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SVT TUBE AMP PROBLEM

ericmonster_tm

ericmonster_tm

Sep 29, 2006
239
2
Oklahoma City
#1
Howdy.

I got an SVT-HD. It is giving me problems.

It was fine at last practice (march 28th), Today I turnbed on the amp and heard what sounds like a distorted sinewave.

When I play through it, it sounds like there is a very heavy chorus. The real tone of the bass is buried behind the nosie and mud.

I checked all my cables, all my switches to make sure it wasnt a cable or Impedance issue. We then tried plugging the amp in to guitar cab with the same impedance and the noise still came through.

We then tried plugging a guitar into the amp, same noise. We tried plugging it into another outlet. Same noise.

We took it apart and checked all the tubes. They all glow orange and look normal, pre-amp tubes included.

I tried changing all preamp tubes. But the noise comes on when I bring it out of standby.

Anyone handy with troubleshooting? Hopefully its not a transformer issue. Maybe a bad capacitor?

Anyone?
 
#5
i_got_a_mohawk said:
Does sound like a microphonic tube.

Try gently tapping all the tubes with a pencil while the amp is on and see if any of them ring out more.

Just because the tubes are glowing doesnt mean they are working. Just means the heater element is ;)

Good luck!
Click to expand...

By tapping on the tubes, we should hear the "ring" through the cab? Or is the sound heard through the tube.

Let me make sure I understand this.

1. Turn the amp on.

2. Take it out of standby.

3. Tap on the tubes with a pencil and listen for a "ring."

Is this correct?
 
#7
hard to say exactly....... microphonic tube maybe........ or possibly something has come loose in teh feedback and it is oscillating at a high frequency......... which suppresses your sound and makes it sound weird, like something is "pressing down" on the sound is the best way I can describe it. Usually cause a lot of power draw, and makes the tubes heat up, often ruining them in a matter of minutes.

When you say glowing orange, I hole you JUST mean the filaments in the very middle of the tube. If the larger metal parts inside the tube look orange or red, immediately turn it off and take to a tech.
 
#8
Jerrold Tiers said:
hard to say exactly....... microphonic tube maybe........ or possibly something has come loose in teh feedback and it is oscillating at a high frequency......... which suppresses your sound and makes it sound weird, like something is "pressing down" on the sound is the best way I can describe it. Usually cause a lot of power draw, and makes the tubes heat up, often ruining them in a matter of minutes.

When you say glowing orange, I hole you JUST mean the filaments in the very middle of the tube. If the larger metal parts inside the tube look orange or red, immediately turn it off and take to a tech.
Click to expand...

Yeah just the filaments.

There is a 6C4 and 4 12AX7'S. I turned the amp on, brought it out of standby and tapped on all the pre-amp tubes. The 6C4 on the far left made noise, the 12AX7 on the far right sounded like I had a mic on my amp. So I tried replacing the 12AX7 on the far right.

Same noise. Sounds like a distorted sinewave. Tapped it with a pencil. Same noise.

So then I decided to tap on all the pre-amp tubes. I could hear it albeit faintly on the 3 in the middle, but I can hear the ring through the cab.

Is it possible something is fried in the preamp circuit? Bad resistor?

I just retubed it with new preamp tubes, all but the 6C4 since no one has them in stock here.

I think Ill just take it to the doc.
 
#10
ericmonster_tm said:
Yeah just the filaments.

There is a 6C4 and 4 12AX7'S. I turned the amp on, brought it out of standby and tapped on all the pre-amp tubes. The 6C4 on the far left made noise, the 12AX7 on the far right sounded like I had a mic on my amp. So I tried replacing the 12AX7 on the far right..
Click to expand...

Doesn't that use 4 12dw7's?
Ther rest should be a 6C4 ,2 12 bh7's a single 12ax7 and the the 6 power tubes. Or is the HD different?
 
#11
The HD was different.

(IIRC the HD is the one produced in the 80s, limited (and numbered) to 500).

The circuit was altered to use 12AX7s in place of 12DW7s because during the 80's there was no (or next to no) major manufacture of 12DW7 tubes. Other than that, it shared a very similar design to the original SVT.

(I think that's how it went anyways, if I'm wrong, feel free to correct me)
 
#13
That 6C4 is an issue..... we had a heck of a time finding ones that were not noisy as anything. The good news is that those we used were NOS JAN 6C4WA and should last a long time.

Yes, the 12AX7 mod...... dates back to the transition to MCI. IIRC you can change some resistors and use the 12DW7. I have the mod document somewhere.

Edit... that would be "MTI"................ not "MCI"... fat fingered it
 
#14
What about the first gen SVT 4? The problems sound the same but all of the tubes are 12ax7's. Everything looks the same as the current SVT 4, controls, outputs, inputs, etc. What do you think, bad cap or resistor or pre-amp tube? I took it in to an Ampeg service center and I'm waiting for an estimate but I'll be pissed if it is a tube!
 
#15
Did you not try swapping a new and (assumably) good 12AX7 in and out of the various slots to see if it was a tube issue?

And just because it looks the same on the outside, doesnt mean it looks the same on the inside ;)

If you are or were having problems with your SVT-4 you'd maybe be better off starting a new thread about it?
 
#16
i_got_a_mohawk said:
The HD was different.

(IIRC the HD is the one produced in the 80s, limited (and numbered) to 500).

The circuit was altered to use 12AX7s in place of 12DW7s because during the 80's there was no (or next to no) major manufacture of 12DW7 tubes. Other than that, it shared a very similar design to the original SVT.

(I think that's how it went anyways, if I'm wrong, feel free to correct me)
Click to expand...
I did not know this . Thanks for the info.
Bill
 
#19
Jerrold Tiers said:
That 6C4 is an issue..... we had a heck of a time finding ones that were not noisy as anything. The good news is that those we used were NOS JAN 6C4WA and should last a long time.
Click to expand...

I called every guitar in my city and no one has a 6C4 in stock. I even tried hunting down some antique radio supply stores, but none appear to exist.

I was tryign to get ti fixed before my gig, TONIGHT. But Ill have to borrow a friends SVT-450.

I went ahead and ordered 1 NOS JAN 6C4WA and 4 JJ ECC83-S.

I'm gonna replace them all and see if the sinewave goes away. I didnt get a chance to replace. If not, then into the shop she goes along with part of my paycheck!

And for the record. I'm not a fan of the SVT-450. I have to damn near turn it all the way up! Never turned my SVT up past 3.
 

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