1969 SVT hum problem

JimmyM · #1 10-10-2011, 08:32 PM

Welp, my SVT developed a problem at a gig a couple weeks ago, and I got it back from the shop today unfixed, so I will turn to you guys and ask what you think...

During soundcheck, it worked fine, and then after a break I went back to tune, and it started to make a low level humming sound. Wasn't too awful so I did the gig with it anyway, and took it to the shop last week. I get a call that the 12DW7 for the midrange control and the two 12BH7 driver tubes were testing poorly. So I bought NOS replacements and took them to the guy, and we tried it out, and I thought I still heard the humming, but could allow for the possibility that it was just something in his shop causing it. He said to bring it back if it was still not working right.

So I tried it, and it's still not working right. I believe it is, indeed, tied to the midrange, as when I put the selector in the number 1 position (I forget the freq center but it's the lowest), I got a huge hum when I turned the mids knob up all the way, and a slightly quieter hum when I turned it all the way down. I switched all my 12DW7's around to see if I could find another culprit, and no matter what order they're in, they all do it. I also tried swapping 6C4's with a couple I had here, and no difference with one and one sounded like I should have thrown it out. I also cleaned all the tube sockets and switches with Deoxit, and no difference, although I did manage to make the midrange selector switch start popping when I switched it. It was loud at first but got quieter, but still never went away.

This amp was fully restored 3 years ago, with every single tube and cap replaced. My tech tested every tube, and those 3 tubes were the only tubes that tested poorly, and other than that hum, the amp still sounds great. So if anyone has any idea what might have gone ka-flooey, I'm all ears. If any techs who are still up would rather discuss it by phone than type out a long post (I'll probably be up till 1 am EST), PM me and I'll give you my phone number.

Thank you in advance very much!

JGR · #2 10-10-2011, 08:42 PM

Sounds like a grounding issue. You can get hum from a loose ground connection or running sensitive ground wires near high voltage. Being that you gig a lot, I would bet something got loose/intermittent/dirty. I know first hand how frustrating it can be chasing this stuff down. My pre has a lot of stuff crammed in a small space. Tech should fire it up and start probing with the chopstick.

johnk_10 · #3 10-10-2011, 08:43 PM

since it is effected by the mid control alot, it sounds like you have it narrowed down to the preamp (so that wouldn't have anything to do with the 12BH7's, but to test it, come out of the extension amp (which can be used as a preamp out) to another amp (pad the input) and see if it's humming the same. if it is, it's inthe preamp.
you can also turn the SVT's preamp all the way down (all knobs fully counterclockwise, and insert another amp's preamp into the extension amp jack and check the SVT's poweramp. the extension amp jacks on a '69 aren't really a send/receive like on modern amps, but rather just a tap to come out of the preamp before the phase inverter, or to insert a signal into the power amp.

JimmyM · #4 10-10-2011, 08:46 PM

Thx Gustav

I have a cheap Radio Shack multimeter and some time...wouldn't know how to check for it, though. Is this something a dolt like me could check?

And thx John...I'll give it a shot. Do I have to have a speaker load plugged into the SVT to check if the pre's the problem?

EDIT: Dumb question...never mind. My mind just goes all haywire when one of these amps isn't working right

JGR · #5 10-10-2011, 09:02 PM

Jimmy,

There is still high voltage in the pre, so it's probably better you take it somewhere. I use something non conductive to probe when the amp is on, but you need to be careful. One thing you can do with the amp off (discharge caps first) is to check for ground continuity at all the various points, but it can be misleading, and it will likely need to be chased down with the amp on with a cab so you can hear when you find the culprit. Your safety is paramount!

**B-string** · #6 10-10-2011, 09:06 PM

Quote:

Originally Posted by JimmyM

Thx Gustav

I have a cheap Radio Shack multimeter and some time...wouldn't know how to check for it, though. Is this something a dolt like me could check?

And thx John...I'll give it a shot. Do I have to have a speaker load plugged into the SVT to check if the pre's the problem?

EDIT: Dumb question...never mind. My mind just goes all haywire when one of these amps isn't working right

YES.

and yeah I understand

There are two 220K resistors going to ground from the midrange multi tapped choke. With power off preamp unplugged (yada yada) check those two with your DVM.

Jerrold Tiers · #7 10-10-2011, 09:08 PM

If the mid still WORKS, the problem is earlier, most likely.

If the mid does NOT work, the mid coil could be open

johnk_10 · #8 10-10-2011, 09:11 PM

Quote:

Originally Posted by JimmyM

And thx John...I'll give it a shot. Do I have to have a speaker load plugged into the SVT to check if the pre's the problem?

there are no dumb questions. and yes, the SVT should always be connected to a load when it's on.

johnk_10 · #9 10-10-2011, 09:14 PM

BTW, here's the schematic for the power amp. if you look at the top left part of it, you'll see the ext amp insert jacks (they're in parallel).

JimmyM · #10 10-10-2011, 09:17 PM

Just checked and the noise is there in both the pre and power amp.

