They won't test them for low noise and microphonics.
This guy does extensive testing and will work with you if you tell him that you need a low noise, non microphonic version. He currently has 12 sets of the 6C4 (not the military version), they are sold in $10 pairs. Worth the extra cost if you can get some that work well.
BTW, the military 6C4WA is also called a 6100. Equivalents are EC90, CV4058, and M8080.
This guy does extensive testing and will work with you if you tell him that you need a low noise, non microphonic version. He currently has 12 sets of the 6C4 (not the military version), they are sold in $10 pairs. Worth the extra cost if you can get some that work well.
BTW, the military 6C4WA is also called a 6100. Equivalents are EC90, CV4058, and M8080.
^ i got the tung sol power tubes for my svtII from TCtubes and my tech said they were well matched...just an fyi
Arghhhh..... 6C4....... 6C4WA, whatever.....
That was the single most troublesome tube in the whole SVT-HD run..... maybe the most troublesome PART. They were made to be radio tubes, not audio, and it was heck finding enough "audio-good" ones to get through the run.
Might not be your problem, though, we mostly just had noise issues, not static-noise.
Static-noise can be lots of things..... mostly of interest to techie-types.
Noisy tube
A flickery plate resistor ("Fenderitis")
Ditto but with a plate supply dropping resistor (usually a bit more muffled)
A leaky, but not yet totally "BAD" coupling capacitor.
etc........
These things can be triggered by the voltage applied (signal level) and with coupling capacitors, can be responsive to frequency.
That was the single most troublesome tube in the whole SVT-HD run..... maybe the most troublesome PART. They were made to be radio tubes, not audio, and it was heck finding enough "audio-good" ones to get through the run.
Might not be your problem, though, we mostly just had noise issues, not static-noise.
Static-noise can be lots of things..... mostly of interest to techie-types.
Noisy tube
A flickery plate resistor ("Fenderitis")
Ditto but with a plate supply dropping resistor (usually a bit more muffled)
A leaky, but not yet totally "BAD" coupling capacitor.
etc........
These things can be triggered by the voltage applied (signal level) and with coupling capacitors, can be responsive to frequency.
Know where to get more of them? My local guy gave me his best ones. Doesn't seem to affect the tone in a bad way when they work, though. Still haven't got to take a look, but if it's beyond tubes, it's beyond me
I think mines a CV133 Brimar or Mullard, not sure on stocks though.
The wire wounds are an easy test, you should get a reading around 8-12 ohms, if not then replace.
For folk who are interested the 6C4 is equivalent to half a 12AU7. If anyone builds a clone pre-amp they can be rid of the 6C4.
