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I got tube problems with my ampeg svt 2

Jake Kinsch

Jake Kinsch

Jun 9, 2015
71
33
Nashville
#1
I have an Ampeg SVT-2 pro that I am trying out to see if I want to buy it. It has only 1 problem and I'm thinking I'll need to re-tube the power section but maybe one of you can help me diagnose a little better. I'm playing along at a reasonable loud volume (master at 1 o'clock) through an ampeg 6x10 cab. I hit a note hard and it makes a bit of a *crack* sound and then stops making sound. The amp is still on but the light that is next to the graphic eq flashes green. I looked this up in the manual and it says that indicates an output tube fault. I take off the hood and make sure they are all seated correctly and even shuffle around their order. I turn out the lights to watch the blue glow. I play some more and one of the tubes turns completely blue and then it powers down same as before. Do I have to replace all of the output tubes? Or just that one? Thanks.
 
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#2
I had something similar happen with my SVT-CL. One output tube was arcing internally. In my case the tube lit up with a kind of pink-purple glow when it faulted like this. Based on my experience, it looked like that tube was just a little gassy (not a good enough vacuum to withstand SVT plate voltage).

I had bought a matched set from a reputable dealer, and was able to replace just that tube with a matched one. Not too long after that, another tube from the original set did much the same thing. I ended up replacing them all.

When the second tube went out, it took out a screen resistor. Would have been a somewhat costly diagnosis and repair, except that fortunately I'm qualified to do it myself.

What kind of tubes do you have? Are they old or new? And is the speaker impedance correct for the amp (2 or 4 ohms, with the amp set correctly)?
 
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#4
Jake Kinsch said:
I have an Ampeg SVT-2 pro that I am trying out to see if I want to buy it. It has only 1 problem and I'm thinking I'll need to re-tube the power section but maybe one of you can help me diagnose a little better. I'm playing along at a reasonable loud volume (master at 1 o'clock) through an ampeg 6x10 cab. I hit a note hard and it makes a bit of a *crack* sound and then stops making sound. The amp is still on but the light that is next to the graphic eq flashes green. I looked this up in the manual and it says that indicates an output tube fault. I take off the hood and make sure they are all seated correctly and even shuffle around their order. I turn out the lights to watch the blue glow. I play some more and one of the tubes turns completely blue and then it powers down same as before. Do I have to replace all of the output tubes? Or just that one? Thanks.
Click to expand...


Whenever I have a tube start to go bad, I frequently just replace it. But I have a basement full of amps, a box full of old tubes, and a meter that allows me to measure how much current is passing through each tube. So when a tube fails, I try to rotate old tubes into the position until I find one that's a good current match to the other tubes. If none of the old tubes is a reasonable current match, then I buy a new set of tubes, and any good tubes from the amp go in the box for the future use.

I also have background as an electronic technician, so when a tube fails I am qualified to troubleshoot and repair the amp.

The amp may just need to have the tube sockets retensioned. When you play a loud note and the amp is sitting on top of the speaker, the vibration can cause intermittent connections if the pins in the tube socket are a bit loose.

I put a brand new set of tubes in one of my amps and played it on the bench for about two weeks and it worked perfectly. When I put it on a speaker, the HV fuse opened. The first time this happened, amp only made a very slight zzt sound that I could barely hear. I put the amp back on the bench and checked all the operating voltages and everything looked good. The next time I tried the amp on the speaker, the amp emitted a bright flash of light and a sound like a cannon shot. I bought a dental pick and retensioned the tube sockets and the amps has been fine since. FYI, it is necessary to drain the filter caps before doing this procedure as the amp contains lethal voltages. My advice is work out a deal with the seller to send the amp to a tech.
 
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#6
rickdog said:
I had something similar happen with my SVT-CL. One output tube was arcing internally. In my case the tube lit up with a kind of pink-purple glow when it faulted like this. Based on my experience, it looked like that tube was just a little gassy (not a good enough vacuum to withstand SVT plate voltage).

I had bought a matched set from a reputable dealer, and was able to replace just that tube with a matched one. Not too long after that, another tube from the original set did much the same thing. I ended up replacing them all.

When the second tube went out, it took out a screen resistor. Would have been a somewhat costly diagnosis and repair, except that fortunately I'm qualified to do it myself.

What kind of tubes do you have? Are they old or new? And is the speaker impedance correct for the amp (2 or 4 ohms, with the amp set correctly)?
Click to expand...
The tube are EHX 6550s, and I'm unsure if they are the origionals that came with the amp. I've had the impedance set at 4 ohms playing through a 4 ohm cab
 
#7
Here's the way I look at prices of used tube amps. I factor in the price the seller is wanting plus $200. That allows my brain to get ready to take it to a qualified tech the moment I buy it.

I bought a late SLM era SVT-CL a couple years ago. Off it went to Mad Science Works in Raleigh for a few weeks. He cleaned it up nice, changed out a few caps, and even put an aluminum plate in it because the amp was picking up some sort of interference.

The tech's bill came in under the $200 I had budgeted. And I take that amp out with complete confidence.
 