Jerrold, the mid does work. It just gets really loud with the hum when you crank it in either direction and have it on position 1, although it didn't seem so noticeable this time with it all the way down.

JimmyM · #11 10-10-2011, 09:21 PM

Quote:

Originally Posted by B-string

[/b]

YES.

and yeah I understand

There are two 220K resistors going to ground from the midrange multi tapped choke. With power off preamp unplugged (yada yada) check those two with your DVM.

Eh, Gustav has me all scared now

I really appreciate all the advice from all of you...I guess I can';t fix it but at least I'll know what to tell my tech. Got to have it for a gig on Tuesday because Peter Noone's bassist has to have an SVT, even though he plays keyboard bass.

**B-string** · #12 10-10-2011, 09:22 PM

You loose the ground to the hum balance pot, or the pot itself fail (100 ohm pot).

**B-string** · #13 10-10-2011, 09:25 PM

Quote:

Originally Posted by JimmyM

Eh, Gustav has me all scared now

I really appreciate all the advice from all of you...I guess I can';t fix it but at least I'll know what to tell my tech.

Between Johnk ,JT and the others here it should get sorted shortly.

You be in good hands.

JimmyM · #14 10-10-2011, 09:27 PM

Quote:

Originally Posted by B-string

You loose the ground to the hum balance pot, or the pot itself fail (100 ohm pot).

Ya, didn't seem to make any difference when I tried it earlier. That sounds like it would affect it system wide, too. Possible good call, Jon! Appreciate it.

This sounds like something I could check myself. OK, so I got this meter. What do I set it to and what do I touch?

**B-string** · #15 10-10-2011, 09:38 PM

Power amp chassis out of the case, tubes removed first (for safety of course don't want broken glass). The center lug on the pot should read less than 0.5 ohms with your meter set to its lowest omhs scale. To check the pot itself you will need to disconnect two leads from the pot (you want to read the pot not the filiment winding of the power trans) it should read 100 ohms +/- probably 10% so 90 to 110 ohms.

**B-string** · #16 10-10-2011, 09:39 PM

While you are at it check from the chassis to the ground lug on the power cord! Should also read less than 0.5 ohms prefer the same reading you would get with your meter leads shorted to each other on the lowest scale.

johnk_10 · #17 10-10-2011, 09:57 PM

i know that you said that the caps were replaced, but i'm kinda wondering about the condition of filter cap C23 (40uf/450V)? (the big blue one on the right.

i'd just check all of the grounds in the preamp first.

you can also try pulling preamp tubes one at a time until you find which one that makes it go away it goes away.

do you have any spare 12DW7's? just rotating the ones in there might not be very helpful in this case.

svtb15 · #18 10-10-2011, 10:01 PM

Jimmy, my 73 is doing the EXACT same thing... Im trying to figure out what it is too. mine has the 12BH7 's and 12DW7 too.
I have already done a complete recap and retube. still there.

Quote:

Originally Posted by JimmyM

Welp, my SVT developed a problem at a gig a couple weeks ago, and I got it back from the shop today unfixed, so I will turn to you guys and ask what you think...

During soundcheck, it worked fine, and then after a break I went back to tune, and it started to make a low level humming sound. Wasn't too awful so I did the gig with it anyway, and took it to the shop last week. I get a call that the 12DW7 for the midrange control and the two 12BH7 driver tubes were testing poorly. So I bought NOS replacements and took them to the guy, and we tried it out, and I thought I still heard the humming, but could allow for the possibility that it was just something in his shop causing it. He said to bring it back if it was still not working right.

So I tried it, and it's still not working right. I believe it is, indeed, tied to the midrange, as when I put the selector in the number 1 position (I forget the freq center but it's the lowest), I got a huge hum when I turned the mids knob up all the way, and a slightly quieter hum when I turned it all the way down. I switched all my 12DW7's around to see if I could find another culprit, and no matter what order they're in, they all do it. I also tried swapping 6C4's with a couple I had here, and no difference with one and one sounded like I should have thrown it out. I also cleaned all the tube sockets and switches with Deoxit, and no difference, although I did manage to make the midrange selector switch start popping when I switched it. It was loud at first but got quieter, but still never went away.

This amp was fully restored 3 years ago, with every single tube and cap replaced. My tech tested every tube, and those 3 tubes were the only tubes that tested poorly, and other than that hum, the amp still sounds great. So if anyone has any idea what might have gone ka-flooey, I'm all ears. If any techs who are still up would rather discuss it by phone than type out a long post (I'll probably be up till 1 am EST), PM me and I'll give you my phone number.

Thank you in advance very much!

**B-string** · #19 10-10-2011, 10:04 PM

I think he fell asleep on us John

JimmyM · #20 10-10-2011, 10:05 PM

Jon, looks like you called it. There is a black ground wire that should be soldered to the hum control pot that's totally off. This actually looks like a job I can handle! One solder, should be good to go...

Fell asleep? No...taking apart those SVT's is a bitch!