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#8
Jake Kinsch said:
The tube are EHX 6550s, and I'm unsure if they are the origionals that came with the amp. I've had the impedance set at 4 ohms playing through a 4 ohm cab
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In my experience EHX 6550 and KT88s are unreliable. From the reading I have done, several of the New Sensor designed 6550s and KT88s tend to have problems with anything approaching 700 volts on the plates, and SVTs run the plates just under 700V.

I have a bunch of amps that I bought used. Four of them came with EHX 6550s or KT88s. The EHX tubes either didn't function properly when I got the amp, or failed shortly thereafter.

Although they are not perfect, I have had way better results with JJ KT88s. I don't have a recommendation for 6550s. I'll try a set of JJ 6550s when my 87 SVT needs retubed. If that doesn't work out, I'll put a set of JJ KT88s in it.
 
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#9
Wasnex said:
...From the reading I have done, several of the New Sensor designed 6550s and KT88s tend to have problems with anything approaching 700 volts on the plates, and SVTs run the plates just under 700V.
...
Although they are not perfect, I have had way better results with JJ KT88s. I don't have a recommendation for 6550s. I'll try a set of JJ 6550s when my 87 SVT needs retubed. If that doesn't work out, I'll put a set of JJ KT88s in it.
Click to expand...

The tubes that failed in my amp were Tung-sol reissues, are they made by New Sensor?

I have JJ KT88s in it now. That amp doesn't see a lot of use because I don't often need that kind of brute power, but on the occasions I've had it out for outdoor gigs with no PA support it has sounded good enough to get noticed by my bandmates, and I haven't had any further trouble. Last I knew, Ampeg was shipping the JJ 6550s in the Heritage SVT-CL, so they can't be too bad.
 
#10
rickdog said:
The tubes that failed in my amp were Tung-sol reissues, are they made by New Sensor?

I have JJ KT88s in it now. That amp doesn't see a lot of use because I don't often need that kind of brute power, but on the occasions I've had it out for outdoor gigs with no PA support it has sounded good enough to get noticed by my bandmates, and I haven't had any further trouble. Last I knew, Ampeg was shipping the JJ 6550s in the Heritage SVT-CL, so they can't be too bad.
Click to expand...

Tung Sol, Mullard, Genalex Gold Lion, Electro Harmonix, Sovtek, and Svetlana are all New Sensor brands.

Link Removed

I am sure the JJ 6550 is a decent tube, but the data sheet shows the plate voltage limit as 600V. JJ Electronic - 6550

KT88 shows a plate voltage limit of 800V. JJ Electronic - KT88
 
#12
Wasnex said:
Tung Sol, Mullard, Genalex Gold Lion, Electro Harmonix, Sovtek, and Svetlana are all New Sensor brands.

Link Removed

I am sure the JJ 6550 is a decent tube, but the data sheet shows the plate voltage limit as 600V. JJ Electronic - 6550
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My Heritage SVT-CL came with JJ's and I still have all the original tubes after 6 years, and it puts out as much volume as my 69 SVT with 6146b's.
 
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#13
JimmyM said:
My Heritage SVT-CL came with JJ's and I still have all the original tubes after 6 years, and it puts out as much volume as my 69 SVT with 6146b's.
Click to expand...

That's good to hear. I remember reading a couple of threads where various users were only getting about 4 months of heavy use out of JJ 6550s and had decided to sell their SVTs.

I don't know if there is a better current production 6550 available. I believe the Sovtek 6550WE are considered tough. These are an older New Sensor design that the company actually produced a datasheet for. The plate voltage limit is listed as 800V and plate dissipation is listed as 42W, which I believe is higher than any other 6550. If I were going to run a New Sensor output tube in an SVT, this is probably the one I would chose.
 
#14
Pretty much why I sold all my SVTs.

Under regular use they eat tubes.

I wouldn't buy new set, if few are bad replace them. Just good tech needs to know where the others sit. And balance the output section

I wouldn't just retube the head and hope for the best. A good tech needs to go through it. Meaning really good tech.

Paying very special attention to the power tube board. Socket pins get loose or connections need touch up.
All the screen resistors should be checked even replaced with higher values if needed according to Ampeg service bulletins.
Basically they raised the value of those resistors so they don't eat tubes as fast.

The tube fault circuit is a great feature. But the cathode resistors are well known to drift and give false readings. Change them ..all of them.

Not a super big deal for a competent tech. After little touch up, few resistors and quick balance of the tubes and bias. Your up and running. Yee hah

At home , at practice there totally fine
Screen resistors only blow in the middle of shows lol. Totally good

Mesa factory tubes and JJs are actually known good survivors. For tube eaters. I always used Mesa tubes in my Traynor musicman tube eaters. And used JJs in my Fender UL tube eaters. And had very good success
 
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#16
I bought an SVT 2 Pro a while back and had some issues with it - I ended up having to replace a few tubes, including the power amp driver tubes and some power tubes. The seller shipped the amp with the tubes in it and I thought several tubes had been damaged in shipping. I learned that the power tubes can be replaced in sets of 3 if you can't find a one-off match (the bias controls in the back each control 3 our of the 6 power tubes). So, if you do have a bad power tube, you can replace three at a time instead of all 6.

My issue ended up being a bad speaker cable. FWIW, I had a bunch of JJ's here that I installed and have been very happy. Sounds great and no issues.
 
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